# SCons User Guide 0.98.0

### Steven Knight

Copyright © 2004, 2005, 2006, 2007 Steven Knight

> SCons User's Guide Copyright (c) 2004, 2005, 2006, 2007 Steven Knight

List of Examples
D-1. Wildcard globbing to create a list of filenames
D-2. Filename extension substitution
D-3. Appending a path prefix to a list of filenames
D-4. Substituting a path prefix with another one
D-5. Filtering a filename list to exclude/retain only a specific set of extensions
D-6. The "backtick function": run a shell command and capture the output

# Preface

Thank you for taking the time to read about SCons. SCons is a next-generation software construction tool, or make tool--that is, a software utility for building software (or other files) and keeping built software up-to-date whenever the underlying input files change.

The most distinctive thing about SCons is that its configuration files are actually scripts, written in the Python programming language. This is in contrast to most alternative build tools, which typically invent a new language to configure the build. SCons still has a learning curve, of course, because you have to know what functions to call to set up your build properly, but the underlying syntax used should be familiar to anyone who has ever looked at a Python script.

Paradoxically, using Python as the configuration file format makes SCons easier for non-programmers to learn than the cryptic languages of other build tools, which are usually invented by programmers for other programmers. This is in no small part due to the consistency and readability that are built in to Python. It just so happens that making a real, live scripting language the basis for the configuration files makes it a snap for more accomplished programmers to do more complicated things with builds, as necessary.

## 1. SCons Principles

There are a few overriding principles we try to live up to in designing and implementing SCons:

Correctness

  First and foremost, by default, SCons guarantees a correct build even if it means sacrificing performance a little. We strive to guarantee the build is correct regardless of how the software being built is structured, how it may have been written, or how unusual the tools are that build it.

Performance

  Given that the build is correct, we try to make SCons build software as quickly as possible. In particular, wherever we may have needed to slow down the default SCons behavior to guarantee a correct build, we also try to make it easy to speed up SCons through optimization options that let you trade off guaranteed correctness in all end cases for a speedier build in the usual cases.

Convenience

  SCons tries to do as much for you out of the box as reasonable, including detecting the right tools on your system and using them correctly to build the software.

In a nutshell, we try hard to make SCons just "do the right thing" and build software correctly, with a minimum of hassles.

## 2. A Caveat About This Guide's Completeness

One word of warning as you read through this Guide: Like too much Open Source software out there, the SCons documentation isn't always kept up-to-date with the available features. In other words, there's a lot that SCons can do that isn't yet covered in this User's Guide. (Come to think of it, that also describes a lot of proprietary software, doesn't it?)

Although this User's Guide isn't as complete as we'd like it to be, our development process does emphasize making sure that the SCons man page is kept up-to-date with new features. So if you're trying to figure out how to do something that SCons supports but can't find enough (or any) information here, it would be worth your while to look at the man page to see if the information is covered there. And if you do, maybe you'd even consider contributing a section to the User's Guide so the next person looking for that information won't have to go through the same thing...?

## 3. Acknowledgements

SCons would not exist without a lot of help from a lot of people, many of whom may not even be aware that they helped or served as inspiration. So in no particular order, and at the risk of leaving out someone:

First and foremost, SCons owes a tremendous debt to Bob Sidebotham, the original author of the classic Perl-based Cons tool which Bob first released to the world back around 1996. Bob's work on Cons classic provided the underlying architecture and model of specifying a build configuration using a real scripting language. My real-world experience working on Cons informed many of the design decisions in SCons, including the improved parallel build support, making Builder objects easily definable by users, and separating the build engine from the wrapping interface.

Greg Wilson was instrumental in getting SCons started as a real project when he initiated the Software Carpentry design competition in February 2000. Without that nudge, marrying the advantages of the Cons classic architecture with the readability of Python might have just stayed no more than a nice idea.

The entire SCons team have been absolutely wonderful to work with, and SCons would be nowhere near as useful a tool without the energy, enthusiasm and time people have contributed over the past few years. The "core team" of Chad Austin, Anthony Roach, Charles Crain, Steve Leblanc, Gary Oberbrunner, Greg Spencer and Christoph Wiedemann have been great about reviewing my (and other) changes and catching problems before they get in the code base. Of particular technical note: Anthony's outstanding and innovative work on the tasking engine has given SCons a vastly superior parallel build model; Charles has been the master of the crucial Node infrastructure; Christoph's work on the Configure infrastructure has added crucial Autoconf-like functionality; and Greg has provided excellent support for Microsoft Visual Studio.

Special thanks to David Snopek for contributing his underlying "Autoscons" code that formed the basis of Christoph's work with the Configure functionality. David was extremely generous in making this code available to SCons, given that he initially released it under the GPL and SCons is released under a less-restrictive MIT-style license.

Thanks to Peter Miller for his splendid change management system, Aegis, which has provided the SCons project with a robust development methodology from day one, and which showed me how you could integrate incremental regression tests into a practical development cycle (years before eXtreme Programming arrived on the scene).

And last, thanks to Guido van Rossum for his elegant scripting language, which is the basis not only for the SCons implementation, but for the interface itself.

## 4. Contact

The best way to contact people involved with SCons, including the author, is through the SCons mailing lists.

If you want to ask general questions about how to use SCons send email to `users@scons.tigris.org`.

If you want to contact the SCons development community directly, send email to `dev@scons.tigris.org`.

If you want to receive announcements about SCons, join the low-volume `announce@scons.tigris.org` mailing list.

# Chapter 1. Building and Installing SCons

This chapter will take you through the basic steps of installing SCons on your system, and building SCons if you don't have a pre-built package available (or simply prefer the flexibility of building it yourself). Before that, however, this chapter will also describe the basic steps involved in installing Python on your system, in case that is necessary. Fortunately, both SCons and Python are very easy to install on almost any system, and Python already comes installed on many systems.

## 1.1. Installing Python

Because SCons is written in Python, you must obviously have Python installed on your system to use SCons Before you try to install Python, you should check to see if Python is already available on your system by typing `python` at your system's command-line prompt. You should see something like the following on a UNIX or Linux system that has Python installed:

           $ python
           Python 2.2.2 (#1, Feb 24 2003, 19:13:11)
           [GCC 3.2.2 20030222 (Red Hat Linux 3.2.2-4)] on linux2
           Type "help", "copyright", "credits" or "license" for more information.
           >>> ^D

And on a Windows system with Python installed:

           C:\>python
           Python 2.2.2 (#34, Apr 9 2002, 19:34:33) [MSC 32 bit (Intel)] on win32
           Type "help", "copyright", "credits" or "license" for more information.
           >>> ^Z

The `>>>` is the input prompt for the Python interpreter. The `^D` and `^Z` represent the CTRL-D and CTRL-Z characters that you will need to type to get out of the interpreter before proceeding to installing SCons.

If Python is not installed on your system, you will see an error message stating something like "command not found" (on UNIX or Linux) or "'python' is not recognized as an internal or external command, operable progam or batch file" (on Windows). In that case, you need to install Python before you can install SCons.

The standard location for information about downloading and installing Python is http://www.python.org/download/. See that page for information about how to download and install Python on your system.

## 1.2. Installing SCons From Pre-Built Packages

SCons comes pre-packaged for installation on a number of systems, including Linux and Windows systems. You do not need to read this entire section, you should only need to read the section appropriate to the type of system you're running on.

### 1.2.1. Installing SCons on Red Hat (and Other RPM-based) Linux Systems

SCons comes in RPM (Red Hat Package Manager) format, pre-built and ready to install on Red Hat Linux, Fedora Core, or any other Linux distribution that uses RPM. Your distribution may already have an SCons RPM built specifically for it; many do, including SuSe, Mandrake and Fedora. You can check for the availability of an SCons RPM on your distribution's download servers, or by consulting an RPM search site like http://www.rpmfind.net/ or http://rpm.pbone.net/.

If your Linux distribution does not already have a specific SCons RPM file, you can download and install from the generic RPM provided by the SCons project. This will install the SCons script(s) in `/usr/bin`, and the SCons library modules in `/usr/lib/scons`.

To install from the command line, simply download the appropriate `.rpm` file, and then run:

            # rpm -Uvh scons-0.96-1.noarch.rpm

Or, you can use a graphical RPM package manager like gnorpm. See your package manager application's documention for specific instructions about how to use it to install a downloaded RPM.

### 1.2.2. Installing SCons on Debian Linux Systems

Debian Linux systems use a different package management format that also makes it very easy to install SCons.

If your system is connected to the Internet, you can install the latest official Debian package by running:

            # apt-get install scons

### 1.2.3. Installing SCons on Windows Systems

SCons provides a Windows installer that makes installation extremely easy. Download the `scons-0.95.win32.exe` file from the SCons download page at http://www.scons.org/download.html. Then all you need to do is execute the file (usually by clicking on its icon in Windows Explorer). These will take you through a small sequence of windows that will install SCons on your system.

## 1.3. Building and Installing SCons on Any System

If a pre-built SCons package is not available for your system, then you can still easily build and install SCons using the native Python `distutils` package.

The first step is to download either the `scons-0.98.0.tar.gz` or `scons-0.98.0.zip`, which are available from the SCons download page at http://www.scons.org/download.html.

Unpack the archive you downloaded, using a utility like tar on Linux or UNIX, or WinZip on Windows. This will create a directory called `scons-0.98.0`, usually in your local directory. Then change your working directory to that directory and install SCons by executing the following commands:

          # cd scons-0.98.0
          # python setup.py install

This will build SCons, install the scons script in the default system scripts directory (`/usr/local/bin` or `C:\Python2.2\Scripts`), and will install the SCons build engine in an appropriate stand-alone library directory (`/usr/local/lib/scons` or `C:\Python2.2\scons`). Because these are system directories, you may need root (on Linux or UNIX) or Administrator (on Windows) privileges to install SCons like this.

### 1.3.1. Building and Installing Multiple Versions of SCons Side-by-Side

The SCons `setup.py` script has some extensions that support easy installation of multiple versions of SCons in side-by-side locations. This makes it easier to download and experiment with different versions of SCons before moving your official build process to a new version, for example.

To install SCons in a version-specific location, add the `--version-lib` option when you call `setup.py`:

            # python setup.py install --version-lib

This will install the SCons build engine in the `/usr/lib/scons-0.98.0` or `C:\Python2.2\scons-0.98.0` directory, for example.

If you use the `--version-lib` option the first time you install SCons, you do not need to specify it each time you install a new version. The SCons `setup.py` script will detect the version-specific directory name(s) and assume you want to install all versions in version-specific directories. You can override that assumption in the future by explicitly specifying the `--standalone-lib` option.

### 1.3.2. Installing SCons in Other Locations

You can install SCons in locations other than the default by specifying the `--prefix=` option:

            # python setup.py install --prefix=/opt/scons

This would install the scons script in `/opt/scons/bin` and the build engine in `/opt/scons/lib/scons`,

Note that you can specify both the `--prefix=` and the `--version-lib` options at the same type, in which case `setup.py` will install the build engine in a version-specific directory relative to the specified prefix. Adding `--version-lib` to the above example would install the build engine in `/opt/scons/lib/scons-0.98.0`.

### 1.3.3. Building and Installing SCons Without Administrative Privileges

If you don't have the right privileges to install SCons in a system location, simply use the `--prefix=` option to install it in a location of your choosing. For example, to install SCons in appropriate locations relative to the user's `$HOME` directory, the scons script in `$HOME/bin` and the build engine in `$HOME/lib/scons`, simply type:

            $ python setup.py install --prefix=$HOME

You may, of course, specify any other location you prefer, and may use the `--version-lib` option if you would like to install version-specific directories relative to the specified prefix.

# Chapter 2. Simple Builds

In this chapter, you will see several examples of very simple build configurations using SCons, which will demonstrate how easy it is to use SCons to build programs from several different programming languages on different types of systems.

## 2.1. Building Simple C / C++ Programs

Here's the famous "Hello, World!" program in C:

          int
          main()
          {
              printf("Hello, world!\n");
          }

And here's how to build it using SCons. Enter the following into a file named `SConstruct`:

          Program('hello.c')

This minimal configuration file gives SCons two pieces of information: what you want to build (an executable program), and the input file from which you want it built (the `hello.c` file). `Program` is a builder_method, a Python call that tells SCons that you want to build an executable program.

That's it. Now run the scons command to build the program. On a POSIX-compliant system like Linux or UNIX, you'll see something like:

          % scons
          scons: Reading SConscript files ...
          scons: done reading SConscript files.
          scons: Building targets ...
          cc -o hello.o -c hello.c
          cc -o hello hello.o
          scons: done building targets.

On a Windows system with the Microsoft Visual C++ compiler, you'll see something like:

          C:\>scons
          scons: Reading SConscript files ...
          scons: done reading SConscript files.
          scons: Building targets ...
          cl /nologo /c hello.c /Fohello.obj
          link /nologo /OUT:hello.exe hello.obj
          scons: done building targets.

First, notice that you only need to specify the name of the source file, and that SCons correctly deduces the names of the object and executable files to be built from the base of the source file name.

Second, notice that the same input `SConstruct` file, without any changes, generates the correct output file names on both systems: `hello.o` and `hello` on POSIX systems, `hello.obj` and `hello.exe` on Windows systems. This is a simple example of how SCons makes it extremely easy to write portable software builds.

(Note that we won't provide duplicate side-by-side POSIX and Windows output for all of the examples in this guide; just keep in mind that, unless otherwise specified, any of the examples should work equally well on both types of systems.)

## 2.2. Building Object Files

The `Program` builder method is only one of many builder methods that SCons provides to build different types of files. Another is the `Object` builder method, which tells SCons to build an object file from the specified source file:

          Object('hello.c')

Now when you run the scons command to build the program, it will build just the `hello.o` object file on a POSIX system:

          % scons
          scons: Reading SConscript files ...
          scons: done reading SConscript files.
          scons: Building targets ...
          cc -o hello.o -c hello.c
          scons: done building targets.

And just the `hello.obj` object file on a Windows system (with the Microsoft Visual C++ compiler):

          C:\>scons
          scons: Reading SConscript files ...
          scons: done reading SConscript files.
          scons: Building targets ...
          cl /nologo /c hello.c /Fohello.obj
          scons: done building targets.

## 2.3. Simple Java Builds

SCons also makes building with Java extremely easy. Unlike the `Program` and `Object` builder methods, however, the `Java` builder method requires that you specify the name of a destination directory in which you want the class files placed, followed by the source directory in which the `.java` files live:

         Java('classes', 'src')

If the `src` directory contains a single `hello.java` file, then the output from running the scons command would look something like this (on a POSIX system):

          % scons
          scons: Reading SConscript files ...
          scons: done reading SConscript files.
          scons: Building targets ...
          javac -d classes -sourcepath src src/hello.java
          scons: done building targets.

We'll cover Java builds in more detail, including building Java archive (`.jar`) and other types of file, in Chapter 24.

## 2.4. Cleaning Up After a Build

When using SCons, it is unnecessary to add special commands or target names to clean up after a build. Instead, you simply use the `-c` or `--clean` option when you invoke SCons, and SCons removes the appropriate built files. So if we build our example above and then invoke `scons -c` afterwards, the output on POSIX looks like:

          % scons
          scons: Reading SConscript files ...
          scons: done reading SConscript files.
          scons: Building targets ...
          cc -o hello.o -c hello.c
          cc -o hello hello.o
          scons: done building targets.
          % scons -c
          scons: Reading SConscript files ...
          scons: done reading SConscript files.
          scons: Cleaning targets ...
          Removed hello.o
          Removed hello
          scons: done cleaning targets.

And the output on Windows looks like:

          C:\>scons
          scons: Reading SConscript files ...
          scons: done reading SConscript files.
          scons: Building targets ...
          cl /nologo /c hello.c /Fohello.obj
          link /nologo /OUT:hello.exe hello.obj
          scons: done building targets.
          C:\>scons -c
          scons: Reading SConscript files ...
          scons: done reading SConscript files.
          scons: Cleaning targets ...
          Removed hello.obj
          Removed hello.exe
          scons: done cleaning targets.

Notice that SCons changes its output to tell you that it is `Cleaning targets ...` and `done cleaning targets.`

## 2.5. The `SConstruct` File

If you're used to build systems like Make you've already figured out that the `SConstruct` file is the SCons equivalent of a `Makefile`. That is, the `SConstruct` file is the input file that SCons reads to control the build.

### 2.5.1. `SConstruct` Files Are Python Scripts

There is, however, an important difference between an `SConstruct` file and a `Makefile`: the `SConstruct` file is actually a Python script. If you're not already familiar with Python, don't worry. This User's Guide will introduce you step-by-step to the relatively small amount of Python you'll need to know to be able to use SCons effectively. And Python is very easy to learn.

One aspect of using Python as the scripting language is that you can put comments in your `SConstruct` file using Python's commenting convention; that is, everything between a '#' and the end of the line will be ignored:

            # Arrange to build the "hello" program.
            Program('hello.c')    # "hello.c" is the source file.

You'll see throughout the remainder of this Guide that being able to use the power of a real scripting language can greatly simplify the solutions to complex requirements of real-world builds.

### 2.5.2. SCons Functions Are Order-Independent

One important way in which the `SConstruct` file is not exactly like a normal Python script, and is more like a `Makefile`, is that the order in which the SCons functions are called in the `SConstruct` file does not affect the order in which SCons actually builds the programs and object files you want it to build.[1] In other words, when you call the `Program` builder (or any other builder method), you're not telling SCons to build the program at the instant the builder method is called. Instead, you're telling SCons to build the program that you want, for example, a program built from a file named `hello.c`, and it's up to SCons to build that program (and any other files) whenever it's necessary. (We'll learn more about how SCons decides when building or rebuilding a file is necessary in Chapter 6, below.)

SCons reflects this distinction between calling a builder method like `Program`> and actually building the program by printing the status messages that indicate when it's "just reading" the `SConstruct` file, and when it's actually building the target files. This is to make it clear when SCons is executing the Python statements that make up the `SConstruct` file, and when SCons is actually executing the commands or other actions to build the necessary files.

Let's clarify this with an example. Python has a `print` statement that prints a string of characters to the screen. If we put `print` statements around our calls to the `Program` builder method:

           print "Calling Program('hello.c')"
           Program('hello.c')
           print "Calling Program('goodbye.c')"
           Program('goodbye.c')
           print "Finished calling Program()"

Then when we execute SCons, we see the output from the `print` statements in between the messages about reading the `SConscript` files, indicating that that is when the Python statements are being executed:

           % scons
           scons: Reading SConscript files ...
           Calling Program('hello.c')
           Calling Program('goodbye.c')
           Finished calling Program()
           scons: done reading SConscript files.
           scons: Building targets ...
           cc -o goodbye.o -c goodbye.c
           cc -o goodbye goodbye.o
           cc -o hello.o -c hello.c
           cc -o hello hello.o
           scons: done building targets.

Notice also that SCons built the goodbye program first, even though the "reading `SConscript`" output shows that we called `Program('hello.c')` first in the `SConstruct` file.

## 2.6. Making the SCons Output Less Verbose

You've already seen how SCons prints some messages about what it's doing, surrounding the actual commands used to build the software:

          C:\>scons
          scons: Reading SConscript files ...
          scons: done reading SConscript files.
          scons: Building targets ...
          cl /nologo /c hello.c /Fohello.obj
          link /nologo /OUT:hello.exe hello.obj
          scons: done building targets.

These messages emphasize the order in which SCons does its work: all of the configuration files (generically referred to as `SConscript` files) are read and executed first, and only then are the target files built. Among other benefits, these messages help to distinguish between errors that occur while the configuration files are read, and errors that occur while targets are being built.

One drawback, of course, is that these messages clutter the output. Fortunately, they're easily disabled by using the `-Q` option when invoking SCons:

          C:\>scons -Q
          cl /nologo /c hello.c /Fohello.obj
          link /nologo /OUT:hello.exe hello.obj

Because we want this User's Guide to focus on what SCons is actually doing, we're going to use the `-Q` option to remove these messages from the output of all the remaining examples in this Guide.

# Chapter 3. Less Simple Things to Do With Builds

In this chapter, you will see several examples of very simple build configurations using SCons, which will demonstrate how easy it is to use SCons to build programs from several different programming languages on different types of systems.

## 3.1. Specifying the Name of the Target (Output) File

You've seen that when you call the `Program` builder method, it builds the resulting program with the same base name as the source file. That is, the following call to build an executable program from the `hello.c` source file will build an executable program named hello on POSIX systems, and an executable program named `hello.exe` on Windows systems:

           Program('hello.c')

If you want to build a program with a different name than the base of the source file name, you simply put the target file name to the left of the source file name:

           Program('new_hello', 'hello.c')

(SCons requires the target file name first, followed by the source file name, so that the order mimics that of an assignment statement in most programming languages, including Python: `"program = source files"`.)

Now SCons will build an executable program named new_hello when run on a POSIX system:

           % scons -Q
           cc -o hello.o -c hello.c
           cc -o new_hello hello.o

And SCons will build an executable program named new_hello.exe when run on a Windows system:

           C:\>scons -Q
           cl /nologo /c hello.c /Fohello.obj
           link /nologo /OUT:new_hello.exe hello.obj

## 3.2. Compiling Multiple Source Files

You've just seen how to configure SCons to compile a program from a single source file. It's more common, of course, that you'll need to build a program from many input source files, not just one. To do this, you need to put the source files in a Python list (enclosed in square brackets), like so:

           Program(['prog.c', 'file1.c', 'file2.c'])

A build of the above example would look like:

           % scons -Q
           cc -o file1.o -c file1.c
           cc -o file2.o -c file2.c
           cc -o prog.o -c prog.c
           cc -o prog prog.o file1.o file2.o

Notice that SCons deduces the output program name from the first source file specified in the list--that is, because the first source file was `prog.c`, SCons will name the resulting program `prog` (or `prog.exe` on a Windows system). If you want to specify a different program name, then (as we've seen in the previous section) you slide the list of source files over to the right to make room for the output program file name. (SCons puts the output file name to the left of the source file names so that the order mimics that of an assignment statement: "program = source files".) This makes our example:

           Program('program', ['prog.c', 'file1.c', 'file2.c'])

On Linux, a build of this example would look like:

           % scons -Q
           cc -o file1.o -c file1.c
           cc -o file2.o -c file2.c
           cc -o prog.o -c prog.c
           cc -o program prog.o file1.o file2.o

Or on Windows:

           C:\>scons -Q
           cl /nologo /c file1.c /Fofile1.obj
           cl /nologo /c file2.c /Fofile2.obj
           cl /nologo /c prog.c /Foprog.obj
           link /nologo /OUT:program.exe prog.obj file1.obj file2.obj

## 3.3. Specifying Single Files Vs. Lists of Files

We've now shown you two ways to specify the source for a program, one with a list of files:

           Program('hello', ['file1.c', 'file2.c'])

And one with a single file:

           Program('hello', 'hello.c')

You could actually put a single file name in a list, too, which you might prefer just for the sake of consistency:

           Program('hello', ['hello.c'])

SCons functions will accept a single file name in either form. In fact, internally, SCons treats all input as lists of files, but allows you to omit the square brackets to cut down a little on the typing when there's only a single file name.

| Although SCons functions are forgiving about whether or not you use a string vs. a list for a single file name, Python itself is more strict about treating lists and strings differently. So where SCons allows either a string or list:
           # The following two calls both work correctly:
           Program('program1', 'program1.c')
           Program('program2', ['program2.c'])
Trying to do "Python things" that mix strings and lists will cause errors or lead to incorrect results:
           common_sources = ['file1.c', 'file2.c']

           # THE FOLLOWING IS INCORRECT AND GENERATES A PYTHON ERROR
           # BECAUSE IT TRIES TO ADD A STRING TO A LIST:
           Program('program1', common_sources + 'program1.c')

           # The following works correctly, because it's adding two
           # lists together to make another list.
           Program('program2', common_sources + ['program2.c'])

## 3.4. Making Lists of Files Easier to Read

One drawback to the use of a Python list for source files is that each file name must be enclosed in quotes (either single quotes or double quotes). This can get cumbersome and difficult to read when the list of file names is long. Fortunately, SCons and Python provide a number of ways to make sure that the `SConstruct` file stays easy to read.

To make long lists of file names easier to deal with, SCons provides a `Split` function that takes a quoted list of file names, with the names separated by spaces or other white-space characters, and turns it into a list of separate file names. Using the `Split` function turns the previous example into:

           Program('program', Split('main.c file1.c file2.c'))

(If you're already familiar with Python, you'll have realized that this is similar to the `split()` method in the Python standard `string` module. Unlike the `string.split()` method, however, the `Split` function does not require a string as input and will wrap up a single non-string object in a list, or return its argument untouched if it's already a list. This comes in handy as a way to make sure arbitrary values can be passed to SCons functions without having to check the type of the variable by hand.)

Putting the call to the `Split` function inside the `Program` call can also be a little unwieldy. A more readable alternative is to assign the output from the `Split` call to a variable name, and then use the variable when calling the `Program` function:

           list = Split('main.c file1.c file2.c')
           Program('program', list)

Lastly, the `Split` function doesn't care how much white space separates the file names in the quoted string. This allows you to create lists of file names that span multiple lines, which often makes for easier editing:

           list = Split("""main.c
                           file1.c
                           file2.c""")
           Program('program', list)

(Note in this example that we used the Python "triple-quote" syntax, which allows a string to contain multiple lines. The three quotes can be either single or double quotes.)

## 3.5. Keyword Arguments

SCons also allows you to identify the output file and input source files using Python keyword arguments. The output file is known as the target, and the source file(s) are known (logically enough) as the source. The Python syntax for this is:

           list = Split('main.c file1.c file2.c')
           Program(target = 'program', source = list)

Because the keywords explicitly identify what each argument is, you can actually reverse the order if you prefer:

           list = Split('main.c file1.c file2.c')
           Program(source = list, target = 'program')

Whether or not you choose to use keyword arguments to identify the target and source files, and the order in which you specify them when using keywords, are purely personal choices; SCons functions the same regardless.

## 3.6. Compiling Multiple Programs

In order to compile multiple programs within the same `SConstruct` file, simply call the `Program` method multiple times, once for each program you need to build:

           Program('foo.c')
           Program('bar', ['bar1.c', 'bar2.c'])

SCons would then build the programs as follows:

           % scons -Q
           cc -o bar1.o -c bar1.c
           cc -o bar2.o -c bar2.c
           cc -o bar bar1.o bar2.o
           cc -o foo.o -c foo.c
           cc -o foo foo.o

Notice that SCons does not necessarily build the programs in the same order in which you specify them in the `SConstruct` file. SCons does, however, recognize that the individual object files must be built before the resulting program can be built. We'll discuss this in greater detail in the "Dependencies" section, below.

## 3.7. Sharing Source Files Between Multiple Programs

It's common to re-use code by sharing source files between multiple programs. One way to do this is to create a library from the common source files, which can then be linked into resulting programs. (Creating libraries is discussed in Chapter 4, below.)

A more straightforward, but perhaps less convenient, way to share source files between multiple programs is simply to include the common files in the lists of source files for each program:

           Program(Split('foo.c common1.c common2.c'))
           Program('bar', Split('bar1.c bar2.c common1.c common2.c'))

SCons recognizes that the object files for the `common1.c` and `common2.c` source files each only need to be built once, even though the resulting object files are each linked in to both of the resulting executable programs:

           % scons -Q
           cc -o bar1.o -c bar1.c
           cc -o bar2.o -c bar2.c
           cc -o common1.o -c common1.c
           cc -o common2.o -c common2.c
           cc -o bar bar1.o bar2.o common1.o common2.o
           cc -o foo.o -c foo.c
           cc -o foo foo.o common1.o common2.o

If two or more programs share a lot of common source files, repeating the common files in the list for each program can be a maintenance problem when you need to change the list of common files. You can simplify this by creating a separate Python list to hold the common file names, and concatenating it with other lists using the Python `+` operator:

           common = ['common1.c', 'common2.c']
           foo_files = ['foo.c'] + common
           bar_files = ['bar1.c', 'bar2.c'] + common
           Program('foo', foo_files)
           Program('bar', bar_files)

This is functionally equivalent to the previous example.

# Chapter 4. Building and Linking with Libraries

It's often useful to organize large software projects by collecting parts of the software into one or more libraries. SCons makes it easy to create libraries and to use them in the programs.

## 4.1. Building Libraries

You build your own libraries by specifying `Library` instead of `Program`:

          Library('foo', ['f1.c', 'f2.c', 'f3.c'])

SCons uses the appropriate library prefix and suffix for your system. So on POSIX or Linux systems, the above example would build as follows (although ranlib may not be called on all systems):

          % scons -Q
          cc -o f1.o -c f1.c
          cc -o f2.o -c f2.c
          cc -o f3.o -c f3.c
          ar rc libfoo.a f1.o f2.o f3.o
          ranlib libfoo.a

On a Windows system, a build of the above example would look like:

          C:\>scons -Q
          cl /nologo /c f1.c /Fof1.obj
          cl /nologo /c f2.c /Fof2.obj
          cl /nologo /c f3.c /Fof3.obj
          lib /nologo /OUT:foo.lib f1.obj f2.obj f3.obj

The rules for the target name of the library are similar to those for programs: if you don't explicitly specify a target library name, SCons will deduce one from the name of the first source file specified, and SCons will add an appropriate file prefix and suffix if you leave them off.

### 4.1.1. Building Libraries From Source Code or Object Files

The previous example shows building a library from a list of source files. You can, however, also give the `Library` call object files, and it will correctly realize In fact, you can arbitrarily mix source code files and object files in the source list:

            Library('foo', ['f1.c', 'f2.o', 'f3.c', 'f4.o'])

And SCons realizes that only the source code files must be compiled into object files before creating the final library:

            % scons -Q
            cc -o f1.o -c f1.c
            cc -o f3.o -c f3.c
            ar rc libfoo.a f1.o f2.o f3.o f4.o
            ranlib libfoo.a

Of course, in this example, the object files must already exist for the build to succeed. See Chapter 5, below, for information about how you can build object files explicitly and include the built files in a library.

### 4.1.2. Building Static Libraries Explicitly: the `StaticLibrary` Builder

The `Library` function builds a traditional static library. If you want to be explicit about the type of library being built, you can use the synonym `StaticLibrary` function instead of `Library`:

            StaticLibrary('foo', ['f1.c', 'f2.c', 'f3.c'])

There is no functional difference between the `StaticLibrary` and `Library` functions.

### 4.1.3. Building Shared (DLL) Libraries: the `SharedLibrary` Builder

If you want to build a shared library (on POSIX systems) or a DLL file (on Windows systems), you use the `SharedLibrary` function:

            SharedLibrary('foo', ['f1.c', 'f2.c', 'f3.c'])

The output on POSIX:

            % scons -Q
            cc -o f1.os -c f1.c
            cc -o f2.os -c f2.c
            cc -o f3.os -c f3.c
            cc -o libfoo.so -shared f1.os f2.os f3.os

And the output on Windows:

            C:\>scons -Q
            cl /nologo /c f1.c /Fof1.obj
            cl /nologo /c f2.c /Fof2.obj
            cl /nologo /c f3.c /Fof3.obj
            link /nologo /dll /out:foo.dll /implib:foo.lib f1.obj f2.obj f3.obj
            RegServerFunc(target, source, env)

Notice again that SCons takes care of building the output file correctly, adding the `-shared` option for a POSIX compilation, and the `/dll` option on Windows.

## 4.2. Linking with Libraries

Usually, you build a library because you want to link it with one or more programs. You link libraries with a program by specifying the libraries in the `$LIBS` construction variable, and by specifying the directory in which the library will be found in the `$LIBPATH` construction variable:

          Library('foo', ['f1.c', 'f2.c', 'f3.c'])
          Program('prog.c', LIBS=['foo', 'bar'], LIBPATH='.')

Notice, of course, that you don't need to specify a library prefix (like `lib`) or suffix (like `.a` or `.lib`). SCons uses the correct prefix or suffix for the current system.

On a POSIX or Linux system, a build of the above example would look like:

          % scons -Q
          cc -o f1.o -c f1.c
          cc -o f2.o -c f2.c
          cc -o f3.o -c f3.c
          ar rc libfoo.a f1.o f2.o f3.o
          ranlib libfoo.a
          cc -o prog.o -c prog.c
          cc -o prog prog.o -L. -lfoo -lbar

On a Windows system, a build of the above example would look like:

          C:\>scons -Q
          cl /nologo /c f1.c /Fof1.obj
          cl /nologo /c f2.c /Fof2.obj
          cl /nologo /c f3.c /Fof3.obj
          lib /nologo /OUT:foo.lib f1.obj f2.obj f3.obj
          cl /nologo /c prog.c /Foprog.obj
          link /nologo /OUT:prog.exe /LIBPATH:. foo.lib bar.lib prog.obj

As usual, notice that SCons has taken care of constructing the correct command lines to link with the specified library on each system.

Note also that, if you only have a single library to link with, you can specify the library name in single string, instead of a Python list, so that:

          Program('prog.c', LIBS='foo', LIBPATH='.')

is equivalent to:

          Program('prog.c', LIBS=['foo'], LIBPATH='.')

This is similar to the way that SCons handles either a string or a list to specify a single source file.

## 4.3. Finding Libraries: the `$LIBPATH` Construction Variable

By default, the linker will only look in certain system-defined directories for libraries. SCons knows how to look for libraries in directories that you specify with the `$LIBPATH` construction variable. `$LIBPATH` consists of a list of directory names, like so:

          Program('prog.c', LIBS = 'm',
                            LIBPATH = ['/usr/lib', '/usr/local/lib'])

Using a Python list is preferred because it's portable across systems. Alternatively, you could put all of the directory names in a single string, separated by the system-specific path separator character: a colon on POSIX systems:

          LIBPATH = '/usr/lib:/usr/local/lib'

or a semi-colon on Windows systems:

          LIBPATH = 'C:\\lib;D:\\lib'

(Note that Python requires that the backslash separators in a Windows path name be escaped within strings.)

When the linker is executed, SCons will create appropriate flags so that the linker will look for libraries in the same directories as SCons. So on a POSIX or Linux system, a build of the above example would look like:

          % scons -Q
          cc -o prog.o -c prog.c
          cc -o prog prog.o -L/usr/lib -L/usr/local/lib -lm

On a Windows system, a build of the above example would look like:

          C:\>scons -Q
          cl /nologo /c prog.c /Foprog.obj
          link /nologo /OUT:prog.exe /LIBPATH:\usr\lib /LIBPATH:\usr\local\lib m.lib prog.obj

Note again that SCons has taken care of the system-specific details of creating the right command-line options.

# Chapter 5. Node Objects

Internally, SCons represents all of the files and directories it knows about as `Nodes`. These internal objects (not object files) can be used in a variety of ways to make your `SConscript` files portable and easy to read.

## 5.1. Builder Methods Return Lists of Target Nodes

All builder methods return a list of `Node` objects that identify the target file or files that will be built. These returned `Nodes` can be passed as source files to other builder methods,

For example, suppose that we want to build the two object files that make up a program with different options. This would mean calling the `Object` builder once for each object file, specifying the desired options:

        Object('hello.c', CCFLAGS='-DHELLO')
        Object('goodbye.c', CCFLAGS='-DGOODBYE')

One way to combine these object files into the resulting program would be to call the `Program` builder with the names of the object files listed as sources:

        Object('hello.c', CCFLAGS='-DHELLO')
        Object('goodbye.c', CCFLAGS='-DGOODBYE')
        Program(['hello.o', 'goodbye.o'])

The problem with listing the names as strings is that our `SConstruct` file is no longer portable across operating systems. It won't, for example, work on Windows because the object files there would be named `hello.obj` and `goodbye.obj`, not `hello.o` and `goodbye.o`.

A better solution is to assign the lists of targets returned by the calls to the `Object` builder to variables, which we can then concatenate in our call to the `Program` builder:

          hello_list = Object('hello.c', CCFLAGS='-DHELLO')
          goodbye_list = Object('goodbye.c', CCFLAGS='-DGOODBYE')
          Program(hello_list + goodbye_list)

This makes our `SConstruct` file portable again, the build output on Linux looking like:

           % scons -Q
           cc -o goodbye.o -c -DGOODBYE goodbye.c
           cc -o hello.o -c -DHELLO hello.c
           cc -o hello hello.o goodbye.o

And on Windows:

           C:\>scons -Q
           cl -DGOODBYE /c goodbye.c /Fogoodbye.obj
           cl -DHELLO /c hello.c /Fohello.obj
           link /nologo /OUT:hello.exe hello.obj goodbye.obj

We'll see examples of using the list of nodes returned by builder methods throughout the rest of this guide.

## 5.2. Explicitly Creating File and Directory Nodes

It's worth mentioning here that SCons maintains a clear distinction between Nodes that represent files and Nodes that represent directories. SCons supports `File` and `Dir` functions that, repectively, return a file or directory Node:

          hello_c = File('hello.c')
          Program(hello_c)

          classes = Dir('classes')
          Java(classes, 'src')

Normally, you don't need to call `File` or `Dir` directly, because calling a builder method automatically treats strings as the names of files or directories, and translates them into the Node objects for you. The `File` and `Dir` functions can come in handy in situations where you need to explicitly instruct SCons about the type of Node being passed to a builder or other function, or unambiguously refer to a specific file in a directory tree.

There are also times when you may need to refer to an entry in a file system without knowing in advance whether it's a file or a directory. For those situations, SCons also supports an `Entry` function, which returns a Node that can represent either a file or a directory.

        xyzzy = Entry('xyzzy')

The returned `xyzzy` Node will be turned into a file or directory Node the first time it is used by a builder method or other function that requires one vs. the other.

## 5.3. Printing `Node` File Names

One of the most common things you can do with a Node is use it to print the file name that the node represents. For example, the following `SConstruct` file:

          hello_c = File('hello.c')
          Program(hello_c)

          classes = Dir('classes')
          Java(classes, 'src')

          object_list = Object('hello.c')
          program_list = Program(object_list)
          print "The object file is:", object_list[0]
          print "The program file is:", program_list[0]

Would print the following file names on a POSIX system:

          % scons -Q
          The object file is: hello.o
          The program file is: hello
          cc -o hello.o -c hello.c
          cc -o hello hello.o

And the following file names on a Windows system:

          C:\>scons -Q
          The object file is: hello.obj
          The program file is: hello.exe
          cl /nologo /c hello.c /Fohello.obj
          link /nologo /OUT:hello.exe hello.obj

## 5.4. Using a `Node`'s File Name as a String

Printing a `Node`'s name as described in the previous section works because the string representation of a `Node` is the name of the file. If you want to do something other than print the name of the file, you can fetch it by using the builtin Python `str` function. For example, if you want to use the Python `os.path.exists` to figure out whether a file exists while the `SConstruct` file is being read and executed, you can fetch the string as follows:

          import os.path
          program_list = Program('hello.c')
          program_name = str(program_list[0])
          if not os.path.exists(program_name):
              print program_name, "does not exist!"

Which executes as follows on a POSIX system:

          % scons -Q
          hello does not exist!
          cc -o hello.o -c hello.c
          cc -o hello hello.o

# Chapter 6. Dependencies

So far we've seen how SCons handles one-time builds. But one of the main functions of a build tool like SCons is to rebuild only the necessary things when source files change--or, put another way, SCons should not waste time rebuilding things that have already been built. You can see this at work simply by re-invoking SCons after building our simple hello example:

         % scons -Q
         cc -o hello.o -c hello.c
         cc -o hello hello.o
         % scons -Q
         scons: `.' is up to date.

The second time it is executed, SCons realizes that the hello program is up-to-date with respect to the current `hello.c` source file, and avoids rebuilding it. You can see this more clearly by naming the hello program explicitly on the command line:

         % scons -Q hello
         cc -o hello.o -c hello.c
         cc -o hello hello.o
         % scons -Q hello
         scons: `hello' is up to date.

Note that SCons reports `"...is up to date"` only for target files named explicitly on the command line, to avoid cluttering the output.

## 6.1. Deciding When an Input File Has Changed: the `Decider` Function

Another aspect of avoiding unnecessary rebuilds is the fundamental build tool behavior of rebuilding things when an input file changes, so that the built software is up to date. By default, SCons keeps track of this through an MD5 `signature`, or checksum, of the contents of each file, although you can easily configure SCons to use the modification times (or time stamps) instead. You can even specify your own Python function for deciding if an input file has changed.

### 6.1.1. Using MD5 Signatures to Decide if a File Has Changed

By default, SCons keeps track of whether a file has changed based on an MD5 checksum of the file's contents, not the file's modification time. This means that you may be surprised by the default SCons behavior if you are used to the Make convention of forcing a rebuild by updating the file's modification time (using the touch command, for example):

             % scons -Q hello
             cc -o hello.o -c hello.c
             cc -o hello hello.o
             % touch hello.c
             % scons -Q hello
             scons: `hello' is up to date.

Even though the file's modification time has changed, SCons realizes that the contents of the `hello.c` file have not changed, and therefore that the hello program need not be rebuilt. This avoids unnecessary rebuilds when, for example, someone rewrites the contents of a file without making a change. But if the contents of the file really do change, then SCons detects the change and rebuilds the program as required:

             % scons -Q hello
             cc -o hello.o -c hello.c
             cc -o hello hello.o
             % edit hello.c
                 [CHANGE THE CONTENTS OF hello.c]
             % scons -Q hello
             cc -o hello.o -c hello.c
             cc -o hello hello.o

Note that you can, if you wish, specify this default behavior (MD5 signatures) explicitly using the `Decider` function as follows:

            Program('hello.c')
            Decider('MD5')

You can also use the string `'content'` as a synonym for `'MD5'` when calling the `Decider` function.

#### 6.1.1.1. Ramifications of Using MD5 Signatures

Using MD5 Signatures to decide if an input file has changed has one surprising benefit: if a source file has been changed in such a way that the contents of the rebuilt target file(s) will be exactly the same as the last time the file was built, then any "downstream" target files that depend on the rebuilt-but-not-changed target file actually need not be rebuilt.

So if, for example, a user were to only change a comment in a `hello.c` file, then the rebuilt `hello.o` file would be exactly the same as the one previously built (assuming the compiler doesn't put any build-specific information in the object file). SCons would then realize that it would not need to rebuild the hello program as follows:

               % scons -Q hello
               cc -o hello.o -c hello.c
               cc -o hello hello.o
               % edit hello.c
                 [CHANGE A COMMENT IN hello.c]
               % scons -Q hello
               cc -o hello.o -c hello.c
               scons: `hello' is up to date.

In essence, SCons "short-circuits" any dependent builds when it realizes that a target file has been rebuilt to exactly the same file as the last build. This does take some extra processing time to read the contents of the target (`hello.o`) file, but often saves time when the rebuild that was avoided would have been time-consuming and expensive.

### 6.1.2. Using Time Stamps to Decide If a File Has Changed

If you prefer, you can configure SCons to use the modification time of a file, not the file contents, when deciding if a target needs to be rebuilt. SCons gives you two ways to use time stamps to decide if an input file has changed since the last time a target has been built.

The most familiar way to use time stamps is the way Make does: that is, have SCons decide and target must be rebuilt if if a source file's modification time is newer than the target file. To do this, call the `Decider` function as follows:

            Program('hello.c')
            Decider('timestamp-newer')

This makes SCons act like Make when a file's modification time is updated (using the touch command, for example):

             % scons -Q hello
             cc -o hello.o -c hello.c
             cc -o hello hello.o
             % touch hello.c
             % scons -Q hello
             cc -o hello.o -c hello.c
             cc -o hello hello.o

And, in fact, because this behavior is the same as the behavior of Make, you can also use the string `'make'` as a synonym for `'timestamp-newer'` when calling the `Decider` function:

            Program('hello.c')
            Decider('make')

One drawback to using times stamps exactly like Make is that if an input file's modification time suddenly becomes older than a target file, the target file will not be rebuilt. This can happen if an old copy of a source file is restored from a backup archive, for example. The contents of the restored file will likely be different than they were the last time a dependent target was built, but the target won't be rebuilt because the modification time of the source file is not newer than the target.

Because SCons actually stores information about the source files' time stamps whenever a target is built, it can handle this situation by checking for an exact match of the source file time stamp, instead of just whether or not the source file is newer than the target file. To do this, specify the argument `'timestamp-match'` when calling the `Decider` function:

            Program('hello.c')
            Decider('timestamp-match')

When configured this way, SCons will rebuild a target whenever a source file's modification time has changed. So if we use the `touch -t` option to change the modification time of `hello.c` to an old date (January 1, 1989), SCons will still rebuild the target file:

             % scons -Q hello
             cc -o hello.o -c hello.c
             cc -o hello hello.o
             % touch -t 198901010000 hello.c
             % scons -Q hello
             cc -o hello.o -c hello.c
             scons: `hello' is up to date.

In general, the only reason to prefer `timestamp-newer` instead of `timestamp-match`, would be if you have some specific reason to require this Make-like behavior of not rebuilding a target when an otherwise-modified source file is older.

### 6.1.3. Deciding If a File Has Changed Using Both MD Signatures and Time Stamps

As a performance enhancement, SCons provides a way to use MD5 checksums of file contents but to only read the contents whenever the file's timestamp has changed. To do this, call the `Decider` function with `'MD5-timestamp'` argument as follows:

            Program('hello.c')
            Decider('MD5-timestamp')

So configured, SCons will still behave like it does when using `Decider('MD5')`:

             % scons -Q hello
             cc -o hello.o -c hello.c
             cc -o hello hello.o
             % touch hello.c
             % scons -Q hello
             scons: `hello' is up to date.
             % edit hello.c
                 [CHANGE THE CONTENTS OF hello.c]
             % scons -Q hello
             scons: `hello' is up to date.

However, the second call to SCons in the above output, when the build is up-to-date, will have been performed by simply looking at the modification time of the `hello.c` file, not by opening it and performing an MD5 checksum calcuation on its contents. This can significantly speed up many up-to-date builds.

The only drawback to using `Decider('MD5-timestamp')` is that SCons will not rebuild a target file if a source file was modified within one second of the last time SCons built the file. While most developers are programming, this isn't a problem in practice, since it's unlikely that someone will have built and then thought quickly enought to make a substantive change to a source file within one second. Certain build scripts or continuous integration tools may, however, rely on the ability to applying changes to files automatically and then rebuild as quickly as possible, in which case use of `Decider('MD5-timestamp')` may not be appropriate.

### 6.1.4. Writing Your Own Custom `Decider` Function

The different string values that we've passed to the `Decider` function are essentially used by SCons to pick one of several specific internal functions that implement various ways of deciding if a dependency (usually a source file) has changed since a target file has been built. As it turns out, you can also supply your own function to decide if a dependency has changed.

For example, suppose we have an input file that contains a lot of data, in some specific regular format, that is used to rebuild a lot of different target files, but each target file really only depends on one particular section of the input file. We'd like to have each target file depend on only its section of the input file. However, since the input file may contain a lot of data, we only want to open the input file if its timestamp has changed. This could done with a custom `Decider` function that might look something like this:

            Program('hello.c')
            def decide_if_changed(dependency, target, prev_ni):
                if self.get_timestamp() != prev_ni.timestamp:
                    dep = str(dependency)
                    tgt = str(target)
                    if specific_part_of_file_has_changed(dep, tgt):
                        return True
                return False
            Decider(decide_if_changed)

Note that in the function definition, the `dependency` (input file) is the first argument, and then the `target`. Both of these are passed to the functions as SCons `Node` objects, which we convert to strings using the Python `str()`. The third argument, `prev_ni`, is an object that holds the signature or timestamp information that was recorded about the dependency the last time the target was built.

Note that ignoring some of the arguments in your custom `Decider` function is a perfectly normal thing to do, if they don't impact the way you want to decide if the dependency file has changed.

### 6.1.5. Mixing Different Ways of Deciding If a File Has Changed

The previous examples have all demonstrated calling the global `Decider` function to configure all dependency decisions that SCons makes. Sometimes, however, you want to be able to configure different decision-making for different targets. When that's necessary, you can use the `env.Decider` method to affect only the configuration decisions for targets built with a specific construction environment.

For example, if we arbitrarily want to build one program using MD5 checkums and another use file modification times from the same source we might configure it this way:

            env1 = Environment(CPPPATH = ['.'])
            env2 = env1.Clone()
            env2.Decider('timestamp-match')
            env1.Program('prog-MD5', 'program1.c')
            env2.Program('prog-timestamp', 'program2.c')

If both of the programs include the same `inc.h` file, then updating the modification time of `inc.h` (using the touch command) will cause only `prog-timestamp` to be rebuilt:

             % scons -Q
             cc -o program1.o -c -I. program1.c
             cc -o prog-MD5 program1.o
             cc -o program2.o -c -I. program2.c
             cc -o prog-timestamp program2.o
             % touch inc.h
             % scons -Q
             cc -o program2.o -c -I. program2.c
             cc -o prog-timestamp program2.o

## 6.2. Older Functions for Deciding When an Input File Has Changed

SCons still supports two functions that used to be the primary methods for configuring the decision about whether or not an input file has changed. Although they're not officially deprecated yet, their use is discouraged, mainly because they rely on a somewhat confusing distinction between how source files and target files are handled. These functions are documented here mainly in case you encounter them in existing `SConscript` files.

### 6.2.1. The `SourceSignatures` Function

The `SourceSignatures` function is fairly straightforward, and supports two different argument values to configure whether source file changes should be decided using MD5 signatures:

            Program('hello.c')
            SourceSignatures('MD5')

Or using time stamps:

            Program('hello.c')
            SourceSignatures('timestamp')

These are roughly equivalent to specifying `Decider('MD5')` or `Decider('timestamp-match')`, respectively, although it only affects how SCons makes decisions about dependencies on source files--that is, files that are not built from any other files.

### 6.2.2. The `TargetSignatures` Function

The `TargetSignatures` function specifies how SCons decides when a target file has changed when it is used as a dependency of (input to) another target--that is, the `TargetSignatures` function configures how the signatures of "intermediate" target files are used when deciding if a "downstream" target file must be rebuilt. [2]

The `TargetSignatures` function supports the same `'MD5'` and `'timestamp'` argument values that are supported by the `SourceSignatures`, with the same meanings, but applied to target files. That is, in the example:

            Program('hello.c')
            TargetSignatures('MD5')

The MD5 checksum of the `hello.o` target file will be used to decide if it has changed since the last time the "downstream" hello target file was built. And in the example:

            Program('hello.c')
            TargetSignatures('timestamp')

The modification time of the `hello.o` target file will be used to decide if it has changed since the last time the "downstream" hello target file was built.

The `TargetSignatures` function supports two additional argument values: `'source'` and `'build'`. The `'source'` argument specifies that decisions involving whether target files have changed since a previous build should use the same behavior for the decisions configured for source files (using the `SourceSignatures` function). So in the example:

            Program('hello.c')
            TargetSignatures('source')
            SourceSignatures('timestamp')

All files, both targets and sources, will use modification times when deciding if an input file has changed since the last time a target was built.

Lastly, the `'build'` argument specifies that SCons should examine the build status of a target file and always rebuild a "downstream" target if the target file was itself rebuilt, without re-examining the contents or timestamp of the newly-built target file. If the target file was not rebuilt during this scons invocation, then the target file will be examined the same way as configured by the `SourceSignature` call to decide if it has changed.

This mimics the behavior of `build signatures` in earlier versions of SCons. A `build signature` re-combined signatures of all the input files that went into making the target file, so that the target file itself did not need to have its contents read to compute an MD5 signature. This can improve performance for some configurations, but is generally not as effective as using `Decider('MD5-timestamp')`.

## 6.3. Implicit Dependencies: The `$CPPPATH` Construction Variable

Now suppose that our "Hello, World!" program actually has an `#include` line to include the `hello.h` file in the compilation:

           #include <hello.h>
           int
           main()
           {
               printf("Hello, %s!\n", string);
           }

And, for completeness, the `hello.h` file looks like this:

           #define string    "world"

In this case, we want SCons to recognize that, if the contents of the `hello.h` file change, the hello program must be recompiled. To do this, we need to modify the `SConstruct` file like so:

           Program('hello.c', CPPPATH = '.')

The `$CPPPATH` value tells SCons to look in the current directory (`'.'`) for any files included by C source files (`.c` or `.h` files). With this assignment in the `SConstruct` file:

           % scons -Q hello
           cc -o hello.o -c -I. hello.c
           cc -o hello hello.o
           % scons -Q hello
           scons: `hello' is up to date.
           % edit hello.h
               [CHANGE THE CONTENTS OF hello.h]
           % scons -Q hello
           cc -o hello.o -c -I. hello.c
           cc -o hello hello.o

First, notice that SCons added the `-I.` argument from the `$CPPPATH` variable so that the compilation would find the `hello.h` file in the local directory.

Second, realize that SCons knows that the hello program must be rebuilt because it scans the contents of the `hello.c` file for the `#include` lines that indicate another file is being included in the compilation. SCons records these as implicit dependencies of the target file, Consequently, when the `hello.h` file changes, SCons realizes that the `hello.c` file includes it, and rebuilds the resulting hello program that depends on both the `hello.c` and `hello.h` files.

Like the `$LIBPATH` variable, the `$CPPPATH` variable may be a list of directories, or a string separated by the system-specific path separation character (':' on POSIX/Linux, ';' on Windows). Either way, SCons creates the right command-line options so that the following example:

           Program('hello.c', CPPPATH = ['include', '/home/project/inc'])

Will look like this on POSIX or Linux:

           % scons -Q hello
           cc -o hello.o -c -Iinclude -I/home/project/inc hello.c
           cc -o hello hello.o

And like this on Windows:

           C:\>scons -Q hello.exe
           cl /nologo /Iinclude /I\home\project\inc /c hello.c /Fohello.obj
           link /nologo /OUT:hello.exe hello.obj

## 6.4. Caching Implicit Dependencies

Scanning each file for `#include` lines does take some extra processing time. When you're doing a full build of a large system, the scanning time is usually a very small percentage of the overall time spent on the build. You're most likely to notice the scanning time, however, when you rebuild all or part of a large system: SCons will likely take some extra time to "think about" what must be built before it issues the first build command (or decides that everything is up to date and nothing must be rebuilt).

In practice, having SCons scan files saves time relative to the amount of potential time lost to tracking down subtle problems introduced by incorrect dependencies. Nevertheless, the "waiting time" while SCons scans files can annoy individual developers waiting for their builds to finish. Consequently, SCons lets you cache the implicit dependencies that its scanners find, for use by later builds. You can do this by specifying the `--implicit-cache` option on the command line:

           % scons -Q --implicit-cache hello
           cc -o hello.o -c hello.c
           cc -o hello hello.o
           % scons -Q hello
           scons: `hello' is up to date.

If you don't want to specify `--implicit-cache` on the command line each time, you can make it the default behavior for your build by setting the `implicit_cache` option in an `SConscript` file:

           SetOption('implicit_cache', 1)

SCons does not cache implicit dependencies like this by default because the `--implicit-cache` causes SCons to simply use the implicit dependencies stored during the last run, without any checking for whether or not those dependencies are still correct. Specifically, this means `--implicit-cache` instructs SCons to not rebuild "correctly" in the following cases:

- When `--implicit-cache` is used, SCons will ignore any changes that may have been made to search paths (like `$CPPPATH` or `$LIBPATH`,). This can lead to SCons not rebuilding a file if a change to `$CPPPATH` would normally cause a different, same-named file from a different directory to be used.

- When `--implicit-cache` is used, SCons will not detect if a same-named file has been added to a directory that is earlier in the search path than the directory in which the file was found last time.

### 6.4.1. The `--implicit-deps-changed` Option

When using cached implicit dependencies, sometimes you want to "start fresh" and have SCons re-scan the files for which it previously cached the dependencies. For example, if you have recently installed a new version of external code that you use for compilation, the external header files will have changed and the previously-cached implicit dependencies will be out of date. You can update them by running SCons with the `--implicit-deps-changed` option:

             % scons -Q --implicit-deps-changed hello
             cc -o hello.o -c hello.c
             cc -o hello hello.o
             % scons -Q hello
             scons: `hello' is up to date.

In this case, SCons will re-scan all of the implicit dependencies and cache updated copies of the information.

### 6.4.2. The `--implicit-deps-unchanged` Option

By default when caching dependencies, SCons notices when a file has been modified and re-scans the file for any updated implicit dependency information. Sometimes, however, you may want to force SCons to use the cached implicit dependencies, even if the source files changed. This can speed up a build for example, when you have changed your source files but know that you haven't changed any `#include` lines. In this case, you can use the `--implicit-deps-unchanged` option:

             % scons -Q --implicit-deps-unchanged hello
             cc -o hello.o -c hello.c
             cc -o hello hello.o
             % scons -Q hello
             scons: `hello' is up to date.

In this case, SCons will assume that the cached implicit dependencies are correct and will not bother to re-scan changed files. For typical builds after small, incremental changes to source files, the savings may not be very big, but sometimes every bit of improved performance counts.

## 6.5. Explicit Dependencies: the `Depends` Function

Sometimes a file depends on another file that is not detected by an SCons scanner. For this situation, SCons allows you to specific explicitly that one file depends on another file, and must be rebuilt whenever that file changes. This is specified using the `Depends` method:

           hello = Program('hello.c')
           Depends(hello, 'other_file')

           % scons -Q hello
           cc -c hello.c -o hello.o
           cc -o hello hello.o
           % scons -Q hello
           scons: `hello' is up to date.
           % edit other_file
               [CHANGE THE CONTENTS OF other_file]
           % scons -Q hello
           cc -c hello.c -o hello.o
           cc -o hello hello.o

## 6.6. Ignoring Dependencies: the `Ignore` Function

Sometimes it makes sense to not rebuild a program, even if a dependency file changes. In this case, you would tell SCons specifically to ignore a dependency as follows:

          hello = Program('hello.c')
          Ignore(hello, 'hello.h')

          % scons -Q hello
          cc -c -o hello.o hello.c
          cc -o hello hello.o
          % scons -Q hello
          scons: `hello' is up to date.
          % edit hello.h
            [CHANGE THE CONTENTS OF hello.h]
          % scons -Q hello
          scons: `hello' is up to date.

Now, the above example is a little contrived, because it's hard to imagine a real-world situation where you wouldn't want to rebuild hello if the `hello.h` file changed. A more realistic example might be if the hello program is being built in a directory that is shared between multiple systems that have different copies of the `stdio.h` include file. In that case, SCons would notice the differences between the different systems' copies of `stdio.h` and would rebuild hello each time you change systems. You could avoid these rebuilds as follows:

           hello = Program('hello.c')
           Ignore(hello, '/usr/include/stdio.h')

## 6.7. The `AlwaysBuild` Function

How SCons handles dependencies can also be affected by the `AlwaysBuild` method. When a file is passed to the `AlwaysBuild` method, like so:

          hello = Program('hello.c')
          AlwaysBuild(hello)

Then the specified target file (hello in our example) will always be considered out-of-date and rebuilt whenever that target file is evaluated while walking the dependency graph:

          % scons -Q
          cc -o hello.o -c hello.c
          cc -o hello hello.o
          % scons -Q
          cc -o hello hello.o

The `AlwaysBuild` function has a somewhat misleading name, because it does not actually mean the target file will be rebuilt every single time SCons is invoked. Instead, it means that the target will, in fact, be rebuilt whenever the target file is encountered while evaluating the targets specified on the command line (and their dependencies). So specifying some other target on the command line, a target that does not itself depend on the `AlwaysBuild` target, will still be rebuilt only if it's out-of-date with respect to its dependencies:

          % scons -Q
          cc -o hello.o -c hello.c
          cc -o hello hello.o
          % scons -Q hello.o
          scons: `hello.o' is up to date.

# Chapter 7. Construction Environments

It is rare that all of the software in a large, complicated system needs to be built the same way. For example, different source files may need different options enabled on the command line, or different executable programs need to be linked with different libraries. SCons accommodates these different build requirements by allowing you to create and configure multiple `construction environments` that control how the software is built. Technically, a `construction environment` is an object that has a number of associated `construction variables`, each with a name and a value. (A construction environment also has an attached set of `Builder` methods, about which we'll learn more later.)

A `construction environment` is created by the `Environment` method:

        env = Environment()

By default, SCons initializes every new construction environment with a set of `construction variables` based on the tools that it finds on your system, plus the default set of builder methods necessary for using those tools. The construction variables are initialized with values describing the C compiler, the Fortran compiler, the linker, etc., as well as the command lines to invoke them.

When you initialize a construction environment you can set the values of the environment's `construction variables` to control how a program is built. For example:

        env = Environment(CC = 'gcc',
                          CCFLAGS = '-O2')

        env.Program('foo.c')

The construction environment in this example is still initialized with the same default construction variable values, except that the user has explicitly specified use of the GNU C compiler gcc, and further specifies that the `-O2` (optimization level two) flag should be used when compiling the object file. In other words, the explicit initializations of `$CC` and `$CCFLAGS` override the default values in the newly-created construction environment. So a run from this example would look like:

        % scons -Q
        gcc -o foo.o -c -O2 foo.c
        gcc -o foo foo.o

## 7.1. Multiple `Construction Environments`

The real advantage of construction environments is that you can create as many different construction environments as you need, each tailored to a different way to build some piece of software or other file. If, for example, we need to build one program with the `-O2` flag and another with the `-g` (debug) flag, we would do this like so:

          opt = Environment(CCFLAGS = '-O2')
          dbg = Environment(CCFLAGS = '-g')

          opt.Program('foo', 'foo.c')

          dbg.Program('bar', 'bar.c')

          % scons -Q
          cc -o bar.o -c -g bar.c
          cc -o bar bar.o
          cc -o foo.o -c -O2 foo.c
          cc -o foo foo.o

We can even use multiple construction environments to build multiple versions of a single program. If you do this by simply trying to use the `Program` builder with both environments, though, like this:

          opt = Environment(CCFLAGS = '-O2')
          dbg = Environment(CCFLAGS = '-g')

          opt.Program('foo', 'foo.c')

          dbg.Program('foo', 'foo.c')

Then SCons generates the following error:

          % scons -Q

          scons: *** Two environments with different actions were specified for the same target: foo.o
          File "/home/my/project/SConstruct", line 6, in <module>

This is because the two `Program` calls have each implicitly told SCons to generate an object file named `foo.o`, one with a `$CCFLAGS` value of `-O2` and one with a `$CCFLAGS` value of `-g`. SCons can't just decide that one of them should take precedence over the other, so it generates the error. To avoid this problem, we must explicitly specify that each environment compile `foo.c` to a separately-named object file using the `Object` builder, like so:

          opt = Environment(CCFLAGS = '-O2')
          dbg = Environment(CCFLAGS = '-g')

          o = opt.Object('foo-opt', 'foo.c')
          opt.Program(o)

          d = dbg.Object('foo-dbg', 'foo.c')
          dbg.Program(d)

Notice that each call to the `Object` builder returns a value, an internal SCons object that represents the object file that will be built. We then use that object as input to the `Program` builder. This avoids having to specify explicitly the object file name in multiple places, and makes for a compact, readable `SConstruct` file. Our SCons output then looks like:

          % scons -Q
          cc -o foo-dbg.o -c -g foo.c
          cc -o foo-dbg foo-dbg.o
          cc -o foo-opt.o -c -O2 foo.c
          cc -o foo-opt foo-opt.o

## 7.2. Copying `Construction Environments`

Sometimes you want more than one construction environment to share the same values for one or more variables. Rather than always having to repeat all of the common variables when you create each construction environment, you can use the `Clone` method to create a copy of a construction environment.

Like the `Environment` call that creates a construction environment, the `Clone` method takes `construction variable` assignments, which will override the values in the copied construction environment. For example, suppose we want to use gcc to create three versions of a program, one optimized, one debug, and one with neither. We could do this by creating a "base" construction environment that sets `$CC` to gcc, and then creating two copies, one which sets `$CCFLAGS` for optimization and the other which sets `$CCFLAGS` for debugging:

          env = Environment(CC = 'gcc')
          opt = env.Clone(CCFLAGS = '-O2')
          dbg = env.Clone(CCFLAGS = '-g')

          env.Program('foo', 'foo.c')

          o = opt.Object('foo-opt', 'foo.c')
          opt.Program(o)

          d = dbg.Object('foo-dbg', 'foo.c')
          dbg.Program(d)

Then our output would look like:

          % scons -Q
          gcc -o foo.o -c foo.c
          gcc -o foo foo.o
          gcc -o foo-dbg.o -c -g foo.c
          gcc -o foo-dbg foo-dbg.o
          gcc -o foo-opt.o -c -O2 foo.c
          gcc -o foo-opt foo-opt.o

## 7.3. Fetching Values From a `Construction Environment`

You can fetch individual construction variables using the normal syntax for accessing individual named items in a Python dictionary:

          env = Environment()
          print "CC is:", env['CC']

This example `SConstruct` file doesn't build anything, but because it's actually a Python script, it will print the value of `$CC` for us:

          % scons -Q
          CC is: cc
          scons: `.' is up to date.

A construction environment, however, is actually an object with associated methods, etc. If you want to have direct access to only the dictionary of construction variables, you can fetch this using the `Dictionary` method:

          env = Environment(FOO = 'foo', BAR = 'bar')
          dict = env.Dictionary()
          for key in ['OBJSUFFIX', 'LIBSUFFIX', 'PROGSUFFIX']:
              print "key = %s, value = %s" % (key, dict[key])

This `SConstruct` file will print the specified dictionary items for us on POSIX systems as follows:

          % scons -Q
          key = OBJSUFFIX, value = .o
          key = LIBSUFFIX, value = .a
          key = PROGSUFFIX, value =
          scons: `.' is up to date.

And on Windows:

          C:\>scons -Q
          key = OBJSUFFIX, value = .obj
          key = LIBSUFFIX, value = .lib
          key = PROGSUFFIX, value = .exe
          scons: `.' is up to date.

If you want to loop through and print the values of all of the construction variables in a construction environment, the Python code to do that in sorted order might look something like:

          env = Environment()
          dict = env.Dictionary()
          keys = dict.keys()
          keys.sort()
          for key in keys:
              print "construction variable = '%s', value = '%s'" % (key, dict[key])

## 7.4. Expanding Values From a `Construction Environment`

Another way to get information from a construction environment. is to use the `subst` method on a string containing $-expansions of construction variable names. As a simple example, the example from the previous section that used `env['CC']` to fetch the value of `$CC` could also be written as:

         env = Environment()
         print "CC is:", env.subst('$CC')

The real advantage of using `subst` to expand strings is that construction variables in the result get re-expanded until there are no expansions left in the string. So a simple fetch of a value like `$CCCOM`:

         env = Environment(CCFLAGS = '-DFOO')
         print "CCCOM is:", env['CCCOM']

Will print the unexpanded value of `$CCCOM`, showing us the construction variables that still need to be expanded:

         % scons -Q
         CCCOM is: $CC $CCFLAGS $CPPFLAGS $_CPPDEFFLAGS $_CPPINCFLAGS -c -o $TARGET $SOURCES
         scons: `.' is up to date.

Calling the `subst` method on `$CCOM`, however:

         env = Environment(CCFLAGS = '-DFOO')
         print "CCCOM is:", env.subst('$CCCOM')

Will recursively expand all of the $-prefixed construction variables, showing us the final output:

         % scons -Q
         CCCOM is: gcc -DFOO -c -o
         scons: `.' is up to date.

(Note that because we're not expanding this in the context of building something there are no target or source files for `$TARGET` and `$SOURCES` to expand.)

## 7.5. Modifying a `Construction Environment`

SCons provides various methods that support modifying existing values in a construction environment.

### 7.5.1. Replacing Values in a `Construction Environment`

You can replace existing construction variable values using the `Replace` method:

            env = Environment(CCFLAGS = '-DDEFINE1')
            env.Replace(CCFLAGS = '-DDEFINE2')
            env.Program('foo.c')

The replacing value (`-DDEFINE2` in the above example) completely replaces the value in the construction environment:

            % scons -Q
            cc -o foo.o -c -DDEFINE2 foo.c
            cc -o foo foo.o

You can safely call `Replace` for construction variables that don't exist in the construction environment:

            env = Environment()
            env.Replace(NEW_VARIABLE = 'xyzzy')
            print "NEW_VARIABLE =", env['NEW_VARIABLE']

In this case, the construction variable simply gets added to the construction environment:

            % scons -Q
            NEW_VARIABLE = xyzzy
            scons: `.' is up to date.

Because the variables aren't expanded until the construction environment is actually used to build the targets, and because SCons function and method calls are order-independent, the last replacement "wins" and is used to build all targets, regardless of the order in which the calls to Replace() are interspersed with calls to builder methods:

            env = Environment(CCFLAGS = '-DDEFINE1')
            print "CCFLAGS =", env['CCFLAGS']
            env.Program('foo.c')

            env.Replace(CCFLAGS = '-DDEFINE2')
            print "CCFLAGS =", env['CCFLAGS']
            env.Program('bar.c')

The timing of when the replacement actually occurs relative to when the targets get built becomes apparent if we run scons without the `-Q` option:

            % scons
            scons: Reading SConscript files ...
            CCFLAGS = -DDEFINE1
            CCFLAGS = -DDEFINE2
            scons: done reading SConscript files.
            scons: Building targets ...
            cc -o bar.o -c -DDEFINE2 bar.c
            cc -o bar bar.o
            cc -o foo.o -c -DDEFINE2 foo.c
            cc -o foo foo.o
            scons: done building targets.

Because the replacement occurs while the `SConscript` files are being read, the `$CCFLAGS` variable has already been set to `-DDEFINE2` by the time the `foo.o` target is built, even though the call to the `Replace` method does not occur until later in the `SConscript` file.

### 7.5.2. Appending to the End of Values in a `Construction Environment`

You can append a value to an existing construction variable using the `Append` method:

            env = Environment(CCFLAGS = '-DMY_VALUE')
            env.Append(CCFLAGS = ' -DLAST')
            env.Program('foo.c')

SCons then supplies both the `-DMY_VALUE` and `-DLAST` flags when compiling the object file:

            % scons -Q
            cc -o foo.o -c -DMY_VALUE -DLAST foo.c
            cc -o foo foo.o

If the construction variable doesn't already exist, the `Append` method will create it:

            env = Environment()
            env.Append(NEW_VARIABLE = 'added')
            print "NEW_VARIABLE =", env['NEW_VARIABLE']

Which yields:

            % scons -Q
            NEW_VARIABLE = added
            scons: `.' is up to date.

### 7.5.3. Appending to the Beginning of Values in a `Construction Environment`

You can append a value to the beginning of an existing construction variable using the `Prepend` method:

            env = Environment(CCFLAGS = '-DMY_VALUE')
            env.Prepend(CCFLAGS = '-DFIRST ')
            env.Program('foo.c')

SCons then supplies both the `-DFIRST` and `-DMY_VALUE` flags when compiling the object file:

            % scons -Q
            cc -o foo.o -c -DFIRST -DMY_VALUE foo.c
            cc -o foo foo.o

If the construction variable doesn't already exist, the `Prepend` method will create it:

            env = Environment()
            env.Prepend(NEW_VARIABLE = 'added')
            print "NEW_VARIABLE =", env['NEW_VARIABLE']

Which yields:

            % scons -Q
            NEW_VARIABLE = added
            scons: `.' is up to date.

# Chapter 8. Controlling the External Environment Used to Execute Build Commands

When SCons builds a target file, it does not execute the commands with the same external environment that you used to execute SCons. Instead, it uses the dictionary stored in the `$ENV` construction variable as the external environment for executing commands.

The most important ramification of this behavior is that the `PATH` environment variable, which controls where the operating system will look for commands and utilities, is not the same as in the external environment from which you called SCons. This means that SCons will not, by default, necessarily find all of the tools that you can execute from the command line.

The default value of the `PATH` environment variable on a POSIX system is `/usr/local/bin:/bin:/usr/bin`. The default value of the `PATH` environment variable on a Windows system comes from the Windows registry value for the command interpreter. If you want to execute any commands--compilers, linkers, etc.--that are not in these default locations, you need to set the `PATH` value in the `$ENV` dictionary in your construction environment.

The simplest way to do this is to initialize explicitly the value when you create the construction environment; this is one way to do that:

        path = ['/usr/local/bin', '/bin', '/usr/bin']
        env = Environment(ENV = {'PATH' : path})

Assign a dictionary to the `$ENV` construction variable in this way completely resets the external environment so that the only variable that will be set when external commands are executed will be the `PATH` value. If you want to use the rest of the values in `$ENV` and only set the value of `PATH`, the most straightforward way is probably:

        env['ENV']['PATH'] = ['/usr/local/bin', '/bin', '/usr/bin']

Note that SCons does allow you to define the directories in the `PATH` in a string, separated by the pathname-separator character for your system (':' on POSIX systems, ';' on Windows):

        env['ENV']['PATH'] = '/usr/local/bin:/bin:/usr/bin'

But doing so makes your `SConscript` file less portable, (although in this case that may not be a huge concern since the directories you list are likley system-specific, anyway).

## 8.1. Propagating `PATH` From the External Environment

You may want to propagate the external `PATH` to the execution environment for commands. You do this by initializing the `PATH` variable with the `PATH` value from the `os.environ` dictionary, which is Python's way of letting you get at the external environment:

          import os
          env = Environment(ENV = {'PATH' : os.environ['PATH']})

Alternatively, you may find it easier to just propagate the entire external environment to the execution environment for commands. This is simpler to code than explicity selecting the `PATH` value:

          import os
          env = Environment(ENV = os.environ)

Either of these will guarantee that SCons will be able to execute any command that you can execute from the command line. The drawback is that the build can behave differently if it's run by people with different `PATH` values in their environment--for example, if both the `/bin` and `/usr/local/bin` directories have different cc commands, then which one will be used to compile programs will depend on which directory is listed first in the user's `PATH` variable.

# Chapter 9. Controlling a Build From the Command Line

SCons provides a number of ways that allow the writer of the `SConscript` files to give users a great deal of control over how to run the builds.

## 9.1. Not Having to Specify Command-Line Options Each Time: the `SCONSFLAGS` Environment Variable

Users may find themselves supplying the same command-line options every time they run SCons. For example, a user might find that it saves time to specify a value of `-j 2` to run the builds in parallel. To avoid having to type `-j 2` by hand every time, you can set the external environment variable `SCONSFLAGS` to a string containing command-line options that you want SCons to use.

If, for example, you're using a POSIX shell that's compatible with the Bourne shell, and you always want SCons to use the `-Q` option, you can set the `SCONSFLAGS` environment as follows:

          % scons
          scons: Reading SConscript files ...
          scons: done reading SConscript files.
          scons: Building targets ...
              ... [build output] ...
          scons: done building targets.
          % export SCONSFLAGS="-Q"
          % scons
              ... [build output] ...

Users of csh-style shells on POSIX systems can set the `SCONSFLAGS` environment as follows:

          $ setenv SCONSFLAGS "-Q"

Windows users may typically want to set the `SCONSFLAGS` in the appropriate tab of the `System Properties` window.

## 9.2. Getting at Command-Line Targets

SCons supports a `COMMAND_LINE_TARGETS` variable that lets you get at the list of targets that the user specified on the command line. You can use the targets to manipulate the build in any way you wish. As a simple example, suppose that you want to print a reminder to the user whenever a specific program is built. You can do this by checking for the target in the `COMMAND_LINE_TARGETS` list:

          if 'bar' in COMMAND_LINE_TARGETS:
              print "Don't forget to copy `bar' to the archive!"
          Default(Program('foo.c'))
          Program('bar.c')

Then, running SCons with the default target works as it always does, but explicity specifying the bar target on the command line generates the warning message:

          % scons -Q
          cc -o foo.o -c foo.c
          cc -o foo foo.o
          % scons -Q bar
          Don't forget to copy `bar' to the archive!
          cc -o bar.o -c bar.c
          cc -o bar bar.o

Another practical use for the `COMMAND_LINE_TARGETS` variable might be to speed up a build by only reading certain subsidiary `SConscript` files if a specific target is requested.

## 9.3. Controlling the Default Targets

One of the most basic things you can control is which targets SCons will build by default--that is, when there are no targets specified on the command line. As mentioned previously, SCons will normally build every target in or below the current directory by default--that is, when you don't explicitly specify one or more targets on the command line. Sometimes, however, you may want to specify explicitly that only certain programs, or programs in certain directories, should be built by default. You do this with the `Default` function:

           env = Environment()
           hello = env.Program('hello.c')
           env.Program('goodbye.c')
           Default(hello)

This `SConstruct` file knows how to build two programs, hello and goodbye, but only builds the hello program by default:

           % scons -Q
           cc -o hello.o -c hello.c
           cc -o hello hello.o
           % scons -Q
           scons: `hello' is up to date.
           % scons -Q goodbye
           cc -o goodbye.o -c goodbye.c
           cc -o goodbye goodbye.o

Note that, even when you use the `Default` function in your `SConstruct` file, you can still explicitly specify the current directory (`.`) on the command line to tell SCons to build everything in (or below) the current directory:

           % scons -Q .
           cc -o goodbye.o -c goodbye.c
           cc -o goodbye goodbye.o
           cc -o hello.o -c hello.c
           cc -o hello hello.o

You can also call the `Default` function more than once, in which case each call adds to the list of targets to be built by default:

           env = Environment()
           prog1 = env.Program('prog1.c')
           Default(prog1)
           prog2 = env.Program('prog2.c')
           prog3 = env.Program('prog3.c')
           Default(prog3)

Or you can specify more than one target in a single call to the `Default` function:

           env = Environment()
           prog1 = env.Program('prog1.c')
           prog2 = env.Program('prog2.c')
           prog3 = env.Program('prog3.c')
           Default(prog1, prog3)

Either of these last two examples will build only the prog1 and prog3 programs by default:

           % scons -Q
           cc -o prog1.o -c prog1.c
           cc -o prog1 prog1.o
           cc -o prog3.o -c prog3.c
           cc -o prog3 prog3.o
           % scons -Q .
           cc -o prog2.o -c prog2.c
           cc -o prog2 prog2.o

You can list a directory as an argument to `Default`:

           env = Environment()
           env.Program(['prog1/main.c', 'prog1/foo.c'])
           env.Program(['prog2/main.c', 'prog2/bar.c'])
           Default('prog1')

In which case only the target(s) in that directory will be built by default:

           % scons -Q
           cc -o prog1/foo.o -c prog1/foo.c
           cc -o prog1/main.o -c prog1/main.c
           cc -o prog1/main prog1/main.o prog1/foo.o
           % scons -Q
           scons: `prog1' is up to date.
           % scons -Q .
           cc -o prog2/bar.o -c prog2/bar.c
           cc -o prog2/main.o -c prog2/main.c
           cc -o prog2/main prog2/main.o prog2/bar.o

Lastly, if for some reason you don't want any targets built by default, you can use the Python `None` variable:

           env = Environment()
           prog1 = env.Program('prog1.c')
           prog2 = env.Program('prog2.c')
           Default(None)

Which would produce build output like:

           % scons -Q
           scons: *** No targets specified and no Default() targets found.  Stop.
           % scons -Q .
           cc -o prog1.o -c prog1.c
           cc -o prog1 prog1.o
           cc -o prog2.o -c prog2.c
           cc -o prog2 prog2.o

### 9.3.1. Getting at the List of Default Targets

SCons supports a `DEFAULT_TARGETS` variable that lets you get at the current list of default targets. The `DEFAULT_TARGETS` variable has two important differences from the `COMMAND_LINE_TARGETS` variable. First, the `DEFAULT_TARGETS` variable is a list of internal SCons nodes, so you need to convert the list elements to strings if you want to print them or look for a specific target name. Fortunately, you can do this easily by using the Python `map` function to run the list through `str`:

             prog1 = Program('prog1.c')
             Default(prog1)
             print "DEFAULT_TARGETS is", map(str, DEFAULT_TARGETS)

(Keep in mind that all of the manipulation of the `DEFAULT_TARGETS` list takes place during the first phase when SCons is reading up the `SConscript` files, which is obvious if we leave off the `-Q` flag when we run SCons:)

             % scons
             scons: Reading SConscript files ...
             DEFAULT_TARGETS is ['prog1']
             scons: done reading SConscript files.
             scons: Building targets ...
             cc -o prog1.o -c prog1.c
             cc -o prog1 prog1.o
             scons: done building targets.

Second, the contents of the `DEFAULT_TARGETS` list change in response to calls to the `Default`: function, as you can see from the following `SConstruct` file:

             prog1 = Program('prog1.c')
             Default(prog1)
             print "DEFAULT_TARGETS is now", map(str, DEFAULT_TARGETS)
             prog2 = Program('prog2.c')
             Default(prog2)
             print "DEFAULT_TARGETS is now", map(str, DEFAULT_TARGETS)

Which yields the output:

             % scons
             scons: Reading SConscript files ...
             DEFAULT_TARGETS is now ['prog1']
             DEFAULT_TARGETS is now ['prog1', 'prog2']
             scons: done reading SConscript files.
             scons: Building targets ...
             cc -o prog1.o -c prog1.c
             cc -o prog1 prog1.o
             cc -o prog2.o -c prog2.c
             cc -o prog2 prog2.o
             scons: done building targets.

In practice, this simply means that you need to pay attention to the order in which you call the `Default` function and refer to the `DEFAULT_TARGETS` list, to make sure that you don't examine the list before you've added the default targets you expect to find in it.

## 9.4. Getting at the List of Build Targets, Regardless of Origin

We've already been introduced to the `COMMAND_LINE_TARGETS` variable, which contains a list of targets specified on the command line, and the `DEFAULT_TARGETS` variable, which contains a list of targets specified via calls to the `Default` method or function. Sometimes, however, you want a list of whatever targets SCons will try to build, regardless of whether the targets came from the command line or a `Default` call. You could code this up by hand, as follows:

          if COMMAND_LINE_TARGETS:
              targets = COMMAND_LINE_TARGETS
          else:
              targets = DEFAULT_TARGETS

SCons, however, provides a convenient `BUILD_TARGETS` variable that eliminates the need for this by-hand manipulation. Essentially, the `BUILD_TARGETS` variable contains a list of the command-line targets, if any were specified, and if no command-line targets were specified, it contains a list of the targets specified via the `Default` method or function.

Because `BUILD_TARGETS` may contain a list of SCons nodes, you must convert the list elements to strings if you want to print them or look for a specific target name, just like the `DEFAULT_TARGETS` list:

          prog1 = Program('prog1.c')
          Program('prog2.c')
          Default(prog1)
          print "BUILD_TARGETS is", map(str, BUILD_TARGETS)

Notice how the value of `BUILD_TARGETS` changes depending on whether a target is specified on the command line:

          % scons -Q
          BUILD_TARGETS is ['prog1']
          cc -o prog1.o -c prog1.c
          cc -o prog1 prog1.o
          % scons -Q prog2
          BUILD_TARGETS is ['prog2']
          cc -o prog2.o -c prog2.c
          cc -o prog2 prog2.o
          % scons -Q -c .
          BUILD_TARGETS is ['.']
          Removed prog1.o
          Removed prog1
          Removed prog2.o
          Removed prog2

## 9.5. Command-Line `variable`=`value` Build Options

You may want to control various aspects of your build by allowing the user to specify `variable`=`value` values on the command line. For example, suppose you want users to be able to build a debug version of a program by running SCons as follows:

          % scons -Q debug=1

SCons provides an `ARGUMENTS` dictionary that stores all of the `variable`=`value` assignments from the command line. This allows you to modify aspects of your build in response to specifications on the command line. (Note that unless you want to require that users always specify an option, you probably want to use the Python `ARGUMENTS.get()` function, which allows you to specify a default value to be used if there is no specification on the command line.)

The following code sets the `$CCFLAGS` construction variable in response to the `debug` flag being set in the `ARGUMENTS` dictionary:

           env = Environment()
           debug = ARGUMENTS.get('debug', 0)
           if int(debug):
               env.Append(CCFLAGS = '-g')
           env.Program('prog.c')

This results in the `-g` compiler option being used when `debug=1` is used on the command line:

           % scons -Q debug=0
           cc -o prog.o -c prog.c
           cc -o prog prog.o
           % scons -Q debug=0
           scons: `.' is up to date.
           % scons -Q debug=1
           cc -o prog.o -c -g prog.c
           cc -o prog prog.o
           % scons -Q debug=1
           scons: `.' is up to date.

Notice that SCons keeps track of the last values used to build the object files, and as a result correctly rebuilds the object and executable files only when the value of the `debug` argument has changed.

## 9.6. Controlling Command-Line Build Options

Being able to use a command-line build option like `debug=1` is handy, but it can be a chore to write specific Python code to recognize each such option and apply the values to a construction variable. To help with this, SCons supports a class to define such build options easily, and a mechanism to apply the build options to a construction environment. This allows you to control how the build options affect construction environments.

For example, suppose that you want users to set a `RELEASE` construction variable on the command line whenever the time comes to build a program for release, and that the value of this variable should be added to the command line with the appropriate `-D` option (or other command line option) to pass the value to the C compiler. Here's how you might do that by setting the appropriate value in a dictionary for the `$CPPDEFINES` construction variable:

             opts = Options()
             opts.Add('RELEASE', 'Set to 1 to build for release', 0)
             env = Environment(options = opts,
                               CPPDEFINES={'RELEASE_BUILD' : '${RELEASE}'})
             env.Program(['foo.c', 'bar.c'])

This `SConstruct` file first creates an `Options` object (the `opts = Options()` call), and then uses the object's `Add` method to indicate that the `RELEASE` option can be set on the command line, and that it's default value will be `0` (the third argument to the `Add` method). The second argument is a line of help text; we'll learn how to use it in the next section.

We then pass the created `Options` object as an `options` keyword argument to the `Environment` call used to create the construction environment. This then allows a user to set the `RELEASE` build option on the command line and have the variable show up in the command line used to build each object from a C source file:

          % scons -Q RELEASE=1
          cc -o bar.o -c -DRELEASE_BUILD=1 bar.c
          cc -o foo.o -c -DRELEASE_BUILD=1 foo.c
          cc -o foo foo.o bar.o

## 9.7. Providing Help for Command-Line Build Options

To make command-line build options most useful, you ideally want to provide some help text that will describe the available options when the user runs `scons -h`. You could write this text by hand, but SCons provides an easier way. `Options` objects support a `GenerateHelpText` method that will, as its name indicates, generate text that describes the various options that have been added to it. You then pass the output from this method to the `Help` function:

             opts = Options('custom.py')
             opts.Add('RELEASE', 'Set to 1 to build for release', 0)
             env = Environment(options = opts)
             Help(opts.GenerateHelpText(env))

SCons will now display some useful text when the `-h` option is used:

          % scons -Q -h

          RELEASE: Set to 1 to build for release
              default: 0
              actual: 0

          Use scons -H for help about command-line options.

Notice that the help output shows the default value, and the current actual value of the build option.

## 9.8. Reading Build Options From a File

Being able to use a command-line build option like `debug=1` is handy, but it can be a chore to write specific Python code to recognize each such option and apply the values to a construction variable. To help with this, SCons supports a class to define such build options easily and to read build option values from a file. This allows you to control how the build options affect construction environments. The way you do this is by specifying a file name when you call `Options`, like `custom.py` in the following example:

             opts = Options('custom.py')
             opts.Add('RELEASE', 'Set to 1 to build for release', 0)
             env = Environment(options = opts,
                               CPPDEFINES={'RELEASE_BUILD' : '${RELEASE}'})
             env.Program(['foo.c', 'bar.c'])
             Help(opts.GenerateHelpText(env))

This then allows us to control the `RELEASE` variable by setting it in the `custom.py` file:

          RELEASE = 1

Note that this file is actually executed like a Python script. Now when we run SCons:

          % scons -Q
          cc -o bar.o -c -DRELEASE_BUILD=1 bar.c
          cc -o foo.o -c -DRELEASE_BUILD=1 foo.c
          cc -o foo foo.o bar.o

And if we change the contents of `custom.py` to:

          RELEASE = 0

The object files are rebuilt appropriately with the new option:

          % scons -Q
          cc -o bar.o -c -DRELEASE_BUILD=0 bar.c
          cc -o foo.o -c -DRELEASE_BUILD=0 foo.c
          cc -o foo foo.o bar.o

## 9.9. Canned Build Options

SCons provides a number of functions that provide ready-made behaviors for various types of command-line build options.

### 9.9.1. True/False Values: the `BoolOption` Build Option

It's often handy to be able to specify an option that controls a simple Boolean variable with a `true` or `false` value. It would be even more handy to accomodate users who have different preferences for how to represent `true` or `false` values. The `BoolOption` function makes it easy to accomodate a variety of common values that represent `true` or `false`.

The `BoolOption` function takes three arguments: the name of the build option, the default value of the build option, and the help string for the option. It then returns appropriate information for passing to the `Add` method of an `Options` object, like so:

               opts = Options('custom.py')
               opts.Add(BoolOption('RELEASE', 'Set to build for release', 0))
               env = Environment(options = opts,
                                 CPPDEFINES={'RELEASE_BUILD' : '${RELEASE}'})
               env.Program('foo.c')

With this build option, the `RELEASE` variable can now be enabled by setting it to the value `yes` or `t`:

            % scons -Q RELEASE=yes foo.o
            cc -o foo.o -c -DRELEASE_BUILD=True foo.c

            % scons -Q RELEASE=t foo.o
            cc -o foo.o -c -DRELEASE_BUILD=True foo.c

Other values that equate to `true` include `y`, `1`, `on` and `all`.

Conversely, `RELEASE` may now be given a `false` value by setting it to `no` or `f`:

            % scons -Q RELEASE=no foo.o
            cc -o foo.o -c -DRELEASE_BUILD=False foo.c

            % scons -Q RELEASE=f foo.o
            cc -o foo.o -c -DRELEASE_BUILD=False foo.c

Other values that equate to `false` include `n`, `0`, `off` and `none`.

Lastly, if a user tries to specify any other value, SCons supplies an appropriate error message:

            % scons -Q RELEASE=bad_value foo.o

            scons: *** Error converting option: RELEASE
            Invalid value for boolean option: bad_value
            File "/home/my/project/SConstruct", line 4, in <module>

### 9.9.2. Single Value From a List: the `EnumOption` Build Option

Suppose that we want a user to be able to set a `COLOR` option that selects a background color to be displayed by an application, but that we want to restrict the choices to a specific set of allowed colors. This can be set up quite easily using the `EnumOption`, which takes a list of `allowed_values` in addition to the variable name, default value, and help text arguments:

               opts = Options('custom.py')
               opts.Add(EnumOption('COLOR', 'Set background color', 'red',
                                   allowed_values=('red', 'green', 'blue')))
               env = Environment(options = opts,
                                 CPPDEFINES={'COLOR' : '"${COLOR}"'})
               env.Program('foo.c')

The user can now explicity set the `COLOR` build option to any of the specified allowed values:

            % scons -Q COLOR=red foo.o
            cc -o foo.o -c -DCOLOR="red" foo.c
            % scons -Q COLOR=blue foo.o
            cc -o foo.o -c -DCOLOR="blue" foo.c
            % scons -Q COLOR=green foo.o
            cc -o foo.o -c -DCOLOR="green" foo.c

But, almost more importantly, an attempt to set `COLOR` to a value that's not in the list generates an error message:

            % scons -Q COLOR=magenta foo.o

            scons: *** Invalid value for option COLOR: magenta
            File "/home/my/project/SConstruct", line 5, in <module>

The `EnumOption` function also supports a way to map alternate names to allowed values. Suppose, for example, that we want to allow the user to use the word `navy` as a synonym for `blue`. We do this by adding a `map` dictionary that will map its key values to the desired legal value:

               opts = Options('custom.py')
               opts.Add(EnumOption('COLOR', 'Set background color', 'red',
                                   allowed_values=('red', 'green', 'blue'),
                                   map={'navy':'blue'}))
               env = Environment(options = opts,
                                 CPPDEFINES={'COLOR' : '"${COLOR}"'})
               env.Program('foo.c')

As desired, the user can then use `navy` on the command line, and SCons will translate it into `blue` when it comes time to use the `COLOR` option to build a target:

            % scons -Q COLOR=navy foo.o
            cc -o foo.o -c -DCOLOR="blue" foo.c

By default, when using the `EnumOption` function, arguments that differ from the legal values only in case are treated as illegal values:

            % scons -Q COLOR=Red foo.o

            scons: *** Invalid value for option COLOR: Red
            File "/home/my/project/SConstruct", line 5, in <module>
            % scons -Q COLOR=BLUE foo.o

            scons: *** Invalid value for option COLOR: BLUE
            File "/home/my/project/SConstruct", line 5, in <module>
            % scons -Q COLOR=nAvY foo.o

            scons: *** Invalid value for option COLOR: nAvY
            File "/home/my/project/SConstruct", line 5, in <module>

The `EnumOption` function can take an additional `ignorecase` keyword argument that, when set to `1`, tells SCons to allow case differences when the values are specified:

               opts = Options('custom.py')
               opts.Add(EnumOption('COLOR', 'Set background color', 'red',
                                   allowed_values=('red', 'green', 'blue'),
                                   map={'navy':'blue'},
                                   ignorecase=1))
               env = Environment(options = opts,
                                 CPPDEFINES={'COLOR' : '"${COLOR}"'})
               env.Program('foo.c')

Which yields the output:

            % scons -Q COLOR=Red foo.o
            cc -o foo.o -c -DCOLOR="Red" foo.c
            % scons -Q COLOR=BLUE foo.o
            cc -o foo.o -c -DCOLOR="BLUE" foo.c
            % scons -Q COLOR=nAvY foo.o
            cc -o foo.o -c -DCOLOR="blue" foo.c
            % scons -Q COLOR=green foo.o
            cc -o foo.o -c -DCOLOR="green" foo.c

Notice that an `ignorecase` value of `1` preserves the case-spelling that the user supplied. If you want SCons to translate the names into lower-case, regardless of the case used by the user, specify an `ignorecase` value of `2`:

               opts = Options('custom.py')
               opts.Add(EnumOption('COLOR', 'Set background color', 'red',
                                   allowed_values=('red', 'green', 'blue'),
                                   map={'navy':'blue'},
                                   ignorecase=2))
               env = Environment(options = opts,
                                 CPPDEFINES={'COLOR' : '"${COLOR}"'})
               env.Program('foo.c')

Now SCons will use values of `red`, `green` or `blue` regardless of how the user spells those values on the command line:

            % scons -Q COLOR=Red foo.o
            cc -o foo.o -c -DCOLOR="red" foo.c
            % scons -Q COLOR=nAvY foo.o
            cc -o foo.o -c -DCOLOR="blue" foo.c
            % scons -Q COLOR=GREEN foo.o
            cc -o foo.o -c -DCOLOR="green" foo.c

### 9.9.3. Multiple Values From a List: the `ListOption` Build Option

Another way in which you might want to allow users to control build option is to specify a list of one or more legal values. SCons supports this through the `ListOption` function. If, for example, we want a user to be able to set a `COLORS` option to one or more of the legal list of values:

               opts = Options('custom.py')
               opts.Add(ListOption('COLORS', 'List of colors', 0,
                                   ['red', 'green', 'blue']))
               env = Environment(options = opts,
                                 CPPDEFINES={'COLORS' : '"${COLORS}"'})
               env.Program('foo.c')

A user can now specify a comma-separated list of legal values, which will get translated into a space-separated list for passing to the any build commands:

            % scons -Q COLORS=red,blue foo.o
            cc -o foo.o -c -DCOLORS="red blue" foo.c
            % scons -Q COLORS=blue,green,red foo.o
            cc -o foo.o -c -DCOLORS="blue green red" foo.c

In addition, the `ListOption` function allows the user to specify explicit keywords of `all` or `none` to select all of the legal values, or none of them, respectively:

            % scons -Q COLORS=all foo.o
            cc -o foo.o -c -DCOLORS="red green blue" foo.c
            % scons -Q COLORS=none foo.o
            cc -o foo.o -c -DCOLORS="" foo.c

And, of course, an illegal value still generates an error message:

            % scons -Q COLORS=magenta foo.o

            scons: *** Error converting option: COLORS
            Invalid value(s) for option: magenta
            File "/home/my/project/SConstruct", line 5, in <module>

### 9.9.4. Path Names: the `PathOption` Build Option

SCons supports a `PathOption` function to make it easy to create a build option to control an expected path name. If, for example, you need to define a variable in the preprocessor that controls the location of a configuration file:

               opts = Options('custom.py')
               opts.Add(PathOption('CONFIG',
                                   'Path to configuration file',
                                   '/etc/my_config'))
               env = Environment(options = opts,
                                 CPPDEFINES={'CONFIG_FILE' : '"$CONFIG"'})
               env.Program('foo.c')

This then allows the user to override the `CONFIG` build option on the command line as necessary:

            % scons -Q foo.o
            cc -o foo.o -c -DCONFIG_FILE="/etc/my_config" foo.c
            % scons -Q CONFIG=/usr/local/etc/other_config foo.o
            scons: `foo.o' is up to date.

By default, `PathOption` checks to make sure that the specified path exists and generates an error if it doesn't:

            % scons -Q CONFIG=/does/not/exist foo.o

            scons: *** Path for option CONFIG does not exist: /does/not/exist
            File "/home/my/project/SConstruct", line 6, in <module>

`PathOption` provides a number of methods that you can use to change this behavior. If you want to ensure that any specified paths are, in fact, files and not directories, use the `PathOption.PathIsFile` method:

               opts = Options('custom.py')
               opts.Add(PathOption('CONFIG',
                                   'Path to configuration file',
                                   '/etc/my_config',
                                   PathOption.PathIsFile))
               env = Environment(options = opts,
                                 CPPDEFINES={'CONFIG_FILE' : '"$CONFIG"'})
               env.Program('foo.c')

Conversely, to ensure that any specified paths are directories and not files, use the `PathOption.PathIsDir` method:

               opts = Options('custom.py')
               opts.Add(PathOption('DBDIR',
                                   'Path to database directory',
                                   '/var/my_dbdir',
                                   PathOption.PathIsDir))
               env = Environment(options = opts,
                                 CPPDEFINES={'DBDIR' : '"$DBDIR"'})
               env.Program('foo.c')

If you want to make sure that any specified paths are directories, and you would like the directory created if it doesn't already exist, use the `PathOption.PathIsDirCreate` method:

               opts = Options('custom.py')
               opts.Add(PathOption('DBDIR',
                                   'Path to database directory',
                                   '/var/my_dbdir',
                                   PathOption.PathIsDirCreate))
               env = Environment(options = opts,
                                 CPPDEFINES={'DBDIR' : '"$DBDIR"'})
               env.Program('foo.c')

Lastly, if you don't care whether the path exists, is a file, or a directory, use the `PathOption.PathAccept` method to accept any path that the user supplies:

               opts = Options('custom.py')
               opts.Add(PathOption('OUTPUT',
                                   'Path to output file or directory',
                                   None,
                                   PathOption.PathAccept))
               env = Environment(options = opts,
                                 CPPDEFINES={'OUTPUT' : '"$OUTPUT"'})
               env.Program('foo.c')

### 9.9.5. Enabled/Disabled Path Names: the `PackageOption` Build Option

Sometimes you want to give users even more control over a path name variable, allowing them to explicitly enable or disable the path name by using `yes` or `no` keywords, in addition to allow them to supply an explicit path name. SCons supports the `PackageOption` function to support this:

               opts = Options('custom.py')
               opts.Add(PackageOption('PACKAGE',
                                      'Location package',
                                      '/opt/location'))
               env = Environment(options = opts,
                                 CPPDEFINES={'PACKAGE' : '"$PACKAGE"'})
               env.Program('foo.c')

When the `SConscript` file uses the `PackageOption` funciton, user can now still use the default or supply an overriding path name, but can now explicitly set the specified variable to a value that indicates the package should be enabled (in which case the default should be used) or disabled:

            % scons -Q foo.o
            cc -o foo.o -c -DPACKAGE="/opt/location" foo.c
            % scons -Q PACKAGE=/usr/local/location foo.o
            cc -o foo.o -c -DPACKAGE="/usr/local/location" foo.c
            % scons -Q PACKAGE=yes foo.o
            cc -o foo.o -c -DPACKAGE="True" foo.c
            % scons -Q PACKAGE=no foo.o
            cc -o foo.o -c -DPACKAGE="False" foo.c

## 9.10. Adding Multiple Command-Line Build Options at Once

Lastly, SCons provides a way to add multiple build options to an `Options` object at once. Instead of having to call the `Add` method multiple times, you can call the `AddOptions` method with a list of build options to be added to the object. Each build option is specified as either a tuple of arguments, just like you'd pass to the `Add` method itself, or as a call to one of the canned functions for pre-packaged command-line build options. in any order:

            opts = Options()
            opts.AddOptions(
                ('RELEASE', 'Set to 1 to build for release', 0),
                ('CONFIG', 'Configuration file', '/etc/my_config'),
                BoolOption('warnings', 'compilation with -Wall and similiar', 1),
                EnumOption('debug', 'debug output and symbols', 'no',
                           allowed_values=('yes', 'no', 'full'),
                           map={}, ignorecase=0),  # case sensitive
                ListOption('shared',
                           'libraries to build as shared libraries',
                           'all',
                           names = list_of_libs),
                PackageOption('x11',
                              'use X11 installed here (yes = search some places)',
                              'yes'),
                PathOption('qtdir', 'where the root of Qt is installed', qtdir),
            )

# Chapter 10. Providing Build Help: the `Help` Function

It's often very useful to be able to give users some help that describes the specific targets, build options, etc., that can be used for your build. SCons provides the `Help` function to allow you to specify this help text:

          Help("""
          Type: 'scons program' to build the production program,
                'scons debug' to build the debug version.
          """)

(Note the above use of the Python triple-quote syntax, which comes in very handy for specifying multi-line strings like help text.)

When the `SConstruct` or `SConscript` files contain such a call to the `Help` function, the specified help text will be displayed in response to the SCons `-h` option:

          % scons -h
          scons: Reading SConscript files ...
          scons: done reading SConscript files.

          Type: 'scons program' to build the production program,
                'scons debug' to build the debug version.

          Use scons -H for help about command-line options.

The `SConscript` files may contain multiple calls to the `Help` function, in which case the specified text(s) will be concatenated when displayed. This allows you to split up the help text across multiple `SConscript` files. In this situation, the order in which the `SConscript` files are called will determine the order in which the `Help` functions are called, which will determine the order in which the various bits of text will get concatenated.

Another use would be to make the help text conditional on some variable. For example, suppose you only want to display a line about building a Windows-only version of a program when actually run on Windows. The following `SConstruct` file:

          env = Environment()

          Help("\nType: 'scons program' to build the production program.\n")

          if env['PLATFORM'] == 'win32':
              Help("\nType: 'scons windebug' to build the Windows debug version.\n")

Will display the complete help text on Windows:

          C:\>scons -h
          scons: Reading SConscript files ...
          scons: done reading SConscript files.

          Type: 'scons program' to build the production program.

          Type: 'scons windebug' to build the Windows debug version.

          Use scons -H for help about command-line options.

But only show the relevant option on a Linux or UNIX system:

          % scons -h
          scons: Reading SConscript files ...
          scons: done reading SConscript files.

          Type: 'scons program' to build the production program.

          Use scons -H for help about command-line options.

If there is no `Help` text in the `SConstruct` or `SConscript` files, SCons will revert to displaying its standard list that describes the SCons command-line options. This list is also always displayed whenever the `-H` option is used.

# Chapter 11. Installing Files in Other Directories: the `Install` Builder

Once a program is built, it is often appropriate to install it in another directory for public use. You use the `Install` method to arrange for a program, or any other file, to be copied into a destination directory:

         env = Environment()
         hello = env.Program('hello.c')
         env.Install('/usr/bin', hello)

Note, however, that installing a file is still considered a type of file "build." This is important when you remember that the default behavior of SCons is to build files in or below the current directory. If, as in the example above, you are installing files in a directory outside of the top-level `SConstruct` file's directory tree, you must specify that directory (or a higher directory, such as `/`) for it to install anything there:

         % scons -Q
         cc -o hello.o -c hello.c
         cc -o hello hello.o
         % scons -Q /usr/bin
         Install file: "hello" as "/usr/bin/hello"

It can, however, be cumbersome to remember (and type) the specific destination directory in which the program (or any other file) should be installed. This is an area where the `Alias` function comes in handy, allowing you, for example, to create a pseudo-target named `install` that can expand to the specified destination directory:

         env = Environment()
         hello = env.Program('hello.c')
         env.Install('/usr/bin', hello)
         env.Alias('install', '/usr/bin')

This then yields the more natural ability to install the program in its destination as follows:

         % scons -Q
         cc -o hello.o -c hello.c
         cc -o hello hello.o
         % scons -Q install
         Install file: "hello" as "/usr/bin/hello"

## 11.1. Installing Multiple Files in a Directory

You can install multiple files into a directory simply by calling the `Install` function multiple times:

           env = Environment()
           hello = env.Program('hello.c')
           goodbye = env.Program('goodbye.c')
           env.Install('/usr/bin', hello)
           env.Install('/usr/bin', goodbye)
           env.Alias('install', '/usr/bin')

Or, more succinctly, listing the multiple input files in a list (just like you can do with any other builder):

           env = Environment()
           hello = env.Program('hello.c')
           goodbye = env.Program('goodbye.c')
           env.Install('/usr/bin', [hello, goodbye])
           env.Alias('install', '/usr/bin')

Either of these two examples yields:

           % scons -Q install
           cc -o goodbye.o -c goodbye.c
           cc -o goodbye goodbye.o
           Install file: "goodbye" as "/usr/bin/goodbye"
           cc -o hello.o -c hello.c
           cc -o hello hello.o
           Install file: "hello" as "/usr/bin/hello"

## 11.2. Installing a File Under a Different Name

The `Install` method preserves the name of the file when it is copied into the destination directory. If you need to change the name of the file when you copy it, use the `InstallAs` function:

           env = Environment()
           hello = env.Program('hello.c')
           env.InstallAs('/usr/bin/hello-new', hello)
           env.Alias('install', '/usr/bin')

This installs the `hello` program with the name `hello-new` as follows:

           % scons -Q install
           cc -o hello.o -c hello.c
           cc -o hello hello.o
           Install file: "hello" as "/usr/bin/hello-new"

## 11.3. Installing Multiple Files Under Different Names

Lastly, if you have multiple files that all need to be installed with different file names, you can either call the `InstallAs` function multiple times, or as a shorthand, you can supply same-length lists for both the target and source arguments:

           env = Environment()
           hello = env.Program('hello.c')
           goodbye = env.Program('goodbye.c')
           env.InstallAs(['/usr/bin/hello-new',
                          '/usr/bin/goodbye-new'],
                         [hello, goodbye])
           env.Alias('install', '/usr/bin')

In this case, the `InstallAs` function loops through both lists simultaneously, and copies each source file into its corresponding target file name:

           % scons -Q install
           cc -o goodbye.o -c goodbye.c
           cc -o goodbye goodbye.o
           Install file: "goodbye" as "/usr/bin/goodbye-new"
           cc -o hello.o -c hello.c
           cc -o hello hello.o
           Install file: "hello" as "/usr/bin/hello-new"

# Chapter 12. Platform-Independent File System Manipulation

SCons provides a number of platform-independent functions, called `factories`, that perform common file system manipulations like copying, moving or deleting files and directories, or making directories. These functions are `factories` because they don't perform the action at the time they're called, they each return an `Action` object that can be executed at the appropriate time.

## 12.1. Copying Files or Directories: The `Copy` Factory

Suppose you want to arrange to make a copy of a file, and the `Install` builder isn't appropriate because it may make a hard link on POSIX systems. One way would be to use the `Copy` action factory in conjunction with the `Command` builder:

            Command("file.out", "file.in", Copy("$TARGET", "$SOURCE"))

Notice that the action returned by the `Copy` factory will expand the `$TARGET` and `$SOURCE` strings at the time `file.out` is built, and that the order of the arguments is the same as that of a builder itself--that is, target first, followed by source:

           % scons -Q
           Copy("file.out", "file.in")

You can, of course, name a file explicitly instead of using `$TARGET` or `$SOURCE`:

          Command("file.out", [], Copy("$TARGET", "file.in"))

Which executes as:

          % scons -Q
          Copy("file.out", "file.in")

The usefulness of the `Copy` factory becomes more apparent when you use it in a list of actions passed to the `Command` builder. For example, suppose you needed to run a file through a utility that only modifies files in-place, and can't "pipe" input to output. One solution is to copy the source file to a temporary file name, run the utility, and then copy the modified temporary file to the target, which the `Copy` factory makes extremely easy:

          Command("file.out", "file.in",
                  [
                    Copy("tempfile", "$SOURCE"),
                    "modify tempfile",
                    Copy("$TARGET", "tempfile"),
                  ])

The output then looks like:

          % scons -Q
          Copy("tempfile", "file.in")
          modify tempfile
          Copy("file.out", "tempfile")

## 12.2. Deleting Files or Directories: The `Delete` Factory

If you need to delete a file, then the `Delete` factory can be used in much the same way as the `Copy` factory. For example, if we want to make sure that the temporary file in our last example doesn't exist before we copy to it, we could add `Delete` to the beginning of the command list:

          Command("file.out", "file.in",
                  [
                    Delete("tempfile"),
                    Copy("tempfile", "$SOURCE"),
                    "modify tempfile",
                    Copy("$TARGET", "tempfile"),
                  ])

When then executes as follows:

          % scons -Q
          Delete("tempfile")
          Copy("tempfile", "file.in")
          modify tempfile
          Copy("file.out", "tempfile")

Of course, like all of these `Action` factories, the `Delete` factory also expands `$TARGET` and `$SOURCE` variables appropriately. For example:

          Command("file.out", "file.in",
                  [
                    Delete("$TARGET"),
                    Copy("$TARGET", "$SOURCE")
                  ])

Executes as:

          % scons -Q
          Delete("file.out")
          Copy("file.out", "file.in")

(Note, however, that you typically don't need to call the `Delete` factory explicitly in this way; by default, SCons deletes its target(s) for you before executing any action.

## 12.3. Moving (Renaming) Files or Directories: The `Move` Factory

The `Move` factory allows you to rename a file or directory. For example, if we don't want to copy the temporary file, we could use:

          Command("file.out", "file.in",
                  [
                    Copy("tempfile", "$SOURCE"),
                    "modify tempfile",
                    Move("$TARGET", "tempfile"),
                  ])

Which would execute as:

          % scons -Q
          Copy("tempfile", "file.in")
          modify tempfile
          Move("file.out", "tempfile")

## 12.4. Updating the Modification Time of a File: The `Touch` Factory

If you just need to update the recorded modification time for a file, use the `Touch` factory:

          Command("file.out", "file.in",
                  [
                    Copy("$TARGET", "$SOURCE"),
                    Touch("$TARGET"),
                  ])

Which executes as:

          % scons -Q
          Copy("file.out", "file.in")
          Touch("file.out")

## 12.5. Creating a Directory: The `Mkdir` Factory

If you need to create a directory, use the `Mkdir` factory. For example, if we need to process a file in a temporary directory in which the processing tool will create other files that we don't care about, you could use:

          Command("file.out", "file.in",
                  [
                    Delete("tempdir"),
                    Mkdir("tempdir"),
                    Copy("tempdir/${SOURCE.file}", "$SOURCE"),
                    "process tempdir",
                    Move("$TARGET", "tempdir/output_file"),
                    Delete("tempdir"),
                  ])

Which executes as:

          % scons -Q
          Delete("tempdir")
          Mkdir("tempdir")
          Copy("tempdir/file.in", "file.in")
          process tempdir
          Move("file.out", "tempdir/output_file")
          scons: *** [file.out] No such file or directory

## 12.6. Changing File or Directory Permissions: The `Chmod` Factory

To change permissions on a file or directory, use the `Chmod` factory. The permission argument uses POSIX-style permission bits and should typically be expressed as an octal, not decimal, number:

          Command("file.out", "file.in",
                  [
                    Copy("$TARGET", "$SOURCE"),
                    Chmod("$TARGET", 0755),
                  ])

Which executes:

          % scons -Q
          Copy("file.out", "file.in")
          Chmod("file.out", 0755)

## 12.7. Executing an action immediately: the `Execute` Function

We've been showing you how to use `Action` factories in the `Command` function. You can also execute an `Action` returned by a factory (or actually, any `Action`) at the time the `SConscript` file is read by wrapping it up in the `Execute` function. For example, if we need to make sure that a directory exists before we build any targets,

          Execute(Mkdir('/tmp/my_temp_directory'))

Notice that this will create the directory while the `SConscript` file is being read:

          % scons
          scons: Reading SConscript files ...
          Mkdir("/tmp/my_temp_directory")
          scons: done reading SConscript files.
          scons: Building targets ...
          scons: `.' is up to date.
          scons: done building targets.

If you're familiar with Python, you may wonder why you would want to use this instead of just calling the native Python `os.mkdir()` function. The advantage here is that the `Mkdir` action will behave appropriately if the user specifies the SCons `-n` or `-q` options--that is, it will print the action but not actually make the directory when `-n` is specified, or make the directory but not print the action when `-q` is specified.

# Chapter 13. Preventing Removal of Targets

There are two occasions when SCons will, by default, remove target files. The first is when SCons determines that an target file needs to be rebuilt and removes the existing version of the target before executing The second is when SCons is invoked with the `-c` option to "clean" a tree of its built targets. These behaviours can be suppressed with the `Precious` and `NoClean` functions, respectively.

## 13.1. Preventing target removal during build: the `Precious` Function

By default, SCons removes targets before building them. Sometimes, however, this is not what you want. For example, you may want to update a library incrementally, not by having it deleted and then rebuilt from all of the constituent object files. In such cases, you can use the `Precious` method to prevent SCons from removing the target before it is built:

            env = Environment(RANLIBCOM='')
            lib = env.Library('foo', ['f1.c', 'f2.c', 'f3.c'])
            env.Precious(lib)

Although the output doesn't look any different, SCons does not, in fact, delete the target library before rebuilding it:

            % scons -Q
            cc -o f1.o -c f1.c
            cc -o f2.o -c f2.c
            cc -o f3.o -c f3.c
            ar rc libfoo.a f1.o f2.o f3.o

SCons will, however, still delete files marked as `Precious` when the `-c` option is used.

## 13.2. Preventing target removal during clean: the `NoClean` Function

By default, SCons removes all built targets when invoked with the `-c` option to clean a source tree of built targets. Sometimes, however, this is not what you want. For example, you may want to remove only intermediate generated files (such as object files), but leave the final targets (the libraries) untouched. In such cases, you can use the `NoClean` method to prevent SCons from removing a target during a clean:

            env = Environment(RANLIBCOM='')
            lib = env.Library('foo', ['f1.c', 'f2.c', 'f3.c'])
            env.NoClean(lib)

Notice that the `libfoo.a` is not listed as a removed file:

            % scons -Q
            cc -o f1.o -c f1.c
            cc -o f2.o -c f2.c
            cc -o f3.o -c f3.c
            ar rc libfoo.a f1.o f2.o f3.o
            % scons -c
            scons: Reading SConscript files ...
            scons: done reading SConscript files.
            scons: Cleaning targets ...
            Removed f1.o
            Removed f2.o
            Removed f3.o
            scons: done cleaning targets.

## 13.3. Removing additional files during clean: the `Clean` Function

There may be additional files that you want removed when the `-c` option is used, but which SCons doesn't know about because they're not normal target files. For example, perhaps a command you invoke creates a log file as part of building the target file you want. You would like the log file cleaned, but you don't want to have to teach SCons that the command "builds" two files.

You can use the `Clean` function to arrange for additional files to be removed when the `-c` option is used. Notice, however, that the `Clean` function takes two arguments, and the second argument is the name of the additional file you want cleaned (`foo.log` in this example):

            t = Command('foo.out', 'foo.in', 'build -o $TARGET $SOURCE')
            Clean(t, 'foo.log')

The first argument is the target with which you want the cleaning of this additional file associated. In the above example, we've used the return value from the `Command` function, which represents the `foo.out` target. Now whenever the `foo.out` target is cleaned by the `-c` option, the `foo.log` file will be removed as well:

            % scons -Q
            build -o foo.out foo.in
            % scons -Q -c
            Removed foo.out
            Removed foo.log

# Chapter 14. Hierarchical Builds

The source code for large software projects rarely stays in a single directory, but is nearly always divided into a hierarchy of directories. Organizing a large software build using SCons involves creating a hierarchy of build scripts using the `SConscript` function.

## 14.1. `SConscript` Files

As we've already seen, the build script at the top of the tree is called `SConstruct`. The top-level `SConstruct` file can use the `SConscript` function to include other subsidiary scripts in the build. These subsidiary scripts can, in turn, use the `SConscript` function to include still other scripts in the build. By convention, these subsidiary scripts are usually named `SConscript`. For example, a top-level `SConstruct` file might arrange for four subsidiary scripts to be included in the build as follows:

          SConscript(['drivers/display/SConscript',
                      'drivers/mouse/SConscript',
                      'parser/SConscript',
                      'utilities/SConscript'])

In this case, the `SConstruct` file lists all of the `SConscript` files in the build explicitly. (Note, however, that not every directory in the tree necessarily has an `SConscript` file.) Alternatively, the `drivers` subdirectory might contain an intermediate `SConscript` file, in which case the `SConscript` call in the top-level `SConstruct` file would look like:

          SConscript(['drivers/SConscript',
                      'parser/SConscript',
                      'utilities/SConscript'])

And the subsidiary `SConscript` file in the `drivers` subdirectory would look like:

          SConscript(['display/SConscript',
                      'mouse/SConscript'])

Whether you list all of the `SConscript` files in the top-level `SConstruct` file, or place a subsidiary `SConscript` file in intervening directories, or use some mix of the two schemes, is up to you and the needs of your software.

## 14.2. Path Names Are Relative to the `SConscript` Directory

Subsidiary `SConscript` files make it easy to create a build hierarchy because all of the file and directory names in a subsidiary `SConscript` files are interpreted relative to the directory in which the `SConscript` file lives. Typically, this allows the `SConscript` file containing the instructions to build a target file to live in the same directory as the source files from which the target will be built, making it easy to update how the software is built whenever files are added or deleted (or other changes are made).

For example, suppose we want to build two programs `prog1` and `prog2` in two separate directories with the same names as the programs. One typical way to do this would be with a top-level `SConstruct` file like this:

          SConscript(['prog1/SConscript',
                      'prog2/SConscript'])

And subsidiary `SConscript` files that look like this:

          env = Environment()
          env.Program('prog1', ['main.c', 'foo1.c', 'foo2.c'])

And this:

          env = Environment()
          env.Program('prog2', ['main.c', 'bar1.c', 'bar2.c'])

Then, when we run SCons in the top-level directory, our build looks like:

           % scons -Q
           cc -o prog1/foo1.o -c prog1/foo1.c
           cc -o prog1/foo2.o -c prog1/foo2.c
           cc -o prog1/main.o -c prog1/main.c
           cc -o prog1/prog1 prog1/main.o prog1/foo1.o prog1/foo2.o
           cc -o prog2/bar1.o -c prog2/bar1.c
           cc -o prog2/bar2.o -c prog2/bar2.c
           cc -o prog2/main.o -c prog2/main.c
           cc -o prog2/prog2 prog2/main.o prog2/bar1.o prog2/bar2.o

Notice the following: First, you can have files with the same names in multiple directories, like main.c in the above example. Second, unlike standard recursive use of Make, SCons stays in the top-level directory (where the `SConstruct` file lives) and issues commands that use the path names from the top-level directory to the target and source files within the hierarchy.

## 14.3. Top-Level Path Names in Subsidiary `SConscript` Files

If you need to use a file from another directory, it's sometimes more convenient to specify the path to a file in another directory from the top-level `SConstruct` directory, even when you're using that file in a subsidiary `SConscript` file in a subdirectory. You can tell SCons to interpret a path name as relative to the top-level `SConstruct` directory, not the local directory of the `SConscript` file, by appending a `#` (hash mark) to the beginning of the path name:

           env = Environment()
           env.Program('prog', ['main.c', '#lib/foo1.c', 'foo2.c'])

In this example, the `lib` directory is directly underneath the top-level `SConstruct` directory. If the above `SConscript` file is in a subdirectory named `src/prog`, the output would look like:

           % scons -Q
           cc -o lib/foo1.o -c lib/foo1.c
           cc -o src/prog/foo2.o -c src/prog/foo2.c
           cc -o src/prog/main.o -c src/prog/main.c
           cc -o src/prog/prog src/prog/main.o lib/foo1.o src/prog/foo2.o

(Notice that the `lib/foo1.o` object file is built in the same directory as its source file. See Chapter 15, below, for information about how to build the object file in a different subdirectory.)

## 14.4. Absolute Path Names

Of course, you can always specify an absolute path name for a file--for example:

           env = Environment()
           env.Program('prog', ['main.c', '/usr/joe/lib/foo1.c', 'foo2.c'])

Which, when executed, would yield:

           % scons -Q
           cc -o src/prog/foo2.o -c src/prog/foo2.c
           cc -o src/prog/main.o -c src/prog/main.c
           cc -o /usr/joe/lib/foo1.o -c /usr/joe/lib/foo1.c
           cc -o src/prog/prog src/prog/main.o /usr/joe/lib/foo1.o src/prog/foo2.o

(As was the case with top-relative path names, notice that the `/usr/joe/lib/foo1.o` object file is built in the same directory as its source file. See Chapter 15, below, for information about how to build the object file in a different subdirectory.)

## 14.5. Sharing Environments (and Other Variables) Between `SConscript` Files

In the previous example, each of the subsidiary `SConscript` files created its own construction environment by calling `Environment` separately. This obviously works fine, but if each program must be built with the same construction variables, it's cumbersome and error-prone to initialize separate construction environments in the same way over and over in each subsidiary `SConscript` file.

SCons supports the ability to export variables from a parent `SConscript` file to its subsidiary `SConscript` files, which allows you to share common initialized values throughout your build hierarchy.

### 14.5.1. Exporting Variables

There are two ways to export a variable, such as a construction environment, from an `SConscript` file, so that it may be used by other `SConscript` files. First, you can call the `Export` function with a list of variables, or a string of white-space separated variable names. Each call to `Export` adds one or more variables to a global list of variables that are available for import by other `SConscript` files.

            env = Environment()
            Export('env')

You may export more than one variable name at a time:

            env = Environment()
            debug = ARGUMENTS['debug']
            Export('env', 'debug')

Because white space is not legal in Python variable names, the `Export` function will even automatically split a string into separate names for you:

            Export('env debug')

Second, you can specify a list of variables to export as a second argument to the `SConscript` function call:

            SConscript('src/SConscript', 'env')

Or as the `exports` keyword argument:

            SConscript('src/SConscript', exports='env')

These calls export the specified variables to only the listed `SConscript` files. You may, however, specify more than one `SConscript` file in a list:

            SConscript(['src1/SConscript',
                        'src2/SConscript'], exports='env')

This is functionally equivalent to calling the `SConscript` function multiple times with the same `exports` argument, one per `SConscript` file.

### 14.5.2. Importing Variables

Once a variable has been exported from a calling `SConscript` file, it may be used in other `SConscript` files by calling the `Import` function:

            Import('env')
            env.Program('prog', ['prog.c'])

The `Import` call makes the `env` construction environment available to the `SConscript` file, after which the variable can be used to build programs, libraries, etc.

Like the `Export` function, the `Import` function can be used with multiple variable names:

            Import('env', 'debug')
            env = env.Clone(DEBUG = debug)
            env.Program('prog', ['prog.c'])

And the `Import` function will similarly split a string along white-space into separate variable names:

            Import('env debug')
            env = env.Clone(DEBUG = debug)
            env.Program('prog', ['prog.c'])

Lastly, as a special case, you may import all of the variables that have been exported by supplying an asterisk to the `Import` function:

            Import('*')
            env = env.Clone(DEBUG = debug)
            env.Program('prog', ['prog.c'])

If you're dealing with a lot of `SConscript` files, this can be a lot simpler than keeping arbitrary lists of imported variables in each file.

### 14.5.3. Returning Values From an `SConscript` File

Sometimes, you would like to be able to use information from a subsidiary `SConscript` file in some way. For example, suppose that you want to create one library from source files scattered throughout a number of subsidiary `SConscript` files. You can do this by using the `Return` function to return values from the subsidiary `SConscript` files to the calling file.

If, for example, we have two subdirectories foo and bar that should each contribute a source file to a Library, what we'd like to be able to do is collect the object files from the subsidiary `SConscript` calls like this:

              env = Environment()
              Export('env')
              objs = []
              for subdir in ['foo', 'bar']:
                  o = SConscript('%s/SConscript' % subdir)
                  objs.append(o)
              env.Library('prog', objs)

We can do this by using the `Return` function in the `foo/SConscript` file like this:

              Import('env')
              obj = env.Object('foo.c')
              Return('obj')

(The corresponding `bar/SConscript` file should be pretty obvious.) Then when we run SCons, the object files from the subsidiary subdirectories are all correctly archived in the desired library:

            % scons -Q
            cc -o bar/bar.o -c bar/bar.c
            cc -o foo/foo.o -c foo/foo.c
            ar rc libprog.a foo/foo.o bar/bar.o
            ranlib libprog.a

# Chapter 15. Separating Source and Build Directories

It's often useful to keep any built files completely separate from the source files. In SCons, this is usually done by creating one or more separate variant directory trees that are used to hold the built objects files, libraries, and executable programs, etc. for a specific flavor, or variant, of build. SCons provides two ways to do this, one through the `SConscript` function that we've already seen, and the second through a more flexible `VariantDir` function.

One historical note: the `VariantDir` function used to be called `BuildDir`. That name is still supported but has been deprecated because the SCons functionality differs from the model of a "build directory" implemented by other build systems like the GNU Autotools.

## 15.1. Specifying a Variant Directory Tree as Part of an `SConscript` Call

The most straightforward way to establish a variant directory tree uses the fact that the usual way to set up a build hierarchy is to have an `SConscript` file in the source subdirectory. If you then pass a `variant_dir` argument to the `SConscript` function call:

          SConscript('src/SConscript', variant_dir='build')

SCons will then build all of the files in the `build` subdirectory:

          % ls src
          SConscript  hello.c
          % scons -Q
          cc -o build/hello.o -c build/hello.c
          cc -o build/hello build/hello.o
          % ls build
          SConscript  hello  hello.c  hello.o

But wait a minute--what's going on here? SCons created the object file `build/hello.o` in the `build` subdirectory, as expected. But even though our `hello.c` file lives in the `src` subdirectory, SCons has actually compiled a `build/hello.c` file to create the object file.

What's happened is that SCons has duplicated the `hello.c` file from the `src` subdirectory to the `build` subdirectory, and built the program from there. The next section explains why SCons does this.

## 15.2. Why SCons Duplicates Source Files in a Variant Directory Tree

SCons duplicates source files in variant directory trees because it's the most straightforward way to guarantee a correct build regardless of include-file directory paths, relative references between files, or tool support for putting files in different locations, and the SCons philosophy is to, by default, guarantee a correct build in all cases.

The most direct reason to duplicate source files in variant directories is simply that some tools (mostly older vesions) are written to only build their output files in the same directory as the source files. In this case, the choices are either to build the output file in the source directory and move it to the variant directory, or to duplicate the source files in the variant directory.

Additionally, relative references between files can cause problems if we don't just duplicate the hierarchy of source files in the variant directory. You can see this at work in use of the C preprocessor `#include` mechanism with double quotes, not angle brackets:

          #include "file.h"

The de facto standard behavior for most C compilers in this case is to first look in the same directory as the source file that contains the `#include` line, then to look in the directories in the preprocessor search path. Add to this that the SCons implementation of support for code repositories (described below) means not all of the files will be found in the same directory hierarchy, and the simplest way to make sure that the right include file is found is to duplicate the source files into the variant directory, which provides a correct build regardless of the original location(s) of the source files.

Although source-file duplication guarantees a correct build even in these end-cases, it can usually be safely disabled. The next section describes how you can disable the duplication of source files in the variant directory.

## 15.3. Telling SCons to Not Duplicate Source Files in the Variant Directory Tree

In most cases and with most tool sets, SCons can place its target files in a build subdirectory without duplicating the source files and everything will work just fine. You can disable the default SCons behavior by specifying `duplicate=0` when you call the `SConscript` function:

          SConscript('src/SConscript', variant_dir='build', duplicate=0)

When this flag is specified, SCons uses the variant directory like most people expect--that is, the output files are placed in the variant directory while the source files stay in the source directory:

          % ls src
          SConscript
          hello.c
          % scons -Q
          cc -c src/hello.c -o build/hello.o
          cc -o build/hello build/hello.o
          % ls build
          hello
          hello.o

## 15.4. The `VariantDir` Function

Use the `VariantDir` function to establish that target files should be built in a separate directory from the source files:

          VariantDir('build', 'src')
          env = Environment()
          env.Program('build/hello.c')

Note that when you're not using an `SConscript` file in the `src` subdirectory, you must actually specify that the program must be built from the `build/hello.c` file that SCons will duplicate in the `build` subdirectory.

When using the `VariantDir` function directly, SCons still duplicates the source files in the variant directory by default:

          % ls src
          hello.c
          % scons -Q
          cc -o build/hello.o -c build/hello.c
          cc -o build/hello build/hello.o
          % ls build
          hello  hello.c  hello.o

You can specify the same `duplicate=0` argument that you can specify for an `SConscript` call:

          VariantDir('build', 'src', duplicate=0)
          env = Environment()
          env.Program('build/hello.c')

In which case SCons will disable duplication of the source files:

          % ls src
          hello.c
          % scons -Q
          cc -o build/hello.o -c src/hello.c
          cc -o build/hello build/hello.o
          % ls build
          hello  hello.o

## 15.5. Using `VariantDir` With an `SConscript` File

Even when using the `VariantDir` function, it's much more natural to use it with a subsidiary `SConscript` file. For example, if the `src/SConscript` looks like this:

          env = Environment()
          env.Program('hello.c')

Then our `SConstruct` file could look like:

          VariantDir('build', 'src')
          SConscript('build/SConscript')

Yielding the following output:

          % ls src
          SConscript  hello.c
          % scons -Q
          cc -o build/hello.o -c build/hello.c
          cc -o build/hello build/hello.o
          % ls build
          SConscript  hello  hello.c  hello.o

Notice that this is completely equivalent to the use of `SConscript` that we learned about in the previous section.

# Chapter 16. Variant Builds

The `variant_dir` keyword argument of the `SConscript` function provides everything we need to show how easy it is to create variant builds using SCons. Suppose, for example, that we want to build a program for both Windows and Linux platforms, but that we want to build it in a shared directory with separate side-by-side build directories for the Windows and Linux versions of the program.

        platform = ARGUMENTS.get('OS', Platform())

        include = "#export/$PLATFORM/include"
        lib = "#export/$PLATFORM/lib"
        bin = "#export/$PLATFORM/bin"

        env = Environment(PLATFORM = platform,
                          BINDIR = bin,
                          INCDIR = include,
                          LIBDIR = lib,
                          CPPPATH = [include],
                          LIBPATH = [lib],
                          LIBS = 'world')

        Export('env')

        env.SConscript('src/SConscript', variant_dir='build/$PLATFORM')

This SConstruct file, when run on a Linux system, yields:

        % scons -Q OS=linux
        Install file: "build/linux/world/world.h" as "export/linux/include/world.h"
        cc -o build/linux/hello/hello.o -c -Iexport/linux/include build/linux/hello/hello.c
        cc -o build/linux/world/world.o -c -Iexport/linux/include build/linux/world/world.c
        ar rc build/linux/world/libworld.a build/linux/world/world.o
        ranlib build/linux/world/libworld.a
        Install file: "build/linux/world/libworld.a" as "export/linux/lib/libworld.a"
        cc -o build/linux/hello/hello build/linux/hello/hello.o -Lexport/linux/lib -lworld
        Install file: "build/linux/hello/hello" as "export/linux/bin/hello"

The same SConstruct file on Windows would build:

        C:\>scons -Q OS=windows
        Install file: "build/windows/world/world.h" as "export/windows/include/world.h"
        cl /nologo /Iexport\windows\include /c build\windows\hello\hello.c /Fobuild\windows\hello\hello.obj
        cl /nologo /Iexport\windows\include /c build\windows\world\world.c /Fobuild\windows\world\world.obj
        lib /nologo /OUT:build\windows\world\world.lib build\windows\world\world.obj
        Install file: "build/windows/world/world.lib" as "export/windows/lib/world.lib"
        link /nologo /OUT:build\windows\hello\hello.exe /LIBPATH:export\windows\lib world.lib build\windows\hello\hello.obj
        Install file: "build/windows/hello/hello.exe" as "export/windows/bin/hello.exe"

# Chapter 17. Writing Your Own Builders

Although SCons provides many useful methods for building common software products: programs, libraries, documents. you frequently want to be able to build some other type of file not supported directly by SCons Fortunately, SCons makes it very easy to define your own `Builder` objects for any custom file types you want to build. (In fact, the SCons interfaces for creating `Builder` objects are flexible enough and easy enough to use that all of the the SCons built-in `Builder` objects are created the mechanisms described in this section.)

## 17.1. Writing Builders That Execute External Commands

The simplest `Builder` to create is one that executes an external command. For example, if we want to build an output file by running the contents of the input file through a command named `foobuild`, creating that `Builder` might look like:

           bld = Builder(action = 'foobuild < $SOURCE > $TARGET')

All the above line does is create a free-standing `Builder` object. The next section will show us how to actually use it.

## 17.2. Attaching a Builder to a `Construction Environment`

A `Builder` object isn't useful until it's attached to a `construction environment` so that we can call it to arrange for files to be built. This is done through the `$BUILDERS` `construction variable` in an environment. The `$BUILDERS` variable is a Python dictionary that maps the names by which you want to call various `Builder` objects to the objects themselves. For example, if we want to call the `Builder` we just defined by the name `Foo`, our `SConstruct` file might look like:

           bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
           env = Environment(BUILDERS = {'Foo' : bld})

With the `Builder` so attached to our `construction environment` we can now actually call it like so:

           env.Foo('file.foo', 'file.input')

Then when we run SCons it looks like:

          % scons -Q
          foobuild < file.input > file.foo

Note, however, that the default `$BUILDERS` variable in a `construction environment` comes with a default set of `Builder` objects already defined: `Program`, `Library`, etc. And when we explicitly set the `$BUILDERS` variable when we create the `construction environment`, the default `Builder`s are no longer part of the environment:

           bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
           env = Environment(BUILDERS = {'Foo' : bld})
           env.Foo('file.foo', 'file.input')
           env.Program('hello.c')

          % scons -Q
          AttributeError: SConsEnvironment instance has no attribute 'Program':
            File "/home/my/project/SConstruct", line 4:
              env.Program('hello.c')

To be able to use both our own defined `Builder` objects and the default `Builder` objects in the same `construction environment`, you can either add to the `$BUILDERS` variable using the `Append` function:

           env = Environment()
           bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
           env.Append(BUILDERS = {'Foo' : bld})
           env.Foo('file.foo', 'file.input')
           env.Program('hello.c')

Or you can explicitly set the appropriately-named key in the `$BUILDERS` dictionary:

           env = Environment()
           bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
           env['BUILDERS']['Foo'] = bld
           env.Foo('file.foo', 'file.input')
           env.Program('hello.c')

Either way, the same `construction environment` can then use both the newly-defined `Foo` `Builder` and the default `Program` `Builder`:

          % scons -Q
          foobuild < file.input > file.foo
          cc -o hello.o -c hello.c
          cc -o hello hello.o

## 17.3. Letting SCons Handle The File Suffixes

By supplying additional information when you create a `Builder`, you can let SCons add appropriate file suffixes to the target and/or the source file. For example, rather than having to specify explicitly that you want the `Foo` `Builder` to build the `file.foo` target file from the `file.input` source file, you can give the `.foo` and `.input` suffixes to the `Builder`, making for more compact and readable calls to the `Foo` `Builder`:

           bld = Builder(action = 'foobuild < $SOURCE > $TARGET',
                         suffix = '.foo',
                         src_suffix = '.input')
           env = Environment(BUILDERS = {'Foo' : bld})
           env.Foo('file1')
           env.Foo('file2')

          % scons -Q
          foobuild < file1.input > file1.foo
          foobuild < file2.input > file2.foo

You can also supply a `prefix` keyword argument if it's appropriate to have SCons append a prefix to the beginning of target file names.

## 17.4. Builders That Execute Python Functions

In SCons, you don't have to call an external command to build a file. You can, instead, define a Python function that a `Builder` object can invoke to build your target file (or files). Such a `builder function` definition looks like:

           def build_function(target, source, env):
               # Code to build "target" from "source"
               return None

The arguments of a `builder function` are:

target

  A list of Node objects representing the target or targets to be built by this builder function. The file names of these target(s) may be extracted using the Python `str` function.

source

  A list of Node objects representing the sources to be used by this builder function to build the targets. The file names of these source(s) may be extracted using the Python `str` function.

env

  The `construction environment` used for building the target(s). The builder function may use any of the environment's construction variables in any way to affect how it builds the targets.

The builder function must return a `0` or `None` value if the target(s) are built successfully. The builder function may raise an exception or return any non-zero value to indicate that the build is unsuccessful,

Once you've defined the Python function that will build your target file, defining a `Builder` object for it is as simple as specifying the name of the function, instead of an external command, as the `Builder`'s `action` argument:

           def build_function(target, source, env):
               # Code to build "target" from "source"
               return None
           bld = Builder(action = build_function,
                         suffix = '.foo',
                         src_suffix = '.input')
           env = Environment(BUILDERS = {'Foo' : bld})
           env.Foo('file')

And notice that the output changes slightly, reflecting the fact that a Python function, not an external command, is now called to build the target file:

          % scons -Q
          build_function(["file.foo"], ["file.input"])

## 17.5. Builders That Create Actions Using a `Generator`

SCons Builder objects can create an action "on the fly" by using a function called a `generator`. This provides a great deal of flexibility to construct just the right list of commands to build your target. A `generator` looks like:

           def generate_actions(source, target, env, for_signature):
               return 'foobuild < %s > %s' % (target[0], source[0])

The arguments of a `generator` are:

source

  A list of Node objects representing the sources to be built by the command or other action generated by this function. The file names of these source(s) may be extracted using the Python `str` function.

target

  A list of Node objects representing the target or targets to be built by the command or other action generated by this function. The file names of these target(s) may be extracted using the Python `str` function.

env

  The `construction environment` used for building the target(s). The generator may use any of the environment's construction variables in any way to determine what command or other action to return.

for_signature

  A flag that specifies whether the generator is being called to contribute to a build signature, as opposed to actually executing the command.

The `generator` must return a command string or other action that will be used to build the specified target(s) from the specified source(s).

Once you've defined a `generator`, you create a `Builder` to use it by specifying the generator keyword argument instead of `action`.

           def generate_actions(source, target, env, for_signature):
               return 'foobuild < %s > %s' % (source[0], target[0])
           bld = Builder(generator = generate_actions,
                         suffix = '.foo',
                         src_suffix = '.input')
           env = Environment(BUILDERS = {'Foo' : bld})
           env.Foo('file')

          % scons -Q
          foobuild < file.input > file.foo

Note that it's illegal to specify both an `action` and a `generator` for a `Builder`.

## 17.6. Builders That Modify the Target or Source Lists Using an `Emitter`

SCons supports the ability for a Builder to modify the lists of target(s) from the specified source(s).

           def modify_targets(target, source, env):
               target.append('new_target')
               source.append('new_source')
               return target, source
           bld = Builder(action = 'foobuild $TARGETS - $SOURCES',
                         suffix = '.foo',
                         src_suffix = '.input',
                         emitter = modify_targets)
           env = Environment(BUILDERS = {'Foo' : bld})
           env.Foo('file')

          % scons -Q
          foobuild file.foo new_target - file.input new_source

           bld = Builder(action = 'my_command',
                         suffix = '.foo',
                         src_suffix = '.input',
                         emitter = 'MY_EMITTER')
           def modify1(target, source, env):
               return target, source
           def modify2(target, source, env):
               return target, source
           env1 = Environment(BUILDERS = {'Foo' : bld},
                              MY_EMITTER = modify1)
           env2 = Environment(BUILDERS = {'Foo' : bld},
                              MY_EMITTER = modify2)
           env1.Foo('file1')
           env2.Foo('file2')

# Chapter 18. Not Writing a Builder: the `Command` Builder

Creating a `Builder` and attaching it to a `construction environment` allows for a lot of flexibility when you want to re-use actions to build multiple files of the same type. This can, however, be cumbersome if you only need to execute one specific command to build a single file (or group of files). For these situations, SCons supports a `Command` `Builder` that arranges for a specific action to be executed to build a specific file or files. This looks a lot like the other builders (like `Program`, `Object`, etc.), but takes as an additional argument the command to be executed to build the file:

         env = Environment()
         env.Command('foo.out', 'foo.in', "sed 's/x/y/' < $SOURCE > $TARGET")

When executed, SCons runs the specified command, substituting `$SOURCE` and `$TARGET` as expected:

        % scons -Q
        sed 's/x/y/' < foo.in > foo.out

This is often more convenient than creating a `Builder` object and adding it to the `$BUILDERS` variable of a `construction environment`

Note that the action you specify to the `Command` `Builder` can be any legal SCons `Action`, such as a Python function:

         env = Environment()
         def build(target, source, env):
             # Whatever it takes to build
             return None
         env.Command('foo.out', 'foo.in', build)

Which executes as follows:

        % scons -Q
        build(["foo.out"], ["foo.in"])

# Chapter 19. Writing Scanners

SCons has built-in scanners that know how to look in C, Fortran and IDL source files for information about other files that targets built from those files depend on--for example, in the case of files that use the C preprocessor, the `.h` files that are specified using `#include` lines in the source. You can use the same mechanisms that SCons uses to create its built-in scanners to write scanners of your own for file types that SCons does not know how to scan "out of the box."

## 19.1. A Simple Scanner Example

Suppose, for example, that we want to create a simple scanner for `.foo` files. A `.foo` file contains some text that will be processed, and can include other files on lines that begin with `include` followed by a file name:

          include filename.foo

Scanning a file will be handled by a Python function that you must supply. Here is a function that will use the Python `re` module to scan for the `include` lines in our example:

          import re

          include_re = re.compile(r'^include\s+(\S+)$', re.M)

          def kfile_scan(node, env, path, arg):
              contents = node.get_contents()
              return include_re.findall(contents)

The scanner function must accept the four specified arguments and return a list of implicit dependencies. Presumably, these would be dependencies found from examining the contents of the file, although the function can perform any manipulation at all to generate the list of dependencies.

node

  An SCons node object representing the file being scanned. The path name to the file can be used by converting the node to a string using the `str()` function, or an internal SCons `get_contents()` object method can be used to fetch the contents.

env

  The construction environment in effect for this scan. The scanner function may choose to use construction variables from this environment to affect its behavior.

path

  A list of directories that form the search path for included files for this scanner. This is how SCons handles the `$CPPPATH` and `$LIBPATH` variables.

arg

  An optional argument that you can choose to have passed to this scanner function by various scanner instances.

A Scanner object is created using the `Scanner` function, which typically takes an `skeys` argument to associate the type of file suffix with this scanner. The Scanner object must then be associated with the `$SCANNERS` construction variable of a construction environment, typically by using the `Append` method:

           kscan = Scanner(function = kfile_scan,
                           skeys = ['.k'])
           env.Append(SCANNERS = kscan)

When we put it all together, it looks like:

            import re

            include_re = re.compile(r'^include\s+(\S+)$', re.M)

            def kfile_scan(node, env, path):
                contents = node.get_contents()
                includes = include_re.findall(contents)
                return includes

            kscan = Scanner(function = kfile_scan,
                            skeys = ['.k'])

            env = Environment(ENV = {'PATH' : '/usr/local/bin'})
            env.Append(SCANNERS = kscan)

            env.Command('foo', 'foo.k', 'kprocess < $SOURCES > $TARGET')

# Chapter 20. Building From Code Repositories

Often, a software project will have one or more central repositories, directory trees that contain source code, or derived files, or both. You can eliminate additional unnecessary rebuilds of files by having SCons use files from one or more code repositories to build files in your local build tree.

## 20.1. The `Repository` Method

It's often useful to allow multiple programmers working on a project to build software from source files and/or derived files that are stored in a centrally-accessible repository, a directory copy of the source code tree. (Note that this is not the sort of repository maintained by a source code management system like BitKeeper, CVS, or Subversion.) You use the `Repository` method to tell SCons to search one or more central code repositories (in order) for any source files and derived files that are not present in the local build tree:

           env = Environment()
           env.Program('hello.c')
           Repository('/usr/repository1', '/usr/repository2')

Multiple calls to the `Repository` method will simply add repositories to the global list that SCons maintains, with the exception that SCons will automatically eliminate the current directory and any non-existent directories from the list.

## 20.2. Finding source files in repositories

The above example specifies that SCons will first search for files under the `/usr/repository1` tree and next under the `/usr/repository2` tree. SCons expects that any files it searches for will be found in the same position relative to the top-level directory. In the above example, if the `hello.c` file is not found in the local build tree, SCons will search first for a `/usr/repository1/hello.c` file and then for a `/usr/repository2/hello.c` file to use in its place.

So given the `SConstruct` file above, if the `hello.c` file exists in the local build directory, SCons will rebuild the hello program as normal:

          % scons -Q
          cc -o hello.o -c hello.c
          cc -o hello hello.o

If, however, there is no local `hello.c` file, but one exists in `/usr/repository1`, SCons will recompile the hello program from the source file it finds in the repository:

          % scons -Q
          cc -o hello.o -c /usr/repository1/hello.c
          cc -o hello hello.o

And similarly, if there is no local `hello.c` file and no `/usr/repository1/hello.c`, but one exists in `/usr/repository2`:

          % scons -Q
          cc -o hello.o -c /usr/repository2/hello.c
          cc -o hello hello.o

## 20.3. Finding `#include` files in repositories

We've already seen that SCons will scan the contents of a source file for `#include` file names and realize that targets built from that source file also depend on the `#include` file(s). For each directory in the `$CPPPATH` list, SCons will actually search the corresponding directories in any repository trees and establish the correct dependencies on any `#include` files that it finds in repository directory.

Unless the C compiler also knows about these directories in the repository trees, though, it will be unable to find the `#include` files. If, for example, the `hello.c` file in our previous example includes the hello.h; in its current directory, and the hello.h; only exists in the repository:

          % scons -Q
          cc -o hello.o -c hello.c
          hello.c:1: hello.h: No such file or directory

In order to inform the C compiler about the repositories, SCons will add appropriate `-I` flags to the compilation commands for each directory in the `$CPPPATH` list. So if we add the current directory to the construction environment `$CPPPATH` like so:

           env = Environment(CPPPATH = ['.'])
           env.Program('hello.c')
           Repository('/usr/repository1')

Then re-executing SCons yields:

          % scons -Q
          cc -o hello.o -c -I. -I/usr/repository1 hello.c
          cc -o hello hello.o

The order of the `-I` options replicates, for the C preprocessor, the same repository-directory search path that SCons uses for its own dependency analysis. If there are multiple repositories and multiple `$CPPPATH` directories, SCons will add the repository directories to the beginning of each `$CPPPATH` directory, rapidly multiplying the number of `-I` flags. If, for example, the `$CPPPATH` contains three directories (and shorter repository path names!):

           env = Environment(CPPPATH = ['dir1', 'dir2', 'dir3'])
           env.Program('hello.c')
           Repository('/r1', '/r2')

Then we'll end up with nine `-I` options on the command line, three (for each of the `$CPPPATH` directories) times three (for the local directory plus the two repositories):

          % scons -Q
          cc -o hello.o -c -Idir1 -I/r1/dir1 -I/r2/dir1 -Idir2 -I/r1/dir2 -I/r2/dir2 -Idir3 -I/r1/dir3 -I/r2/dir3 hello.c
          cc -o hello hello.o

### 20.3.1. Limitations on `#include` files in repositories

SCons relies on the C compiler's `-I` options to control the order in which the preprocessor will search the repository directories for `#include` files. This causes a problem, however, with how the C preprocessor handles `#include` lines with the file name included in double-quotes.

As we've seen, SCons will compile the `hello.c` file from the repository if it doesn't exist in the local directory. If, however, the `hello.c` file in the repository contains a `#include` line with the file name in double quotes:

            #include "hello.h"
            int
            main(int argc, char *argv[])
            {
                printf(HELLO_MESSAGE);
                return (0);
            }

Then the C preprocessor will always use a `hello.h` file from the repository directory first, even if there is a `hello.h` file in the local directory, despite the fact that the command line specifies `-I` as the first option:

            % scons -Q
            cc -o hello.o -c -I. -I/usr/repository1 /usr/repository1/hello.c
            cc -o hello hello.o

This behavior of the C preprocessor--always search for a `#include` file in double-quotes first in the same directory as the source file, and only then search the `-I`--can not, in general, be changed. In other words, it's a limitation that must be lived with if you want to use code repositories in this way. There are three ways you can possibly work around this C preprocessor behavior:

1. Some modern versions of C compilers do have an option to disable or control this behavior. If so, add that option to `$CFLAGS` (or `$CXXFLAGS` or both) in your construction environment(s). Make sure the option is used for all construction environments that use C preprocessing!

2. Change all occurrences of `#include "file.h"` to `#include <file.h>`. Use of `#include` with angle brackets does not have the same behavior--the `-I` directories are searched first for `#include` files--which gives SCons direct control over the list of directories the C preprocessor will search.

3. Require that everyone working with compilation from repositories check out and work on entire directories of files, not individual files. (If you use local wrapper scripts around your source code control system's command, you could add logic to enforce this restriction there.

## 20.4. Finding the `SConstruct` file in repositories

SCons will also search in repositories for the `SConstruct` file and any specified `SConscript` files. This poses a problem, though: how can SCons search a repository tree for an `SConstruct` file if the `SConstruct` file itself contains the information about the pathname of the repository? To solve this problem, SCons allows you to specify repository directories on the command line using the `-Y` option:

          % scons -Q -Y /usr/repository1 -Y /usr/repository2

When looking for source or derived files, SCons will first search the repositories specified on the command line, and then search the repositories specified in the `SConstruct` or `SConscript` files.

## 20.5. Finding derived files in repositories

If a repository contains not only source files, but also derived files (such as object files, libraries, or executables), SCons will perform its normal MD5 signature calculation to decide if a derived file in a repository is up-to-date, or the derived file must be rebuilt in the local build directory. For the SCons signature calculation to work correctly, a repository tree must contain the `.sconsign` files that SCons uses to keep track of signature information.

Usually, this would be done by a build integrator who would run SCons in the repository to create all of its derived files and `.sconsign` files, or who would run SCons in a separate build directory and copy the resulting tree to the desired repository:

          % cd /usr/repository1
          % scons -Q
          cc -o file1.o -c file1.c
          cc -o file2.o -c file2.c
          cc -o hello.o -c hello.c
          cc -o hello hello.o file1.o file2.o

(Note that this is safe even if the `SConstruct` file lists `/usr/repository1` as a repository, because SCons will remove the current build directory from its repository list for that invocation.)

Now, with the repository populated, we only need to create the one local source file we're interested in working with at the moment, and use the `-Y` option to tell SCons to fetch any other files it needs from the repository:

          % cd $HOME/build
          % edit hello.c
          % scons -Q -Y /usr/repository1
          cc -c -o hello.o hello.c
          cc -o hello hello.o /usr/repository1/file1.o /usr/repository1/file2.o

Notice that SCons realizes that it does not need to rebuild local copies `file1.o` and `file2.o` files, but instead uses the already-compiled files from the repository.

## 20.6. Guaranteeing local copies of files

If the repository tree contains the complete results of a build, and we try to build from the repository without any files in our local tree, something moderately surprising happens:

          % mkdir $HOME/build2
          % cd $HOME/build2
          % scons -Q -Y /usr/all/repository hello
          scons: `hello' is up-to-date.

Why does SCons say that the hello program is up-to-date when there is no hello program in the local build directory? Because the repository (not the local directory) contains the up-to-date hello program, and SCons correctly determines that nothing needs to be done to rebuild that up-to-date copy of the file.

There are, however, many times when you want to ensure that a local copy of a file always exists. A packaging or testing script, for example, may assume that certain generated files exist locally. To tell SCons to make a copy of any up-to-date repository file in the local build directory, use the `Local` function:

           env = Environment()
           hello = env.Program('hello.c')
           Local(hello)

If we then run the same command, SCons will make a local copy of the program from the repository copy, and tell you that it is doing so:

          % scons -Y /usr/all/repository hello
          Local copy of hello from /usr/all/repository/hello
          scons: `hello' is up-to-date.

(Notice that, because the act of making the local copy is not considered a "build" of the hello file, SCons still reports that it is up-to-date.)

# Chapter 21. Multi-Platform Configuration (Autoconf Functionality)

SCons has integrated support for multi-platform build configuration similar to that offered by GNU Autoconf, such as figuring out what libraries or header files are available on the local system. This section describes how to use this SCons feature.

| This chapter is still under development, so not everything is explained as well as it should be. See the SCons man page for additional information.

## 21.1. `Configure Contexts`

The basic framework for multi-platform build configuration in SCons is to attach a `configure context` to a construction environment by calling the `Configure` function, perform a number of checks for libraries, functions, header files, etc., and to then call the configure context's `Finish` method to finish off the configuration:

        env = Environment()
        conf = Configure(env)
        # Checks for libraries, header files, etc. go here!
        env = conf.Finish()

SCons provides a number of basic checks, as well as a mechanism for adding your own custom checks.

Note that SCons uses its own dependency mechanism to determine when a check needs to be run--that is, SCons does not run the checks every time it is invoked, but caches the values returned by previous checks and uses the cached values unless something has changed. This saves a tremendous amount of developer time while working on cross-platform build issues.

The next sections describe the basic checks that SCons supports, as well as how to add your own custom checks.

## 21.2. Checking for the Existence of Header Files

Testing the existence of a header file requires knowing what language the header file is. A configure context has a `CheckCHeader` method that checks for the existence of a C header file:

        env = Environment()
        conf = Configure(env)
        if not conf.CheckCHeader('math.h'):
            print 'Math.h must be installed!'
            Exit(1)
        if conf.CheckCHeader('foo.h'):
            conf.env.Append('-DHAS_FOO_H')
        env = conf.Finish()

Note that you can choose to terminate the build if a given header file doesn't exist, or you can modify the construction environment based on the existence of a header file.

If you need to check for the existence a C++ header file, use the `CheckCXXHeader` method:

        env = Environment()
        conf = Configure(env)
        if not conf.CheckCXXHeader('vector.h'):
            print 'vector.h must be installed!'
            Exit(1)
        env = conf.Finish()

## 21.3. Checking for the Availability of a Function

Check for the availability of a specific function using the `CheckFunc` method:

        env = Environment()
        conf = Configure(env)
        if not conf.CheckFunc('strcpy'):
            print 'Did not find strcpy(), using local version'
            conf.env.Append('-Dstrcpy=my_local_strcpy')
        env = conf.Finish()

## 21.4. Checking for the Availability of a Library

Check for the availability of a library using the `CheckLib` method. You only specify the basename of the library, you don't need to add a `lib` prefix or a `.a` or `.lib` suffix:

        env = Environment()
        conf = Configure(env)
        if not conf.CheckLib('m'):
            print 'Did not find libm.a or m.lib, exiting!'
            Exit(1)
        env = conf.Finish()

Because the ability to use a library successfully often depends on having access to a header file that describes the library's interface, you can check for a library and a header file at the same time by using the `CheckLibWithHeader` method:

        env = Environment()
        conf = Configure(env)
        if not conf.CheckLibWithHeader('m', 'math.h'):
            print 'Did not find libm.a or m.lib, exiting!'
            Exit(1)
        env = conf.Finish()

This is essentially shorthand for separate calls to the `CheckHeader` and `CheckLib` functions.

## 21.5. Checking for the Availability of a `typedef`

Check for the availability of a `typedef` by using the `CheckType` method:

        env = Environment()
        conf = Configure(env)
        if not conf.CheckType('off_t'):
            print 'Did not find off_t typedef, assuming int'
            conf.env.Append(CCFLAGS = '-Doff_t=int')
        env = conf.Finish()

You can also add a string that will be placed at the beginning of the test file that will be used to check for the `typedef`. This provide a way to specify files that must be included to find the `typedef`:

        env = Environment()
        conf = Configure(env)
        if not conf.CheckType('off_t', '#include <sys/types.h>\n'):
            print 'Did not find off_t typedef, assuming int'
            conf.env.Append(CCFLAGS = '-Doff_t=int')
        env = conf.Finish()

## 21.6. Adding Your Own Custom Checks

A custom check is a Python function that checks for a certain condition to exist on the running system, usually using methods that SCons supplies to take care of the details of checking whether a compilation succeeds, a link succeeds, a program is runnable, etc. A simple custom check for the existence of a specific library might look as follows:

        mylib_test_source_file = """
        #include <mylib.h>
        int main(int argc, char **argv)
        {
            MyLibrary mylib(argc, argv);
            return 0;
        }
        """

        def CheckMyLibrary(context):
            context.Message('Checking for MyLibrary...')
            result = context.TryLink(mylib_test_source_file, '.c')
            context.Result(result)
            return result

The `Message` and `Result` methods should typically begin and end a custom check to let the user know what's going on: the `Message` call prints the specified message (with no trailing newline) and the `Result` call prints `ok` if the check succeeds and `failed` if it doesn't. The `TryLink` method actually tests for whether the specified program text will successfully link.

(Note that a custom check can modify its check based on any arguments you choose to pass it, or by using or modifying the configure context environment in the `context.env` attribute.)

This custom check function is then attached to the `configure context` by passing a dictionary to the `Configure` call that maps a name of the check to the underlying function:

        env = Environment()
        conf = Configure(env, custom_tests = {'CheckMyLibrary' : CheckMyLibrary})

You'll typically want to make the check and the function name the same, as we've done here, to avoid potential confusion.

We can then put these pieces together and actually call the `CheckMyLibrary` check as follows:

        mylib_test_source_file = """
        #include <mylib.h>
        int main(int argc, char **argv)
        {
            MyLibrary mylib(argc, argv);
            return 0;
        }
        """

        def CheckMyLibrary(context):
            context.Message('Checking for MyLibrary... ')
            result = context.TryLink(mylib_test_source_file, '.c')
            context.Result(result)
            return result

        env = Environment()
        conf = Configure(env, custom_tests = {'CheckMyLibrary' : CheckMyLibrary})
        if not conf.CheckMyLibrary():
            print 'MyLibrary is not installed!'
            Exit(1)
        env = conf.Finish()

        # We would then add actual calls like Program() to build
        # something using the "env" construction environment.

If MyLibrary is not installed on the system, the output will look like:

        % scons
        scons: Reading SConscript file ...
        Checking for MyLibrary... failed
        MyLibrary is not installed!

If MyLibrary is installed, the output will look like:

        % scons
        scons: Reading SConscript file ...
        Checking for MyLibrary... failed
        scons: done reading SConscript
        scons: Building targets ...
            .
            .
            .

## 21.7. Not Configuring When Cleaning Targets

Using multi-platform configuration as described in the previous sections will run the configuration commands even when invoking `scons -c` to clean targets:

        % scons -Q -c
        Checking for MyLibrary... ok
        Removed foo.o
        Removed foo

Although running the platform checks when removing targets doesn't hurt anything, it's usually unnecessary. You can avoid this by using the `GetOption`(); method to check whether the `-c` (clean) option has been invoked on the command line:

        env = Environment()
        if not env.GetOption('clean'):
            conf = Configure(env, custom_tests = {'CheckMyLibrary' : CheckMyLibrary})
            if not conf.CheckMyLibrary():
                print 'MyLibrary is not installed!'
                Exit(1)
            env = conf.Finish()

        % scons -Q -c
        Removed foo.o
        Removed foo

# Chapter 22. Caching Built Files

On multi-developer software projects, you can sometimes speed up every developer's builds a lot by allowing them to share the derived files that they build. SCons makes this easy, as well as reliable.

## 22.1. Specifying the Shared Cache Directory

To enable sharing of derived files, use the `CacheDir` function in any `SConscript` file:

           CacheDir('/usr/local/build_cache')

Note that the directory you specify must already exist and be readable and writable by all developers who will be sharing derived files. It should also be in some central location that all builds will be able to access. In environments where developers are using separate systems (like individual workstations) for builds, this directory would typically be on a shared or NFS-mounted file system.

Here's what happens: When a build has a `CacheDir` specified, every time a file is built, it is stored in the shared cache directory along with its MD5 build signature. [3] On subsequent builds, before an action is invoked to build a file, SCons will check the shared cache directory to see if a file with the exact same build signature already exists. If so, the derived file will not be built locally, but will be copied into the local build directory from the shared cache directory, like so:

          % scons -Q
          cc -o hello.o -c hello.c
          cc -o hello hello.o
          % scons -Q -c
          Removed hello.o
          Removed hello
          % scons -Q
          Retrieved `hello.o' from cache
          Retrieved `hello' from cache

## 22.2. Keeping Build Output Consistent

One potential drawback to using a shared cache is that the output printed by SCons can be inconsistent from invocation to invocation, because any given file may be rebuilt one time and retrieved from the shared cache the next time. This can make analyzing build output more difficult, especially for automated scripts that expect consistent output each time.

If, however, you use the `--cache-show` option, SCons will print the command line that it would have executed to build the file, even when it is retrieving the file from the shared cache. This makes the build output consistent every time the build is run:

          % scons -Q
          cc -o hello.o -c hello.c
          cc -o hello hello.o
          % scons -Q -c
          Removed hello.o
          Removed hello
          % scons -Q --cache-show
          cc -o hello.o -c hello.c
          cc -o hello hello.o

The trade-off, of course, is that you no longer know whether or not SCons has retrieved a derived file from cache or has rebuilt it locally.

## 22.3. Not Using the Shared Cache for Specific Files

You may want to disable caching for certain specific files in your configuration. For example, if you only want to put executable files in a central cache, but not the intermediate object files, you can use the `NoCache` function to specify that the object files should not be cached:

           env = Environment()
           obj = env.Object('hello.c')
           env.Program('hello.c')
           CacheDir('cache')
           NoCache('hello.o')

Then when you run scons after cleaning the built targets, it will recompile the object file locally (since it doesn't exist in the shared cache directory), but still realize that the shared cache directory contains an up-to-date executable program that can be retrieved instead of re-linking:

          % scons -Q
          cc -o hello.o -c hello.c
          cc -o hello hello.o
          % scons -Q -c
          Removed hello.o
          Removed hello
          % scons -Q
          cc -o hello.o -c hello.c
          Retrieved `hello' from cache

## 22.4. Disabling the Shared Cache

Retrieving an already-built file from the shared cache is usually a significant time-savings over rebuilding the file, but how much of a savings (or even whether it saves time at all) can depend a great deal on your system or network configuration. For example, retrieving cached files from a busy server over a busy network might end up being slower than rebuilding the files locally.

In these cases, you can specify the `--cache-disable` command-line option to tell SCons to not retrieve already-built files from the shared cache directory:

          % scons -Q
          cc -o hello.o -c hello.c
          cc -o hello hello.o
          % scons -Q -c
          Removed hello.o
          Removed hello
          % scons -Q
          Retrieved `hello.o' from cache
          Retrieved `hello' from cache
          % scons -Q -c
          Removed hello.o
          Removed hello
          % scons -Q --cache-disable
          cc -o hello.o -c hello.c
          cc -o hello hello.o

## 22.5. Populating a Shared Cache With Already-Built Files

Sometimes, you may have one or more derived files already built in your local build tree that you wish to make available to other people doing builds. For example, you may find it more effective to perform integration builds with the cache disabled (per the previous section) and only populate the shared cache directory with the built files after the integration build has completed successfully. This way, the cache will only get filled up with derived files that are part of a complete, successful build not with files that might be later overwritten while you debug integration problems.

In this case, you can use the the `--cache-force` option to tell SCons to put all derived files in the cache, even if the files already exist in your local tree from having been built by a previous invocation:

          % scons -Q --cache-disable
          cc -o hello.o -c hello.c
          cc -o hello hello.o
          % scons -Q -c
          Removed hello.o
          Removed hello
          % scons -Q --cache-disable
          cc -o hello.o -c hello.c
          cc -o hello hello.o
          % scons -Q --cache-force
          scons: `.' is up to date.
          % scons -Q
          scons: `.' is up to date.

Notice how the above sample run demonstrates that the `--cache-disable` option avoids putting the built `hello.o` and `hello` files in the cache, but after using the `--cache-force` option, the files have been put in the cache for the next invocation to retrieve.

## 22.6. Minimizing Cache Contention: the `--random` Option

If you allow multiple builds to update the shared cache directory simultaneously, two builds that occur at the same time can sometimes start "racing" with one another to build the same files in the same order. If, for example, you are linking multiple files into an executable program:

           Program('prog',
                   ['f1.c', 'f2.c', 'f3.c', 'f4.c', 'f5.c'])

SCons will normally build the input object files on which the program depends in their normal, sorted order:

          % scons -Q
          cc -o f1.o -c f1.c
          cc -o f2.o -c f2.c
          cc -o f3.o -c f3.c
          cc -o f4.o -c f4.c
          cc -o f5.o -c f5.c
          cc -o prog f1.o f2.o f3.o f4.o f5.o

But if two such builds take place simultaneously, they may each look in the cache at nearly the same time and both decide that `f1.o` must be rebuilt and pushed into the shared cache directory, then both decide that `f2.o` must be rebuilt (and pushed into the shared cache directory), then both decide that `f3.o` must be rebuilt... This won't cause any actual build problems--both builds will succeed, generate correct output files, and populate the cache--but it does represent wasted effort.

To alleviate such contention for the cache, you can use the `--random` command-line option to tell SCons to build dependencies in a random order:

          % scons -Q --random
          cc -o f3.o -c f3.c
          cc -o f1.o -c f1.c
          cc -o f5.o -c f5.c
          cc -o f2.o -c f2.c
          cc -o f4.o -c f4.c
          cc -o prog f1.o f2.o f3.o f4.o f5.o

Multiple builds using the `--random` option will usually build their dependencies in different, random orders, which minimizes the chances for a lot of contention for same-named files in the shared cache directory. Multiple simultaneous builds might still race to try to build the same target file on occasion, but long sequences of inefficient contention should be rare.

Note, of course, the `--random` option will cause the output that SCons prints to be inconsistent from invocation to invocation, which may be an issue when trying to compare output from different build runs.

If you want to make sure dependencies will be built in a random order without having to specify the `--random` on very command line, you can use the `SetOption` function to set the `random` option within any `SConscript` file:

           Program('prog',
                   ['f1.c', 'f2.c', 'f3.c', 'f4.c', 'f5.c'])

           SetOption('random', 1)
           Program('prog',
                   ['f1.c', 'f2.c', 'f3.c', 'f4.c', 'f5.c'])

# Chapter 23. Alias Targets

We've already seen how you can use the `Alias` function to create a target named `install`:

         env = Environment()
         hello = env.Program('hello.c')
         env.Install('/usr/bin', hello)
         env.Alias('install', '/usr/bin')

You can then use this alias on the command line to tell SCons more naturally that you want to install files:

         % scons -Q install
         cc -o hello.o -c hello.c
         cc -o hello hello.o
         Install file: "hello" as "/usr/bin/hello"

Like other `Builder` methods, though, the `Alias` method returns an object representing the alias being built. You can then use this object as input to anothother `Builder`. This is especially useful if you use such an object as input to another call to the `Alias` `Builder`, allowing you to create a hierarchy of nested aliases:

         env = Environment()
         p = env.Program('foo.c')
         l = env.Library('bar.c')
         env.Install('/usr/bin', p)
         env.Install('/usr/lib', l)
         ib = env.Alias('install-bin', '/usr/bin')
         il = env.Alias('install-lib', '/usr/lib')
         env.Alias('install', [ib, il])

This example defines separate `install`, `install-bin`, and `install-lib` aliases, allowing you finer control over what gets installed:

         % scons -Q install-bin
         cc -o foo.o -c foo.c
         cc -o foo foo.o
         Install file: "foo" as "/usr/bin/foo"
         % scons -Q install-lib
         cc -o bar.o -c bar.c
         ar rc libbar.a bar.o
         ranlib libbar.a
         Install file: "libbar.a" as "/usr/lib/libbar.a"
         % scons -Q -c /
         Removed foo.o
         Removed foo
         Removed /usr/bin/foo
         Removed bar.o
         Removed libbar.a
         Removed /usr/lib/libbar.a
         % scons -Q install
         cc -o foo.o -c foo.c
         cc -o foo foo.o
         Install file: "foo" as "/usr/bin/foo"
         cc -o bar.o -c bar.c
         ar rc libbar.a bar.o
         ranlib libbar.a
         Install file: "libbar.a" as "/usr/lib/libbar.a"

# Chapter 24. Java Builds

So far, we've been using examples of building C and C++ programs to demonstrate the features of SCons. SCons also supports building Java programs, but Java builds are handled slightly differently, which reflects the ways in which the Java compiler and tools build programs differently than other languages' tool chains.

## 24.1. Building Java Class Files: the `Java` Builder

The basic activity when programming in Java, of course, is to take one or more `.java` files containing Java source code and to call the Java compiler to turn them into one or more `.class` files. In SCons, you do this by giving the `Java` Builder a target directory in which to put the `.class` files, and a source directory that contains the `.java` files:

          Java('classes', 'src')

If the `src` directory contains three `.java` source files, then running SCons might look like this:

          % scons -Q
          javac -d classes -sourcepath src src/Example1.java src/Example2.java src/Example3.java

SCons will actually search the `src` directory tree for all of the `.java` files. The Java compiler will then create the necessary class files in the `classes` subdirectory, based on the class names found in the `.java` files.

## 24.2. How SCons Handles Java Dependencies

In addition to searching the source directory for `.java` files, SCons actually runs the `.java` files through a stripped-down Java parser that figures out what classes are defined. In other words, SCons knows, without you having to tell it, what `.class` files will be produced by the javac call. So our one-liner example from the preceding section:

          Java('classes', 'src')

Will not only tell you reliably that the `.class` files in the `classes` subdirectory are up-to-date:

          % scons -Q
          javac -d classes -sourcepath src src/Example1.java src/Example2.java src/Example3.java
          % scons -Q classes
          scons: `classes' is up to date.

But it will also remove all of the generated `.class` files, even for inner classes, without you having to specify them manually. For example, if our `Example1.java` and `Example3.java` files both define additional classes, and the class defined in `Example2.java` has an inner class, running `scons -c` will clean up all of those `.class` files as well:

          % scons -Q
          javac -d classes -sourcepath src src/Example1.java src/Example2.java src/Example3.java
          % scons -Q -c classes
          Removed classes/Example1.class
          Removed classes/AdditionalClass1.class
          Removed classes/Example2$Inner2.class
          Removed classes/Example2.class
          Removed classes/Example3.class
          Removed classes/AdditionalClass3.class

## 24.3. Building Java Archive (`.jar`) Files: the `Jar` Builder

After building the class files, it's common to collect them into a Java archive (`.jar`) file, which you do by calling the `Jar` Builder method. If you want to just collect all of the class files within a subdirectory, you can just specify that subdirectory as the `Jar` source:

          Java(target = 'classes', source = 'src')
          Jar(target = 'test.jar', source = 'classes')

SCons will then pass that directory to the jar command, which will collect all of the underlying `.class` files:

          % scons -Q
          javac -d classes -sourcepath src src/Example1.java src/Example2.java src/Example3.java
          jar cf test.jar classes

If you want to keep all of the `.class` files for multiple programs in one location, and only archive some of them in each `.jar` file, you can pass the `Jar` builder a list of files as its source. It's extremely simple to create multiple `.jar` files this way, using the lists of target class files created by calls to the `Java` builder as sources to the various `Jar` calls:

          prog1_class_files = Java(target = 'classes', source = 'prog1')
          prog2_class_files = Java(target = 'classes', source = 'prog2')
          Jar(target = 'prog1.jar', source = prog1_class_files)
          Jar(target = 'prog2.jar', source = prog2_class_files)

This will then create `prog1.jar` and `prog2.jar` next to the subdirectories that contain their `.java` files:

          % scons -Q
          javac -d classes -sourcepath prog1 prog1/Example1.java prog1/Example2.java
          javac -d classes -sourcepath prog2 prog2/Example3.java prog2/Example4.java
          jar cf prog1.jar -C classes Example1.class -C classes Example2.class
          jar cf prog2.jar -C classes Example3.class -C classes Example4.class

## 24.4. Building C Header and Stub Files: the `JavaH` Builder

You can generate C header and source files for implementing native methods, by using the `JavaH` Builder. There are several ways of using the `JavaH` Builder. One typical invocation might look like:

          classes = Java(target = 'classes', source = 'src/pkg/sub')
          JavaH(target = 'native', source = classes)

The source is a list of class files generated by the call to the `Java` Builder, and the target is the output directory in which we want the C header files placed. The target gets converted into the `-d` when SCons runs javah:

          % scons -Q
          javac -d classes -sourcepath src/pkg/sub src/pkg/sub/Example1.java src/pkg/sub/Example2.java src/pkg/sub/Example3.java
          javah -d native -classpath classes pkg.sub.Example1 pkg.sub.Example2 pkg.sub.Example3

In this case, the call to javah will generate the header files `native/pkg_sub_Example1.h`, `native/pkg_sub_Example2.h` and `native/pkg_sub_Example3.h`. Notice that SCons remembered that the class files were generated with a target directory of `classes`, and that it then specified that target directory as the `-classpath` option to the call to javah.

Although it's more convenient to use the list of class files returned by the `Java` Builder as the source of a call to the `JavaH` Builder, you can specify the list of class files by hand, if you prefer. If you do, you need to set the `$JAVACLASSDIR` construction variable when calling `JavaH`:

          Java(target = 'classes', source = 'src/pkg/sub')
          class_file_list = ['classes/pkg/sub/Example1.class',
                             'classes/pkg/sub/Example2.class',
                             'classes/pkg/sub/Example3.class']
          JavaH(target = 'native', source = class_file_list, JAVACLASSDIR = 'classes')

The `$JAVACLASSDIR` value then gets converted into the `-classpath` when SCons runs javah:

          % scons -Q
          javac -d classes -sourcepath src/pkg/sub src/pkg/sub/Example1.java src/pkg/sub/Example2.java src/pkg/sub/Example3.java
          javah -d native -classpath classes pkg.sub.Example1 pkg.sub.Example2 pkg.sub.Example3

Lastly, if you don't want a separate header file generated for each source file, you can specify an explicit File Node as the target of the `JavaH` Builder:

          classes = Java(target = 'classes', source = 'src/pkg/sub')
          JavaH(target = File('native.h'), source = classes)

Because SCons assumes by default that the target of the `JavaH` builder is a directory, you need to use the `File` function to make sure that SCons doesn't create a directory named `native.h`. When a file is used, though, SCons correctly converts the file name into the javah `-o` option:

          % scons -Q
          javac -d classes -sourcepath src/pkg/sub src/pkg/sub/Example1.java src/pkg/sub/Example2.java src/pkg/sub/Example3.java
          javah -o native.h -classpath classes pkg.sub.Example1 pkg.sub.Example2 pkg.sub.Example3

## 24.5. Building RMI Stub and Skeleton Class Files: the `RMIC` Builder

You can generate Remote Method Invocation stubs by using the `RMIC` Builder. The source is a list of directories, typically returned by a call to the `Java` Builder, and the target is an output directory where the `_Stub.class` and `_Skel.class` files will be placed:

          classes = Java(target = 'classes', source = 'src/pkg/sub')
          RMIC(target = 'outdir', source = classes)

As it did with the `JavaH` Builder, SCons remembers the class directory and passes it as the `-classpath` option to rmic:

          % scons -Q
          javac -d classes -sourcepath src/pkg/sub src/pkg/sub/Example1.java src/pkg/sub/Example2.java
          rmic -d outdir -classpath classes pkg.sub.Example1 pkg.sub.Example2

This example would generate the files `outdir/pkg/sub/Example1_Skel.class`, `outdir/pkg/sub/Example1_Stub.class`, `outdir/pkg/sub/Example2_Skel.class` and `outdir/pkg/sub/Example2_Stub.class`.

# Chapter 25. Troubleshooting

The experience of configuring any software build tool to build a large code base usually, at some point, involves trying to figure out why the tool is behaving a certain way, and how to get it to behave the way you want. SCons is no different. This appendix contains a number of different ways in which you can get some additional insight into SCons' behavior.

Note that we're always interested in trying to improve how you can troubleshoot configuration problems. If you run into a problem that has you scratching your head, and which there just doesn't seem to be a good way to debug, odds are pretty good that someone else will run into the same problem, too. If so, please let the SCons development team know (preferably by filing a bug report or feature request at our project pages at tigris.org) so that we can use your feedback to try to come up with a better way to help you, and others, get the necessary insight into SCons behavior to help identify and fix configuration issues.

## 25.1. Why is That Target Being Rebuilt? the `--debug=explain` Option

Let's look at a simple example of a misconfigured build that causes a target to be rebuilt every time SCons is run:

          # Intentionally misspell the output file name in the
          # command used to create the file:
          Command('file.out', 'file.in', 'cp $SOURCE file.oout')

(Note to Windows users: The POSIX cp command copies the first file named on the command line to the second file. In our example, it copies the `file.in` file to the `file.out` file.)

Now if we run SCons multiple times on this example, we see that it re-runs the cp command every time:

          % scons -Q
          cp file.in file.oout
          % scons -Q
          cp file.in file.oout
          % scons -Q
          cp file.in file.oout

In this example, the underlying cause is obvious: we've intentionally misspelled the output file name in the cp command, so the command doesn't actually build the `file.out` file that we've told SCons to expect. But if the problem weren't obvious, it would be helpful to specify the `--debug=explain` option on the command line to have SCons tell us very specifically why it's decided to rebuild the target:

          % scons -Q --debug=explain
          scons: building `file.out' because it doesn't exist
          cp file.in file.oout

If this had been a more complicated example involving a lot of build output, having SCons tell us that it's trying to rebuild the target file because it doesn't exist would be an important clue that something was wrong with the command that we invoked to build it.

The `--debug=explain` option also comes in handy to help figure out what input file changed. Given a simple configuration that builds a program from three source files, changing one of the source files and rebuilding with the `--debug=explain` option shows very specifically why SCons rebuilds the files that it does:

          % scons -Q
          cc -o file1.o -c file1.c
          cc -o file2.o -c file2.c
          cc -o file3.o -c file3.c
          cc -o prog file1.o file2.o file3.o
          % edit file2.c
              [CHANGE THE CONTENTS OF file2.c]
          % scons -Q --debug=explain
          scons: rebuilding `file2.o' because `file2.c' changed
          cc -o file2.o -c file2.c
          scons: rebuilding `prog' because `file2.o' changed
          cc -o prog file1.o file2.o file3.o

This becomes even more helpful in identifying when a file is rebuilt due to a change in an implicit dependency, such as an incuded `.h` file. If the `file1.c` and `file3.c` files in our example both included a `hello.h` file, then changing that included file and re-running SCons with the `--debug=explain` option will pinpoint that it's the change to the included file that starts the chain of rebuilds:

          % scons -Q
          cc -o file1.o -c -I. file1.c
          cc -o file2.o -c -I. file2.c
          cc -o file3.o -c -I. file3.c
          cc -o prog file1.o file2.o file3.o
          % edit hello.h
              [CHANGE THE CONTENTS OF hello.h]
          % scons -Q --debug=explain
          scons: rebuilding `file1.o' because `hello.h' changed
          cc -o file1.o -c -I. file1.c
          scons: rebuilding `file3.o' because `hello.h' changed
          cc -o file3.o -c -I. file3.c
          scons: rebuilding `prog' because:
                     `file1.o' changed
                     `file3.o' changed
          cc -o prog file1.o file2.o file3.o

(Note that the `--debug=explain` option will only tell you why SCons decided to rebuild necessary targets. It does not tell you what files it examined when deciding not to rebuild a target file, which is often a more valuable question to answer.)

## 25.2. What's in That Construction Environment? the `Dump` Method

When you create a construction environment, SCons populates it with construction variables that are set up for various compilers, linkers and utilities that it finds on your system. Although this is usually helpful and what you want, it might be frustrating if SCons doesn't set certain variables that you expect to be set. In situations like this, it's sometimes helpful to use the construction environment `Dump` method to print all or some of the construction variables. Note that the `Dump` method returns the representation of the variables in the environment for you to print (or otherwise manipulate):

             env = Environment()
             print env.Dump()

On a POSIX system with gcc installed, this might generate:

          % scons
          scons: Reading SConscript files ...
          { 'BUILDERS': {'_InternalInstall': <function InstallBuilderWrapper at 0x700000>, '_InternalInstallAs': <function InstallAsBuilderWrapper at 0x700000>},
            'CONFIGUREDIR': '#/.sconf_temp',
            'CONFIGURELOG': '#/config.log',
            'CPPSUFFIXES': [ '.c',
                             '.C',
                             '.cxx',
                             '.cpp',
                             '.c++',
                             '.cc',
                             '.h',
                             '.H',
                             '.hxx',
                             '.hpp',
                             '.hh',
                             '.F',
                             '.fpp',
                             '.FPP',
                             '.m',
                             '.mm',
                             '.S',
                             '.spp',
                             '.SPP'],
            'DSUFFIXES': ['.d'],
            'Dir': <SCons.Defaults.Variable_Method_Caller instance at 0x700000>,
            'Dirs': <SCons.Defaults.Variable_Method_Caller instance at 0x700000>,
            'ENV': {'PATH': '/usr/local/bin:/opt/bin:/bin:/usr/bin'},
            'ESCAPE': <function escape at 0x700000>,
            'File': <SCons.Defaults.Variable_Method_Caller instance at 0x700000>,
            'IDLSUFFIXES': ['.idl', '.IDL'],
            'INSTALL': <function copyFunc at 0x700000>,
            'LATEXSUFFIXES': ['.tex', '.ltx', '.latex'],
            'LIBPREFIX': 'lib',
            'LIBPREFIXES': ['$LIBPREFIX'],
            'LIBSUFFIX': '.a',
            'LIBSUFFIXES': ['$LIBSUFFIX', '$SHLIBSUFFIX'],
            'MAXLINELENGTH': 128072,
            'OBJPREFIX': '',
            'OBJSUFFIX': '.o',
            'PLATFORM': 'posix',
            'PROGPREFIX': '',
            'PROGSUFFIX': '',
            'PSPAWN': <function piped_env_spawn at 0x700000>,
            'RDirs': <SCons.Defaults.Variable_Method_Caller instance at 0x700000>,
            'SCANNERS': [],
            'SHELL': 'sh',
            'SHLIBPREFIX': '$LIBPREFIX',
            'SHLIBSUFFIX': '.so',
            'SHOBJPREFIX': '$OBJPREFIX',
            'SHOBJSUFFIX': '$OBJSUFFIX',
            'SPAWN': <function spawnvpe_spawn at 0x700000>,
            'TEMPFILE': <class SCons.Platform.TempFileMunge at 0x700000>,
            'TEMPFILEPREFIX': '@',
            'TOOLS': ['install', 'install'],
            '_CPPDEFFLAGS': '${_defines(CPPDEFPREFIX, CPPDEFINES, CPPDEFSUFFIX, __env__)}',
            '_CPPINCFLAGS': '$( ${_concat(INCPREFIX, CPPPATH, INCSUFFIX, __env__, RDirs, TARGET, SOURCE)} $)',
            '_LIBDIRFLAGS': '$( ${_concat(LIBDIRPREFIX, LIBPATH, LIBDIRSUFFIX, __env__, RDirs, TARGET, SOURCE)} $)',
            '_LIBFLAGS': '${_concat(LIBLINKPREFIX, LIBS, LIBLINKSUFFIX, __env__)}',
            '__RPATH': '$_RPATH',
            '_concat': <function _concat at 0x700000>,
            '_defines': <function _defines at 0x700000>,
            '_stripixes': <function _stripixes at 0x700000>}
          scons: done reading SConscript files.
          scons: Building targets ...
          scons: `.' is up to date.
          scons: done building targets.

On a Windows system with Visual C++ the output might look like:

          C:\>scons
          scons: Reading SConscript files ...
          { 'BUILDERS': {'_InternalInstall': <function InstallBuilderWrapper at 0x700000>, 'Object': <SCons.Builder.CompositeBuilder instance at 0x700000>, 'PCH': <SCons.Builder.BuilderBase instance at 0x700000>, 'RES': <SCons.Builder.BuilderBase instance at 0x700000>, 'SharedObject': <SCons.Builder.CompositeBuilder instance at 0x700000>, 'StaticObject': <SCons.Builder.CompositeBuilder instance at 0x700000>, '_InternalInstallAs': <function InstallAsBuilderWrapper at 0x700000>},
            'CC': 'cl',
            'CCCOM': <SCons.Action.FunctionAction instance at 0x700000>,
            'CCCOMFLAGS': '$CPPFLAGS $_CPPDEFFLAGS $_CPPINCFLAGS /c $SOURCES /Fo$TARGET $CCPCHFLAGS $CCPDBFLAGS',
            'CCFLAGS': ['/nologo'],
            'CCPCHFLAGS': ['${(PCH and "/Yu%s /Fp%s"%(PCHSTOP or "",File(PCH))) or ""}'],
            'CCPDBFLAGS': ['${(PDB and "/Z7") or ""}'],
            'CFILESUFFIX': '.c',
            'CFLAGS': [],
            'CONFIGUREDIR': '#/.sconf_temp',
            'CONFIGURELOG': '#/config.log',
            'CPPDEFPREFIX': '/D',
            'CPPDEFSUFFIX': '',
            'CPPSUFFIXES': [ '.c',
                             '.C',
                             '.cxx',
                             '.cpp',
                             '.c++',
                             '.cc',
                             '.h',
                             '.H',
                             '.hxx',
                             '.hpp',
                             '.hh',
                             '.F',
                             '.fpp',
                             '.FPP',
                             '.m',
                             '.mm',
                             '.S',
                             '.spp',
                             '.SPP'],
            'CXX': '$CC',
            'CXXCOM': '$CXX $CXXFLAGS $CCCOMFLAGS',
            'CXXFILESUFFIX': '.cc',
            'CXXFLAGS': ['$CCFLAGS', '$(', '/TP', '$)'],
            'DSUFFIXES': ['.d'],
            'Dir': <SCons.Defaults.Variable_Method_Caller instance at 0x700000>,
            'Dirs': <SCons.Defaults.Variable_Method_Caller instance at 0x700000>,
            'ENV': { 'INCLUDE': 'C:\\Program Files\\Microsoft Visual Studio/VC98\\include',
                     'LIB': 'C:\\Program Files\\Microsoft Visual Studio/VC98\\lib',
                     'PATH': 'C:\\Program Files\\Microsoft Visual Studio\\Common\\tools\\WIN95;C:\\Program Files\\Microsoft Visual Studio\\Common\\MSDev98\\bin;C:\\Program Files\\Microsoft Visual Studio\\Common\\tools;C:\\Program Files\\Microsoft Visual Studio/VC98\\bin',
                     'PATHEXT': '.COM;.EXE;.BAT;.CMD',
                     'SystemRoot': 'C:/WINDOWS'},
            'ESCAPE': <function escape at 0x700000>,
            'File': <SCons.Defaults.Variable_Method_Caller instance at 0x700000>,
            'IDLSUFFIXES': ['.idl', '.IDL'],
            'INCPREFIX': '/I',
            'INCSUFFIX': '',
            'INSTALL': <function copyFunc at 0x700000>,
            'LATEXSUFFIXES': ['.tex', '.ltx', '.latex'],
            'LIBPREFIX': '',
            'LIBPREFIXES': ['$LIBPREFIX'],
            'LIBSUFFIX': '.lib',
            'LIBSUFFIXES': ['$LIBSUFFIX'],
            'MAXLINELENGTH': 2048,
            'MSVS': {'VERSION': '6.0', 'VERSIONS': ['6.0']},
            'MSVS_VERSION': '6.0',
            'OBJPREFIX': '',
            'OBJSUFFIX': '.obj',
            'PCHCOM': '$CXX $CXXFLAGS $CPPFLAGS $_CPPDEFFLAGS $_CPPINCFLAGS /c $SOURCES /Fo${TARGETS[1]} /Yc$PCHSTOP /Fp${TARGETS[0]} $CCPDBFLAGS $PCHPDBFLAGS',
            'PCHPDBFLAGS': ['${(PDB and "/Yd") or ""}'],
            'PLATFORM': 'win32',
            'PROGPREFIX': '',
            'PROGSUFFIX': '.exe',
            'PSPAWN': <function piped_spawn at 0x700000>,
            'RC': 'rc',
            'RCCOM': '$RC $_CPPDEFFLAGS $_CPPINCFLAGS $RCFLAGS /fo$TARGET $SOURCES',
            'RCFLAGS': [],
            'RDirs': <SCons.Defaults.Variable_Method_Caller instance at 0x700000>,
            'SCANNERS': [],
            'SHCC': '$CC',
            'SHCCCOM': <SCons.Action.FunctionAction instance at 0x700000>,
            'SHCCFLAGS': ['$CCFLAGS'],
            'SHCFLAGS': ['$CFLAGS'],
            'SHCXX': '$CXX',
            'SHCXXCOM': '$SHCXX $SHCXXFLAGS $CCCOMFLAGS',
            'SHCXXFLAGS': ['$CXXFLAGS'],
            'SHELL': None,
            'SHLIBPREFIX': '',
            'SHLIBSUFFIX': '.dll',
            'SHOBJPREFIX': '$OBJPREFIX',
            'SHOBJSUFFIX': '$OBJSUFFIX',
            'SPAWN': <function spawn at 0x700000>,
            'STATIC_AND_SHARED_OBJECTS_ARE_THE_SAME': 1,
            'TEMPFILE': <class SCons.Platform.TempFileMunge at 0x700000>,
            'TEMPFILEPREFIX': '@',
            'TOOLS': ['msvc', 'install', 'install'],
            '_CPPDEFFLAGS': '${_defines(CPPDEFPREFIX, CPPDEFINES, CPPDEFSUFFIX, __env__)}',
            '_CPPINCFLAGS': '$( ${_concat(INCPREFIX, CPPPATH, INCSUFFIX, __env__, RDirs, TARGET, SOURCE)} $)',
            '_LIBDIRFLAGS': '$( ${_concat(LIBDIRPREFIX, LIBPATH, LIBDIRSUFFIX, __env__, RDirs, TARGET, SOURCE)} $)',
            '_LIBFLAGS': '${_concat(LIBLINKPREFIX, LIBS, LIBLINKSUFFIX, __env__)}',
            '_concat': <function _concat at 0x700000>,
            '_defines': <function _defines at 0x700000>,
            '_stripixes': <function _stripixes at 0x700000>}
          scons: done reading SConscript files.
          scons: Building targets ...
          scons: `.' is up to date.
          scons: done building targets.

The construction environments in these examples have actually been restricted to just gcc and Visual C++, respectively. In a real-life situation, the construction environments will likely contain a great many more variables. Also note that we've massaged the example output above to make the memory address of all objects a constant 0x700000. In reality, you would see a different hexadecimal number for each object.

To make it easier to see just what you're interested in, the `Dump` method allows you to specify a specific constrcution variable that you want to disply. For example, it's not unusual to want to verify the external environment used to execute build commands, to make sure that the PATH and other environment variables are set up the way they should be. You can do this as follows:

             env = Environment()
             print env.Dump('ENV')

Which might display the following when executed on a POSIX system:

          % scons
          scons: Reading SConscript files ...
          {'PATH': '/usr/local/bin:/opt/bin:/bin:/usr/bin'}
          scons: done reading SConscript files.
          scons: Building targets ...
          scons: `.' is up to date.
          scons: done building targets.

And the following when executed on a Windows system:

          C:\>scons
          scons: Reading SConscript files ...
          { 'INCLUDE': 'C:\\Program Files\\Microsoft Visual Studio/VC98\\include',
            'LIB': 'C:\\Program Files\\Microsoft Visual Studio/VC98\\lib',
            'PATH': 'C:\\Program Files\\Microsoft Visual Studio\\Common\\tools\\WIN95;C:\\Program Files\\Microsoft Visual Studio\\Common\\MSDev98\\bin;C:\\Program Files\\Microsoft Visual Studio\\Common\\tools;C:\\Program Files\\Microsoft Visual Studio/VC98\\bin',
            'PATHEXT': '.COM;.EXE;.BAT;.CMD',
            'SystemRoot': 'C:/WINDOWS'}
          scons: done reading SConscript files.
          scons: Building targets ...
          scons: `.' is up to date.
          scons: done building targets.

## 25.3. What Dependencies Does SCons Know About? the `--tree` Option

Sometimes the best way to try to figure out what SCons is doing is simply to take a look at the dependency graph that it constructs based on your `SConscript` files. The `--tree` option will display all or part of the SCons dependency graph in an "ASCII art" graphical format that shows the dependency hierarchy.

For example, given the following input `SConstruct` file:

             env = Environment(CPPPATH = ['.'])
             env.Program('prog', ['f1.c', 'f2.c', 'f3.c'])

Running SCons with the `--tree=all` option yields:

          % scons -Q --tree=all
          cc -o f1.o -c -I. f1.c
          cc -o f2.o -c -I. f2.c
          cc -o f3.o -c -I. f3.c
          cc -o prog f1.o f2.o f3.o
          +-.
            +-SConstruct
            +-f1.c
            +-f1.o
            | +-f1.c
            | +-inc.h
            +-f2.c
            +-f2.o
            | +-f2.c
            | +-inc.h
            +-f3.c
            +-f3.o
            | +-f3.c
            | +-inc.h
            +-inc.h
            +-prog
              +-f1.o
              | +-f1.c
              | +-inc.h
              +-f2.o
              | +-f2.c
              | +-inc.h
              +-f3.o
                +-f3.c
                +-inc.h

The tree will also be printed when the `-n` (no execute) option is used, which allows you to examine the dependency graph for a configuration without actually rebuilding anything in the tree.

The `--tree` option only prints the dependency graph for the specified targets (or the default target(s) if none are specified on the command line). So if you specify a target like `f2.o` on the command line, the `--tree` option will only print the dependency graph for that file:

          % scons -Q --tree=all f2.o
          cc -o f2.o -c -I. f2.c
          +-f2.o
            +-f2.c
            +-inc.h

This is, of course, useful for restricting the output from a very large build configuration to just a portion in which you're interested. Multiple targets are fine, in which case a tree will be printed for each specified target:

          % scons -Q --tree=all f1.o f3.o
          cc -o f1.o -c -I. f1.c
          +-f1.o
            +-f1.c
            +-inc.h
          cc -o f3.o -c -I. f3.c
          +-f3.o
            +-f3.c
            +-inc.h

The `status` argument may be used to tell SCons to print status information about each file in the dependency graph:

          % scons -Q --tree=status
          cc -o f1.o -c -I. f1.c
          cc -o f2.o -c -I. f2.c
          cc -o f3.o -c -I. f3.c
          cc -o prog f1.o f2.o f3.o
           E         = exists
            R        = exists in repository only
             b       = implicit builder
             B       = explicit builder
              S      = side effect
               P     = precious
                A    = always build
                 C   = current
                  N  = no clean
                   H = no cache

          [E b      ]+-.
          [E     C  ]  +-SConstruct
          [E     C  ]  +-f1.c
          [E B   C  ]  +-f1.o
          [E     C  ]  | +-f1.c
          [E     C  ]  | +-inc.h
          [E     C  ]  +-f2.c
          [E B   C  ]  +-f2.o
          [E     C  ]  | +-f2.c
          [E     C  ]  | +-inc.h
          [E     C  ]  +-f3.c
          [E B   C  ]  +-f3.o
          [E     C  ]  | +-f3.c
          [E     C  ]  | +-inc.h
          [E     C  ]  +-inc.h
          [E B   C  ]  +-prog
          [E B   C  ]    +-f1.o
          [E     C  ]    | +-f1.c
          [E     C  ]    | +-inc.h
          [E B   C  ]    +-f2.o
          [E     C  ]    | +-f2.c
          [E     C  ]    | +-inc.h
          [E B   C  ]    +-f3.o
          [E     C  ]      +-f3.c
          [E     C  ]      +-inc.h

Note that `--tree=all,status` is equivalent; the `all` is assumed if only `status` is present. As an alternative to `all`, you can specify `--tree=derived` to have SCons only print derived targets in the tree output, skipping source files (like `.c` and `.h` files):

          % scons -Q --tree=derived
          cc -o f1.o -c -I. f1.c
          cc -o f2.o -c -I. f2.c
          cc -o f3.o -c -I. f3.c
          cc -o prog f1.o f2.o f3.o
          +-.
            +-f1.o
            +-f2.o
            +-f3.o
            +-prog
              +-f1.o
              +-f2.o
              +-f3.o

You can use the `status` modifier with `derived` as well:

          % scons -Q --tree=derived,status
          cc -o f1.o -c -I. f1.c
          cc -o f2.o -c -I. f2.c
          cc -o f3.o -c -I. f3.c
          cc -o prog f1.o f2.o f3.o
           E         = exists
            R        = exists in repository only
             b       = implicit builder
             B       = explicit builder
              S      = side effect
               P     = precious
                A    = always build
                 C   = current
                  N  = no clean
                   H = no cache

          [E b      ]+-.
          [E B   C  ]  +-f1.o
          [E B   C  ]  +-f2.o
          [E B   C  ]  +-f3.o
          [E B   C  ]  +-prog
          [E B   C  ]    +-f1.o
          [E B   C  ]    +-f2.o
          [E B   C  ]    +-f3.o

Note that the order of the `--tree=` arguments doesn't matter; `--tree=status,derived` is completely equivalent.

The default behavior of the `--tree` option is to repeat all of the dependencies each time the library dependency (or any other dependency file) is encountered in the tree. If certain target files share other target files, such as two programs that use the same library:

             env = Environment(CPPPATH = ['.'],
                               LIBS = ['foo'],
                               LIBPATH = ['.'])
             env.Library('foo', ['f1.c', 'f2.c', 'f3.c'])
             env.Program('prog1.c')
             env.Program('prog2.c')

Then there can be a lot of repetition in the `--tree=` output:

          % scons -Q --tree=all
          cc -o f1.o -c -I. f1.c
          cc -o f2.o -c -I. f2.c
          cc -o f3.o -c -I. f3.c
          ar rc libfoo.a f1.o f2.o f3.o
          ranlib libfoo.a
          cc -o prog1.o -c -I. prog1.c
          cc -o prog1 prog1.o -L. -lfoo
          cc -o prog2.o -c -I. prog2.c
          cc -o prog2 prog2.o -L. -lfoo
          +-.
            +-SConstruct
            +-f1.c
            +-f1.o
            | +-f1.c
            | +-inc.h
            +-f2.c
            +-f2.o
            | +-f2.c
            | +-inc.h
            +-f3.c
            +-f3.o
            | +-f3.c
            | +-inc.h
            +-inc.h
            +-libfoo.a
            | +-f1.o
            | | +-f1.c
            | | +-inc.h
            | +-f2.o
            | | +-f2.c
            | | +-inc.h
            | +-f3.o
            |   +-f3.c
            |   +-inc.h
            +-prog1
            | +-prog1.o
            | | +-prog1.c
            | | +-inc.h
            | +-libfoo.a
            |   +-f1.o
            |   | +-f1.c
            |   | +-inc.h
            |   +-f2.o
            |   | +-f2.c
            |   | +-inc.h
            |   +-f3.o
            |     +-f3.c
            |     +-inc.h
            +-prog1.c
            +-prog1.o
            | +-prog1.c
            | +-inc.h
            +-prog2
            | +-prog2.o
            | | +-prog2.c
            | | +-inc.h
            | +-libfoo.a
            |   +-f1.o
            |   | +-f1.c
            |   | +-inc.h
            |   +-f2.o
            |   | +-f2.c
            |   | +-inc.h
            |   +-f3.o
            |     +-f3.c
            |     +-inc.h
            +-prog2.c
            +-prog2.o
              +-prog2.c
              +-inc.h

In a large configuration with many internal libraries and include files, this can very quickly lead to huge output trees. To help make this more manageable, a `prune` modifier may be added to the option list, in which case SCons will print the name of a target that has already been visited during the tree-printing in `[square brackets]` as an indication that the dependencies of the target file may be found by looking farther up the tree:

          % scons -Q --tree=prune
          cc -o f1.o -c -I. f1.c
          cc -o f2.o -c -I. f2.c
          cc -o f3.o -c -I. f3.c
          ar rc libfoo.a f1.o f2.o f3.o
          ranlib libfoo.a
          cc -o prog1.o -c -I. prog1.c
          cc -o prog1 prog1.o -L. -lfoo
          cc -o prog2.o -c -I. prog2.c
          cc -o prog2 prog2.o -L. -lfoo
          +-.
            +-SConstruct
            +-f1.c
            +-f1.o
            | +-f1.c
            | +-inc.h
            +-f2.c
            +-f2.o
            | +-f2.c
            | +-inc.h
            +-f3.c
            +-f3.o
            | +-f3.c
            | +-inc.h
            +-inc.h
            +-libfoo.a
            | +-[f1.o]
            | +-[f2.o]
            | +-[f3.o]
            +-prog1
            | +-prog1.o
            | | +-prog1.c
            | | +-inc.h
            | +-[libfoo.a]
            +-prog1.c
            +-[prog1.o]
            +-prog2
            | +-prog2.o
            | | +-prog2.c
            | | +-inc.h
            | +-[libfoo.a]
            +-prog2.c
            +-[prog2.o]

Like the `status` keyword, the `prune` argument by itself is equivalent to `--tree=all,prune`.

## 25.4. How is SCons Constructing the Command Lines It Executes? the `--debug=presub` Option

Sometimes it's useful to look at the pre-substitution string that SCons uses to generate the command lines it executes. This can be done with the `--debug=presub` option:

          % scons -Q --debug=presub
          Building prog.o with action:
            $CC -o $TARGET -c $CFLAGS $CCFLAGS $_CCOMCOM $SOURCES
          cc -o prog.o -c -I. prog.c
          Building prog with action:
            $SMART_LINKCOM
          cc -o prog prog.o

## 25.5. Where is SCons Searching for Libraries? the `--debug=findlibs` Option

To get some insight into what library names SCons is searching for, and in which directories it is searching, Use the `--debug=findlibs` option. Given the following input `SConstruct` file:

            env = Environment(LIBPATH = ['libs1', 'libs2'])
            env.Program('prog.c', LIBS=['foo', 'bar'])

And the libraries `libfoo.a` and `libbar.a` in `libs1` and `libs2`, respectively, use of the `--debug=findlibs` option yields:

          % scons -Q --debug=findlibs
            findlibs: looking for 'libfoo.a' in 'libs1' ...
            findlibs: ... FOUND 'libfoo.a' in 'libs1'
            findlibs: looking for 'libfoo.so' in 'libs1' ...
            findlibs: looking for 'libfoo.so' in 'libs2' ...
            findlibs: looking for 'libbar.a' in 'libs1' ...
            findlibs: looking for 'libbar.a' in 'libs2' ...
            findlibs: ... FOUND 'libbar.a' in 'libs2'
            findlibs: looking for 'libbar.so' in 'libs1' ...
            findlibs: looking for 'libbar.so' in 'libs2' ...
          cc -o prog.o -c prog.c
          cc -o prog prog.o -Llibs1 -Llibs2 -lfoo -lbar

## 25.6. Where is SCons Blowing Up? the `--debug=stacktrace` Option

In general, SCons tries to keep its error messages short and informative. That means we usually try to avoid showing the stack traces that are familiar to experienced Python programmers, since they usually contain much more information than is useful to most people.

For example, the following `SConstruct` file:

             Program('prog.c')

Generates the following error if the `prog.c` file does not exist:

          % scons -Q
          scons: *** Source `prog.c' not found, needed by target `prog.o'.  Stop.

In this case, the error is pretty obvious. But if it weren't, and you wanted to try to get more information about the error, the `--debug=stacktrace` option would show you exactly where in the SCons source code the problem occurs:

          % scons -Q --debug=stacktrace
          scons: *** Source `prog.c' not found, needed by target `prog.o'.  Stop.
          scons: internal stack trace:
            File "bootstrap/src/engine/SCons/Job.py", line 131, in start
            File "bootstrap/src/engine/SCons/Script/Main.py", line 169, in prepare
            File "bootstrap/src/engine/SCons/Taskmaster.py", line 169, in prepare
            File "bootstrap/src/engine/SCons/Node/FS.py", line 2551, in prepare
            File "bootstrap/src/engine/SCons/Node/__init__.py", line 349, in prepare

Of course, if you do need to dive into the SCons source code, we'd like to know if, or how, the error messages or troubleshooting options could have been improved to avoid that. Not everyone has the necessary time or Python skill to dive into the source code, and we'd like to improve SCons for those people as well...

## 25.7. How is SCons Making Its Decisions? the `--taskmastertrace` Option

The internal SCons subsystem that handles walking the dependency graph and controls the decision-making about what to rebuild is the `Taskmaster`. SCons supports a `--taskmastertrace` option that tells the Taskmaster to print information about the children (dependencies) of the various Nodes on its walk down the graph, which specific dependent Nodes are being evaluated, and in what order.

The `--taskmastertrace` option takes as an argument the name of a file in which to put the trace output, with `-` (a single hyphen) indicating that the trace messages should be printed to the standard output:

          env = Environment(CPPPATH = ['.'])
          env.Program('prog.c')

          % scons -Q --taskmastertrace=- prog
          Taskmaster: 'prog': children:
              ['prog.o']
              waiting on unfinished children:
              ['prog.o']
          Taskmaster: 'prog.o': children:
              ['inc.h', 'prog.c']
              waiting on unfinished children:
              ['inc.h', 'prog.c']
          Taskmaster: 'prog.c': evaluating prog.c
          Taskmaster: 'inc.h': evaluating inc.h
          Taskmaster: 'prog.o': children:
              ['inc.h', 'prog.c']
              evaluating prog.o
          cc -o prog.o -c -I. prog.c
          Taskmaster: 'prog': children:
              ['prog.o']
              evaluating prog
          cc -o prog prog.o

The `--taskmastertrace` option doesn't provide information about the actual calculations involved in deciding if a file is up-to-date, but it does show all of the dependencies it knows about for each Node, and the order in which those dependencies are evaluated. This can be useful as an alternate way to determine whether or not your SCons configuration, or the implicit dependency scan, has actually identified all the correct dependencies you want it to.

# Appendix A. Construction Variables

This appendix contains descriptions of all of the construction variables that are potentially available "out of the box" in this version of SCons. Whether or not setting a construction variable in a construction environment will actually have an effect depends on whether any of the Tools and/or Builders that use the variable have been included in the construction environment.

In this appendix, we have appended the initial `$` (dollar sign) to the beginning of each variable name when it appears in the text, but left off the dollar sign in the left-hand column where the name appears for each entry.

`AR`

  The static library archiver.

`ARCHITECTURE`

  Specifies the system architecture for which the package is being built. The default is the system architecture of the machine on which SCons is running. This is used to fill in the `Architecture:` field in an Ipkg `control` file, and as part of the name of a generated RPM file.

`ARCOM`

  The command line used to generate a static library from object files.

`ARCOMSTR`

  The string displayed when an object file is generated from an assembly-language source file. If this is not set, then `$ARCOM` (the command line) is displayed.

      env = Environment(ARCOMSTR = "Archiving $TARGET")

`ARFLAGS`

  General options passed to the static library archiver.

`AS`

  The assembler.

`ASCOM`

  The command line used to generate an object file from an assembly-language source file.

`ASCOMSTR`

  The string displayed when an object file is generated from an assembly-language source file. If this is not set, then `$ASCOM` (the command line) is displayed.

      env = Environment(ASCOMSTR = "Assembling $TARGET")

`ASFLAGS`

  General options passed to the assembler.

`ASPPCOM`

  The command line used to assemble an assembly-language source file into an object file after first running the file through the C preprocessor. Any options specified in the `$ASFLAGS` and `$CPPFLAGS` construction variables are included on this command line.

`ASPPCOMSTR`

  The string displayed when an object file is generated from an assembly-language source file after first running the file through the C preprocessor. If this is not set, then `$ASPPCOM` (the command line) is displayed.

      env = Environment(ASPPCOMSTR = "Assembling $TARGET")

`ASPPFLAGS`

  General options when an assembling an assembly-language source file into an object file after first running the file through the C preprocessor. The default is to use the value of `$ASFLAGS`.

`BIBTEX`

  The bibliography generator for the TeX formatter and typesetter and the LaTeX structured formatter and typesetter.

`BIBTEXCOM`

  The command line used to call the bibliography generator for the TeX formatter and typesetter and the LaTeX structured formatter and typesetter.

`BIBTEXCOMSTR`

  The string displayed when generating a bibliography for TeX or LaTeX. If this is not set, then `$BIBTEXCOM` (the command line) is displayed.

      env = Environment(BIBTEXCOMSTR = "Generating bibliography $TARGET")

`BIBTEXFLAGS`

  General options passed to the bibliography generator for the TeX formatter and typesetter and the LaTeX structured formatter and typesetter.

`BITKEEPER`

  The BitKeeper executable.

`BITKEEPERCOM`

  The command line for fetching source files using BitKeeper.

`BITKEEPERCOMSTR`

  The string displayed when fetching a source file using BitKeeper. If this is not set, then `$BITKEEPERCOM` (the command line) is displayed.

`BITKEEPERGET`

  The command (`$BITKEEPER`) and subcommand for fetching source files using BitKeeper.

`BITKEEPERGETFLAGS`

  Options that are passed to the BitKeeper get subcommand.

`BUILDERS`

  A dictionary mapping the names of the builders available through this environment to underlying Builder objects. Builders named Alias, CFile, CXXFile, DVI, Library, Object, PDF, PostScript, and Program are available by default. If you initialize this variable when an Environment is created:

      env = Environment(BUILDERS = {'NewBuilder' : foo})

  the default Builders will no longer be available. To use a new Builder object in addition to the default Builders, add your new Builder object like this:

      env = Environment()
      env.Append(BUILDERS = {'NewBuilder' : foo})

  or this:

      env = Environment()
      env['BUILDERS]['NewBuilder'] = foo

`CC`

  The C compiler.

`CCCOM`

  The command line used to compile a C source file to a (static) object file. Any options specified in the `$CFLAGS`, `$CCFLAGS` and `$CPPFLAGS` construction variables are included on this command line.

`CCCOMSTR`

  The string displayed when a C source file is compiled to a (static) object file. If this is not set, then `$CCCOM` (the command line) is displayed.

      env = Environment(CCCOMSTR = "Compiling static object $TARGET")

`CCFLAGS`

  General options that are passed to the C and C++ compilers.

`CCPCHFLAGS`

  Options added to the compiler command line to support building with precompiled headers. The default value expands expands to the appropriate Microsoft Visual C++ command-line options when the `$PCH` construction variable is set.

`CCPDBFLAGS`

  Options added to the compiler command line to support storing debugging information in a Microsoft Visual C++ PDB file. The default value expands expands to appropriate Microsoft Visual C++ command-line options when the `$PDB` construction variable is set.

  The Visual C++ compiler option that SCons uses by default to generate PDB information is `/Z7`. This works correctly with parallel (`-j`) builds because it embeds the debug information in the intermediate object files, as opposed to sharing a single PDB file between multiple object files. This is also the only way to get debug information embedded into a static library. Using the `/Zi` instead may yield improved link-time performance, although parallel builds will no longer work.

  You can generate PDB files with the `/Zi` switch by overriding the default `$CCPDBFLAGS` variable as follows:

      import SCons.Util
      env['CCPDBFLAGS'] = SCons.Util.CLVar(['${(PDB and "/Zi /Fd%s" % File(PDB)) or ""}'])

  An alternative would be to use the `/Zi` to put the debugging information in a separate `.pdb` file for each object file by overriding the `$CCPDBFLAGS` variable as follows:

      env['CCPDBFLAGS'] = '/Zi /Fd${TARGET}.pdb'

`CCVERSION`

  The version number of the C compiler. This may or may not be set, depending on the specific C compiler being used.

`CFILESUFFIX`

  The suffix for C source files. This is used by the internal CFile builder when generating C files from Lex (.l) or YACC (.y) input files. The default suffix, of course, is `.c` (lower case). On case-insensitive systems (like Windows), SCons also treats `.C` (upper case) files as C files.

`CFLAGS`

  General options that are passed to the C compiler (C only; not C++).

`CHANGE_SPECFILE`

  A hook for modifying the file that controls the packaging build (the `.spec` for RPM, the `control` for Ipkg, the `.wxs` for MSI). If set, the function will be called after the SCons template for the file has been written. XXX

`CHANGELOG`

  The name of a file containing the change log text to be included in the package. This is included as the `%changelog` section of the RPM `.spec` file.

`_concat`

  A function used to produce variables like `$_CPPINCFLAGS`. It takes four or five arguments: a prefix to concatenate onto each element, a list of elements, a suffix to concatenate onto each element, an environment for variable interpolation, and an optional function that will be called to transform the list before concatenation.

      env['_CPPINCFLAGS'] = '$( ${_concat(INCPREFIX, CPPPATH, INCSUFFIX, __env__, RDirs)} $)',

`CONFIGUREDIR`

  The name of the directory in which Configure context test files are written. The default is `.sconf_temp` in the top-level directory containing the `SConstruct` file.

`CONFIGURELOG`

  The name of the Configure context log file. The default is `config.log` in the top-level directory containing the `SConstruct` file.

`_CPPDEFFLAGS`

  An automatically-generated construction variable containing the C preprocessor command-line options to define values. The value of `$_CPPDEFFLAGS` is created by appending `$CPPDEFPREFIX` and `$CPPDEFSUFFIX` to the beginning and end of each directory in `$CPPDEFINES`.

`CPPDEFINES`

  A platform independent specification of C preprocessor definitions. The definitions will be added to command lines through the automatically-generated `$_CPPDEFFLAGS` construction variable (see above), which is constructed according to the type of value of `$CPPDEFINES`:

  If `$CPPDEFINES` is a string, the values of the `$CPPDEFPREFIX` and `$CPPDEFSUFFIX` construction variables will be added to the beginning and end.

      # Will add -Dxyz to POSIX compiler command lines,
      # and /Dxyz to Microsoft Visual C++ command lines.
      env = Environment(CPPDEFINES='xyz')

  If `$CPPDEFINES` is a list, the values of the `$CPPDEFPREFIX` and `$CPPDEFSUFFIX` construction variables will be appended to the beginning and end of each element in the list. If any element is a list or tuple, then the first item is the name being defined and the second item is its value:

      # Will add -DB=2 -DA to POSIX compiler command lines,
      # and /DB=2 /DA to Microsoft Visual C++ command lines.
      env = Environment(CPPDEFINES=[('B', 2), 'A'])

  If `$CPPDEFINES` is a dictionary, the values of the `$CPPDEFPREFIX` and `$CPPDEFSUFFIX` construction variables will be appended to the beginning and end of each item from the dictionary. The key of each dictionary item is a name being defined to the dictionary item's corresponding value; if the value is `None`, then the name is defined without an explicit value. Note that the resulting flags are sorted by keyword to ensure that the order of the options on the command line is consistent each time scons is run.

      # Will add -DA -DB=2 to POSIX compiler command lines,
      # and /DA /DB=2 to Microsoft Visual C++ command lines.
      env = Environment(CPPDEFINES={'B':2, 'A':None})

`CPPDEFPREFIX`

  The prefix used to specify preprocessor definitions on the C compiler command line. This will be appended to the beginning of each definition in the `$CPPDEFINES` construction variable when the `$_CPPDEFFLAGS` variable is automatically generated.

`CPPDEFSUFFIX`

  The suffix used to specify preprocessor definitions on the C compiler command line. This will be appended to the end of each definition in the `$CPPDEFINES` construction variable when the `$_CPPDEFFLAGS` variable is automatically generated.

`CPPFLAGS`

  User-specified C preprocessor options. These will be included in any command that uses the C preprocessor, including not just compilation of C and C++ source files via the `$CCCOM`, `$SHCCCOM`, `$CXXCOM` and `$SHCXXCOM` command lines, but also the `$FORTRANPPCOM`, `$SHFORTRANPPCOM`, `$F77PPCOM` and `$SHF77PPCOM` command lines used to compile a Fortran source file, and the `$ASPPCOM` command line used to assemble an assembly language source file, after first running each file through the C preprocessor. Note that this variable does not contain `-I` (or similar) include search path options that scons generates automatically from `$CPPPATH`. See `$_CPPINCFLAGS`, below, for the variable that expands to those options.

`_CPPINCFLAGS`

  An automatically-generated construction variable containing the C preprocessor command-line options for specifying directories to be searched for include files. The value of `$_CPPINCFLAGS` is created by appending `$INCPREFIX` and `$INCSUFFIX` to the beginning and end of each directory in `$CPPPATH`.

`CPPPATH`

  The list of directories that the C preprocessor will search for include directories. The C/C++ implicit dependency scanner will search these directories for include files. Don't explicitly put include directory arguments in CCFLAGS or CXXFLAGS because the result will be non-portable and the directories will not be searched by the dependency scanner. Note: directory names in CPPPATH will be looked-up relative to the SConscript directory when they are used in a command. To force scons to look-up a directory relative to the root of the source tree use #:

      env = Environment(CPPPATH='#/include')

  The directory look-up can also be forced using the `Dir`() function:

      include = Dir('include')
      env = Environment(CPPPATH=include)

  The directory list will be added to command lines through the automatically-generated `$_CPPINCFLAGS` construction variable, which is constructed by appending the values of the `$INCPREFIX` and `$INCSUFFIX` construction variables to the beginning and end of each directory in `$CPPPATH`. Any command lines you define that need the CPPPATH directory list should include `$_CPPINCFLAGS`:

      env = Environment(CCCOM="my_compiler $_CPPINCFLAGS -c -o $TARGET $SOURCE")

`CPPSUFFIXES`

  The list of suffixes of files that will be scanned for C preprocessor implicit dependencies (#include lines). The default list is:

      [".c", ".C", ".cxx", ".cpp", ".c++", ".cc",
       ".h", ".H", ".hxx", ".hpp", ".hh",
       ".F", ".fpp", ".FPP",
       ".m", ".mm",
       ".S", ".spp", ".SPP"]

`CVS`

  The CVS executable.

`CVSCOFLAGS`

  Options that are passed to the CVS checkout subcommand.

`CVSCOM`

  The command line used to fetch source files from a CVS repository.

`CVSCOMSTR`

  The string displayed when fetching a source file from a CVS repository. If this is not set, then `$CVSCOM` (the command line) is displayed.

`CVSFLAGS`

  General options that are passed to CVS. By default, this is set to `-d $CVSREPOSITORY` to specify from where the files must be fetched.

`CVSREPOSITORY`

  The path to the CVS repository. This is referenced in the default `$CVSFLAGS` value.

`CXX`

  The C++ compiler.

`CXXCOM`

  The command line used to compile a C++ source file to an object file. Any options specified in the `$CXXFLAGS` and `$CPPFLAGS` construction variables are included on this command line.

`CXXCOMSTR`

  The string displayed when a C++ source file is compiled to a (static) object file. If this is not set, then `$CXXCOM` (the command line) is displayed.

      env = Environment(CXXCOMSTR = "Compiling static object $TARGET")

`CXXFILESUFFIX`

  The suffix for C++ source files. This is used by the internal CXXFile builder when generating C++ files from Lex (.ll) or YACC (.yy) input files. The default suffix is `.cc`. SCons also treats files with the suffixes `.cpp`, `.cxx`, `.c++`, and `.C++` as C++ files, and files with `.mm` suffixes as Objective C++ files. On case-sensitive systems (Linux, UNIX, and other POSIX-alikes), SCons also treats `.C` (upper case) files as C++ files.

`CXXFLAGS`

  General options that are passed to the C++ compiler. By default, this includes the value of `$CCFLAGS`, so that setting `$CCFLAGS` affects both C and C++ compilation. If you want to add C++-specific flags, you must set or override the value of `$CXXFLAGS`.

`CXXVERSION`

  The version number of the C++ compiler. This may or may not be set, depending on the specific C++ compiler being used.

`DESCRIPTION`

  A long description of the project being packaged. This is included in the relevant section of the file that controls the packaging build.

`DESCRIPTION_lang`

  A language-specific long description for the specified `lang`. This is used to populate a `%description -l` section of an RPM `.spec` file.

`Dir`

  A function that converts a string into a Dir instance relative to the target being built.

`Dirs`

  A function that converts a list of strings into a list of Dir instances relative to the target being built.

`DSUFFIXES`

  The list of suffixes of files that will be scanned for imported D package files. The default list is:

      ['.d']

`DVIPDF`

  The TeX DVI file to PDF file converter.

`DVIPDFCOM`

  The command line used to convert TeX DVI files into a PDF file.

`DVIPDFCOMSTR`

  The string displayed when a TeX DVI file is converted into a PDF file. If this is not set, then `$DVIPDFCOM` (the command line) is displayed.

`DVIPDFFLAGS`

  General options passed to the TeX DVI file to PDF file converter.

`DVIPS`

  The TeX DVI file to PostScript converter.

`DVIPSFLAGS`

  General options passed to the TeX DVI file to PostScript converter.

`ENV`

  A dictionary of environment variables to use when invoking commands. When `$ENV` is used in a command all list values will be joined using the path separator and any other non-string values will simply be coerced to a string. Note that, by default, scons does not propagate the environment in force when you execute scons to the commands used to build target files. This is so that builds will be guaranteed repeatable regardless of the environment variables set at the time scons is invoked.

  If you want to propagate your environment variables to the commands executed to build target files, you must do so explicitly:

      import os
      env = Environment(ENV = os.environ)

  Note that you can choose only to propagate certain environment variables. A common example is the system `PATH` environment variable, so that scons uses the same utilities as the invoking shell (or other process):

      import os
      env = Environment(ENV = {'PATH' : os.environ['PATH']})

`ESCAPE`

  A function that will be called to escape shell special characters in command lines. The function should take one argument: the command line string to escape; and should return the escaped command line.

`F77`

  The Fortran 77 compiler. You should normally set the `$FORTRAN` variable, which specifies the default Fortran compiler for all Fortran versions. You only need to set `$F77` if you need to use a specific compiler or compiler version for Fortran 77 files.

`F77COM`

  The command line used to compile a Fortran 77 source file to an object file. You only need to set `$F77COM` if you need to use a specific command line for Fortran 77 files. You should normally set the `$FORTRANCOM` variable, which specifies the default command line for all Fortran versions.

`F77COMSTR`

  The string displayed when a Fortran 77 source file is compiled to an object file. If this is not set, then `$F77COM` or `$FORTRANCOM` (the command line) is displayed.

`F77FLAGS`

  General user-specified options that are passed to the Fortran 77 compiler. Note that this variable does not contain `-I` (or similar) include search path options that scons generates automatically from `$F77PATH`. See `$_F77INCFLAGS` below, for the variable that expands to those options. You only need to set `$F77FLAGS` if you need to define specific user options for Fortran 77 files. You should normally set the `$FORTRANFLAGS` variable, which specifies the user-specified options passed to the default Fortran compiler for all Fortran versions.

`_F77INCFLAGS`

  An automatically-generated construction variable containing the Fortran 77 compiler command-line options for specifying directories to be searched for include files. The value of `$_F77INCFLAGS` is created by appending `$INCPREFIX` and `$INCSUFFIX` to the beginning and end of each directory in `$F77PATH`.

`F77PATH`

  The list of directories that the Fortran 77 compiler will search for include directories. The implicit dependency scanner will search these directories for include files. Don't explicitly put include directory arguments in `$F77FLAGS` because the result will be non-portable and the directories will not be searched by the dependency scanner. Note: directory names in `$F77PATH` will be looked-up relative to the SConscript directory when they are used in a command. To force scons to look-up a directory relative to the root of the source tree use #: You only need to set `$F77PATH` if you need to define a specific include path for Fortran 77 files. You should normally set the `$FORTRANPATH` variable, which specifies the include path for the default Fortran compiler for all Fortran versions.

      env = Environment(F77PATH='#/include')

  The directory look-up can also be forced using the `Dir`() function:

      include = Dir('include')
      env = Environment(F77PATH=include)

  The directory list will be added to command lines through the automatically-generated `$_F77INCFLAGS` construction variable, which is constructed by appending the values of the `$INCPREFIX` and `$INCSUFFIX` construction variables to the beginning and end of each directory in `$F77PATH`. Any command lines you define that need the F77PATH directory list should include `$_F77INCFLAGS`:

      env = Environment(F77COM="my_compiler $_F77INCFLAGS -c -o $TARGET $SOURCE")

`F77PPCOM`

  The command line used to compile a Fortran 77 source file to an object file after first running the file through the C preprocessor. Any options specified in the `$F77FLAGS` and `$CPPFLAGS` construction variables are included on this command line. You only need to set `$F77PPCOM` if you need to use a specific C-preprocessor command line for Fortran 77 files. You should normally set the `$FORTRANPPCOM` variable, which specifies the default C-preprocessor command line for all Fortran versions.

`F77PPCOMSTR`

  The string displayed when a Fortran 77 source file is compiled to an object file after first running the file through the C preprocessor. If this is not set, then `$F77PPCOM` or `$FORTRANPPCOM` (the command line) is displayed.

`F90`

  The Fortran 90 compiler. You should normally set the `$FORTRAN` variable, which specifies the default Fortran compiler for all Fortran versions. You only need to set `$F90` if you need to use a specific compiler or compiler version for Fortran 90 files.

`F90COM`

  The command line used to compile a Fortran 90 source file to an object file. You only need to set `$F90COM` if you need to use a specific command line for Fortran 90 files. You should normally set the `$FORTRANCOM` variable, which specifies the default command line for all Fortran versions.

`F90COMSTR`

  The string displayed when a Fortran 90 source file is compiled to an object file. If this is not set, then `$F90COM` or `$FORTRANCOM` (the command line) is displayed.

`F90FLAGS`

  General user-specified options that are passed to the Fortran 90 compiler. Note that this variable does not contain `-I` (or similar) include search path options that scons generates automatically from `$F90PATH`. See `$_F90INCFLAGS` below, for the variable that expands to those options. You only need to set `$F90FLAGS` if you need to define specific user options for Fortran 90 files. You should normally set the `$FORTRANFLAGS` variable, which specifies the user-specified options passed to the default Fortran compiler for all Fortran versions.

`_F90INCFLAGS`

  An automatically-generated construction variable containing the Fortran 90 compiler command-line options for specifying directories to be searched for include files. The value of `$_F90INCFLAGS` is created by appending `$INCPREFIX` and `$INCSUFFIX` to the beginning and end of each directory in `$F90PATH`.

`F90PATH`

  The list of directories that the Fortran 90 compiler will search for include directories. The implicit dependency scanner will search these directories for include files. Don't explicitly put include directory arguments in `$F90FLAGS` because the result will be non-portable and the directories will not be searched by the dependency scanner. Note: directory names in `$F90PATH` will be looked-up relative to the SConscript directory when they are used in a command. To force scons to look-up a directory relative to the root of the source tree use #: You only need to set `$F90PATH` if you need to define a specific include path for Fortran 90 files. You should normally set the `$FORTRANPATH` variable, which specifies the include path for the default Fortran compiler for all Fortran versions.

      env = Environment(F90PATH='#/include')

  The directory look-up can also be forced using the `Dir`() function:

      include = Dir('include')
      env = Environment(F90PATH=include)

  The directory list will be added to command lines through the automatically-generated `$_F90INCFLAGS` construction variable, which is constructed by appending the values of the `$INCPREFIX` and `$INCSUFFIX` construction variables to the beginning and end of each directory in `$F90PATH`. Any command lines you define that need the F90PATH directory list should include `$_F90INCFLAGS`:

      env = Environment(F90COM="my_compiler $_F90INCFLAGS -c -o $TARGET $SOURCE")

`F90PPCOM`

  The command line used to compile a Fortran 90 source file to an object file after first running the file through the C preprocessor. Any options specified in the `$F90FLAGS` and `$CPPFLAGS` construction variables are included on this command line. You only need to set `$F90PPCOM` if you need to use a specific C-preprocessor command line for Fortran 90 files. You should normally set the `$FORTRANPPCOM` variable, which specifies the default C-preprocessor command line for all Fortran versions.

`F90PPCOMSTR`

  The string displayed when a Fortran 90 source file is compiled after first running the file through the C preprocessor. If this is not set, then `$F90PPCOM` or `$FORTRANPPCOM` (the command line) is displayed.

`F95`

  The Fortran 95 compiler. You should normally set the `$FORTRAN` variable, which specifies the default Fortran compiler for all Fortran versions. You only need to set `$F95` if you need to use a specific compiler or compiler version for Fortran 95 files.

`F95COM`

  The command line used to compile a Fortran 95 source file to an object file. You only need to set `$F95COM` if you need to use a specific command line for Fortran 95 files. You should normally set the `$FORTRANCOM` variable, which specifies the default command line for all Fortran versions.

`F95COMSTR`

  The string displayed when a Fortran 95 source file is compiled to an object file. If this is not set, then `$F95COM` or `$FORTRANCOM` (the command line) is displayed.

`F95FLAGS`

  General user-specified options that are passed to the Fortran 95 compiler. Note that this variable does not contain `-I` (or similar) include search path options that scons generates automatically from `$F95PATH`. See `$_F95INCFLAGS` below, for the variable that expands to those options. You only need to set `$F95FLAGS` if you need to define specific user options for Fortran 95 files. You should normally set the `$FORTRANFLAGS` variable, which specifies the user-specified options passed to the default Fortran compiler for all Fortran versions.

`_F95INCFLAGS`

  An automatically-generated construction variable containing the Fortran 95 compiler command-line options for specifying directories to be searched for include files. The value of `$_F95INCFLAGS` is created by appending `$INCPREFIX` and `$INCSUFFIX` to the beginning and end of each directory in `$F95PATH`.

`F95PATH`

  The list of directories that the Fortran 95 compiler will search for include directories. The implicit dependency scanner will search these directories for include files. Don't explicitly put include directory arguments in `$F95FLAGS` because the result will be non-portable and the directories will not be searched by the dependency scanner. Note: directory names in `$F95PATH` will be looked-up relative to the SConscript directory when they are used in a command. To force scons to look-up a directory relative to the root of the source tree use #: You only need to set `$F95PATH` if you need to define a specific include path for Fortran 95 files. You should normally set the `$FORTRANPATH` variable, which specifies the include path for the default Fortran compiler for all Fortran versions.

      env = Environment(F95PATH='#/include')

  The directory look-up can also be forced using the `Dir`() function:

      include = Dir('include')
      env = Environment(F95PATH=include)

  The directory list will be added to command lines through the automatically-generated `$_F95INCFLAGS` construction variable, which is constructed by appending the values of the `$INCPREFIX` and `$INCSUFFIX` construction variables to the beginning and end of each directory in `$F95PATH`. Any command lines you define that need the F95PATH directory list should include `$_F95INCFLAGS`:

      env = Environment(F95COM="my_compiler $_F95INCFLAGS -c -o $TARGET $SOURCE")

`F95PPCOM`

  The command line used to compile a Fortran 95 source file to an object file after first running the file through the C preprocessor. Any options specified in the `$F95FLAGS` and `$CPPFLAGS` construction variables are included on this command line. You only need to set `$F95PPCOM` if you need to use a specific C-preprocessor command line for Fortran 95 files. You should normally set the `$FORTRANPPCOM` variable, which specifies the default C-preprocessor command line for all Fortran versions.

`F95PPCOMSTR`

  The string displayed when a Fortran 95 source file is compiled to an object file after first running the file through the C preprocessor. If this is not set, then `$F95PPCOM` or `$FORTRANPPCOM` (the command line) is displayed.

`File`

  A function that converts a string into a File instance relative to the target being built.

`FORTRAN`

  The default Fortran compiler for all versions of Fortran.

`FORTRANCOM`

  The command line used to compile a Fortran source file to an object file. By default, any options specified in the `$FORTRANFLAGS`, `$CPPFLAGS`, `$_CPPDEFFLAGS`, `$_FORTRANMODFLAG`, and `$_FORTRANINCFLAGS` construction variables are included on this command line.

`FORTRANCOMSTR`

  The string displayed when a Fortran source file is compiled to an object file. If this is not set, then `$FORTRANCOM` (the command line) is displayed.

`FORTRANFLAGS`

  General user-specified options that are passed to the Fortran compiler. Note that this variable does not contain `-I` (or similar) include or module search path options that scons generates automatically from `$FORTRANPATH`. See `$_FORTRANINCFLAGS` and `$_FORTRANMODFLAG`, below, for the variables that expand those options.

`_FORTRANINCFLAGS`

  An automatically-generated construction variable containing the Fortran compiler command-line options for specifying directories to be searched for include files and module files. The value of `$_FORTRANINCFLAGS` is created by prepending/appending `$INCPREFIX` and `$INCSUFFIX` to the beginning and end of each directory in `$FORTRANPATH`.

`FORTRANMODDIR`

  Directory location where the Fortran compiler should place any module files it generates. This variable is empty, by default. Some Fortran compilers will internally append this directory in the search path for module files, as well.

`FORTRANMODDIRPREFIX`

  The prefix used to specify a module directory on the Fortran compiler command line. This will be appended to the beginning of the directory in the `$FORTRANMODDIR` construction variables when the `$_FORTRANMODFLAG` variables is automatically generated.

`FORTRANMODDIRSUFFIX`

  The suffix used to specify a module directory on the Fortran compiler command line. This will be appended to the beginning of the directory in the `$FORTRANMODDIR` construction variables when the `$_FORTRANMODFLAG` variables is automatically generated.

`_FORTRANMODFLAG`

  An automatically-generated construction variable containing the Fortran compiler command-line option for specifying the directory location where the Fortran compiler should place any module files that happen to get generated during compilation. The value of `$_FORTRANMODFLAG` is created by prepending/appending `$FORTRANMODDIRPREFIX` and `$FORTRANMODDIRSUFFIX` to the beginning and end of the directory in `$FORTRANMODDIR`.

`FORTRANMODPREFIX`

  The module file prefix used by the Fortran compiler. SCons assumes that the Fortran compiler follows the quasi-standard naming convention for module files of `module_name.mod`. As a result, this variable is left empty, by default. For situations in which the compiler does not necessarily follow the normal convention, the user may use this variable. Its value will be appended to every module file name as scons attempts to resolve dependencies.

`FORTRANMODSUFFIX`

  The module file suffix used by the Fortran compiler. SCons assumes that the Fortran compiler follows the quasi-standard naming convention for module files of `module_name.mod`. As a result, this variable is set to ".mod", by default. For situations in which the compiler does not necessarily follow the normal convention, the user may use this variable. Its value will be appended to every module file name as scons attempts to resolve dependencies.

`FORTRANPATH`

  The list of directories that the Fortran compiler will search for include files and (for some compilers) module files. The Fortran implicit dependency scanner will search these directories for include files (but not module files since they are autogenerated and, as such, may not actually exist at the time the scan takes place). Don't explicitly put include directory arguments in FORTRANFLAGS because the result will be non-portable and the directories will not be searched by the dependency scanner. Note: directory names in FORTRANPATH will be looked-up relative to the SConscript directory when they are used in a command. To force scons to look-up a directory relative to the root of the source tree use #:

      env = Environment(FORTRANPATH='#/include')

  The directory look-up can also be forced using the `Dir`() function:

      include = Dir('include')
      env = Environment(FORTRANPATH=include)

  The directory list will be added to command lines through the automatically-generated `$_FORTRANINCFLAGS` construction variable, which is constructed by appending the values of the `$INCPREFIX` and `$INCSUFFIX` construction variables to the beginning and end of each directory in `$FORTRANPATH`. Any command lines you define that need the FORTRANPATH directory list should include `$_FORTRANINCFLAGS`:

      env = Environment(FORTRANCOM="my_compiler $_FORTRANINCFLAGS -c -o $TARGET $SOURCE")

`FORTRANPPCOM`

  The command line used to compile a Fortran source file to an object file after first running the file through the C preprocessor. By default, any options specified in the `$FORTRANFLAGS`, `$CPPFLAGS`, `$_CPPDEFFLAGS`, `$_FORTRANMODFLAG`, and `$_FORTRANINCFLAGS` construction variables are included on this command line.

`FORTRANPPCOMSTR`

  The string displayed when a Fortran source file is compiled to an object file after first running the file throught the C preprocessor. If this is not set, then `$FORTRANPPCOM` (the command line) is displayed.

`FORTRANSUFFIXES`

  The list of suffixes of files that will be scanned for Fortran implicit dependencies (INCLUDE lines and USE statements). The default list is:

      [".f", ".F", ".for", ".FOR", ".ftn", ".FTN", ".fpp", ".FPP",
      ".f77", ".F77", ".f90", ".F90", ".f95", ".F95"]

`FRAMEWORKPATH`

  On Mac OS X with gcc, a list containing the paths to search for frameworks. Used by the compiler to find framework-style includes like #include <Fmwk/Header.h>. Used by the linker to find user-specified frameworks when linking (see `$FRAMEWORKS`). For example:

       env.AppendUnique(FRAMEWORKPATH='#myframeworkdir')

  will add

        ... -Fmyframeworkdir

  to the compiler and linker command lines.

`_FRAMEWORKPATH`

  On Mac OS X with gcc, an automatically-generated construction variable containing the linker command-line options corresponding to `$FRAMEWORKPATH`.

`FRAMEWORKPATHPREFIX`

  On Mac OS X with gcc, the prefix to be used for the FRAMEWORKPATH entries. (see `$FRAMEWORKPATH`). The default value is `-F`.

`FRAMEWORKPREFIX`

  On Mac OS X with gcc, the prefix to be used for linking in frameworks (see `$FRAMEWORKS`). The default value is `-framework`.

`_FRAMEWORKS`

  On Mac OS X with gcc, an automatically-generated construction variable containing the linker command-line options for linking with FRAMEWORKS.

`FRAMEWORKS`

  On Mac OS X with gcc, a list of the framework names to be linked into a program or shared library or bundle. The default value is the empty list. For example:

       env.AppendUnique(FRAMEWORKS=Split('System Cocoa SystemConfiguration'))

`FRAMEWORKSFLAGS`

  On Mac OS X with gcc, general user-supplied frameworks options to be added at the end of a command line building a loadable module. (This has been largely superceded by the `$FRAMEWORKPATH`, `$FRAMEWORKPATHPREFIX`, `$FRAMEWORKPREFIX` and `$FRAMEWORKS` variables described above.)

`GS`

  The Ghostscript program used to convert PostScript to PDF files.

`GSCOM`

  The Ghostscript command line used to convert PostScript to PDF files.

`GSCOMSTR`

  The string displayed when Ghostscript is used to convert a PostScript file to a PDF file. If this is not set, then `$GSCOM` (the command line) is displayed.

`GSFLAGS`

  General options passed to the Ghostscript program when converting PostScript to PDF files.

`IDLSUFFIXES`

  The list of suffixes of files that will be scanned for IDL implicit dependencies (#include or import lines). The default list is:

      [".idl", ".IDL"]

`IMPLICIT_COMMAND_DEPENDENCIES`

  Controls whether or not SCons will add implicit dependencies for the commands executed to build targets.

  By default, SCons will add to each target an implicit dependency on the command represented by the first argument on any command line it executes. The specific file for the dependency is found by searching the `PATH` variable in the `ENV` environment used to execute the command.

  If the construction variable `$IMPLICIT_COMMAND_DEPENDENCIES` is set to a false value (`None`, `False`, `0`, etc.), then the implicit dependency will not be added to the targets built with that construction environment.

      env = Environment(IMPLICIT_COMMAND_DEPENDENCIES = 0)

`INCPREFIX`

  The prefix used to specify an include directory on the C compiler command line. This will be appended to the beginning of each directory in the `$CPPPATH` and `$FORTRANPATH` construction variables when the `$_CPPINCFLAGS` and `$_FORTRANINCFLAGS` variables are automatically generated.

`INCSUFFIX`

  The suffix used to specify an include directory on the C compiler command line. This will be appended to the end of each directory in the `$CPPPATH` and `$FORTRANPATH` construction variables when the `$_CPPINCFLAGS` and `$_FORTRANINCFLAGS` variables are automatically generated.

`INSTALL`

  A function to be called to install a file into a destination file name. The default function copies the file into the destination (and sets the destination file's mode and permission bits to match the source file's). The function takes the following arguments:

      def install(dest, source, env):

  `dest` is the path name of the destination file. `source` is the path name of the source file. `env` is the construction environment (a dictionary of construction values) in force for this file installation.

`INSTALLSTR`

  The string displayed when a file is installed into a destination file name. The default is:

      Install file: "$SOURCE" as "$TARGET"

`INTEL_C_COMPILER_VERSION`

  Set by the "intelc" Tool to the major version number of the Intel C compiler selected for use.

`JAR`

  The Java archive tool.

`JARCHDIR`

  The directory to which the Java archive tool should change (using the `-C` option).

`JARCOM`

  The command line used to call the Java archive tool.

`JARCOMSTR`

  The string displayed when the Java archive tool is called If this is not set, then `$JARCOM` (the command line) is displayed.

      env = Environment(JARCOMSTR = "JARchiving $SOURCES into $TARGET")

`JARFLAGS`

  General options passed to the Java archive tool. By default this is set to `cf` to create the necessary jar file.

`JARSUFFIX`

  The suffix for Java archives: `.jar` by default.

`JAVABOOTCLASSPATH`

  Specifies the list of directories that will be added to the javac command line via the `-bootclasspath` option. The individual directory names will be separated by the operating system's path separate character (`:` on UNIX/Linux/POSIX, `;` on Windows).

`JAVAC`

  The Java compiler.

`JAVACCOM`

  The command line used to compile a directory tree containing Java source files to corresponding Java class files. Any options specified in the `$JAVACFLAGS` construction variable are included on this command line.

`JAVACCOMSTR`

  The string displayed when compiling a directory tree of Java source files to corresponding Java class files. If this is not set, then `$JAVACCOM` (the command line) is displayed.

      env = Environment(JAVACCOMSTR = "Compiling class files $TARGETS from $SOURCES")

`JAVACFLAGS`

  General options that are passed to the Java compiler.

`JAVACLASSDIR`

  The directory in which Java class files may be found. This is stripped from the beginning of any Java .class file names supplied to the `JavaH` builder.

`JAVACLASSPATH`

  Specifies the list of directories that will be searched for Java `.class` file. The directories in this list will be added to the javac and javah command lines via the `-classpath` option. The individual directory names will be separated by the operating system's path separate character (`:` on UNIX/Linux/POSIX, `;` on Windows).

  Note that this currently just adds the specified directory via the `-classpath` option. SCons does not currently search the `$JAVACLASSPATH` directories for dependency `.class` files.

`JAVACLASSSUFFIX`

  The suffix for Java class files; `.class` by default.

`JAVAH`

  The Java generator for C header and stub files.

`JAVAHCOM`

  The command line used to generate C header and stub files from Java classes. Any options specified in the `$JAVAHFLAGS` construction variable are included on this command line.

`JAVAHCOMSTR`

  The string displayed when C header and stub files are generated from Java classes. If this is not set, then `$JAVAHCOM` (the command line) is displayed.

      env = Environment(JAVAHCOMSTR = "Generating header/stub file(s) $TARGETS from $SOURCES")

`JAVAHFLAGS`

  General options passed to the C header and stub file generator for Java classes.

`JAVASOURCEPATH`

  Specifies the list of directories that will be searched for input `.java` file. The directories in this list will be added to the javac command line via the `-sourcepath` option. The individual directory names will be separated by the operating system's path separate character (`:` on UNIX/Linux/POSIX, `;` on Windows).

  Note that this currently just adds the specified directory via the `-sourcepath` option. SCons does not currently search the `$JAVASOURCEPATH` directories for dependency `.java` files.

`JAVASUFFIX`

  The suffix for Java files; `.java` by default.

`JAVAVERSION`

  Specifies the Java version being used by the `Java` builder. This is not currently used to select one version of the Java compiler vs. another. Instead, you should set this to specify the version of Java supported by your javac compiler. The default is `1.4`.

  This is sometimes necessary because Java 1.5 changed the file names that are created for nested anonymous inner classes, which can cause a mismatch with the files that SCons expects will be generated by the javac compiler. Setting `$JAVAVERSION` to `1.5` (or `1.6`, as appropriate) can make SCons realize that a Java 1.5 or 1.6 build is actually up to date.

`LATEX`

  The LaTeX structured formatter and typesetter.

`LATEXCOM`

  The command line used to call the LaTeX structured formatter and typesetter.

`LATEXCOMSTR`

  The string displayed when calling the LaTeX structured formatter and typesetter. If this is not set, then `$LATEXCOM` (the command line) is displayed.

      env = Environment(LATEXCOMSTR = "Building $TARGET from LaTeX input $SOURCES")

`LATEXFLAGS`

  General options passed to the LaTeX structured formatter and typesetter.

`LATEXRETRIES`

  The maximum number of times that LaTeX will be re-run if the `.log` generated by the `$LATEXCOM` command indicates that there are undefined references. The default is to try to resolve undefined references by re-running LaTeX up to three times.

`LATEXSUFFIXES`

  The list of suffixes of files that will be scanned for LaTeX implicit dependencies (`\include` or `\import` files). The default list is:

      [".tex", ".ltx", ".latex"]

`LDMODULE`

  The linker for building loadable modules. By default, this is the same as `$SHLINK`.

`LDMODULECOM`

  The command line for building loadable modules. On Mac OS X, this uses the `$LDMODULE`, `$LDMODULEFLAGS` and `$FRAMEWORKSFLAGS` variables. On other systems, this is the same as `$SHLINK`.

`LDMODULECOMSTR`

  The string displayed when building loadable modules. If this is not set, then `$LDMODULECOM` (the command line) is displayed.

`LDMODULEFLAGS`

  General user options passed to the linker for building loadable modules.

`LDMODULEPREFIX`

  The prefix used for loadable module file names. On Mac OS X, this is null; on other systems, this is the same as `$SHLIBPREFIX`.

`LDMODULESUFFIX`

  The suffix used for loadable module file names. On Mac OS X, this is null; on other systems, this is the same as $SHLIBSUFFIX.

`LEX`

  The lexical analyzer generator.

`LEXCOM`

  The command line used to call the lexical analyzer generator to generate a source file.

`LEXCOMSTR`

  The string displayed when generating a source file using the lexical analyzer generator. If this is not set, then `$LEXCOM` (the command line) is displayed.

      env = Environment(LEXCOMSTR = "Lex'ing $TARGET from $SOURCES")

`LEXFLAGS`

  General options passed to the lexical analyzer generator.

`_LIBDIRFLAGS`

  An automatically-generated construction variable containing the linker command-line options for specifying directories to be searched for library. The value of `$_LIBDIRFLAGS` is created by appending `$LIBDIRPREFIX` and `$LIBDIRSUFFIX` to the beginning and end of each directory in `$LIBPATH`.

`LIBDIRPREFIX`

  The prefix used to specify a library directory on the linker command line. This will be appended to the beginning of each directory in the `$LIBPATH` construction variable when the `$_LIBDIRFLAGS` variable is automatically generated.

`LIBDIRSUFFIX`

  The suffix used to specify a library directory on the linker command line. This will be appended to the end of each directory in the `$LIBPATH` construction variable when the `$_LIBDIRFLAGS` variable is automatically generated.

`_LIBFLAGS`

  An automatically-generated construction variable containing the linker command-line options for specifying libraries to be linked with the resulting target. The value of `$_LIBFLAGS` is created by appending `$LIBLINKPREFIX` and `$LIBLINKSUFFIX` to the beginning and end of each filename in `$LIBS`.

`LIBLINKPREFIX`

  The prefix used to specify a library to link on the linker command line. This will be appended to the beginning of each library in the `$LIBS` construction variable when the `$_LIBFLAGS` variable is automatically generated.

`LIBLINKSUFFIX`

  The suffix used to specify a library to link on the linker command line. This will be appended to the end of each library in the `$LIBS` construction variable when the `$_LIBFLAGS` variable is automatically generated.

`LIBPATH`

  The list of directories that will be searched for libraries. The implicit dependency scanner will search these directories for include files. Don't explicitly put include directory arguments in `$LINKFLAGS` or `$SHLINKFLAGS` because the result will be non-portable and the directories will not be searched by the dependency scanner. Note: directory names in LIBPATH will be looked-up relative to the SConscript directory when they are used in a command. To force scons to look-up a directory relative to the root of the source tree use #:

      env = Environment(LIBPATH='#/libs')

  The directory look-up can also be forced using the `Dir`() function:

      libs = Dir('libs')
      env = Environment(LIBPATH=libs)

  The directory list will be added to command lines through the automatically-generated `$_LIBDIRFLAGS` construction variable, which is constructed by appending the values of the `$LIBDIRPREFIX` and `$LIBDIRSUFFIX` construction variables to the beginning and end of each directory in `$LIBPATH`. Any command lines you define that need the LIBPATH directory list should include `$_LIBDIRFLAGS`:

      env = Environment(LINKCOM="my_linker $_LIBDIRFLAGS $_LIBFLAGS -o $TARGET $SOURCE")

`LIBPREFIX`

  The prefix used for (static) library file names. A default value is set for each platform (posix, win32, os2, etc.), but the value is overridden by individual tools (ar, mslib, sgiar, sunar, tlib, etc.) to reflect the names of the libraries they create.

`LIBPREFIXES`

  A list of all legal prefixes for library file names. When searching for library dependencies, SCons will look for files with these prefixes, the base library name, and suffixes in the `$LIBSUFFIXES` list.

`LIBS`

  A list of one or more libraries that will be linked with any executable programs created by this environment.

  The library list will be added to command lines through the automatically-generated `$_LIBFLAGS` construction variable, which is constructed by appending the values of the `$LIBLINKPREFIX` and `$LIBLINKSUFFIX` construction variables to the beginning and end of each filename in `$LIBS`. Any command lines you define that need the LIBS library list should include `$_LIBFLAGS`:

      env = Environment(LINKCOM="my_linker $_LIBDIRFLAGS $_LIBFLAGS -o $TARGET $SOURCE")

  If you add a File object to the `$LIBS` list, the name of that file will be added to `$_LIBFLAGS`, and thus the link line, as is, without `$LIBLINKPREFIX` or `$LIBLINKSUFFIX`. For example:

      env.Append(LIBS=File('/tmp/mylib.so'))

  In all cases, scons will add dependencies from the executable program to all the libraries in this list.

`LIBSUFFIX`

  The suffix used for (static) library file names. A default value is set for each platform (posix, win32, os2, etc.), but the value is overridden by individual tools (ar, mslib, sgiar, sunar, tlib, etc.) to reflect the names of the libraries they create.

`LIBSUFFIXES`

  A list of all legal suffixes for library file names. When searching for library dependencies, SCons will look for files with prefixes, in the `$LIBPREFIXES` list, the base library name, and these suffixes.

`LICENSE`

  The abbreviated name of the license under which this project is released (gpl, lpgl, bsd etc.). See http://www.opensource.org/licenses/alphabetical for a list of license names.

`LINK`

  The linker.

`LINKCOM`

  The command line used to link object files into an executable.

`LINKCOMSTR`

  The string displayed when object files are linked into an executable. If this is not set, then `$LINKCOM` (the command line) is displayed.

      env = Environment(LINKCOMSTR = "Linking $TARGET")

`LINKFLAGS`

  General user options passed to the linker. Note that this variable should not contain `-l` (or similar) options for linking with the libraries listed in `$LIBS`, nor `-L` (or similar) library search path options that scons generates automatically from `$LIBPATH`. See `$_LIBFLAGS` above, for the variable that expands to library-link options, and `$_LIBDIRFLAGS` above, for the variable that expands to library search path options.

`M4`

  The M4 macro preprocessor.

`M4COM`

  The command line used to pass files through the M4 macro preprocessor.

`M4COMSTR`

  The string displayed when a file is passed through the M4 macro preprocessor. If this is not set, then `$M4COM` (the command line) is displayed.

`M4FLAGS`

  General options passed to the M4 macro preprocessor.

`MAKEINDEX`

  The makeindex generator for the TeX formatter and typesetter and the LaTeX structured formatter and typesetter.

`MAKEINDEXCOM`

  The command line used to call the makeindex generator for the TeX formatter and typesetter and the LaTeX structured formatter and typesetter.

`MAKEINDEXCOMSTR`

  The string displayed when calling the makeindex generator for the TeX formatter and typesetter and the LaTeX structured formatter and typesetter. If this is not set, then `$MAKEINDEXCOM` (the command line) is displayed.

`MAKEINDEXFLAGS`

  General options passed to the makeindex generator for the TeX formatter and typesetter and the LaTeX structured formatter and typesetter.

`MAXLINELENGTH`

  The maximum number of characters allowed on an external command line. On Win32 systems, link lines longer than this many characters are linked via a temporary file name.

`MIDL`

  The Microsoft IDL compiler.

`MIDLCOM`

  The command line used to pass files to the Microsoft IDL compiler.

`MIDLCOMSTR`

  The string displayed when the Microsoft IDL copmiler is called. If this is not set, then `$MIDLCOM` (the command line) is displayed.

`MIDLFLAGS`

  General options passed to the Microsoft IDL compiler.

`MSVS`

  When the Microsoft Visual Studio tools are initialized, they set up this dictionary with the following keys:

  `VERSION`: the version of MSVS being used (can be set via MSVS_VERSION)

  `VERSIONS`: the available versions of MSVS installed

  `VCINSTALLDIR`: installed directory of Visual C++

  `VSINSTALLDIR`: installed directory of Visual Studio

  `FRAMEWORKDIR`: installed directory of the .NET framework

  `FRAMEWORKVERSIONS`: list of installed versions of the .NET framework, sorted latest to oldest.

  `FRAMEWORKVERSION`: latest installed version of the .NET framework

  `FRAMEWORKSDKDIR`: installed location of the .NET SDK.

  `PLATFORMSDKDIR`: installed location of the Platform SDK.

  `PLATFORMSDK_MODULES`: dictionary of installed Platform SDK modules, where the dictionary keys are keywords for the various modules, and the values are 2-tuples where the first is the release date, and the second is the version number.

  If a value isn't set, it wasn't available in the registry.

`MSVS_IGNORE_IDE_PATHS`

  Tells the MS Visual Studio tools to use minimal INCLUDE, LIB, and PATH settings, instead of the settings from the IDE.

  For Visual Studio, SCons will (by default) automatically determine where MSVS is installed, and use the LIB, INCLUDE, and PATH variables set by the IDE. You can override this behavior by setting these variables after Environment initialization, or by setting `MSVS_IGNORE_IDE_PATHS = 1` in the Environment initialization. Specifying this will not leave these unset, but will set them to a minimal set of paths needed to run the tools successfully.

  For VS6, the mininimal set is:

         INCLUDE:'<VSDir>\VC98\ATL\include;<VSDir>\VC98\MFC\include;<VSDir>\VC98\include'
         LIB:'<VSDir>\VC98\MFC\lib;<VSDir>\VC98\lib'
         PATH:'<VSDir>\Common\MSDev98\bin;<VSDir>\VC98\bin'

  For VS7, it is:

         INCLUDE:'<VSDir>\Vc7\atlmfc\include;<VSDir>\Vc7\include'
         LIB:'<VSDir>\Vc7\atlmfc\lib;<VSDir>\Vc7\lib'
         PATH:'<VSDir>\Common7\Tools\bin;<VSDir>\Common7\Tools;<VSDir>\Vc7\bin'

  Where '<VSDir>' is the installed location of Visual Studio.

`MSVS_PROJECT_BASE_PATH`

  The string placed in a generated Microsoft Visual Studio solution file as the value of the `SccProjectFilePathRelativizedFromConnection0` and `SccProjectFilePathRelativizedFromConnection1` attributes of the `GlobalSection(SourceCodeControl)` section. There is no default value.

`MSVS_PROJECT_GUID`

  The string placed in a generated Microsoft Visual Studio project file as the value of the `ProjectGUID` attribute. The string is also placed in the `SolutionUniqueID` attribute of the `GlobalSection(SourceCodeControl)` section of the Microsoft Visual Studio solution file. There is no default value.

`MSVS_SCC_AUX_PATH`

  The path name placed in a generated Microsoft Visual Studio project file as the value of the `SccAuxPath` attribute if the `MSVS_SCC_PROVIDER` construction variable is also set. There is no default value.

`MSVS_SCC_LOCAL_PATH`

  The path name placed in a generated Microsoft Visual Studio project file as the value of the `SccLocalPath` attribute if the `MSVS_SCC_PROVIDER` construction variable is also set. The path name is also placed in the `SccLocalPath0` and `SccLocalPath1` attributes of the `GlobalSection(SourceCodeControl)` section of the Microsoft Visual Studio solution file. There is no default value.

`MSVS_SCC_PROJECT_NAME`

  The project name placed in a generated Microsoft Visual Studio project file as the value of the `SccProjectName` attribute. There is no default value.

`MSVS_SCC_PROVIDER`

  The string placed in a generated Microsoft Visual Studio project file as the value of the `SccProvider` attribute. The string is also placed in the `SccProvider1` attribute of the `GlobalSection(SourceCodeControl)` section of the Microsoft Visual Studio solution file. There is no default value.

`MSVS_USE_MFC_DIRS`

  Tells the MS Visual Studio tool(s) to use the MFC directories in its default paths for compiling and linking. The `$MSVS_USE_MFC_DIRS` variable has no effect if the `INCLUDE` or `LIB` environment variables are set explictly.

  Under Visual Studio version 6, setting `$MSVS_USE_MFC_DIRS` to a non-zero value adds the `ATL\include` and `MFC\include` directories to the default `INCLUDE` external environment variable, and adds the `MFC\lib` directory to the default `LIB` external environment variable.

  Under Visual Studio version 7, setting `$MSVS_USE_MFC_DIRS` to a non-zero value adds the `atlmfc\include` directory to the default `INCLUDE` external environment variable, and adds the `atlmfc\lib` directory to the default `LIB` external environment variable.

  Under Visual Studio version 8, setting `$MSVS_USE_MFC_DIRS` to a non-zero value will, by default, add the `atlmfc\include` directory to the default `INCLUDE` external environment variable, and the `atlmfc\lib` directory to the default `LIB` external environment variable. If, however, the `['MSVS']['PLATFORMSDKDIR']` variable is set, then the `mfc` and the `atl` subdirectories of the `PLATFORMSDKDIR` are added to the default value of the `INCLUDE` external environment variable, and the default value of the `LIB` external environment variable is left untouched.

`MSVS_VERSION`

  Sets the preferred version of MSVS to use.

  SCons will (by default) select the latest version of MSVS installed on your machine. So, if you have version 6 and version 7 (MSVS .NET) installed, it will prefer version 7. You can override this by specifying the `MSVS_VERSION` variable in the Environment initialization, setting it to the appropriate version ('6.0' or '7.0', for example). If the given version isn't installed, tool initialization will fail.

`MSVSBUILDCOM`

  The build command line placed in a generated Microsoft Visual Studio project file. The default is to have Visual Studio invoke SCons with any specified build targets.

`MSVSCLEANCOM`

  The clean command line placed in a generated Microsoft Visual Studio project file. The default is to have Visual Studio invoke SCons with the -c option to remove any specified targets.

`MSVSENCODING`

  The encoding string placed in a generated Microsoft Visual Studio project file. The default is encoding `Windows-1252`.

`MSVSPROJECTCOM`

  The action used to generate Microsoft Visual Studio project files.

`MSVSPROJECTSUFFIX`

  The suffix used for Microsoft Visual Studio project (DSP) files. The default value is `.vcproj` when using Visual Studio version 7.x (.NET) or later version, and `.dsp` when using earlier versions of Visual Studio.

`MSVSREBUILDCOM`

  The rebuild command line placed in a generated Microsoft Visual Studio project file. The default is to have Visual Studio invoke SCons with any specified rebuild targets.

`MSVSSCONS`

  The SCons used in generated Microsoft Visual Studio project files. The default is the version of SCons being used to generate the project file.

`MSVSSCONSCOM`

  The default SCons command used in generated Microsoft Visual Studio project files.

`MSVSSCONSCRIPT`

  The sconscript file (that is, `SConstruct` or `SConscript` file) that will be invoked by Visual Studio project files (through the `$MSVSSCONSCOM` variable). The default is the same sconscript file that contains the call to `MSVSProject` to build the project file.

`MSVSSCONSFLAGS`

  The SCons flags used in generated Microsoft Visual Studio project files.

`MSVSSOLUTIONCOM`

  The action used to generate Microsoft Visual Studio solution files.

`MSVSSOLUTIONSUFFIX`

  The suffix used for Microsoft Visual Studio solution (DSW) files. The default value is `.sln` when using Visual Studio version 7.x (.NET), and `.dsw` when using earlier versions of Visual Studio.

`MWCW_VERSION`

  The version number of the MetroWerks CodeWarrior C compiler to be used.

`MWCW_VERSIONS`

  A list of installed versions of the MetroWerks CodeWarrior C compiler on this system.

`NAME`

  Specfies the name of the project to package.

`no_import_lib`

  When set to non-zero, suppresses creation of a corresponding Windows static import lib by the `SharedLibrary` builder when used with MinGW, Microsoft Visual Studio or Metrowerks. This also suppresses creation of an export (.exp) file when using Microsoft Visual Studio.

`OBJPREFIX`

  The prefix used for (static) object file names.

`OBJSUFFIX`

  The suffix used for (static) object file names.

`P4`

  The Perforce executable.

`P4COM`

  The command line used to fetch source files from Perforce.

`P4COMSTR`

  The string displayed when fetching a source file from Perforce. If this is not set, then `$P4COM` (the command line) is displayed.

`P4FLAGS`

  General options that are passed to Perforce.

`PACKAGEROOT`

  Specifies the directory where all files in resulting archive will be placed if applicable. The default value is "$NAME-$VERSION".

`PACKAGETYPE`

  Selects the package type to build. Currently these are available:

  * msi - Microsoft Installer * rpm - Redhat Package Manger * ipkg - Itsy Package Management System * tarbz2 - compressed tar * targz - compressed tar * zip - zip file * src_tarbz2 - compressed tar source * src_targz - compressed tar source * src_zip - zip file source

  This may be overridden with the "package_type" command line option.

`PACKAGEVERSION`

  The version of the package (not the underlying project). This is currently only used by the rpm packager and should reflect changes in the packaging, not the underlying project code itself.

`PCH`

  The Microsoft Visual C++ precompiled header that will be used when compiling object files. This variable is ignored by tools other than Microsoft Visual C++. When this variable is defined SCons will add options to the compiler command line to cause it to use the precompiled header, and will also set up the dependencies for the PCH file. Example:

      env['PCH'] = 'StdAfx.pch'

`PCHCOM`

  The command line used by the `PCH` builder to generated a precompiled header.

`PCHCOMSTR`

  The string displayed when generating a precompiled header. If this is not set, then `$PCHCOM` (the command line) is displayed.

`PCHPDBFLAGS`

  A construction variable that, when expanded, adds the `/yD` flag to the command line only if the `$PDB` construction variable is set.

`PCHSTOP`

  This variable specifies how much of a source file is precompiled. This variable is ignored by tools other than Microsoft Visual C++, or when the PCH variable is not being used. When this variable is define it must be a string that is the name of the header that is included at the end of the precompiled portion of the source files, or the empty string if the "#pragma hrdstop" construct is being used:

      env['PCHSTOP'] = 'StdAfx.h'

`PDB`

  The Microsoft Visual C++ PDB file that will store debugging information for object files, shared libraries, and programs. This variable is ignored by tools other than Microsoft Visual C++. When this variable is defined SCons will add options to the compiler and linker command line to cause them to generate external debugging information, and will also set up the dependencies for the PDB file. Example:

      env['PDB'] = 'hello.pdb'

  The Visual C++ compiler switch that SCons uses by default to generate PDB information is `/Z7`. This works correctly with parallel (`-j`) builds because it embeds the debug information in the intermediate object files, as opposed to sharing a single PDB file between multiple object files. This is also the only way to get debug information embedded into a static library. Using the `/Zi` instead may yield improved link-time performance, although parallel builds will no longer work. You can generate PDB files with the `/Zi` switch by overriding the default `$CCPDBFLAGS` variable; see the entry for that variable for specific examples.

`PDFCOM`

  A deprecated synonym for `$DVIPDFCOM`.

`PDFLATEX`

  The pdflatex utility.

`PDFLATEXCOM`

  The command line used to call the pdflatex utility.

`PDFLATEXCOMSTR`

  The string displayed when calling the pdflatex utility. If this is not set, then `$PDFLATEXCOM` (the command line) is displayed.

      env = Environment(PDFLATEX;COMSTR = "Building $TARGET from LaTeX input $SOURCES")

`PDFLATEXFLAGS`

  General options passed to the pdflatex utility.

`PDFPREFIX`

  The prefix used for PDF file names.

`PDFSUFFIX`

  The suffix used for PDF file names.

`PDFTEX`

  The pdftex utility.

`PDFTEXCOM`

  The command line used to call the pdftex utility.

`PDFTEXCOMSTR`

  The string displayed when calling the pdftex utility. If this is not set, then `$PDFTEXCOM` (the command line) is displayed.

      env = Environment(PDFTEXCOMSTR = "Building $TARGET from TeX input $SOURCES")

`PDFTEXFLAGS`

  General options passed to the pdftex utility.

`PKGCHK`

  On Solaris systems, the package-checking program that will be used (along with `$PKGINFO`) to look for installed versions of the Sun PRO C++ compiler. The default is `/usr/sbin/pgkchk`.

`PKGINFO`

  On Solaris systems, the package information program that will be used (along with `$PKGCHK`) to look for installed versions of the Sun PRO C++ compiler. The default is `pkginfo`.

`PLATFORM`

  The name of the platform used to create the Environment. If no platform is specified when the Environment is created, scons autodetects the platform.

      env = Environment(tools = [])
      if env['PLATFORM'] == 'cygwin':
          Tool('mingw')(env)
      else:
          Tool('msvc')(env)

`PRINT_CMD_LINE_FUNC`

  A Python function used to print the command lines as they are executed (assuming command printing is not disabled by the `-q` or `-s` options or their equivalents). The function should take four arguments: `s`, the command being executed (a string), `target`, the target being built (file node, list, or string name(s)), `source`, the source(s) used (file node, list, or string name(s)), and `env`, the environment being used.

  The function must do the printing itself. The default implementation, used if this variable is not set or is None, is:

      def print_cmd_line(s, target, source, env):
        sys.stdout.write(s + "\n")

  Here's an example of a more interesting function:

      def print_cmd_line(s, target, source, env):
         sys.stdout.write("Building %s -> %s...\n" %
          (' and '.join([str(x) for x in source]),
           ' and '.join([str(x) for x in target])))
      env=Environment(PRINT_CMD_LINE_FUNC=print_cmd_line)
      env.Program('foo', 'foo.c')

  This just prints "Building `targetname` from `sourcename`..." instead of the actual commands. Such a function could also log the actual commands to a log file, for example.

`PROGPREFIX`

  The prefix used for executable file names.

`PROGSUFFIX`

  The suffix used for executable file names.

`PSCOM`

  The command line used to convert TeX DVI files into a PostScript file.

`PSCOMSTR`

  The string displayed when a TeX DVI file is converted into a PostScript file. If this is not set, then `$PSCOM` (the command line) is displayed.

`PSPREFIX`

  The prefix used for PostScript file names.

`PSSUFFIX`

  The prefix used for PostScript file names.

`QT_AUTOSCAN`

  Turn off scanning for mocable files. Use the Moc Builder to explicitely specify files to run moc on.

`QT_BINPATH`

  The path where the qt binaries are installed. The default value is '`$QTDIR`/bin'.

`QT_CPPPATH`

  The path where the qt header files are installed. The default value is '`$QTDIR`/include'. Note: If you set this variable to None, the tool won't change the `$CPPPATH` construction variable.

`QT_DEBUG`

  Prints lots of debugging information while scanning for moc files.

`QT_LIB`

  Default value is 'qt'. You may want to set this to 'qt-mt'. Note: If you set this variable to None, the tool won't change the `$LIBS` variable.

`QT_LIBPATH`

  The path where the qt libraries are installed. The default value is '`$QTDIR`/lib'. Note: If you set this variable to None, the tool won't change the `$LIBPATH` construction variable.

`QT_MOC`

  Default value is '`$QT_BINPATH`/moc'.

`QT_MOCCXXPREFIX`

  Default value is ''. Prefix for moc output files, when source is a cxx file.

`QT_MOCCXXSUFFIX`

  Default value is '.moc'. Suffix for moc output files, when source is a cxx file.

`QT_MOCFROMCXXCOM`

  Command to generate a moc file from a cpp file.

`QT_MOCFROMCXXCOMSTR`

  The string displayed when generating a moc file from a cpp file. If this is not set, then `$QT_MOCFROMCXXCOM` (the command line) is displayed.

`QT_MOCFROMCXXFLAGS`

  Default value is '-i'. These flags are passed to moc, when moccing a C++ file.

`QT_MOCFROMHCOM`

  Command to generate a moc file from a header.

`QT_MOCFROMHCOMSTR`

  The string displayed when generating a moc file from a cpp file. If this is not set, then `$QT_MOCFROMHCOM` (the command line) is displayed.

`QT_MOCFROMHFLAGS`

  Default value is ''. These flags are passed to moc, when moccing a header file.

`QT_MOCHPREFIX`

  Default value is 'moc_'. Prefix for moc output files, when source is a header.

`QT_MOCHSUFFIX`

  Default value is '`$CXXFILESUFFIX`'. Suffix for moc output files, when source is a header.

`QT_UIC`

  Default value is '`$QT_BINPATH`/uic'.

`QT_UICCOM`

  Command to generate header files from .ui files.

`QT_UICCOMSTR`

  The string displayed when generating header files from .ui files. If this is not set, then `$QT_UICCOM` (the command line) is displayed.

`QT_UICDECLFLAGS`

  Default value is ''. These flags are passed to uic, when creating a a h file from a .ui file.

`QT_UICDECLPREFIX`

  Default value is ''. Prefix for uic generated header files.

`QT_UICDECLSUFFIX`

  Default value is '.h'. Suffix for uic generated header files.

`QT_UICIMPLFLAGS`

  Default value is ''. These flags are passed to uic, when creating a cxx file from a .ui file.

`QT_UICIMPLPREFIX`

  Default value is 'uic_'. Prefix for uic generated implementation files.

`QT_UICIMPLSUFFIX`

  Default value is '`$CXXFILESUFFIX`'. Suffix for uic generated implementation files.

`QT_UISUFFIX`

  Default value is '.ui'. Suffix of designer input files.

`QTDIR`

  The qt tool tries to take this from os.environ. It also initializes all QT_* construction variables listed below. (Note that all paths are constructed with python's os.path.join() method, but are listed here with the '/' separator for easier reading.) In addition, the construction environment variables `$CPPPATH`, `$LIBPATH` and `$LIBS` may be modified and the variables PROGEMITTER, SHLIBEMITTER and LIBEMITTER are modified. Because the build-performance is affected when using this tool, you have to explicitly specify it at Environment creation:

      Environment(tools=['default','qt'])

  The qt tool supports the following operations:

  Automatic moc file generation from header files. You do not have to specify moc files explicitly, the tool does it for you. However, there are a few preconditions to do so: Your header file must have the same filebase as your implementation file and must stay in the same directory. It must have one of the suffixes .h, .hpp, .H, .hxx, .hh. You can turn off automatic moc file generation by setting QT_AUTOSCAN to 0. See also the corresponding builder method .B Moc()

  Automatic moc file generation from cxx files. As stated in the qt documentation, include the moc file at the end of the cxx file. Note that you have to include the file, which is generated by the transformation ${QT_MOCCXXPREFIX}<basename>${QT_MOCCXXSUFFIX}, by default <basename>.moc. A warning is generated after building the moc file, if you do not include the correct file. If you are using VariantDir, you may need to specify duplicate=1. You can turn off automatic moc file generation by setting QT_AUTOSCAN to 0. See also the corresponding `Moc` builder method.

  Automatic handling of .ui files. The implementation files generated from .ui files are handled much the same as yacc or lex files. Each .ui file given as a source of Program, Library or SharedLibrary will generate three files, the declaration file, the implementation file and a moc file. Because there are also generated headers, you may need to specify duplicate=1 in calls to VariantDir. See also the corresponding `Uic` builder method.

`RANLIB`

  The archive indexer.

`RANLIBCOM`

  The command line used to index a static library archive.

`RANLIBCOMSTR`

  The string displayed when a static library archive is indexed. If this is not set, then `$RANLIBCOM` (the command line) is displayed.

      env = Environment(RANLIBCOMSTR = "Indexing $TARGET")

`RANLIBFLAGS`

  General options passed to the archive indexer.

`RC`

  The resource compiler used to build a Microsoft Visual C++ resource file.

`RCCOM`

  The command line used to build a Microsoft Visual C++ resource file.

`RCCOMSTR`

  The string displayed when invoking the resource compiler to build a Microsoft Visual C++ resource file. If this is not set, then `$RCCOM` (the command line) is displayed.

`RCFLAGS`

  The flags passed to the resource compiler by the RES builder.

`RCINCFLAGS`

  An automatically-generated construction variable containing the command-line options for specifying directories to be searched by the resource compiler. The value of `$RCINCFLAGS` is created by appending `$RCINCPREFIX` and `$RCINCSUFFIX` to the beginning and end of each directory in `$CPPPATH`.

`RCINCPREFIX`

  The prefix (flag) used to specify an include directory on the resource compiler command line. This will be appended to the beginning of each directory in the `$CPPPATH` construction variable when the `$RCINCFLAGS` variable is expanded.

`RCINCSUFFIX`

  The suffix used to specify an include directory on the resource compiler command line. This will be appended to the end of each directory in the `$CPPPATH` construction variable when the `$RCINCFLAGS` variable is expanded.

`RCS`

  The RCS executable. Note that this variable is not actually used for the command to fetch source files from RCS; see the `$RCS_CO` construction variable, below.

`RCS_CO`

  The RCS "checkout" executable, used to fetch source files from RCS.

`RCS_COCOM`

  The command line used to fetch (checkout) source files from RCS.

`RCS_COCOMSTR`

  The string displayed when fetching a source file from RCS. If this is not set, then `$RCS_COCOM` (the command line) is displayed.

`RCS_COFLAGS`

  Options that are passed to the `$RCS_CO` command.

`RDirs`

  A function that converts a string into a list of Dir instances by searching the repositories.

`REGSVR`

  The program used on Windows systems to register a newly-built DLL library whenever the `SharedLibrary` builder is passed a keyword argument of `register=1`.

`REGSVRCOM`

  The command line used on Windows systems to register a newly-built DLL library whenever the `SharedLibrary` builder is passed a keyword argument of `register=1`.

`REGSVRCOMSTR`

  The string displayed when registering a newly-built DLL file. If this is not set, then `$REGSVRCOM` (the command line) is displayed.

`REGSVRFLAGS`

  Flags passed to the DLL registration program on Windows systems when a newly-built DLL library is registered. By default, this includes the `/s` that prevents dialog boxes from popping up and requiring user attention.

`RMIC`

  The Java RMI stub compiler.

`RMICCOM`

  The command line used to compile stub and skeleton class files from Java classes that contain RMI implementations. Any options specified in the `$RMICFLAGS` construction variable are included on this command line.

`RMICCOMSTR`

  The string displayed when compiling stub and skeleton class files from Java classes that contain RMI implementations. If this is not set, then `$RMICCOM` (the command line) is displayed.

      env = Environment(RMICCOMSTR = "Generating stub/skeleton class files $TARGETS from $SOURCES")

`RMICFLAGS`

  General options passed to the Java RMI stub compiler.

`_RPATH`

  An automatically-generated construction variable containing the rpath flags to be used when linking a program with shared libraries. The value of `$_RPATH` is created by appending `$RPATHPREFIX` and `$RPATHSUFFIX` to the beginning and end of each directory in `$RPATH`.

`RPATH`

  A list of paths to search for shared libraries when running programs. Currently only used in the GNU (gnulink), IRIX (sgilink) and Sun (sunlink) linkers. Ignored on platforms and toolchains that don't support it. Note that the paths added to RPATH are not transformed by scons in any way: if you want an absolute path, you must make it absolute yourself.

`RPATHPREFIX`

  The prefix used to specify a directory to be searched for shared libraries when running programs. This will be appended to the beginning of each directory in the `$RPATH` construction variable when the `$_RPATH` variable is automatically generated.

`RPATHSUFFIX`

  The suffix used to specify a directory to be searched for shared libraries when running programs. This will be appended to the end of each directory in the `$RPATH` construction variable when the `$_RPATH` variable is automatically generated.

`RPCGEN`

  The RPC protocol compiler.

`RPCGENCLIENTFLAGS`

  Options passed to the RPC protocol compiler when generating client side stubs. These are in addition to any flags specified in the `$RPCGENFLAGS` construction variable.

`RPCGENFLAGS`

  General options passed to the RPC protocol compiler.

`RPCGENHEADERFLAGS`

  Options passed to the RPC protocol compiler when generating a header file. These are in addition to any flags specified in the `$RPCGENFLAGS` construction variable.

`RPCGENSERVICEFLAGS`

  Options passed to the RPC protocol compiler when generating server side stubs. These are in addition to any flags specified in the `$RPCGENFLAGS` construction variable.

`RPCGENXDRFLAGS`

  Options passed to the RPC protocol compiler when generating XDR routines. These are in addition to any flags specified in the `$RPCGENFLAGS` construction variable.

`SCANNERS`

  A list of the available implicit dependency scanners. New file scanners may be added by appending to this list, although the more flexible approach is to associate scanners with a specific Builder. See the sections "Builder Objects" and "Scanner Objects," below, for more information.

`SCCS`

  The SCCS executable.

`SCCSCOM`

  The command line used to fetch source files from SCCS.

`SCCSCOMSTR`

  The string displayed when fetching a source file from a CVS repository. If this is not set, then `$SCCSCOM` (the command line) is displayed.

`SCCSFLAGS`

  General options that are passed to SCCS.

`SCCSGETFLAGS`

  Options that are passed specifically to the SCCS "get" subcommand. This can be set, for example, to `-e` to check out editable files from SCCS.

`SCONS_HOME`

  The (optional) path to the SCons library directory, initialized from the external environment. If set, this is used to construct a shorter and more efficient search path in the `$MSVSSCONS` command line executed from Microsoft Visual Studio project files.

`SHCC`

  The C compiler used for generating shared-library objects.

`SHCCCOM`

  The command line used to compile a C source file to a shared-library object file. Any options specified in the `$SHCFLAGS`, `$SHCCFLAGS` and `$CPPFLAGS` construction variables are included on this command line.

`SHCCCOMSTR`

  The string displayed when a C source file is compiled to a shared object file. If this is not set, then `$SHCCCOM` (the command line) is displayed.

      env = Environment(SHCCCOMSTR = "Compiling shared object $TARGET")

`SHCCFLAGS`

  Options that are passed to the C and C++ compilers to generate shared-library objects.

`SHCFLAGS`

  Options that are passed to the C compiler (only; not C++) to generate shared-library objects.

`SHCXX`

  The C++ compiler used for generating shared-library objects.

`SHCXXCOM`

  The command line used to compile a C++ source file to a shared-library object file. Any options specified in the `$SHCXXFLAGS` and `$CPPFLAGS` construction variables are included on this command line.

`SHCXXCOMSTR`

  The string displayed when a C++ source file is compiled to a shared object file. If this is not set, then `$SHCXXCOM` (the command line) is displayed.

      env = Environment(SHCXXCOMSTR = "Compiling shared object $TARGET")

`SHCXXFLAGS`

  Options that are passed to the C++ compiler to generate shared-library objects.

`SHELL`

  A string naming the shell program that will be passed to the `$SPAWN` function. See the `$SPAWN` construction variable for more information.

`SHF77`

  The Fortran 77 compiler used for generating shared-library objects. You should normally set the `$SHFORTRAN` variable, which specifies the default Fortran compiler for all Fortran versions. You only need to set `$SHF77` if you need to use a specific compiler or compiler version for Fortran 77 files.

`SHF77COM`

  The command line used to compile a Fortran 77 source file to a shared-library object file. You only need to set `$SHF77COM` if you need to use a specific command line for Fortran 77 files. You should normally set the `$SHFORTRANCOM` variable, which specifies the default command line for all Fortran versions.

`SHF77COMSTR`

  The string displayed when a Fortran 77 source file is compiled to a shared-library object file. If this is not set, then `$SHF77COM` or `$SHFORTRANCOM` (the command line) is displayed.

`SHF77FLAGS`

  Options that are passed to the Fortran 77 compiler to generated shared-library objects. You only need to set `$SHF77FLAGS` if you need to define specific user options for Fortran 77 files. You should normally set the `$SHFORTRANFLAGS` variable, which specifies the user-specified options passed to the default Fortran compiler for all Fortran versions.

`SHF77PPCOM`

  The command line used to compile a Fortran 77 source file to a shared-library object file after first running the file through the C preprocessor. Any options specified in the `$SHF77FLAGS` and `$CPPFLAGS` construction variables are included on this command line. You only need to set `$SHF77PPCOM` if you need to use a specific C-preprocessor command line for Fortran 77 files. You should normally set the `$SHFORTRANPPCOM` variable, which specifies the default C-preprocessor command line for all Fortran versions.

`SHF77PPCOMSTR`

  The string displayed when a Fortran 77 source file is compiled to a shared-library object file after first running the file through the C preprocessor. If this is not set, then `$SHF77PPCOM` or `$SHFORTRANPPCOM` (the command line) is displayed.

`SHF90`

  The Fortran 90 compiler used for generating shared-library objects. You should normally set the `$SHFORTRAN` variable, which specifies the default Fortran compiler for all Fortran versions. You only need to set `$SHF90` if you need to use a specific compiler or compiler version for Fortran 90 files.

`SHF90COM`

  The command line used to compile a Fortran 90 source file to a shared-library object file. You only need to set `$SHF90COM` if you need to use a specific command line for Fortran 90 files. You should normally set the `$SHFORTRANCOM` variable, which specifies the default command line for all Fortran versions.

`SHF90COMSTR`

  The string displayed when a Fortran 90 source file is compiled to a shared-library object file. If this is not set, then `$SHF90COM` or `$SHFORTRANCOM` (the command line) is displayed.

`SHF90FLAGS`

  Options that are passed to the Fortran 90 compiler to generated shared-library objects. You only need to set `$SHF90FLAGS` if you need to define specific user options for Fortran 90 files. You should normally set the `$SHFORTRANFLAGS` variable, which specifies the user-specified options passed to the default Fortran compiler for all Fortran versions.

`SHF90PPCOM`

  The command line used to compile a Fortran 90 source file to a shared-library object file after first running the file through the C preprocessor. Any options specified in the `$SHF90FLAGS` and `$CPPFLAGS` construction variables are included on this command line. You only need to set `$SHF90PPCOM` if you need to use a specific C-preprocessor command line for Fortran 90 files. You should normally set the `$SHFORTRANPPCOM` variable, which specifies the default C-preprocessor command line for all Fortran versions.

`SHF90PPCOMSTR`

  The string displayed when a Fortran 90 source file is compiled to a shared-library object file after first running the file through the C preprocessor. If this is not set, then `$SHF90PPCOM` or `$SHFORTRANPPCOM` (the command line) is displayed.

`SHF95`

  The Fortran 95 compiler used for generating shared-library objects. You should normally set the `$SHFORTRAN` variable, which specifies the default Fortran compiler for all Fortran versions. You only need to set `$SHF95` if you need to use a specific compiler or compiler version for Fortran 95 files.

`SHF95COM`

  The command line used to compile a Fortran 95 source file to a shared-library object file. You only need to set `$SHF95COM` if you need to use a specific command line for Fortran 95 files. You should normally set the `$SHFORTRANCOM` variable, which specifies the default command line for all Fortran versions.

`SHF95COMSTR`

  The string displayed when a Fortran 95 source file is compiled to a shared-library object file. If this is not set, then `$SHF95COM` or `$SHFORTRANCOM` (the command line) is displayed.

`SHF95FLAGS`

  Options that are passed to the Fortran 95 compiler to generated shared-library objects. You only need to set `$SHF95FLAGS` if you need to define specific user options for Fortran 95 files. You should normally set the `$SHFORTRANFLAGS` variable, which specifies the user-specified options passed to the default Fortran compiler for all Fortran versions.

`SHF95PPCOM`

  The command line used to compile a Fortran 95 source file to a shared-library object file after first running the file through the C preprocessor. Any options specified in the `$SHF95FLAGS` and `$CPPFLAGS` construction variables are included on this command line. You only need to set `$SHF95PPCOM` if you need to use a specific C-preprocessor command line for Fortran 95 files. You should normally set the `$SHFORTRANPPCOM` variable, which specifies the default C-preprocessor command line for all Fortran versions.

`SHF95PPCOMSTR`

  The string displayed when a Fortran 95 source file is compiled to a shared-library object file after first running the file through the C preprocessor. If this is not set, then `$SHF95PPCOM` or `$SHFORTRANPPCOM` (the command line) is displayed.

`SHFORTRAN`

  The default Fortran compiler used for generating shared-library objects.

`SHFORTRANCOM`

  The command line used to compile a Fortran source file to a shared-library object file.

`SHFORTRANCOMSTR`

  The string displayed when a Fortran source file is compiled to a shared-library object file. If this is not set, then `$SHFORTRANCOM` (the command line) is displayed.

`SHFORTRANFLAGS`

  Options that are passed to the Fortran compiler to generate shared-library objects.

`SHFORTRANPPCOM`

  The command line used to compile a Fortran source file to a shared-library object file after first running the file through the C preprocessor. Any options specified in the `$SHFORTRANFLAGS` and `$CPPFLAGS` construction variables are included on this command line.

`SHFORTRANPPCOMSTR`

  The string displayed when a Fortran source file is compiled to a shared-library object file after first running the file throught the C preprocessor. If this is not set, then `$SHFORTRANPPCOM` (the command line) is displayed.

`SHLIBPREFIX`

  The prefix used for shared library file names.

`SHLIBSUFFIX`

  The suffix used for shared library file names.

`SHLINK`

  The linker for programs that use shared libraries.

`SHLINKCOM`

  The command line used to link programs using shared libaries.

`SHLINKCOMSTR`

  The string displayed when programs using shared libraries are linked. If this is not set, then `$SHLINKCOM` (the command line) is displayed.

      env = Environment(SHLINKCOMSTR = "Linking shared $TARGET")

`SHLINKFLAGS`

  General user options passed to the linker for programs using shared libraries. Note that this variable should not contain `-l` (or similar) options for linking with the libraries listed in `$LIBS`, nor `-L` (or similar) include search path options that scons generates automatically from `$LIBPATH`. See `$_LIBFLAGS` above, for the variable that expands to library-link options, and `$_LIBDIRFLAGS` above, for the variable that expands to library search path options.

`SHOBJPREFIX`

  The prefix used for shared object file names.

`SHOBJSUFFIX`

  The suffix used for shared object file names.

`SOURCE`

  A reserved variable name that may not be set or used in a construction environment. (See "Variable Substitution," below.)

`SOURCE_URL`

  The URL (web address) of the location from which the project was retrieved. This is used to fill in the `Source:` field in the controlling information for Ipkg and RPM packages.

`SOURCES`

  A reserved variable name that may not be set or used in a construction environment. (See "Variable Substitution," below.)

`SPAWN`

  A command interpreter function that will be called to execute command line strings. The function must expect the following arguments:

      def spawn(shell, escape, cmd, args, env):

  `sh` is a string naming the shell program to use. `escape` is a function that can be called to escape shell special characters in the command line. `cmd` is the path to the command to be executed. `args` is the arguments to the command. `env` is a dictionary of the environment variables in which the command should be executed.

`SUMMARY`

  A short summary of what the project is about. This is used to fill in the `Summary:` field in the controlling information for Ipkg and RPM packages, and as the `Description:` field in MSI packages.

`SWIG`

  The scripting language wrapper and interface generator.

`SWIGCFILESUFFIX`

  The suffix that will be used for intermediate C source files generated by the scripting language wrapper and interface generator. The default value is `_wrap``$CFILESUFFIX`. By default, this value is used whenever the `-c++` option is not specified as part of the `$SWIGFLAGS` construction variable.

`SWIGCOM`

  The command line used to call the scripting language wrapper and interface generator.

`SWIGCOMSTR`

  The string displayed when calling the scripting language wrapper and interface generator. If this is not set, then `$SWIGCOM` (the command line) is displayed.

`SWIGCXXFILESUFFIX`

  The suffix that will be used for intermediate C++ source files generated by the scripting language wrapper and interface generator. The default value is `_wrap``$CFILESUFFIX`. By default, this value is used whenever the `-c++` option is specified as part of the `$SWIGFLAGS` construction variable.

`SWIGFLAGS`

  General options passed to the scripting language wrapper and interface generator. This is where you should set `-python`, `-perl5`, `-tcl`, or whatever other options you want to specify to SWIG. If you set the `-c++` option in this variable, scons will, by default, generate a C++ intermediate source file with the extension that is specified as the `$CXXFILESUFFIX` variable.

`_SWIGINCFLAGS`

  An automatically-generated construction variable containing the SWIG command-line options for specifying directories to be searched for included files. The value of `$_SWIGINCFLAGS` is created by appending `$SWIGINCPREFIX` and `$SWIGINCSUFFIX` to the beginning and end of each directory in `$SWIGPATH`.

`SWIGINCPREFIX`

  The prefix used to specify an include directory on the SWIG command line. This will be appended to the beginning of each directory in the `$SWIGPATH` construction variable when the `$_SWIGINCFLAGS` variable is automatically generated.

`SWIGINCSUFFIX`

  The suffix used to specify an include directory on the SWIG command line. This will be appended to the end of each directory in the `$SWIGPATH` construction variable when the `$_SWIGINCFLAGS` variable is automatically generated.

`SWIGOUTDIR`

  Specifies the output directory in which the scripting language wrapper and interface generator should place generated language-specific files. This will be used by SCons to identify the files that will be generated by the swig call, and translated into the `swig -outdir` option on the command line.

`SWIGPATH`

  The list of directories that the scripting language wrapper and interface generate will search for included files. The SWIG implicit dependency scanner will search these directories for include files. The default is to use the same path specified as `$CPPPATH`.

  Don't explicitly put include directory arguments in SWIGFLAGS; the result will be non-portable and the directories will not be searched by the dependency scanner. Note: directory names in SWIGPATH will be looked-up relative to the SConscript directory when they are used in a command. To force scons to look-up a directory relative to the root of the source tree use #:

      env = Environment(SWIGPATH='#/include')

  The directory look-up can also be forced using the `Dir`() function:

      include = Dir('include')
      env = Environment(SWIGPATH=include)

  The directory list will be added to command lines through the automatically-generated `$_SWIGINCFLAGS` construction variable, which is constructed by appending the values of the `$SWIGINCPREFIX` and `$SWIGINCSUFFIX` construction variables to the beginning and end of each directory in `$SWIGPATH`. Any command lines you define that need the SWIGPATH directory list should include `$_SWIGINCFLAGS`:

      env = Environment(SWIGCOM="my_swig -o $TARGET $_SWIGINCFLAGS $SORUCES")

`TAR`

  The tar archiver.

`TARCOM`

  The command line used to call the tar archiver.

`TARCOMSTR`

  The string displayed when archiving files using the tar archiver. If this is not set, then `$TARCOM` (the command line) is displayed.

      env = Environment(TARCOMSTR = "Archiving $TARGET")

`TARFLAGS`

  General options passed to the tar archiver.

`TARGET`

  A reserved variable name that may not be set or used in a construction environment. (See "Variable Substitution," below.)

`TARGETS`

  A reserved variable name that may not be set or used in a construction environment. (See "Variable Substitution," below.)

`TARSUFFIX`

  The suffix used for tar file names.

`TEMPFILEPREFIX`

  The prefix for a temporary file used to execute lines longer than $MAXLINELENGTH. The default is '@'. This may be set for toolchains that use other values, such as '-@' for the diab compiler or '-via' for ARM toolchain.

`TEX`

  The TeX formatter and typesetter.

`TEXCOM`

  The command line used to call the TeX formatter and typesetter.

`TEXCOMSTR`

  The string displayed when calling the TeX formatter and typesetter. If this is not set, then `$TEXCOM` (the command line) is displayed.

      env = Environment(TEXCOMSTR = "Building $TARGET from TeX input $SOURCES")

`TEXFLAGS`

  General options passed to the TeX formatter and typesetter.

`TEXINPUTS`

  List of directories that the LaTeX programm will search for include directories. The LaTeX implicit dependency scanner will search these directories for \include and \import files.

`TOOLS`

  A list of the names of the Tool specifications that are part of this construction environment.

`VENDOR`

  The person or organization who supply the packaged software. This is used to fill in the `Vendor:` field in the controlling information for RPM packages, and the `Manufacturer:` field in the controlling information for MSI packages.

`VERSION`

  The version of the project, specified as a string.

`WIN32_INSERT_DEF`

  A deprecated synonym for `$WINDOWS_INSERT_DEF`.

`WIN32DEFPREFIX`

  A deprecated synonym for `$WINDOWSDEFPREFIX`.

`WIN32DEFSUFFIX`

  A deprecated synonym for `$WINDOWSDEFSUFFIX`.

`WIN32EXPPREFIX`

  A deprecated synonym for `$WINDOWSEXPSUFFIX`.

`WIN32EXPSUFFIX`

  A deprecated synonym for `$WINDOWSEXPSUFFIX`.

`WINDOWS_INSERT_DEF`

  When this is set to true, a library build of a Windows shared library (`.dll`file) will also build a corresponding `.def` file at the same time, if a `.def` file is not already listed as a build target. The default is 0 (do not build a `.def` file).

`WINDOWS_INSERT_MANIFEST`

  When this is set to true, scons will be aware of the `.manifest` files generated by Microsoft Visua C/C++ 8.

`WINDOWSDEFPREFIX`

  The prefix used for Windows `.def`file names.

`WINDOWSDEFSUFFIX`

  The suffix used for Windows `.def` file names.

`WINDOWSEXPPREFIX`

  The prefix used for Windows `.exp` file names.

`WINDOWSEXPSUFFIX`

  The suffix used for Windows `.exp` file names.

`WINDOWSPROGMANIFESTPREFIX`

  The prefix used for executable program `.manifest` files generated by Microsoft Visual C/C++.

`WINDOWSPROGMANIFESTSUFFIX`

  The suffix used for executable program `.manifest` files generated by Microsoft Visual C/C++.

`WINDOWSSHLIBMANIFESTPREFIX`

  The prefix used for shared library `.manifest` files generated by Microsoft Visual C/C++.

`WINDOWSSHLIBMANIFESTSUFFIX`

  The suffix used for shared library `.manifest` files generated by Microsoft Visual C/C++.

`X_IPK_DEPENDS`

  This is used to fill in the `Depends:` field in the controlling information for Ipkg packages.

`X_IPK_DESCRIPTION`

  This is used to fill in the `Description:` field in the controlling information for Ipkg packages. The default value is `$SUMMARY\n$DESCRIPTION`

`X_IPK_MAINTAINER`

  This is used to fill in the `Maintainer:` field in the controlling information for Ipkg packages.

`X_IPK_PRIORITY`

  This is used to fill in the `Priority:` field in the controlling information for Ipkg packages.

`X_IPK_SECTION`

  This is used to fill in the `Section:` field in the controlling information for Ipkg packages.

`X_MSI_LANGUAGE`

  This is used to fill in the `Language:` attribute in the controlling information for MSI packages.

`X_MSI_LICENSE_TEXT`

  The text of the software license in RTF format. Carriage return characters will be replaced with the RTF equivalent \\par.

`X_MSI_UPGRADE_CODE`

  TODO

`X_RPM_AUTOREQPROV`

  This is used to fill in the `AutoReqProv:` field in the RPM `.spec` file.

`X_RPM_BUILD`

  internal, but overridable

`X_RPM_BUILDREQUIRES`

  This is used to fill in the `BuildRequires:` field in the RPM `.spec` file.

`X_RPM_BUILDROOT`

  internal, but overridable

`X_RPM_CLEAN`

  internal, but overridable

`X_RPM_CONFLICTS`

  This is used to fill in the `Conflicts:` field in the RPM `.spec` file.

`X_RPM_DEFATTR`

  This value is used as the default attributes for the files in the RPM package. The default value is `(-,root,root)`.

`X_RPM_DISTRIBUTION`

  This is used to fill in the `Distribution:` field in the RPM `.spec` file.

`X_RPM_EPOCH`

  This is used to fill in the `Epoch:` field in the controlling information for RPM packages.

`X_RPM_EXCLUDEARCH`

  This is used to fill in the `ExcludeArch:` field in the RPM `.spec` file.

`X_RPM_EXLUSIVEARCH`

  This is used to fill in the `ExclusiveArch:` field in the RPM `.spec` file.

`X_RPM_GROUP`

  This is used to fill in the `Group:` field in the RPM `.spec` file.

`X_RPM_GROUP_lang`

  This is used to fill in the `Group(lang):` field in the RPM `.spec` file. Note that `lang` is not literal and should be replaced by the appropriate language code.

`X_RPM_ICON`

  This is used to fill in the `Icon:` field in the RPM `.spec` file.

`X_RPM_INSTALL`

  internal, but overridable

`X_RPM_PACKAGER`

  This is used to fill in the `Packager:` field in the RPM `.spec` file.

`X_RPM_POSTINSTALL`

  This is used to fill in the `%post:` section in the RPM `.spec` file.

`X_RPM_POSTUNINSTALL`

  This is used to fill in the `%postun:` section in the RPM `.spec` file.

`X_RPM_PREFIX`

  This is used to fill in the `Prefix:` field in the RPM `.spec` file.

`X_RPM_PREINSTALL`

  This is used to fill in the `%pre:` section in the RPM `.spec` file.

`X_RPM_PREP`

  internal, but overridable

`X_RPM_PREUNINSTALL`

  This is used to fill in the `%preun:` section in the RPM `.spec` file.

`X_RPM_PROVIDES`

  This is used to fill in the `Provides:` field in the RPM `.spec` file.

`X_RPM_REQUIRES`

  This is used to fill in the `Requires:` field in the RPM `.spec` file.

`X_RPM_SERIAL`

  This is used to fill in the `Serial:` field in the RPM `.spec` file.

`X_RPM_URL`

  This is used to fill in the `Url:` field in the RPM `.spec` file.

`YACC`

  The parser generator.

`YACCCOM`

  The command line used to call the parser generator to generate a source file.

`YACCCOMSTR`

  The string displayed when generating a source file using the parser generator. If this is not set, then `$YACCCOM` (the command line) is displayed.

      env = Environment(YACCCOMSTR = "Yacc'ing $TARGET from $SOURCES")

`YACCFLAGS`

  General options passed to the parser generator. If `$YACCFLAGS` contains a `-d` option, SCons assumes that the call will also create a .h file (if the yacc source file ends in a .y suffix) or a .hpp file (if the yacc source file ends in a .yy suffix)

`YACCHFILESUFFIX`

  The suffix of the C header file generated by the parser generator when the `-d` option is used. Note that setting this variable does not cause the parser generator to generate a header file with the specified suffix, it exists to allow you to specify what suffix the parser generator will use of its own accord. The default value is `.h`.

`YACCHXXFILESUFFIX`

  The suffix of the C++ header file generated by the parser generator when the `-d` option is used. Note that setting this variable does not cause the parser generator to generate a header file with the specified suffix, it exists to allow you to specify what suffix the parser generator will use of its own accord. The default value is `.hpp`, except on Mac OS X, where the default is `${TARGET.suffix}.h`. because the default bison parser generator just appends `.h` to the name of the generated C++ file.

`YACCVCGFILESUFFIX`

  The suffix of the file containing the VCG grammar automaton definition when the `--graph=` option is used. Note that setting this variable does not cause the parser generator to generate a VCG file with the specified suffix, it exists to allow you to specify what suffix the parser generator will use of its own accord. The default value is `.vcg`.

`ZIP`

  The zip compression and file packaging utility.

`ZIPCOM`

  The command line used to call the zip utility, or the internal Python function used to create a zip archive.

`ZIPCOMPRESSION`

  The `compression` flag from the Python `zipfile` module used by the internal Python function to control whether the zip archive is compressed or not. The default value is `zipfile.ZIP_DEFLATED`, which creates a compressed zip archive. This value has no effect when using Python 1.5.2 or if the `zipfile` module is otherwise unavailable.

`ZIPCOMSTR`

  The string displayed when archiving files using the zip utility. If this is not set, then `$ZIPCOM` (the command line or internal Python function) is displayed.

      env = Environment(ZIPCOMSTR = "Zipping $TARGET")

`ZIPFLAGS`

  General options passed to the zip utility.

`ZIPSUFFIX`

  The suffix used for zip file names.

# Appendix B. Builders

This appendix contains descriptions of all of the Builders that are potentially available "out of the box" in this version of SCons.

`CFile()`, `env.CFile()`

  Builds a C source file given a lex (`.l`) or yacc (`.y`) input file. The suffix specified by the `$CFILESUFFIX` construction variable (`.c` by default) is automatically added to the target if it is not already present. Example:

      # builds foo.c
      env.CFile(target = 'foo.c', source = 'foo.l')
      # builds bar.c
      env.CFile(target = 'bar', source = 'bar.y')

`CXXFile()`, `env.CXXFile()`

  Builds a C++ source file given a lex (`.ll`) or yacc (`.yy`) input file. The suffix specified by the `$CXXFILESUFFIX` construction variable (`.cc` by default) is automatically added to the target if it is not already present. Example:

      # builds foo.cc
      env.CXXFile(target = 'foo.cc', source = 'foo.ll')
      # builds bar.cc
      env.CXXFile(target = 'bar', source = 'bar.yy')

`DVI()`, `env.DVI()`

  Builds a `.dvi` file from a `.tex`, `.ltx` or `.latex` input file. If the source file suffix is `.tex`, scons will examine the contents of the file; if the string `\documentclass` or `\documentstyle` is found, the file is assumed to be a LaTeX file and the target is built by invoking the `$LATEXCOM` command line; otherwise, the `$TEXCOM` command line is used. If the file is a LaTeX file, the `DVI` builder method will also examine the contents of the `.aux` file and invoke the `$BIBTEX` command line if the string `bibdata` is found, start `$MAKEINDEX` to generate an index if a `.ind` file is found and will examine the contents `.log` file and re-run the `$LATEXCOM` command if the log file says it is necessary.

  The suffix `.dvi` (hard-coded within TeX itself) is automatically added to the target if it is not already present. Examples:

      # builds from aaa.tex
      env.DVI(target = 'aaa.dvi', source = 'aaa.tex')
      # builds bbb.dvi
      env.DVI(target = 'bbb', source = 'bbb.ltx')
      # builds from ccc.latex
      env.DVI(target = 'ccc.dvi', source = 'ccc.latex')

`Install()`, `env.Install()`

  Installs one or more source files or directories in the specified target, which must be a directory. The names of the specified source files or directories remain the same within the destination directory.

      env.Install('/usr/local/bin', source = ['foo', 'bar'])

`InstallAs()`, `env.InstallAs()`

  Installs one or more source files or directories to specific names, allowing changing a file or directory name as part of the installation. It is an error if the target and source arguments list different numbers of files or directories.

      env.InstallAs(target = '/usr/local/bin/foo',
                    source = 'foo_debug')
      env.InstallAs(target = ['../lib/libfoo.a', '../lib/libbar.a'],
                    source = ['libFOO.a', 'libBAR.a'])

`Jar()`, `env.Jar()`

  Builds a Java archive (`.jar`) file from the specified list of sources. Any directories in the source list will be searched for `.class` files). Any `.java` files in the source list will be compiled to `.class` files by calling the `Java` Builder.

  If the `$JARCHDIR` value is set, the jar command will change to the specified directory using the `-C` option. If `$JARCHDIR` is not set explicitly, SCons will use the top of any subdirectory tree in which Java `.class` were built by the `Java` Builder.

  If the contents any of the source files begin with the string `Manifest-Version`, the file is assumed to be a manifest and is passed to the jar command with the `m` option set.

      env.Jar(target = 'foo.jar', source = 'classes')

      env.Jar(target = 'bar.jar',
              source = ['bar1.java', 'bar2.java'])

`Java()`, `env.Java()`

  Builds one or more Java class files. The sources may be any combination of explicit `.java` files, or directory trees which will be scanned for `.java` files.

  SCons will parse each source `.java` file to find the classes (including inner classes) defined within that file, and from that figure out the target `.class` files that will be created. The class files will be placed underneath the specified target directory.

  SCons will also search each Java file for the Java package name, which it assumes can be found on a line beginning with the string `package` in the first column; the resulting `.class` files will be placed in a directory reflecting the specified package name. For example, the file `Foo.java` defining a single public `Foo` class and containing a package name of `sub.dir` will generate a corresponding `sub/dir/Foo.class` class file.

  Example:

      env.Java(target = 'classes', source = 'src')
      env.Java(target = 'classes', source = ['src1', 'src2'])
      env.Java(target = 'classes', source = ['File1.java', 'File2.java'])

`JavaH()`, `env.JavaH()`

  Builds C header and source files for implementing Java native methods. The target can be either a directory in which the header files will be written, or a header file name which will contain all of the definitions. The source can be the names of `.class` files, the names of `.java` files to be compiled into `.class` files by calling the `Java` builder method, or the objects returned from the `Java` builder method.

  If the construction variable `$JAVACLASSDIR` is set, either in the environment or in the call to the `JavaH` builder method itself, then the value of the variable will be stripped from the beginning of any `.class` file names.

  Examples:

      # builds java_native.h
      classes = env.Java(target = 'classdir', source = 'src')
      env.JavaH(target = 'java_native.h', source = classes)

      # builds include/package_foo.h and include/package_bar.h
      env.JavaH(target = 'include',
                source = ['package/foo.class', 'package/bar.class'])

      # builds export/foo.h and export/bar.h
      env.JavaH(target = 'export',
                source = ['classes/foo.class', 'classes/bar.class'],
                JAVACLASSDIR = 'classes')

`Library()`, `env.Library()`

  A synonym for the `StaticLibrary` builder method.

`LoadableModule()`, `env.LoadableModule()`

  On most systems, this is the same as `SharedLibrary`. On Mac OS X (Darwin) platforms, this creates a loadable module bundle.

`M4()`, `env.M4()`

  Builds an output file from an M4 input file. This uses a default `$M4FLAGS` value of `-E`, which considers all warnings to be fatal and stops on the first warning when using the GNU version of m4. Example:

      env.M4(target = 'foo.c', source = 'foo.c.m4')

`Moc()`, `env.Moc()`

  Builds an output file from a moc input file. Moc input files are either header files or cxx files. This builder is only available after using the tool 'qt'. See the `$QTDIR` variable for more information. Example:

      env.Moc('foo.h') # generates moc_foo.cc
      env.Moc('foo.cpp') # generates foo.moc

`MSVSProject()`, `env.MSVSProject()`

  Builds a Microsoft Visual Studio project file, and by default builds a solution file as well.

  This builds a Visual Studio project file, based on the version of Visual Studio that is configured (either the latest installed version, or the version specified by `$MSVS_VERSION` in the Environment constructor). For Visual Studio 6, it will generate a `.dsp` file. For Visual Studio 7 (.NET) and later versions, it will generate a `.vcproj` file.

  By default, this also generates a solution file for the specified project, a `.dsw` file for Visual Studio 6 or a `.sln` file for Visual Studio 7 (.NET). This behavior may be disabled by specifying `auto_build_solution=0` when you call `MSVSProject`, in which case you presumably want to build the solution file(s) by calling the `MSVSSolution` Builder (see below).

  It takes several lists of filenames to be placed into the project file. These are currently limited to `srcs`, `incs`, `localincs`, `resources`, and `misc`. These are pretty self-explanatory, but it should be noted that these lists are added to the `$SOURCES` construction variable as strings, NOT as SCons File Nodes. This is because they represent file names to be added to the project file, not the source files used to build the project file.

  The above filename lists are all optional, although at least one must be specified for the resulting project file to be non-empty.

  In addition to the above lists of values, the following values may be specified:

  `target`: The name of the target `.dsp` or `.vcproj` file. The correct suffix for the version of Visual Studio must be used, but the `$MSVSPROJECTSUFFIX` construction variable will be defined to the correct value (see example below).

  `variant`: The name of this particular variant. For Visual Studio 7 projects, this can also be a list of variant names. These are typically things like "Debug" or "Release", but really can be anything you want. For Visual Studio 7 projects, they may also specify a target platform separated from the variant name by a `|` (vertical pipe) character: `Debug|Xbox`. The default target platform is Win32. Multiple calls to `MSVSProject` with different variants are allowed; all variants will be added to the project file with their appropriate build targets and sources.

  `buildtarget`: An optional string, node, or list of strings or nodes (one per build variant), to tell the Visual Studio debugger what output target to use in what build variant. The number of `buildtarget` entries must match the number of `variant` entries.

  `runfile`: The name of the file that Visual Studio 7 and later will run and debug. This appears as the value of the `Output` field in the resutling Visual Studio project file. If this is not specified, the default is the same as the specified `buildtarget` value.

  Example usage:

      barsrcs = ['bar.cpp'],
      barincs = ['bar.h'],
      barlocalincs = ['StdAfx.h']
      barresources = ['bar.rc','resource.h']
      barmisc = ['bar_readme.txt']

      dll = env.SharedLibrary(target = 'bar.dll',
                              source = barsrcs)

      env.MSVSProject(target = 'Bar' + env['MSVSPROJECTSUFFIX'],
                      srcs = barsrcs,
                      incs = barincs,
                      localincs = barlocalincs,
                      resources = barresources,
                      misc = barmisc,
                      buildtarget = dll,
                      variant = 'Release')

`MSVSSolution()`, `env.MSVSSolution()`

  Builds a Microsoft Visual Studio solution file.

  This builds a Visual Studio solution file, based on the version of Visual Studio that is configured (either the latest installed version, or the version specified by `$MSVS_VERSION` in the construction environment). For Visual Studio 6, it will generate a `.dsw` file. For Visual Studio 7 (.NET), it will generate a `.sln` file.

  The following values must be specified:

  `target`: The name of the target .dsw or .sln file. The correct suffix for the version of Visual Studio must be used, but the value `$MSVSSOLUTIONSUFFIX` will be defined to the correct value (see example below).

  `variant`: The name of this particular variant, or a list of variant names (the latter is only supported for MSVS 7 solutions). These are typically things like "Debug" or "Release", but really can be anything you want. For MSVS 7 they may also specify target platform, like this "Debug|Xbox". Default platform is Win32.

  `projects`: A list of project file names, or Project nodes returned by calls to the `MSVSProject` Builder, to be placed into the solution file. It should be noted that these file names are NOT added to the $SOURCES environment variable in form of files, but rather as strings. This is because they represent file names to be added to the solution file, not the source files used to build the solution file.

  (NOTE: Currently only one project is supported per solution.)

  Example Usage:

      env.MSVSSolution(target = 'Bar' + env['MSVSSOLUTIONSUFFIX'],
                       projects = ['bar' + env['MSVSPROJECTSUFFIX']],
                       variant = 'Release')

`Object()`, `env.Object()`

  A synonym for the `StaticObject` builder method.

`Package()`, `env.Package()`

  Builds software distribution packages. Packages consist of files to install and packaging information. The former may be specified with the `source` parameter and may be left out, in which case the `FindInstalledFiles` function will collect all files that have an `Install` or `InstallAs` Builder attached. If the `target`, is not specified it will be deduced from additional information given to this Builder.

  The packaging information is specified with the help of construction variables documented below. This information is called a tag to stress that some of them can also be attached to files with the `Tag` function. The mandatory ones will complain if they were not specified. They vary depending on chosen target packager.

  The target packager may be selected with the "PACKAGETYPE" command line option or with the `$PACKAGETYPE` construction variable. Currently the following packagers available:

  * msi - Microsoft Installer * rpm - Redhat Package Manger * ipkg - Itsy Package Management System * tarbz2 - compressed tar * targz - compressed tar * zip - zip file * src_tarbz2 - compressed tar source * src_targz - compressed tar source * src_zip - zip file source

  An updated list is always available under the "package_type" option when running "scons --help" on a project that has packaging activated.

      env = Environment(tools=['default', 'packaging'])
      env.Install('/bin/', 'my_program')
      env.Package( NAME           = 'foo',
                   VERSION        = '1.2.3',
                   PACKAGEVERSION = 0,
                   PACKAGETYPE    = 'rpm',
                   LICENSE        = 'gpl',
                   SUMMARY        = 'balalalalal',
                   DESCRIPTION    = 'this should be really really long',
                   X_RPM_GROUP    = 'Application/fu',
                   SOURCE_URL     = 'http://foo.org/foo-1.2.3.tar.gz'
              )

`PCH()`, `env.PCH()`

  Builds a Microsoft Visual C++ precompiled header. Calling this builder method returns a list of two targets: the PCH as the first element, and the object file as the second element. Normally the object file is ignored. This builder method is only provided when Microsoft Visual C++ is being used as the compiler. The PCH builder method is generally used in conjuction with the PCH construction variable to force object files to use the precompiled header:

      env['PCH'] = env.PCH('StdAfx.cpp')[0]

`PDF()`, `env.PDF()`

  Builds a `.pdf` file from a `.dvi` input file (or, by extension, a `.tex`, `.ltx`, or `.latex` input file). The suffix specified by the `$PDFSUFFIX` construction variable (`.pdf` by default) is added automatically to the target if it is not already present. Example:

      # builds from aaa.tex
      env.PDF(target = 'aaa.pdf', source = 'aaa.tex')
      # builds bbb.pdf from bbb.dvi
      env.PDF(target = 'bbb', source = 'bbb.dvi')

`PostScript()`, `env.PostScript()`

  Builds a `.ps` file from a `.dvi` input file (or, by extension, a `.tex`, `.ltx`, or `.latex` input file). The suffix specified by the `$PSSUFFIX` construction variable (`.ps` by default) is added automatically to the target if it is not already present. Example:

      # builds from aaa.tex
      env.PostScript(target = 'aaa.ps', source = 'aaa.tex')
      # builds bbb.ps from bbb.dvi
      env.PostScript(target = 'bbb', source = 'bbb.dvi')

`Program()`, `env.Program()`

  Builds an executable given one or more object files or C, C++, D, or Fortran source files. If any C, C++, D or Fortran source files are specified, then they will be automatically compiled to object files using the `Object` builder method; see that builder method's description for a list of legal source file suffixes and how they are interpreted. The target executable file prefix (specified by the `$PROGPREFIX` construction variable; nothing by default) and suffix (specified by the `$PROGSUFFIX` construction variable; by default, `.exe` on Windows systems, nothing on POSIX systems) are automatically added to the target if not already present. Example:

      env.Program(target = 'foo', source = ['foo.o', 'bar.c', 'baz.f'])

`RES()`, `env.RES()`

  Builds a Microsoft Visual C++ resource file. This builder method is only provided when Microsoft Visual C++ or MinGW is being used as the compiler. The `.res` (or `.o` for MinGW) suffix is added to the target name if no other suffix is given. The source file is scanned for implicit dependencies as though it were a C file. Example:

      env.RES('resource.rc')

`RMIC()`, `env.RMIC()`

  Builds stub and skeleton class files for remote objects from Java `.class` files. The target is a directory relative to which the stub and skeleton class files will be written. The source can be the names of `.class` files, or the objects return from the `Java` builder method.

  If the construction variable `$JAVACLASSDIR` is set, either in the environment or in the call to the `RMIC` builder method itself, then the value of the variable will be stripped from the beginning of any `.class` file names.

      classes = env.Java(target = 'classdir', source = 'src')
      env.RMIC(target = 'outdir1', source = classes)

      env.RMIC(target = 'outdir2',
               source = ['package/foo.class', 'package/bar.class'])

      env.RMIC(target = 'outdir3',
               source = ['classes/foo.class', 'classes/bar.class'],
               JAVACLASSDIR = 'classes')

`RPCGenClient()`, `env.RPCGenClient()`

  Generates an RPC client stub (`_clnt.c`) file from a specified RPC (`.x`) source file. Because rpcgen only builds output files in the local directory, the command will be executed in the source file's directory by default.

      # Builds src/rpcif_clnt.c
      env.RPCGenClient('src/rpcif.x')

`RPCGenHeader()`, `env.RPCGenHeader()`

  Generates an RPC header (`.h`) file from a specified RPC (`.x`) source file. Because rpcgen only builds output files in the local directory, the command will be executed in the source file's directory by default.

      # Builds src/rpcif.h
      env.RPCGenHeader('src/rpcif.x')

`RPCGenService()`, `env.RPCGenService()`

  Generates an RPC server-skeleton (`_svc.c`) file from a specified RPC (`.x`) source file. Because rpcgen only builds output files in the local directory, the command will be executed in the source file's directory by default.

      # Builds src/rpcif_svc.c
      env.RPCGenClient('src/rpcif.x')

`RPCGenXDR()`, `env.RPCGenXDR()`

  Generates an RPC XDR routine (`_xdr.c`) file from a specified RPC (`.x`) source file. Because rpcgen only builds output files in the local directory, the command will be executed in the source file's directory by default.

      # Builds src/rpcif_xdr.c
      env.RPCGenClient('src/rpcif.x')

`SharedLibrary()`, `env.SharedLibrary()`

  Builds a shared library (`.so` on a POSIX system, `.dll` on Windows) given one or more object files or C, C++, D or Fortran source files. If any source files are given, then they will be automatically compiled to object files. The static library prefix and suffix (if any) are automatically added to the target. The target library file prefix (specified by the `$SHLIBPREFIX` construction variable; by default, `lib` on POSIX systems, nothing on Windows systems) and suffix (specified by the `$SHLIBSUFFIX` construction variable; by default, `.dll` on Windows systems, `.so` on POSIX systems) are automatically added to the target if not already present. Example:

      env.SharedLibrary(target = 'bar', source = ['bar.c', 'foo.o'])

  On Windows systems, the `SharedLibrary` builder method will always build an import (`.lib`) library in addition to the shared (`.dll`) library, adding a `.lib` library with the same basename if there is not already a `.lib` file explicitly listed in the targets.

  Any object files listed in the `source` must have been built for a shared library (that is, using the `SharedObject` builder method). scons will raise an error if there is any mismatch.

  On Windows systems, specifying `register=1` will cause the `.dll` to be registered after it is built using REGSVR32. The command that is run ("regsvr32" by default) is determined by `$REGSVR` construction variable, and the flags passed are determined by `$REGSVRFLAGS`. By default, `$REGSVRFLAGS` includes the `/s` option, to prevent dialogs from popping up and requiring user attention when it is run. If you change `$REGSVRFLAGS`, be sure to include the `/s` option. For example,

      env.SharedLibrary(target = 'bar',
                        source = ['bar.cxx', 'foo.obj'],
                        register=1)

  will register `bar.dll` as a COM object when it is done linking it.

`SharedObject()`, `env.SharedObject()`

  Builds an object file for inclusion in a shared library. Source files must have one of the same set of extensions specified above for the `StaticObject` builder method. On some platforms building a shared object requires additional compiler option (e.g. `-fPIC` for gcc) in addition to those needed to build a normal (static) object, but on some platforms there is no difference between a shared object and a normal (static) one. When there is a difference, SCons will only allow shared objects to be linked into a shared library, and will use a different suffix for shared objects. On platforms where there is no difference, SCons will allow both normal (static) and shared objects to be linked into a shared library, and will use the same suffix for shared and normal (static) objects. The target object file prefix (specified by the `$SHOBJPREFIX` construction variable; by default, the same as `$OBJPREFIX`) and suffix (specified by the `$SHOBJSUFFIX` construction variable) are automatically added to the target if not already present. Examples:

      env.SharedObject(target = 'ddd', source = 'ddd.c')
      env.SharedObject(target = 'eee.o', source = 'eee.cpp')
      env.SharedObject(target = 'fff.obj', source = 'fff.for')

  Note that the source files will be scanned according to the suffix mappings in the `SourceFileScanner` object. See the section "Scanner Objects," below, for a more information.

`StaticLibrary()`, `env.StaticLibrary()`

  Builds a static library given one or more object files or C, C++, D or Fortran source files. If any source files are given, then they will be automatically compiled to object files. The static library prefix and suffix (if any) are automatically added to the target. The target library file prefix (specified by the `$LIBPREFIX` construction variable; by default, `lib` on POSIX systems, nothing on Windows systems) and suffix (specified by the `$LIBSUFFIX` construction variable; by default, `.lib` on Windows systems, `.a` on POSIX systems) are automatically added to the target if not already present. Example:

      env.StaticLibrary(target = 'bar', source = ['bar.c', 'foo.o'])

  Any object files listed in the `source` must have been built for a static library (that is, using the `StaticObject` builder method). scons will raise an error if there is any mismatch.

`StaticObject()`, `env.StaticObject()`

  Builds a static object file from one or more C, C++, D, or Fortran source files. Source files must have one of the following extensions:

        .asm    assembly language file
        .ASM    assembly language file
        .c      C file
        .C      Windows:  C file
                POSIX:  C++ file
        .cc     C++ file
        .cpp    C++ file
        .cxx    C++ file
        .cxx    C++ file
        .c++    C++ file
        .C++    C++ file
        .d      D file
        .f      Fortran file
        .F      Windows:  Fortran file
                POSIX:  Fortran file + C pre-processor
        .for    Fortran file
        .FOR    Fortran file
        .fpp    Fortran file + C pre-processor
        .FPP    Fortran file + C pre-processor
        .m      Object C file
        .mm     Object C++ file
        .s      assembly language file
        .S      Windows:  assembly language file
                POSIX:  assembly language file + C pre-processor
        .spp    assembly language file + C pre-processor
        .SPP    assembly language file + C pre-processor

  The target object file prefix (specified by the `$OBJPREFIX` construction variable; nothing by default) and suffix (specified by the `$OBJSUFFIX` construction variable; `.obj` on Windows systems, `.o` on POSIX systems) are automatically added to the target if not already present. Examples:

      env.StaticObject(target = 'aaa', source = 'aaa.c')
      env.StaticObject(target = 'bbb.o', source = 'bbb.c++')
      env.StaticObject(target = 'ccc.obj', source = 'ccc.f')

  Note that the source files will be scanned according to the suffix mappings in `SourceFileScanner` object. See the section "Scanner Objects," below, for a more information.

`Tar()`, `env.Tar()`

  Builds a tar archive of the specified files and/or directories. Unlike most builder methods, the `Tar` builder method may be called multiple times for a given target; each additional call adds to the list of entries that will be built into the archive. Any source directories will be scanned for changes to any on-disk files, regardless of whether or not scons knows about them from other Builder or function calls.

      env.Tar('src.tar', 'src')

      # Create the stuff.tar file.
      env.Tar('stuff', ['subdir1', 'subdir2'])
      # Also add "another" to the stuff.tar file.
      env.Tar('stuff', 'another')

      # Set TARFLAGS to create a gzip-filtered archive.
      env = Environment(TARFLAGS = '-c -z')
      env.Tar('foo.tar.gz', 'foo')

      # Also set the suffix to .tgz.
      env = Environment(TARFLAGS = '-c -z',
                        TARSUFFIX = '.tgz')
      env.Tar('foo')

`TypeLibrary()`, `env.TypeLibrary()`

  Builds a Windows type library (`.tlb`) file from an input IDL file (`.idl`). In addition, it will build the associated inteface stub and proxy source files, naming them according to the base name of the `.idl` file. For example,

      env.TypeLibrary(source="foo.idl")

  Will create `foo.tlb`, `foo.h`, `foo_i.c`, `foo_p.c` and `foo_data.c` files.

`Uic()`, `env.Uic()`

  Builds a header file, an implementation file and a moc file from an ui file. and returns the corresponding nodes in the above order. This builder is only available after using the tool 'qt'. Note: you can specify `.ui` files directly as source files to the `Program`, `Library` and `SharedLibrary` builders without using this builder. Using this builder lets you override the standard naming conventions (be careful: prefixes are always prepended to names of built files; if you don't want prefixes, you may set them to ``). See the `$QTDIR` variable for more information. Example:

      env.Uic('foo.ui') # -> ['foo.h', 'uic_foo.cc', 'moc_foo.cc']
      env.Uic(target = Split('include/foo.h gen/uicfoo.cc gen/mocfoo.cc'),
              source = 'foo.ui') # -> ['include/foo.h', 'gen/uicfoo.cc', 'gen/mocfoo.cc']

`Zip()`, `env.Zip()`

  Builds a zip archive of the specified files and/or directories. Unlike most builder methods, the `Zip` builder method may be called multiple times for a given target; each additional call adds to the list of entries that will be built into the archive. Any source directories will be scanned for changes to any on-disk files, regardless of whether or not scons knows about them from other Builder or function calls.

      env.Zip('src.zip', 'src')

      # Create the stuff.zip file.
      env.Zip('stuff', ['subdir1', 'subdir2'])
      # Also add "another" to the stuff.tar file.
      env.Zip('stuff', 'another')

# Appendix C. Tools

This appendix contains descriptions of all of the Tools modules that are available "out of the box" in this version of SCons.

`386asm`

  Sets construction variables for the 386ASM assembler for the Phar Lap ETS embedded operating system.

  Sets: `$AS`, `$ASCOM`, `$ASFLAGS`, `$ASPPCOM`, `$ASPPFLAGS`.

  Uses: `$CC`, `$CPPFLAGS`, `$_CPPDEFFLAGS`, `$_CPPINCFLAGS`.

`aixc++`

  Sets construction variables for the IMB xlc / Visual Age C++ compiler.

  Sets: `$CXX`, `$CXXVERSION`, `$SHCXX`, `$SHOBJSUFFIX`.

`aixcc`

  Sets construction variables for the IBM xlc / Visual Age C compiler.

  Sets: `$CC`, `$CCVERSION`, `$SHCC`.

`aixf77`

  Sets construction variables for the IBM Visual Age f77 Fortran compiler.

  Sets: `$F77`, `$SHF77`.

`aixlink`

  Sets construction variables for the IBM Visual Age linker.

  Sets: `$LINKFLAGS`, `$SHLIBSUFFIX`, `$SHLINKFLAGS`.

`applelink`

  Sets construction variables for the Apple linker (similar to the GNU linker).

  Sets: `$FRAMEWORKPATHPREFIX`, `$LDMODULECOM`, `$LDMODULEFLAGS`, `$LDMODULEPREFIX`, `$LDMODULESUFFIX`, `$LINKCOM`, `$SHLINKCOM`, `$SHLINKFLAGS`, `$_FRAMEWORKPATH`, `$_FRAMEWORKS`.

  Uses: `$FRAMEWORKSFLAGS`.

`ar`

  Sets construction variables for the ar library archiver.

  Sets: `$AR`, `$ARCOM`, `$ARFLAGS`, `$LIBPREFIX`, `$LIBSUFFIX`, `$RANLIB`, `$RANLIBCOM`, `$RANLIBFLAGS`.

`as`

  Sets construction variables for the as assembler.

  Sets: `$AS`, `$ASCOM`, `$ASFLAGS`, `$ASPPCOM`, `$ASPPFLAGS`.

  Uses: `$CC`, `$CPPFLAGS`, `$_CPPDEFFLAGS`, `$_CPPINCFLAGS`.

`bcc32`

  Sets construction variables for the bcc32 compiler.

  Sets: `$CC`, `$CCCOM`, `$CCFLAGS`, `$CFILESUFFIX`, `$CFLAGS`, `$CPPDEFPREFIX`, `$CPPDEFSUFFIX`, `$INCPREFIX`, `$INCSUFFIX`, `$SHCC`, `$SHCCCOM`, `$SHCCFLAGS`, `$SHCFLAGS`, `$SHOBJSUFFIX`.

  Uses: `$_CPPDEFFLAGS`, `$_CPPINCFLAGS`.

`BitKeeper`

  Sets construction variables for the BitKeeper source code control system.

  Sets: `$BITKEEPER`, `$BITKEEPERCOM`, `$BITKEEPERGET`, `$BITKEEPERGETFLAGS`.

  Uses: `$BITKEEPERCOMSTR`.

`cc`

  Sets construction variables for generic POSIX C copmilers.

  Sets: `$CC`, `$CCCOM`, `$CCFLAGS`, `$CFILESUFFIX`, `$CFLAGS`, `$CPPDEFPREFIX`, `$CPPDEFSUFFIX`, `$FRAMEWORKPATH`, `$FRAMEWORKS`, `$INCPREFIX`, `$INCSUFFIX`, `$SHCC`, `$SHCCCOM`, `$SHCCFLAGS`, `$SHCFLAGS`, `$SHOBJSUFFIX`.

  Uses: `$PLATFORM`.

`cvf`

  Sets construction variables for the Compaq Visual Fortran compiler.

  Sets: `$FORTRAN`, `$FORTRANCOM`, `$FORTRANMODDIR`, `$FORTRANMODDIRPREFIX`, `$FORTRANMODDIRSUFFIX`, `$FORTRANPPCOM`, `$OBJSUFFIX`, `$SHFORTRANCOM`, `$SHFORTRANPPCOM`.

  Uses: `$CPPFLAGS`, `$FORTRANFLAGS`, `$SHFORTRANFLAGS`, `$_CPPDEFFLAGS`, `$_FORTRANINCFLAGS`, `$_FORTRANMODFLAG`.

`CVS`

  Sets construction variables for the CVS source code management system.

  Sets: `$CVS`, `$CVSCOFLAGS`, `$CVSCOM`, `$CVSFLAGS`.

  Uses: `$CVSCOMSTR`.

`cXX`

  Sets construction variables for generic POSIX C++ compilers.

  Sets: `$CPPDEFPREFIX`, `$CPPDEFSUFFIX`, `$CXX`, `$CXXCOM`, `$CXXFILESUFFIX`, `$CXXFLAGS`, `$INCPREFIX`, `$INCSUFFIX`, `$OBJSUFFIX`, `$SHCXX`, `$SHCXXCOM`, `$SHCXXFLAGS`, `$SHOBJSUFFIX`.

  Uses: `$CXXCOMSTR`.

`default`

  Sets variables by calling a default list of Tool modules for the platform on which SCons is running.

`dmd`

  Sets construction variables for the Digital Mars D compiler.

`dvi`

  Attaches the `DVI` builder to the construction environment.

`dvipdf`

  Sets construction variables for the dvipdf utility.

  Sets: `$DVIPDF`, `$DVIPDFCOM`, `$DVIPDFFLAGS`.

  Uses: `$DVIPDFCOMSTR`.

`dvips`

  Sets construction variables for the dvips utility.

  Sets: `$DVIPS`, `$DVIPSFLAGS`, `$PSCOM`, `$PSPREFIX`, `$PSSUFFIX`.

  Uses: `$PSCOMSTR`.

`f77`

  Set construction variables for generic POSIX Fortran 77 compilers.

  Sets: `$F77`, `$F77COM`, `$F77FLAGS`, `$F77PPCOM`, `$FORTRAN`, `$FORTRANCOM`, `$FORTRANFLAGS`, `$SHF77`, `$SHF77COM`, `$SHF77FLAGS`, `$SHF77PPCOM`, `$SHFORTRAN`, `$SHFORTRANCOM`, `$SHFORTRANFLAGS`, `$SHFORTRANPPCOM`, `$_F77INCFLAGS`.

  Uses: `$F77COMSTR`, `$F77PPCOMSTR`, `$FORTRANCOMSTR`, `$FORTRANPPCOMSTR`, `$SHF77COMSTR`, `$SHF77PPCOMSTR`, `$SHFORTRANCOMSTR`, `$SHFORTRANPPCOMSTR`.

`f90`

  Set construction variables for generic POSIX Fortran 90 compilers.

  Sets: `$F90`, `$F90COM`, `$F90FLAGS`, `$F90PPCOM`, `$SHF90`, `$SHF90COM`, `$SHF90FLAGS`, `$SHF90PPCOM`, `$_F90INCFLAGS`.

  Uses: `$F90COMSTR`, `$F90PPCOMSTR`, `$SHF90COMSTR`, `$SHF90PPCOMSTR`.

`f95`

  Set construction variables for generic POSIX Fortran 95 compilers.

  Sets: `$F95`, `$F95COM`, `$F95FLAGS`, `$F95PPCOM`, `$SHF95`, `$SHF95COM`, `$SHF95FLAGS`, `$SHF95PPCOM`, `$_F95INCFLAGS`.

  Uses: `$F95COMSTR`, `$F95PPCOMSTR`, `$SHF95COMSTR`, `$SHF95PPCOMSTR`.

`fortran`

  Set construction variables for generic POSIX Fortran compilers.

  Sets: `$FORTRAN`, `$FORTRANCOM`, `$FORTRANFLAGS`, `$SHFORTRAN`, `$SHFORTRANCOM`, `$SHFORTRANFLAGS`, `$SHFORTRANPPCOM`.

  Uses: `$FORTRANCOMSTR`, `$FORTRANPPCOMSTR`, `$SHFORTRANCOMSTR`, `$SHFORTRANPPCOMSTR`.

`g++`

  Set construction variables for the gXX C++ compiler.

  Sets: `$CXX`, `$CXXVERSION`, `$SHCXXFLAGS`, `$SHOBJSUFFIX`.

`g77`

  Set construction variables for the g77 Fortran compiler. Calls the `f77` Tool module to set variables.

`gas`

  Sets construction variables for the gas assembler. Calls the `as` module.

  Sets: `$AS`.

`gcc`

  Set construction variables for the gcc C compiler.

  Sets: `$CC`, `$CCVERSION`, `$SHCCFLAGS`.

`gnulink`

  Set construction variables for GNU linker/loader.

  Sets: `$RPATHPREFIX`, `$RPATHSUFFIX`, `$SHLINKFLAGS`.

`gs`

  Set construction variables for Ghostscript.

  Sets: `$GS`, `$GSCOM`, `$GSFLAGS`.

  Uses: `$GSCOMSTR`.

`hpc++`

  Set construction variables for the compilers aCC on HP/UX systems.

`hpcc`

  Set construction variables for the aCC on HP/UX systems. Calls the `cXX` tool for additional variables.

  Sets: `$CXX`, `$CXXVERSION`, `$SHCXXFLAGS`.

`hplink`

  Sets construction variables for the linker on HP/UX systems.

  Sets: `$LINKFLAGS`, `$SHLIBSUFFIX`, `$SHLINKFLAGS`.

`icc`

  Sets construction variables for the icc compiler on OS/2 systems.

  Sets: `$CC`, `$CCCOM`, `$CFILESUFFIX`, `$CPPDEFPREFIX`, `$CPPDEFSUFFIX`, `$CXXCOM`, `$CXXFILESUFFIX`, `$INCPREFIX`, `$INCSUFFIX`.

  Uses: `$CCFLAGS`, `$CFLAGS`, `$CPPFLAGS`, `$_CPPDEFFLAGS`, `$_CPPINCFLAGS`.

`icl`

  Sets construction variables for the Intel C/C++ compiler. Calls the `intelc` Tool module to set its variables.

`ifl`

  Sets construction variables for the Intel Fortran compiler.

  Sets: `$FORTRAN`, `$FORTRANCOM`, `$FORTRANPPCOM`, `$SHFORTRANCOM`, `$SHFORTRANPPCOM`.

  Uses: `$CPPFLAGS`, `$FORTRANFLAGS`, `$_CPPDEFFLAGS`, `$_FORTRANINCFLAGS`.

`ifort`

  Sets construction variables for newer versions of the Intel Fortran compiler for Linux.

  Sets: `$SHLINK`, `$SHLINKFLAGS`.

`ilink`

  Sets construction variables for the ilink linker on OS/2 systems.

  Sets: `$LIBDIRPREFIX`, `$LIBDIRSUFFIX`, `$LIBLINKPREFIX`, `$LIBLINKSUFFIX`, `$LINK`, `$LINKCOM`, `$LINKFLAGS`.

`ilink32`

  Sets construction variables for the Borland ilink32 linker.

  Sets: `$LIBDIRPREFIX`, `$LIBDIRSUFFIX`, `$LIBLINKPREFIX`, `$LIBLINKSUFFIX`, `$LINK`, `$LINKCOM`, `$LINKFLAGS`.

`install`

  Sets construction variables for file and directory installation.

  Sets: `$INSTALL`, `$INSTALLSTR`.

`intelc`

  Sets construction variables for the Intel C/C++ compiler (Linux and Windows, version 7 and later). Calls the `gcc` or `msvc` (on Linux and Windows, respectively) to set underlying variables.

  Sets: `$AR`, `$CC`, `$CXX`, `$INTEL_C_COMPILER_VERSION`, `$LINK`.

`jar`

  Sets construction variables for the jar utility.

  Sets: `$JAR`, `$JARCOM`, `$JARFLAGS`, `$JARSUFFIX`.

  Uses: `$JARCOMSTR`.

`javac`

  Sets construction variables for the javac compiler.

  Sets: `$JAVABOOTCLASSPATH`, `$JAVAC`, `$JAVACCOM`, `$JAVACFLAGS`, `$JAVACLASSPATH`, `$JAVACLASSSUFFIX`, `$JAVASOURCEPATH`, `$JAVASUFFIX`.

  Uses: `$JAVACCOMSTR`.

`javah`

  Sets construction variables for the javah tool.

  Sets: `$JAVACLASSSUFFIX`, `$JAVAH`, `$JAVAHCOM`, `$JAVAHFLAGS`.

  Uses: `$JAVACLASSPATH`, `$JAVAHCOMSTR`.

`latex`

  Sets construction variables for the latex utility.

  Sets: `$LATEX`, `$LATEXCOM`, `$LATEXFLAGS`.

  Uses: `$LATEXCOMSTR`.

`lex`

  Sets construction variables for the lex lexical analyser.

  Sets: `$LEX`, `$LEXCOM`, `$LEXFLAGS`.

  Uses: `$LEXCOMSTR`.

`link`

  Sets construction variables for generic POSIX linkers.

  Sets: `$LDMODULE`, `$LDMODULECOM`, `$LDMODULEFLAGS`, `$LDMODULEPREFIX`, `$LDMODULESUFFIX`, `$LIBDIRPREFIX`, `$LIBDIRSUFFIX`, `$LIBLINKPREFIX`, `$LIBLINKSUFFIX`, `$LINK`, `$LINKCOM`, `$LINKFLAGS`, `$SHLIBSUFFIX`, `$SHLINK`, `$SHLINKCOM`, `$SHLINKFLAGS`.

  Uses: `$LDMODULECOMSTR`, `$LINKCOMSTR`, `$SHLINKCOMSTR`.

`linkloc`

  Sets construction variables for the LinkLoc linker for the Phar Lap ETS embedded operating system.

  Sets: `$LIBDIRPREFIX`, `$LIBDIRSUFFIX`, `$LIBLINKPREFIX`, `$LIBLINKSUFFIX`, `$LINK`, `$LINKCOM`, `$LINKFLAGS`, `$SHLINK`, `$SHLINKCOM`, `$SHLINKFLAGS`.

  Uses: `$LINKCOMSTR`, `$SHLINKCOMSTR`.

`m4`

  Sets construction variables for the m4 macro processor.

  Sets: `$M4`, `$M4COM`, `$M4FLAGS`.

  Uses: `$M4COMSTR`.

`masm`

  Sets construction variables for the Microsoft assembler.

  Sets: `$AS`, `$ASCOM`, `$ASFLAGS`, `$ASPPCOM`, `$ASPPFLAGS`.

  Uses: `$ASCOMSTR`, `$ASPPCOMSTR`, `$CPPFLAGS`, `$_CPPDEFFLAGS`, `$_CPPINCFLAGS`.

`midl`

  Sets construction variables for the Microsoft IDL compiler.

  Sets: `$MIDL`, `$MIDLCOM`, `$MIDLFLAGS`.

  Uses: `$MIDLCOMSTR`.

`mingw`

  Sets construction variables for MinGW (Minimal Gnu on Windows).

  Sets: `$AS`, `$CC`, `$CXX`, `$LIBPREFIX`, `$LIBSUFFIX`, `$OBJSUFFIX`, `$RC`, `$RCCOM`, `$RCFLAGS`, `$RCINCFLAGS`, `$RCINCPREFIX`, `$RCINCSUFFIX`, `$SHCCFLAGS`, `$SHCXXFLAGS`, `$SHLINKCOM`, `$SHLINKFLAGS`, `$SHOBJSUFFIX`, `$WINDOWSDEFPREFIX`, `$WINDOWSDEFSUFFIX`.

  Uses: `$RCCOMSTR`, `$SHLINKCOMSTR`.

`mslib`

  Sets construction variables for the Microsoft mslib library archiver.

  Sets: `$AR`, `$ARCOM`, `$ARFLAGS`, `$LIBPREFIX`, `$LIBSUFFIX`.

  Uses: `$ARCOMSTR`.

`mslink`

  Sets construction variables for the Microsoft linker.

  Sets: `$LDMODULE`, `$LDMODULECOM`, `$LDMODULEFLAGS`, `$LDMODULEPREFIX`, `$LDMODULESUFFIX`, `$LIBDIRPREFIX`, `$LIBDIRSUFFIX`, `$LIBLINKPREFIX`, `$LIBLINKSUFFIX`, `$LINK`, `$LINKCOM`, `$LINKFLAGS`, `$REGSVR`, `$REGSVRCOM`, `$REGSVRFLAGS`, `$SHLINK`, `$SHLINKCOM`, `$SHLINKFLAGS`, `$WIN32DEFPREFIX`, `$WIN32DEFSUFFIX`, `$WIN32EXPPREFIX`, `$WIN32EXPSUFFIX`, `$WINDOWSDEFPREFIX`, `$WINDOWSDEFSUFFIX`, `$WINDOWSEXPPREFIX`, `$WINDOWSEXPSUFFIX`, `$WINDOWSPROGMANIFESTPREFIX`, `$WINDOWSPROGMANIFESTSUFFIX`, `$WINDOWSSHLIBMANIFESTPREFIX`, `$WINDOWSSHLIBMANIFESTSUFFIX`, `$WINDOWS_INSERT_DEF`.

  Uses: `$LDMODULECOMSTR`, `$LINKCOMSTR`, `$MSVS_IGNORE_IDE_PATHS`, `$REGSVRCOMSTR`, `$SHLINKCOMSTR`.

`msvc`

  Sets construction variables for the Microsoft Visual C/C++ compiler.

  Sets: `$BUILDERS`, `$CC`, `$CCCOM`, `$CCFLAGS`, `$CCPCHFLAGS`, `$CCPDBFLAGS`, `$CFILESUFFIX`, `$CFLAGS`, `$CPPDEFPREFIX`, `$CPPDEFSUFFIX`, `$CXX`, `$CXXCOM`, `$CXXFILESUFFIX`, `$CXXFLAGS`, `$INCPREFIX`, `$INCSUFFIX`, `$OBJPREFIX`, `$OBJSUFFIX`, `$PCHCOM`, `$PCHPDBFLAGS`, `$RC`, `$RCCOM`, `$RCFLAGS`, `$SHCC`, `$SHCCCOM`, `$SHCCFLAGS`, `$SHCFLAGS`, `$SHCXX`, `$SHCXXCOM`, `$SHCXXFLAGS`, `$SHOBJPREFIX`, `$SHOBJSUFFIX`.

  Uses: `$CCCOMSTR`, `$CXXCOMSTR`, `$SHCCCOMSTR`, `$SHCXXCOMSTR`.

`msvs`

  Sets construction variables for Microsoft Visual Studio.

  Sets: `$MSVSBUILDCOM`, `$MSVSCLEANCOM`, `$MSVSENCODING`, `$MSVSPROJECTCOM`, `$MSVSREBUILDCOM`, `$MSVSSCONS`, `$MSVSSCONSCOM`, `$MSVSSCONSCRIPT`, `$MSVSSCONSFLAGS`, `$MSVSSOLUTIONCOM`.

`mwcc`

  Sets construction variables for the Metrowerks CodeWarrior compiler.

  Sets: `$CC`, `$CCCOM`, `$CFILESUFFIX`, `$CPPDEFPREFIX`, `$CPPDEFSUFFIX`, `$CXX`, `$CXXCOM`, `$CXXFILESUFFIX`, `$INCPREFIX`, `$INCSUFFIX`, `$MWCW_VERSION`, `$MWCW_VERSIONS`, `$SHCC`, `$SHCCCOM`, `$SHCCFLAGS`, `$SHCFLAGS`, `$SHCXX`, `$SHCXXCOM`, `$SHCXXFLAGS`.

  Uses: `$CCCOMSTR`, `$CXXCOMSTR`, `$SHCCCOMSTR`, `$SHCXXCOMSTR`.

`mwld`

  Sets construction variables for the Metrowerks CodeWarrior linker.

  Sets: `$AR`, `$ARCOM`, `$LIBDIRPREFIX`, `$LIBDIRSUFFIX`, `$LIBLINKPREFIX`, `$LIBLINKSUFFIX`, `$LINK`, `$LINKCOM`, `$SHLINK`, `$SHLINKCOM`, `$SHLINKFLAGS`.

`nasm`

  Sets construction variables for the nasm Netwide Assembler.

  Sets: `$AS`, `$ASCOM`, `$ASFLAGS`, `$ASPPCOM`, `$ASPPFLAGS`.

  Uses: `$ASCOMSTR`, `$ASPPCOMSTR`.

`packaging`

  A framework for building binary and source packages.

`Packaging`

  Sets construction variables for the `Package` Builder.

`pdf`

  Sets construction variables for the Portable Document Format builder.

  Sets: `$PDFPREFIX`, `$PDFSUFFIX`.

`pdflatex`

  Sets construction variables for the pdflatex utility.

  Sets: `$LATEXRETRIES`, `$PDFLATEX`, `$PDFLATEXCOM`, `$PDFLATEXFLAGS`.

  Uses: `$PDFLATEXCOMSTR`.

`pdftex`

  Sets construction variables for the pdftex utility.

  Sets: `$LATEXRETRIES`, `$PDFLATEX`, `$PDFLATEXCOM`, `$PDFLATEXFLAGS`, `$PDFTEX`, `$PDFTEXCOM`, `$PDFTEXFLAGS`.

  Uses: `$PDFLATEXCOMSTR`, `$PDFTEXCOMSTR`.

`Perforce`

  Sets construction variables for interacting with the Perforce source code management system.

  Sets: `$P4`, `$P4COM`, `$P4FLAGS`.

  Uses: `$P4COMSTR`.

`qt`

  Sets construction variables for building Qt applications.

  Sets: `$QTDIR`, `$QT_AUTOSCAN`, `$QT_BINPATH`, `$QT_CPPPATH`, `$QT_LIB`, `$QT_LIBPATH`, `$QT_MOC`, `$QT_MOCCXXPREFIX`, `$QT_MOCCXXSUFFIX`, `$QT_MOCFROMCXXCOM`, `$QT_MOCFROMCXXFLAGS`, `$QT_MOCFROMHCOM`, `$QT_MOCFROMHFLAGS`, `$QT_MOCHPREFIX`, `$QT_MOCHSUFFIX`, `$QT_UIC`, `$QT_UICCOM`, `$QT_UICDECLFLAGS`, `$QT_UICDECLPREFIX`, `$QT_UICDECLSUFFIX`, `$QT_UICIMPLFLAGS`, `$QT_UICIMPLPREFIX`, `$QT_UICIMPLSUFFIX`, `$QT_UISUFFIX`.

`RCS`

  Sets construction variables for the interaction with the Revision Control System.

  Sets: `$RCS`, `$RCS_CO`, `$RCS_COCOM`, `$RCS_COFLAGS`.

  Uses: `$RCS_COCOMSTR`.

`rmic`

  Sets construction variables for the rmic utility.

  Sets: `$JAVACLASSSUFFIX`, `$RMIC`, `$RMICCOM`, `$RMICFLAGS`.

  Uses: `$RMICCOMSTR`.

`rpcgen`

  Sets construction variables for building with RPCGEN.

  Sets: `$RPCGEN`, `$RPCGENCLIENTFLAGS`, `$RPCGENFLAGS`, `$RPCGENHEADERFLAGS`, `$RPCGENSERVICEFLAGS`, `$RPCGENXDRFLAGS`.

`SCCS`

  Sets construction variables for interacting with the Source Code Control System.

  Sets: `$SCCS`, `$SCCSCOM`, `$SCCSFLAGS`, `$SCCSGETFLAGS`.

  Uses: `$SCCSCOMSTR`.

`sgiar`

  Sets construction variables for the SGI library archiver.

  Sets: `$AR`, `$ARCOMSTR`, `$ARFLAGS`, `$LIBPREFIX`, `$LIBSUFFIX`, `$SHLINK`, `$SHLINKFLAGS`.

  Uses: `$ARCOMSTR`, `$SHLINKCOMSTR`.

`sgic++`

  Sets construction variables for the SGI C++ compiler.

  Sets: `$CXX`, `$CXXFLAGS`, `$SHCXX`, `$SHOBJSUFFIX`.

`sgicc`

  Sets construction variables for the SGI C compiler.

  Sets: `$CXX`, `$SHOBJSUFFIX`.

`sgilink`

  Sets construction variables for the SGI linker.

  Sets: `$LINK`, `$RPATHPREFIX`, `$RPATHSUFFIX`, `$SHLINKFLAGS`.

`sunar`

  Sets construction variables for the Sun library archiver.

  Sets: `$AR`, `$ARCOM`, `$ARFLAGS`, `$LIBPREFIX`, `$LIBSUFFIX`, `$SHLINK`, `$SHLINKCOM`, `$SHLINKFLAGS`.

  Uses: `$ARCOMSTR`, `$SHLINKCOMSTR`.

`sunc++`

  Sets construction variables for the Sun C++ compiler.

  Sets: `$CXX`, `$CXXVERSION`, `$SHCXX`, `$SHCXXFLAGS`, `$SHOBJPREFIX`, `$SHOBJSUFFIX`.

`suncc`

  Sets construction variables for the Sun C compiler.

  Sets: `$CXX`, `$SHCCFLAGS`, `$SHOBJPREFIX`, `$SHOBJSUFFIX`.

`sunlink`

  Sets construction variables for the Sun linker.

  Sets: `$RPATHPREFIX`, `$RPATHSUFFIX`, `$SHLINKFLAGS`.

`swig`

  Sets construction variables for the SWIG interface generator.

  Sets: `$SWIG`, `$SWIGCFILESUFFIX`, `$SWIGCOM`, `$SWIGCXXFILESUFFIX`, `$SWIGFLAGS`, `$SWIGINCPREFIX`, `$SWIGINCSUFFIX`, `$SWIGPATH`, `$_SWIGINCFLAGS`.

  Uses: `$SWIGCOMSTR`.

`tar`

  Sets construction variables for the tar archiver.

  Sets: `$TAR`, `$TARCOM`, `$TARFLAGS`, `$TARSUFFIX`.

  Uses: `$TARCOMSTR`.

`tex`

  Sets construction variables for the TeX formatter and typesetter.

  Sets: `$BIBTEX`, `$BIBTEXCOM`, `$BIBTEXFLAGS`, `$LATEX`, `$LATEXCOM`, `$LATEXFLAGS`, `$MAKEINDEX`, `$MAKEINDEXCOM`, `$MAKEINDEXFLAGS`, `$TEX`, `$TEXCOM`, `$TEXFLAGS`.

  Uses: `$BIBTEXCOMSTR`, `$LATEXCOMSTR`, `$MAKEINDEXCOMSTR`, `$TEXCOMSTR`.

`tlib`

  Sets construction variables for the Borlan tib library archiver.

  Sets: `$AR`, `$ARCOM`, `$ARFLAGS`, `$LIBPREFIX`, `$LIBSUFFIX`.

  Uses: `$ARCOMSTR`.

`yacc`

  Sets construction variables for the yacc parse generator.

  Sets: `$YACC`, `$YACCCOM`, `$YACCFLAGS`, `$YACCHFILESUFFIX`, `$YACCHXXFILESUFFIX`, `$YACCVCGFILESUFFIX`.

  Uses: `$YACCCOMSTR`.

`zip`

  Sets construction variables for the zip archiver.

  Sets: `$ZIP`, `$ZIPCOM`, `$ZIPCOMPRESSION`, `$ZIPFLAGS`, `$ZIPSUFFIX`.

  Uses: `$ZIPCOMSTR`.

# Appendix D. Handling Common Tasks

There is a common set of simple tasks that many build configurations rely on as they become more complex. Most build tools have special purpose constructs for performing these tasks, but since `SConscript` files are Python scripts, you can use more flexible built-in Python services to perform these tasks. This appendix lists a number of these tasks and how to implement them in Python.

Example D-1. Wildcard globbing to create a list of filenames

    import glob
    files = glob.glob(wildcard)

Example D-2. Filename extension substitution

    import os.path
    filename = os.path.splitext(filename)[0]+extension

Example D-3. Appending a path prefix to a list of filenames

    import os.path
    filenames = [os.path.join(prefix, x) for x in filenames]

or in Python 1.5.2:

    import os.path
    new_filenames = []
    for x in filenames:
        new_filenames.append(os.path.join(prefix, x))

Example D-4. Substituting a path prefix with another one

    if filename.find(old_prefix) == 0:
        filename = filename.replace(old_prefix, new_prefix)

or in Python 1.5.2:

    import string
    if string.find(filename, old_prefix) == 0:
        filename = string.replace(filename, old_prefix, new_prefix)

Example D-5. Filtering a filename list to exclude/retain only a specific set of extensions

    import os.path
    filenames = [x for x in filenames if os.path.splitext(x)[1] in extensions]

or in Python 1.5.2:

    import os.path
    new_filenames = []
    for x in filenames:
        if os.path.splitext(x)[1] in extensions:
            new_filenames.append(x)

Example D-6. The "backtick function": run a shell command and capture the output

    import os
    output = os.popen(command).read()

### Notes

[1] | In programming parlance, the `SConstruct` file is declarative, meaning you tell SCons what you want done and let it figure out the order in which to do it, rather than strictly imperative, where you specify explicitly the order in which to do things.
[2] | This easily-overlooked distinction between how SCons decides if the target itself must be rebuilt and how the target is then used to decide if a different target must be rebuilt is one of the confusing things that has led to the `TargetSignatures` and `SourceSignatures` functions being replaced by the simpler `Decider` function.
[3] | Actually, the MD5 signature is used as the name of the file in the shared cache directory in which the contents are stored.