Linux Frequently Asked Questions with Answers: Porting, compiling and obtaining programs

## 5. Porting, compiling and obtaining programs

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## 5.1 How do I compile programs?

Most Linux software is written in C and compiled with the GNU C compiler. GCC is a part of every Linux distribution. The latest compiler version, documentation, and patches are on ftp://ftp.gnu.org/pub/gnu/.

Programs that are written in C++ must be compiled with the GNU G++ compiler, which is also included in Linux distributions and available from the same place as GCC.

To build version 2.0.x kernels, you will need GCC version 2.7.2.x. Trying to build a Linux kernel with a different compiler, like GCC 2.8.x, EGCS, or PGCC, may cause problems until code dependencies of the 2.7.2.x compilers are fixed.

Information on the EGCS compiler is at htmlurl url="http://egcs.cygnus.com" name="http://egcs.cygnus.com">.

Note that at this time, the kernel developers are not answering bug requests for 2.0.x version kernels, but instead are concentrating on developing 2.1.x version kernels.

[J.H.M. Dassen]

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## 5.2 How do I port XXX to Linux?

In general, Unix programs need very little porting. Simply follow the installation instructions. If you don't know--and don't know how to find out--the answers to some of the questions asked during the installation procedure, you can guess, but this tends to produce buggy programs. In this case, you're probably better off asking someone else to do the port.

If you have a BSD-ish program, you should try using `-I/usr/include/bsd` and `-lbsd` on the appropriate parts of the compilation lines.

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## 5.3 What is ld.so and where do I get it?

`Ld.so` is the dynamic library loader. Each binary using shared libraries used to have about 3K of start-up code to find and load the shared libraries. Now that code has been put in a special shared library, `/lib/ld.so`, where all binaries can look for it, so that it wastes less disk space, and can be upgraded more easily.

`Ld.so` can be obtained from tsx-11.mit.edu/pub/linux/packages/GCC/ and mirror sites. The latest version at the time of writing is `ld.so.1.9.5.tar.gz`.

`/lib/ld-linux.so.1` is the same thing for ELF (`` What's all this about ELF? '') and comes in the same package as the `a.out` loader.

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## 5.4 How do I upgrade the libraries withough trashing my system?

Note: You should always have a rescue disk set ready when you perform this procedure, in the likely event that something goes wrong!

This procedure is especially difficult if you're upgrading very old libraries like libc4. But you should be able to keep libc4 on the same system with libc5 libraries for the programs that still need them. The same holds true for upgrading from libc5 to the newer-yet glibc2 libraries.

The problem with upgrading dynamic libraries is that, the moment you remove the old libraries, the utilities that you need to upgrade to the new version of the libraries don't work. There are ways around around this. One is to temporarily place a spare copy of the run time libraries, which are in /lib/, in /usr/lib/, or /usr/local/lib/, or another directory that is listed in the /etc/ld.so.conf file.

For example, when upgrading libc5 libraries, the files in /lib/ might look something like:

    libc.so.5
    libc.so.5.4.33
    libm.so.5
    libm.so.5.0.9

These are the C libraries and the math libraries. Copy them to another directory that is listed in /etc/ld.so.conf, like /usr/lib/.

    cp -df /lib/libc.so.5* /usr/lib/
    cp -df /lib/libm.so.5* /usr/lib/
    ldconfig

Be sure to run ldconfig to upgrade the library configuration.

The files libc.so.5 and libm.so.5 are symbolic links to the actual library files. When you upgrade, the new links will not be created if the old links are still there, unless you use the -f flag with cp. The -d flag to cp will copy the symbolic link itself, and not the file it points to.

If you need to overwrite the link to the library directly, use the -f flag with ln.

For example, to copy new libraries over the old ones, try this. Make a symbolic link to the new libraries first, then copy both the libraries and the links to /lib/, with the following commands.

    ln -sf ./libm.so.5.0.48 libm.so.5
    ln -sf ./libc.so.5.0.48 libc.so.5
    cp -df libm.so.5* /lib
    cp -df libc.so.5* /lib

Again, remember to run ldconfig after you copy the libraries.

If you are satisfied that everything is working correctly, you can remove the temporary copies of the old libraries from /usr/lib/ or wherever you copied them.

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## 5.5 Has anyone ported / compiled / written XXX for Linux?

First, look in the Linux Software Map--it's at sunsite.unc.edu/pub/Linux/docs/linux-software-map, and on the other FTP sites. A search engine is available on the World Wide Web at http://www.boutell.com/lsm/.

Check the FTP sites (`` Where can I get Linux material by FTP? '') first--search the `ls-lR` or `INDEX` files for appropriate strings.

Also look at the Linux Projects Map, ftp.ix.de/pub/ix/Linux/docs/Projects-Map.gz.

There's a search engine for Linux FTP archives at http://lfw.linuxhq.com/

Also check out the Freshmeat Web site http://www.freshmeat.org, which is really cool. (`` What online/free periodicals exist for Linux? ''

If you don't find anything, you could download the sources to the program yourself and compile them. See `` How do I port XXX to Linux? '' If it's a large package that may require some porting, post a message to `comp.os.linux.development.apps`.

If you compile a large-ish program, please upload it to one or more of the FTP sites, and post a message to `comp.os.linux.announce` (submit your posting to linux-announce@news.ornl.gov).

If you're looking for an application program, the chances are that someone has already written a free version. The `comp.sources.wanted` FAQ has instructions for finding the source code.

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## 5.6 Can I use code or a compiler compiled for a 486 on my 386?

Yes, unless it's the kernel.

The -m486 option to GCC, which is used to compile binaries for x486 machines, merely changes certain optimizations. This makes for slightly larger binaries that run somewhat faster on a 486. They still work fine on a 386, though, with a small performance hit.

However, from version 1.3.35 the kernel uses 486 or Pentium-specific instructions if configured for a 486 or Pentium, thus making it unusable on a 386.

GCC can be configured for a 386 or 486; the only difference is that configuring it for a 386 makes -m386 the default and configuring for a 486 makes -m486 the default. In either case, these can be overridden on a per-compilation basis or by editing /usr/lib/gcc-lib/i*-linux/n.n.n/specs.

There is an Alpha version of GCC which knows how to do optimization well for the 586, but it is quite unreliable, especially at high optimization settings. The Pentium GCC can be found on tsx-11.mit.edu in /pub/linux/ALPHA/pentium-gcc. I'd recommend using the ordinary 486 GCC instead; word has it that using -m386 produces code that's better for the Pentium, or at least slightly smaller.

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## 5.7 What does gcc -O6 do?

Currently, the same as -O2 (GCC 2.5) or -O3 (GCC 2.6, 2.7). Any number greater than that does the same thing. The Makefiles of newer kernels use -O2, and you should probably do the same.

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## 5.8 Where are linux/*.h and asm/*.h?

The files `/usr/include/linux/` and `/usr/include/asm/` are often soft links to the directories where the kernel headers are. They are usually under /usr/src/kernel*/.

If you don't have the kernel sources, download them--see, `` How do I upgrade/recompile my kernel? ''

Then, use `rm` to remove any garbage, and `ln` to create the links:

    rm -rf /usr/include/linux /usr/include/asm
    ln -sf /usr/src/linux/include/linux /usr/include/linux
    ln -sf /usr/src/linux/include/asm /usr/include/asm

`/usr/src/linux/include/asm/` is a symbolic link to an architecture-specific asm directory--if you have a freshly unpacked kernel source tree, you must make symlinks. You'll also find that you may need to do `make config' in a newly-unpacked kernel source tree, to create linux/autoconf.h.

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## 5.9 I get errors when I try to compile the kernel.

See the previous question regarding the header files.

Remember that when you apply a patch to the kernel, you must use the `-p0` or `-p1` option: otherwise, the patch may be misapplied. See the `patch` manual page for details.

```ld: unrecognized option `-qmagic'`'' means that you should get a newer linker, from ftp://tsx-11.mit.edu/pub/linux/packages/GCC/, in the file `binutils-2.8.1.0.1.bin.tar.gz`.

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## 5.10 How do I make a shared library?

For ELF,

       gcc -fPIC -c *.c
       gcc -shared -Wl,-soname,libfoo.so.1 -o libfoo.so.1.0 *.o

For `a.out`, get tools-n.nn.tar.gz from tsx-11.mit.edu, in /pub/linux/packages/GCC/src/. It comes with documentation that will tell you what to do. Note that `a.out` shared libraries are a very tricky business. Consider upgrading your libraries to ELF shared libraries. See the ELF HOWTO, at sunsite.unc.edu/pub/Linux/docs/HOWTO/

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## 5.11 My executables are (very) large.

With an ELF compiler (`` What's all this about ELF? ''), the most common cause of large executables is the lack of an appropriate .so library link for one of the libraries you're using. There should be a link like libc.so for every library like libc.so.5.2.18.

With an `a.out compiler` the most common cause of large executables is the `-g` linker (compiler) flag. This produces (as well as debugging information in the output file) a program which is statically linked--one which includes a copy of the C library instead of a dynamically linked copy.

Other things worth investigating are `-O` and `-O2`, which enable optimization (check the GCC documentation), and `-s` (or the strip command) which strip the symbol information from the resulting binary (making debugging totally impossible).

You may wish to use `-N` on very small executables (less than 8K with the `-N`), but you shouldn't do this unless you understand its performance implications, and definitely never with daemons.

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## 5.12 Does Linux support threads or lightweight processes?

As well as the Unix multiprocessing model involving heavyweight processes, which is of course part of the standard Linux kernel, there are several implementations of lightweight processes or threads. Recent kernels implement a thread model, kthreads. In addition, there are the following packages available for Linux.

- GNU glibc2 for Linux has optional support for threads. The archive is available from the same place as glibc2, ftp://ftp.gnu.org/pub/gnu
- In sipb.mit.edu:/pub/pthread or ftp.ibp.fr:/pub/unix/threads/pthreads. Documentation isn't in the package, but is available on the World Wide Web at http://www.mit.edu:8001/people/proven/home_page.html. Newer Linux libc's contain the pthreads source. The GNU Ada compiler on sunsite.unc.edu in /pub/Linux/devel/lang/ada/gnat-3.01-linux+elf.tar.gz contains binaries made from that source code.
- In ftp.cs.washington.edu:/pub/qt-001.tar.Z is QuickThreads. More information can be found in the technical report, available on the same site as /tr/1993/05/UW-CSE-93-05-06.PS.Z.
- In gummo.doc.ic.ac.uk/rex/ is lwp, a very minimal implementation.
- In ftp.cs.fsu.edu:/pub/PART/, an Ada implementation. This is useful mainly because it has a lot of Postscript papers that you'll find useful in learning more about threads. This is not directly usable under Linux.

Please contact the authors of the packages in question for details.

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## 5.13 Where can I get `lint' for Linux?

Roughly equivalent functionality is built into GCC. Use the -Wall option to turn on most of the useful extra warnings. See the GCC manual for more details (type control-h followed by i in Emacs and select the entry for GCC).

There is a freely available program called `lclint' that does much the same thing as traditional lint. The announcement and source code are available at on larch.lcs.mit.edu in /pub/Larch/lclint/; on the World Wide Web, look at http://larch-www.lcs.mit.edu:8001/larch/lclint.html.

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## 5.14 Where can I find kermit for Linux?

Kermit is distributed under a non-GPL copyright that makes its terms of distribution somewhat different. The sources and some binaries are available on kermit.columbia.edu.

The WWW Home Page of the Columbia University Kermit project is http://www.columbia.edu/kermit/.

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