## 2. Using and understanding the Valgrind core

This section describes the Valgrind core services, flags and behaviours. That means it is relevant regardless of what particular tool you are using. A point of terminology: most references to "valgrind" in the rest of this section (Section 2) refer to the valgrind core services.

## 2.1. What Valgrind does with your program

Valgrind is designed to be as non-intrusive as possible. It works directly with existing executables. You don't need to recompile, relink, or otherwise modify, the program to be checked.

Simply put `valgrind --tool=tool_name` at the start of the command line normally used to run the program. For example, if want to run the command `ls -l` using the heavyweight memory-checking tool Memcheck, issue the command:

    valgrind --tool=memcheck ls -l

(Memcheck is the default, so if you want to use it you can actually omit the `--tool` flag.

Regardless of which tool is in use, Valgrind takes control of your program before it starts. Debugging information is read from the executable and associated libraries, so that error messages and other outputs can be phrased in terms of source code locations (if that is appropriate).

Your program is then run on a synthetic CPU provided by the Valgrind core. As new code is executed for the first time, the core hands the code to the selected tool. The tool adds its own instrumentation code to this and hands the result back to the core, which coordinates the continued execution of this instrumented code.

The amount of instrumentation code added varies widely between tools. At one end of the scale, Memcheck adds code to check every memory access and every value computed, increasing the size of the code at least 12 times, and making it run 25-50 times slower than natively. At the other end of the spectrum, the ultra-trivial "none" tool (a.k.a. Nulgrind) adds no instrumentation at all and causes in total "only" about a 4 times slowdown.

Valgrind simulates every single instruction your program executes. Because of this, the active tool checks, or profiles, not only the code in your application but also in all supporting dynamically-linked (`.so`-format) libraries, including the GNU C library, the X client libraries, Qt, if you work with KDE, and so on.

If you're using one of the error-detection tools, Valgrind will often detect errors in libraries, for example the GNU C or X11 libraries, which you have to use. You might not be interested in these errors, since you probably have no control over that code. Therefore, Valgrind allows you to selectively suppress errors, by recording them in a suppressions file which is read when Valgrind starts up. The build mechanism attempts to select suppressions which give reasonable behaviour for the libc and XFree86 versions detected on your machine. To make it easier to write suppressions, you can use the `--gen-suppressions=yes` option which tells Valgrind to print out a suppression for each error that appears, which you can then copy into a suppressions file.

Different error-checking tools report different kinds of errors. The suppression mechanism therefore allows you to say which tool or tool(s) each suppression applies to.

## 2.2. Getting started

First off, consider whether it might be beneficial to recompile your application and supporting libraries with debugging info enabled (the `-g` flag). Without debugging info, the best Valgrind tools will be able to do is guess which function a particular piece of code belongs to, which makes both error messages and profiling output nearly useless. With `-g`, you'll hopefully get messages which point directly to the relevant source code lines.

Another flag you might like to consider, if you are working with C++, is `-fno-inline`. That makes it easier to see the function-call chain, which can help reduce confusion when navigating around large C++ apps. For whatever it's worth, debugging OpenOffice.org with Memcheck is a bit easier when using this flag. You don't have to do this, but doing so helps Valgrind produce more accurate and less confusing error reports. Chances are you're set up like this already, if you intended to debug your program with GNU gdb, or some other debugger.

This paragraph applies only if you plan to use Memcheck: On rare occasions, optimisation levels at `-O2` and above have been observed to generate code which fools Memcheck into wrongly reporting uninitialised value errors. We have looked in detail into fixing this, and unfortunately the result is that doing so would give a further significant slowdown in what is already a slow tool. So the best solution is to turn off optimisation altogether. Since this often makes things unmanagably slow, a plausible compromise is to use `-O`. This gets you the majority of the benefits of higher optimisation levels whilst keeping relatively small the chances of false complaints from Memcheck. All other tools (as far as we know) are unaffected by optimisation level.

Valgrind understands both the older "stabs" debugging format, used by gcc versions prior to 3.1, and the newer DWARF2 format used by gcc 3.1 and later. We continue to refine and debug our debug-info readers, although the majority of effort will naturally enough go into the newer DWARF2 reader.

When you're ready to roll, just run your application as you would normally, but place `valgrind --tool=tool_name` in front of your usual command-line invocation. Note that you should run the real (machine-code) executable here. If your application is started by, for example, a shell or perl script, you'll need to modify it to invoke Valgrind on the real executables. Running such scripts directly under Valgrind will result in you getting error reports pertaining to `/bin/sh`, `/usr/bin/perl`, or whatever interpreter you're using. This may not be what you want and can be confusing. You can force the issue by giving the flag `--trace-children=yes`, but confusion is still likely.

## 2.3. The commentary

Valgrind tools write a commentary, a stream of text, detailing error reports and other significant events. All lines in the commentary have following form:

    ==12345== some-message-from-Valgrind

The `12345` is the process ID. This scheme makes it easy to distinguish program output from Valgrind commentary, and also easy to differentiate commentaries from different processes which have become merged together, for whatever reason.

By default, Valgrind tools write only essential messages to the commentary, so as to avoid flooding you with information of secondary importance. If you want more information about what is happening, re-run, passing the `-v` flag to Valgrind. A second `-v` gives yet more detail.

You can direct the commentary to three different places:

1. The default: send it to a file descriptor, which is by default 2 (stderr). So, if you give the core no options, it will write commentary to the standard error stream. If you want to send it to some other file descriptor, for example number 9, you can specify `--log-fd=9`.

   This is the simplest and most common arrangement, but can cause problems when Valgrinding entire trees of processes which expect specific file descriptors, particularly stdin/stdout/stderr, to be available for their own use.

2. A less intrusive option is to write the commentary to a file, which you specify by `--log-file=filename`. Note carefully that the commentary is not written to the file you specify, but instead to one called `filename.12345`, if for example the pid of the traced process is 12345. This is helpful when valgrinding a whole tree of processes at once, since it means that each process writes to its own logfile, rather than the result being jumbled up in one big logfile. If `filename.12345` already exists, then it will name new files `filename.12345.1` and so on.

   If you want to specify precisely the file name to use, without the trailing `.12345` part, you can instead use `--log-file-exactly=filename`.

   You can also use the `--log-file-qualifier=<VAR>` option to modify the filename via according to the environment variable `VAR`. This is rarely needed, but very useful in certain circumstances (eg. when running MPI programs). In this case, the trailing `.12345` part is replaced by the contents of `$VAR`. The idea is that you specify a variable which will be set differently for each process in the job, for example `BPROC_RANK` or whatever is applicable in your MPI setup.

3. The least intrusive option is to send the commentary to a network socket. The socket is specified as an IP address and port number pair, like this: `--log-socket=192.168.0.1:12345` if you want to send the output to host IP 192.168.0.1 port 12345 (I have no idea if 12345 is a port of pre-existing significance). You can also omit the port number: `--log-socket=192.168.0.1`, in which case a default port of 1500 is used. This default is defined by the constant `VG_CLO_DEFAULT_LOGPORT` in the sources.

   Note, unfortunately, that you have to use an IP address here, rather than a hostname.

   Writing to a network socket is pretty useless if you don't have something listening at the other end. We provide a simple listener program, `valgrind-listener`, which accepts connections on the specified port and copies whatever it is sent to stdout. Probably someone will tell us this is a horrible security risk. It seems likely that people will write more sophisticated listeners in the fullness of time.

   valgrind-listener can accept simultaneous connections from up to 50 valgrinded processes. In front of each line of output it prints the current number of active connections in round brackets.

   valgrind-listener accepts two command-line flags:

   - `-e` or `--exit-at-zero`: when the number of connected processes falls back to zero, exit. Without this, it will run forever, that is, until you send it Control-C.

   - `portnumber`: changes the port it listens on from the default (1500). The specified port must be in the range 1024 to 65535. The same restriction applies to port numbers specified by a `--log-socket=` to Valgrind itself.

   If a valgrinded process fails to connect to a listener, for whatever reason (the listener isn't running, invalid or unreachable host or port, etc), Valgrind switches back to writing the commentary to stderr. The same goes for any process which loses an established connection to a listener. In other words, killing the listener doesn't kill the processes sending data to it.

Here is an important point about the relationship between the commentary and profiling output from tools. The commentary contains a mix of messages from the Valgrind core and the selected tool. If the tool reports errors, it will report them to the commentary. However, if the tool does profiling, the profile data will be written to a file of some kind, depending on the tool, and independent of what `--log-*` options are in force. The commentary is intended to be a low-bandwidth, human-readable channel. Profiling data, on the other hand, is usually voluminous and not meaningful without further processing, which is why we have chosen this arrangement.

## 2.4. Reporting of errors

When one of the error-checking tools (Memcheck, Addrcheck, Helgrind) detects something bad happening in the program, an error message is written to the commentary. For example:

    ==25832== Invalid read of size 4
    ==25832==    at 0x8048724: BandMatrix::ReSize(int, int, int) (bogon.cpp:45)
    ==25832==    by 0x80487AF: main (bogon.cpp:66)
    ==25832==  Address 0xBFFFF74C is not stack'd, malloc'd or free'd

This message says that the program did an illegal 4-byte read of address 0xBFFFF74C, which, as far as Memcheck can tell, is not a valid stack address, nor corresponds to any currently malloc'd or free'd blocks. The read is happening at line 45 of `bogon.cpp`, called from line 66 of the same file, etc. For errors associated with an identified malloc'd/free'd block, for example reading free'd memory, Valgrind reports not only the location where the error happened, but also where the associated block was malloc'd/free'd.

Valgrind remembers all error reports. When an error is detected, it is compared against old reports, to see if it is a duplicate. If so, the error is noted, but no further commentary is emitted. This avoids you being swamped with bazillions of duplicate error reports.

If you want to know how many times each error occurred, run with the `-v` option. When execution finishes, all the reports are printed out, along with, and sorted by, their occurrence counts. This makes it easy to see which errors have occurred most frequently.

Errors are reported before the associated operation actually happens. If you're using a tool (Memcheck, Addrcheck) which does address checking, and your program attempts to read from address zero, the tool will emit a message to this effect, and the program will then duly die with a segmentation fault.

In general, you should try and fix errors in the order that they are reported. Not doing so can be confusing. For example, a program which copies uninitialised values to several memory locations, and later uses them, will generate several error messages, when run on Memcheck. The first such error message may well give the most direct clue to the root cause of the problem.

The process of detecting duplicate errors is quite an expensive one and can become a significant performance overhead if your program generates huge quantities of errors. To avoid serious problems, Valgrind will simply stop collecting errors after 1000 different errors have been seen, or 100000 errors in total have been seen. In this situation you might as well stop your program and fix it, because Valgrind won't tell you anything else useful after this. Note that the 1000/100000 limits apply after suppressed errors are removed. These limits are defined in `m_errormgr.c` and can be increased if necessary.

To avoid this cutoff you can use the `--error-limit=no` flag. Then Valgrind will always show errors, regardless of how many there are. Use this flag carefully, since it may have a bad effect on performance.

## 2.5. Suppressing errors

The error-checking tools detect numerous problems in the base libraries, such as the GNU C library, and the XFree86 client libraries, which come pre-installed on your GNU/Linux system. You can't easily fix these, but you don't want to see these errors (and yes, there are many!) So Valgrind reads a list of errors to suppress at startup. A default suppression file is cooked up by the `./configure` script when the system is built.

You can modify and add to the suppressions file at your leisure, or, better, write your own. Multiple suppression files are allowed. This is useful if part of your project contains errors you can't or don't want to fix, yet you don't want to continuously be reminded of them.

Note: By far the easiest way to add suppressions is to use the `--gen-suppressions=yes` flag described in Command-line flags for the Valgrind core.

Each error to be suppressed is described very specifically, to minimise the possibility that a suppression-directive inadvertantly suppresses a bunch of similar errors which you did want to see. The suppression mechanism is designed to allow precise yet flexible specification of errors to suppress.

If you use the `-v` flag, at the end of execution, Valgrind prints out one line for each used suppression, giving its name and the number of times it got used. Here's the suppressions used by a run of `valgrind --tool=memcheck ls -l`:

    --27579-- supp: 1 socketcall.connect(serv_addr)/__libc_connect/__nscd_getgrgid_r
    --27579-- supp: 1 socketcall.connect(serv_addr)/__libc_connect/__nscd_getpwuid_r
    --27579-- supp: 6 strrchr/_dl_map_object_from_fd/_dl_map_object

Multiple suppressions files are allowed. By default, Valgrind uses `$PREFIX/lib/valgrind/default.supp`. You can ask to add suppressions from another file, by specifying `--suppressions=/path/to/file.supp`.

If you want to understand more about suppressions, look at an existing suppressions file whilst reading the following documentation. The file `glibc-2.2.supp`, in the source distribution, provides some good examples.

Each suppression has the following components:

- First line: its name. This merely gives a handy name to the suppression, by which it is referred to in the summary of used suppressions printed out when a program finishes. It's not important what the name is; any identifying string will do.

- Second line: name of the tool(s) that the suppression is for (if more than one, comma-separated), and the name of the suppression itself, separated by a colon (Nb: no spaces are allowed), eg:

      tool_name1,tool_name2:suppression_name

  Recall that Valgrind-2.0.X is a modular system, in which different instrumentation tools can observe your program whilst it is running. Since different tools detect different kinds of errors, it is necessary to say which tool(s) the suppression is meaningful to.

  Tools will complain, at startup, if a tool does not understand any suppression directed to it. Tools ignore suppressions which are not directed to them. As a result, it is quite practical to put suppressions for all tools into the same suppression file.

  Valgrind's core can detect certain PThreads API errors, for which this line reads:

      core:PThread

- Next line: a small number of suppression types have extra information after the second line (eg. the `Param` suppression for Memcheck)

- Remaining lines: This is the calling context for the error -- the chain of function calls that led to it. There can be up to four of these lines.

  Locations may be either names of shared objects/executables or wildcards matching function names. They begin `obj:` and `fun:` respectively. Function and object names to match against may use the wildcard characters `*` and `?`.

  Important note: C++ function names must be mangled. If you are writing suppressions by hand, use the `--demangle=no` option to get the mangled names in your error messages.

- Finally, the entire suppression must be between curly braces. Each brace must be the first character on its own line.

A suppression only suppresses an error when the error matches all the details in the suppression. Here's an example:

    {
      __gconv_transform_ascii_internal/__mbrtowc/mbtowc
      Memcheck:Value4
      fun:__gconv_transform_ascii_internal
      fun:__mbr*toc
      fun:mbtowc
    }

What it means is: for Memcheck only, suppress a use-of-uninitialised-value error, when the data size is 4, when it occurs in the function `__gconv_transform_ascii_internal`, when that is called from any function of name matching `__mbr*toc`, when that is called from `mbtowc`. It doesn't apply under any other circumstances. The string by which this suppression is identified to the user is `__gconv_transform_ascii_internal/__mbrtowc/mbtowc`.

(See Writing suppression files for more details on the specifics of Memcheck's suppression kinds.)

Another example, again for the Memcheck tool:

    {
      libX11.so.6.2/libX11.so.6.2/libXaw.so.7.0
      Memcheck:Value4
      obj:/usr/X11R6/lib/libX11.so.6.2
      obj:/usr/X11R6/lib/libX11.so.6.2
      obj:/usr/X11R6/lib/libXaw.so.7.0
    }

Suppress any size 4 uninitialised-value error which occurs anywhere in `libX11.so.6.2`, when called from anywhere in the same library, when called from anywhere in `libXaw.so.7.0`. The inexact specification of locations is regrettable, but is about all you can hope for, given that the X11 libraries shipped with Red Hat 7.2 have had their symbol tables removed.

Note: since the above two examples did not make it clear, you can freely mix the `obj:` and `fun:` styles of description within a single suppression record.

## 2.6. Command-line flags for the Valgrind core

As mentioned above, Valgrind's core accepts a common set of flags. The tools also accept tool-specific flags, which are documented seperately for each tool.

You invoke Valgrind like this:

    valgrind [valgrind-options] your-prog [your-prog options]

Valgrind's default settings succeed in giving reasonable behaviour in most cases. We group the available options by rough categories.

### 2.6.1. Tool-selection option

The single most important option.

- `--tool=<name>` [default=memcheck]

  Run the Valgrind tool called name, e.g. Memcheck, Addrcheck, Cachegrind, etc.

### 2.6.2. Basic Options

These options work with all tools.

- `--help`

  Show help for all options, both for the core and for the selected tool.

- `--help-debug`

  Same as `--help`, but also lists debugging options which usually are only of use to Valgrind's developers.

- `--version`

  Show the version number of the Valgrind core. Tools can have their own version numbers. There is a scheme in place to ensure that tools only execute when the core version is one they are known to work with. This was done to minimise the chances of strange problems arising from tool-vs-core version incompatibilities.

- `-q --quiet`

  Run silently, and only print error messages. Useful if you are running regression tests or have some other automated test machinery.

- `-v --verbose`

  Be more verbose. Gives extra information on various aspects of your program, such as: the shared objects loaded, the suppressions used, the progress of the instrumentation and execution engines, and warnings about unusual behaviour. Repeating the flag increases the verbosity level.

- `-d`

  Emit information for debugging Valgrind itself. This is usually only of interest to the Valgrind developers. Repeating the flag produces more detailed output. If you want to send us a bug report, a log of the output generated by `-v -v -d -d` will make your report more useful.

- `--trace-children=no` [default]

  `--trace-children=yes`

  When enabled, Valgrind will trace into child processes. This is confusing and usually not what you want, so is disabled by default.

- `--child-silent-after-fork=yes` [default]

  `--child-silent-after-fork=yes`

  When enabled, Valgrind will omit child output between fork and exec. This is normally disabled in Valgrind, but it must be enabled for clean XML, which Valkyrie requires.

- `--track-fds=no` [default]

  `--track-fds=yes`

  When enabled, Valgrind will print out a list of open file descriptors on exit. Along with each file descriptor, Valgrind prints out a stack backtrace of where the file was opened and any details relating to the file descriptor such as the file name or socket details.

- `--time-stamp=no` [default]

  `--time-stamp=yes`

  When enabled, Valgrind will precede each message with an indication of the elapsed wallclock time since startup, expressed as days, hours, minutes, seconds and milliseconds.

- `--log-fd=<number>` [default: 2, stderr]

  Specifies that Valgrind should send all of its messages to the specified file descriptor. The default, 2, is the standard error channel (stderr). Note that this may interfere with the client's own use of stderr.

- `--log-file=<filename>`

  Specifies that Valgrind should send all of its messages to the specified file. The specified file name may not be an empty string. If you trace multiple processes with Valgrind when using this option the log file may get all messed up.

- `--log-socket=<ip-address:port-number>`

  Specifies that Valgrind should send all of its messages to the specified port at the specified IP address. The port may be omitted, in which case port 1500 is used. If a connection cannot be made to the specified socket, Valgrind falls back to writing output to the standard error (stderr). This option is intended to be used in conjunction with the `valgrind-listener` program. For further details, see The commentary.

### 2.6.3. Error-related options

These options are used by all tools that can report errors, e.g. Memcheck, but not Cachegrind.

- `--xml=no` [default]

  `--xml=yes`

  When enabled, output will be in XML format. This is aimed at making life easier for tools that consume Valgrind's output as input, such as GUI front ends. Currently this option only works with Memcheck and Nulgrind.

- `--xml-user-comment=<string>` [default=""]

  Embeds an extra user comment string in the XML output. Only works with `--xml=yes` is specified; ignored otherwise.

- `--demangle=no`

  `--demangle=yes` [default]

  Disable/enable automatic demangling (decoding) of C++ names. Enabled by default. When enabled, Valgrind will attempt to translate encoded C++ names back to something approaching the original. The demangler handles symbols mangled by g++ versions 2.X and 3.X.

  An important fact about demangling is that function names mentioned in suppressions files should be in their mangled form. Valgrind does not demangle function names when searching for applicable suppressions, because to do otherwise would make suppressions file contents dependent on the state of Valgrind's demangling machinery, and would also be slow and pointless.

- `--num-callers=<number>` [default=12]

  By default, Valgrind shows twelve levels of function call names to help you identify program locations. You can change that number with this option. This can help in determining the program's location in deeply-nested call chains. Note that errors are commoned up using only the top three function locations (the place in the current function, and that of its two immediate callers). So this doesn't affect the total number of errors reported.

  The maximum value for this is 50. Note that higher settings will make Valgrind run a bit more slowly and take a bit more memory, but can be useful when working with programs with deeply-nested call chains.

- `--error-limit=yes` [default]

  `--error-limit=no`

  When enabled, Valgrind stops reporting errors after 100000 in total, or 1000 different ones, have been seen. This is to stop the error tracking machinery from becoming a huge performance overhead in programs with many errors.

- `--show-below-main=yes`

  `--show-below-main=no` [default]

  By default, stack traces for errors do not show any functions that appear beneath `main()` (or similar functions such as glibc's `__libc_start_main()`, if `main()` is not present in the stack trace); most of the time it's uninteresting C library stuff. If this option is enabled, these entries below `main()` will be shown.

- `--suppressions=<filename>` [default: $PREFIX/lib/valgrind/default.supp]

  Specifies an extra file from which to read descriptions of errors to suppress. You may use as many extra suppressions files as you like.

- `--gen-suppressions=no` [default]

  `--gen-suppressions=yes`

  `--gen-suppressions=all`

  When set to `yes`, Valgrind will pause after every error shown, and print the line: `---- Print suppression ? --- [Return/N/n/Y/y/C/c] ----`

  The prompt's behaviour is the same as for the `--db-attach` option.

  If you choose to, Valgrind will print out a suppression for this error. You can then cut and paste it into a suppression file if you don't want to hear about the error in the future.

  When set to `all`, Valgrind will print a suppression for every reported error, without querying the user.

  This option is particularly useful with C++ programs, as it prints out the suppressions with mangled names, as required.

  Note that the suppressions printed are as specific as possible. You may want to common up similar ones, eg. by adding wildcards to function names. Also, sometimes two different errors are suppressed by the same suppression, in which case Valgrind will output the suppression more than once, but you only need to have one copy in your suppression file (but having more than one won't cause problems). Also, the suppression name is given as `<insert a suppression name here>`; the name doesn't really matter, it's only used with the `-v` option which prints out all used suppression records.

- `--db-attach=no` [default]

  `--db-attach=yes`

  When enabled, Valgrind will pause after every error shown, and print the line: `---- Attach to debugger ? --- [Return/N/n/Y/y/C/c] ----`

  Pressing `Ret`, or `N Ret` or `n Ret`, causes Valgrind not to start a debugger for this error.

  `Y Ret` or `y Ret` causes Valgrind to start a debugger, for the program at this point. When you have finished with the debugger, quit from it, and the program will continue. Trying to continue from inside the debugger doesn't work.

  `C Ret` or `c Ret` causes Valgrind not to start a debugger, and not to ask again.

  Note: `--db-attach=yes` conflicts with `--trace-children=yes`. You can't use them together. Valgrind refuses to start up in this situation.

  1 May 2002: this is a historical relic which could be easily fixed if it gets in your way. Mail us and complain if this is a problem for you.

  Nov 2002: if you're sending output to a logfile or to a network socket, I guess this option doesn't make any sense. Caveat emptor.

- `--db-command=<command>` [default: gdb -nw %f %p]

  This specifies how Valgrind will invoke the debugger. By default it will use whatever GDB is detected at build time, which is usually `/usr/bin/gdb`. Using this command, you can specify some alternative command to invoke the debugger you want to use.

  The command string given can include one or instances of the `%p` and `%f` expansions. Each instance of `%p` expands to the PID of the process to be debugged and each instance of `%f` expands to the path to the executable for the process to be debugged.

- `--input-fd=<number>` [default=0, stdin]

  When using `--db-attach=yes` and `--gen-suppressions=yes`, Valgrind will stop so as to read keyboard input from you, when each error occurs. By default it reads from the standard input (stdin), which is problematic for programs which close stdin. This option allows you to specify an alternative file descriptor from which to read input.

- `--max-stackframe=<number>` [default=2000000]

  You may need to use this option if your program has large stack-allocated arrays. Valgrind keeps track of your program's stack pointer. If it changes by more than the threshold amount, Valgrind assumes your program is switching to a different stack, and Memcheck behaves differently than it would for a stack pointer change smaller than the threshold. Usually this heuristic works well. However, if your program allocates large structures on the stack, this heuristic will be fooled, and Memcheck will subsequently report large numbers of invalid stack accesses. This option allows you to change the threshold to a different value.

  You should only consider use of this flag if Valgrind's debug output directs you to do so. In that case it will tell you the new threshold you should specify.

  In general, allocating large structures on the stack is a bad idea, because (1) you can easily run out of stack space, especially on systems with limited memory or which expect to support large numbers of threads each with a small stack, and (2) because the error checking performed by Memcheck is more effective for heap-allocated data than for stack-allocated data. If you have to use this flag, you may wish to consider rewriting your code to allocate on the heap rather than on the stack.

### 2.6.4. `malloc()`-related Options

For tools that use their own version of `malloc()` (e.g. Memcheck and Addrcheck), the following options apply.

- `--alignment=<number>` [default: 8]

  By default Valgrind's `malloc`, `realloc`, etc, return 8-byte aligned addresses. This is standard for most processors. Some programs might however assume that `malloc` et al return 16- or more aligned memory. The supplied value must be between 4 and 4096 inclusive, and must be a power of two.

### 2.6.5. Uncommon Options

These options apply to all tools, as they affect certain obscure workings of the Valgrind core. Most people won't need to use these.

- `--run-libc-freeres=yes` [default]

  `--run-libc-freeres=no`

  The GNU C library (`libc.so`), which is used by all programs, may allocate memory for its own uses. Usually it doesn't bother to free that memory when the program ends - there would be no point, since the Linux kernel reclaims all process resources when a process exits anyway, so it would just slow things down.

  The glibc authors realised that this behaviour causes leak checkers, such as Valgrind, to falsely report leaks in glibc, when a leak check is done at exit. In order to avoid this, they provided a routine called `__libc_freeres` specifically to make glibc release all memory it has allocated. Memcheck and Addrcheck therefore try and run `__libc_freeres` at exit.

  Unfortunately, in some versions of glibc, `__libc_freeres` is sufficiently buggy to cause segmentation faults. This is particularly noticeable on Red Hat 7.1. So this flag is provided in order to inhibit the run of `__libc_freeres`. If your program seems to run fine on Valgrind, but segfaults at exit, you may find that `--run-libc-freeres=no` fixes that, although at the cost of possibly falsely reporting space leaks in `libc.so`.

- `--sim-hints=hint1,hint2,...`

  Pass miscellaneous hints to Valgrind which slightly modify the simulated behaviour in nonstandard or dangerous ways, possibly to help the simulation of strange features. By default no hints are enabled. Use with caution! Currently known hints are:

  - `lax-ioctls`

    Be very lax about ioctl handling; the only assumption is that the size is correct. Doesn't require the full buffer to be initialized when writing. Without this, using some device drivers with a large number of strange ioctl commands becomes very tiresome.

  - `enable-inner`

    Enable some special magic needed when the program being run is itself Valgrind.

- `--kernel-variant=variant1,variant2,...`

  Handle system calls and ioctls arising from minor variants of the default kernel for this platform. This is useful for running on hacked kernels or with kernel modules which support nonstandard ioctls, for example. Use with caution. If you don't understand what this option does then you almost certainly don't need it. Currently known variants are:

  - `bproc`

    Support the sys_broc system call on x86. This is for running on BProc, which is a minor variant of standard Linux which is sometimes used for building clusters.

- `--show-emwarns=no` [default]

  `--show-emwarns=yes`

  When enabled, Valgrind will emit warnings about its CPU emulation in certain cases. These are usually not interesting.

- `--smc-check=none`

  `--smc-check=stack` [default]

  `--smc-check=all`

  This option controls Valgrind's detection of self-modifying code. Valgrind can do no detection, detect self-modifying code on the stack, or detect self-modifying code anywhere. Note that the default option will catch the vast majority of cases, as far as we know. Running with `all` will slow Valgrind down greatly (but running with `none` will rarely speed things up, since very little code gets put on the stack for most programs).

### 2.6.6. Debugging Valgrind Options

There are also some options for debugging Valgrind itself. You shouldn't need to use them in the normal run of things. If you wish to see the list, use the `--help-debug` option.

### 2.6.7. Setting default Options

Note that Valgrind also reads options from three places:

1. The file `~/.valgrindrc`

2. The environment variable `$VALGRIND_OPTS`

3. The file `./.valgrindrc`

These are processed in the given order, before the command-line options. Options processed later override those processed earlier; for example, options in `./.valgrindrc` will take precedence over those in `~/.valgrindrc`. The first two are particularly useful for setting the default tool to use.

Any tool-specific options put in `$VALGRIND_OPTS` or the `.valgrindrc` files should be prefixed with the tool name and a colon. For example, if you want Memcheck to always do leak checking, you can put the following entry in `~/.valgrindrc`:

    --memcheck:leak-check=yes

This will be ignored if any tool other than Memcheck is run. Without the `memcheck:` part, this will cause problems if you select other tools that don't understand `--leak-check=yes`.

## 2.7. The Client Request mechanism

Valgrind has a trapdoor mechanism via which the client program can pass all manner of requests and queries to Valgrind and the current tool. Internally, this is used extensively to make malloc, free, etc, work, although you don't see that.

For your convenience, a subset of these so-called client requests is provided to allow you to tell Valgrind facts about the behaviour of your program, and also to make queries. In particular, your program can tell Valgrind about changes in memory range permissions that Valgrind would not otherwise know about, and so allows clients to get Valgrind to do arbitrary custom checks.

Clients need to include a header file to make this work. Which header file depends on which client requests you use. Some client requests are handled by the core, and are defined in the header file `valgrind/valgrind.h`. Tool-specific header files are named after the tool, e.g. `valgrind/memcheck.h`. All header files can be found in the `include/valgrind` directory of wherever Valgrind was installed.

The macros in these header files have the magical property that they generate code in-line which Valgrind can spot. However, the code does nothing when not run on Valgrind, so you are not forced to run your program under Valgrind just because you use the macros in this file. Also, you are not required to link your program with any extra supporting libraries.

The code left in your binary has negligible performance impact: on x86, amd64 and ppc32, the overhead is 6 simple integer instructions and is probably undetectable except in tight loops. However, if you really wish to compile out the client requests, you can compile with `-DNVALGRIND` (analogous to `-DNDEBUG`'s effect on `assert()`).

You are encouraged to copy the `valgrind/*.h` headers into your project's include directory, so your program doesn't have a compile-time dependency on Valgrind being installed. The Valgrind headers, unlike the rest of the code, are under a BSD-style license so you may include them without worrying about license incompatibility.

Here is a brief description of the macros available in `valgrind.h`, which work with more than one tool (see the tool-specific documentation for explanations of the tool-specific macros).

`RUNNING_ON_VALGRIND`:

  returns 1 if running on Valgrind, 0 if running on the real CPU. If you are running Valgrind on itself, it will return the number of layers of Valgrind emulation we're running on.

`VALGRIND_DISCARD_TRANSLATIONS`:

  discard translations of code in the specified address range. Useful if you are debugging a JITter or some other dynamic code generation system. After this call, attempts to execute code in the invalidated address range will cause Valgrind to make new translations of that code, which is probably the semantics you want. Note that code invalidations are expensive because finding all the relevant translations quickly is very difficult. So try not to call it often. Note that you can be clever about this: you only need to call it when an area which previously contained code is overwritten with new code. You can choose to write code into fresh memory, and just call this occasionally to discard large chunks of old code all at once.

  Alternatively, for transparent self-modifying-code support, use`--smc-check=all`.

`VALGRIND_COUNT_ERRORS`:

  returns the number of errors found so far by Valgrind. Can be useful in test harness code when combined with the `--log-fd=-1` option; this runs Valgrind silently, but the client program can detect when errors occur. Only useful for tools that report errors, e.g. it's useful for Memcheck, but for Cachegrind it will always return zero because Cachegrind doesn't report errors.

`VALGRIND_MALLOCLIKE_BLOCK`:

  If your program manages its own memory instead of using the standard `malloc()` / `new` / `new[]`, tools that track information about heap blocks will not do nearly as good a job. For example, Memcheck won't detect nearly as many errors, and the error messages won't be as informative. To improve this situation, use this macro just after your custom allocator allocates some new memory. See the comments in `valgrind.h` for information on how to use it.

`VALGRIND_FREELIKE_BLOCK`:

  This should be used in conjunction with `VALGRIND_MALLOCLIKE_BLOCK`. Again, see `memcheck/memcheck.h` for information on how to use it.

`VALGRIND_CREATE_MEMPOOL`:

  This is similar to `VALGRIND_MALLOCLIKE_BLOCK`, but is tailored towards code that uses memory pools. See the comments in `valgrind.h` for information on how to use it.

`VALGRIND_DESTROY_MEMPOOL`:

  This should be used in conjunction with `VALGRIND_CREATE_MEMPOOL` Again, see the comments in `valgrind.h` for information on how to use it.

`VALGRIND_MEMPOOL_ALLOC`:

  This should be used in conjunction with `VALGRIND_CREATE_MEMPOOL` Again, see the comments in `valgrind.h` for information on how to use it.

`VALGRIND_MEMPOOL_FREE`:

  This should be used in conjunction with `VALGRIND_CREATE_MEMPOOL` Again, see the comments in `valgrind.h` for information on how to use it.

`VALGRIND_NON_SIMD_CALL[0123]`:

  executes a function of 0, 1, 2 or 3 args in the client program on the real CPU, not the virtual CPU that Valgrind normally runs code on. These are used in various ways internally to Valgrind. They might be useful to client programs.

  Warning: Only use these if you really know what you are doing.

`VALGRIND_PRINTF(format, ...)`:

  printf a message to the log file when running under Valgrind. Nothing is output if not running under Valgrind. Returns the number of characters output.

`VALGRIND_PRINTF_BACKTRACE(format, ...)`:

  printf a message to the log file along with a stack backtrace when running under Valgrind. Nothing is output if not running under Valgrind. Returns the number of characters output.

`VALGRIND_STACK_REGISTER(start, end)`:

  Register a new stack. Informs Valgrind that the memory range between start and end is a unique stack. Returns a stack identifier that can be used with other `VALGRIND_STACK_*` calls.

  Valgrind will use this information to determine if a change to the stack pointer is an item pushed onto the stack or a change over to a new stack. Use this if you're using a user-level thread package and are noticing spurious errors from Valgrind about uninitialized memory reads.

`VALGRIND_STACK_DEREGISTER(id)`:

  Deregister a previously registered stack. Informs Valgrind that previously registered memory range with stack id `id` is no longer a stack.

`VALGRIND_STACK_CHANGE(id, start, end)`:

  Change a previously registered stack. Informs Valgrind that the previously registerer stack with stack id `id` has changed it's start and end values. Use this if your user-level thread package implements stack growth.

Note that `valgrind.h` is included by all the tool-specific header files (such as `memcheck.h`), so you don't need to include it in your client if you include a tool-specific header.

## 2.8. Support for Threads

Valgrind supports programs which use POSIX pthreads. Getting this to work was technically challenging but it all works well enough for significant threaded applications to work.

The main thing to point out is that although Valgrind works with the built-in threads system (eg. NPTL or LinuxThreads), it serialises execution so that only one thread is running at a time. This approach avoids the horrible implementation problems of implementing a truly multiprocessor version of Valgrind, but it does mean that threaded apps run only on one CPU, even if you have a multiprocessor machine.

Valgrind schedules your program's threads in a round-robin fashion, with all threads having equal priority. It switches threads every 50000 basic blocks (on x86, typically around 300000 instructions), which means you'll get a much finer interleaving of thread executions than when run natively. This in itself may cause your program to behave differently if you have some kind of concurrency, critical race, locking, or similar, bugs.

Your program will use the native `libpthread`, but not all of its facilities will work. In particular, synchonisation of processes via shared-memory segments will not work. This relies on special atomic instruction sequences which Valgrind does not emulate in a way which works between processes. Unfortunately there's no way for Valgrind to warn when this is happening, and such calls will mostly work; it's only when there's a race that it will fail.

Valgrind also supports direct use of the `clone()` system call, `futex()` and so on. `clone()` is supported where either everything is shared (a thread) or nothing is shared (fork-like); partial sharing will fail. Again, any use of atomic instruction sequences in shared memory between processes will not work reliably.

## 2.9. Handling of Signals

Valgrind has a fairly complete signal implementation. It should be able to cope with any valid use of signals.

If you're using signals in clever ways (for example, catching SIGSEGV, modifying page state and restarting the instruction), you're probably relying on precise exceptions. In this case, you will need to use `--vex-iropt-precise-memory-exns=yes`.

If your program dies as a result of a fatal core-dumping signal, Valgrind will generate its own core file (`vgcore.NNNNN`) containing your program's state. You may use this core file for post-mortem debugging with gdb or similar. (Note: it will not generate a core if your core dump size limit is 0.) At the time of writing the core dumps do not include all the floating point register information.

If Valgrind itself crashes (hopefully not) the operating system will create a core dump in the usual way.

## 2.10. Building and Installing

We use the standard Unix `./configure`, `make`, `make install` mechanism, and we have attempted to ensure that it works on machines with kernel 2.4 or 2.6 and glibc 2.2.X or 2.3.X. You may then want to run the regression tests with `make regtest`.

There are three options (in addition to the usual `--prefix=` which affect how Valgrind is built:

- `--enable-inner`

  This builds Valgrind with some special magic hacks which make it possible to run it on a standard build of Valgrind (what the developers call "self-hosting"). Ordinarily you should not use this flag as various kinds of safety checks are disabled.

- `--enable-tls`

  TLS (Thread Local Storage) is a relatively new mechanism which requires compiler, linker and kernel support. Valgrind automatically test if TLS is supported and enable this option. Sometimes it cannot test for TLS, so this option allows you to override the automatic test.

- `--with-vex=`

  Specifies the path to the underlying VEX dynamic-translation library. By default this is taken to be in the VEX directory off the root of the source tree.

The `configure` script tests the version of the X server currently indicated by the current `$DISPLAY`. This is a known bug. The intention was to detect the version of the current XFree86 client libraries, so that correct suppressions could be selected for them, but instead the test checks the server version. This is just plain wrong.

If you are building a binary package of Valgrind for distribution, please read `README_PACKAGERS` Readme Packagers. It contains some important information.

Apart from that, there's not much excitement here. Let us know if you have build problems.

## 2.11. If You Have Problems

Contact us at http://www.valgrind.org/.

See Limitations for the known limitations of Valgrind, and for a list of programs which are known not to work on it.

All parts of the system make heavy use of assertions and internal self-checks. They are permanently enabled, and we have no plans to disable them. If one of them breaks, please mail us!

If you get an assertion failure on the expression `blockSane(ch)` in `VG_(free)()` in `m_mallocfree.c`, this may have happened because your program wrote off the end of a malloc'd block, or before its beginning. Valgrind hopefully will have emitted a proper message to that effect before dying in this way. This is a known problem which we should fix.

Read the Valgrind FAQ for more advice about common problems, crashes, etc.

## 2.12. Limitations

The following list of limitations seems long. However, most programs actually work fine.

Valgrind will run Linux ELF binaries, on a kernel 2.4.X or 2.6.X system, on the x86, amd64 and ppc32 architectures, subject to the following constraints:

- On x86 and amd64, there is no support for 3DNow! instructions. If the translator encounters these, Valgrind will generate a SIGILL when the instruction is executed. Apart from that, on x86 and amd64, essentially all instructions are supported, up to and including SSE2. Version 3.1.0 includes limited support for SSE3 on x86. This could be improved if necessary.

  On ppc32, almost all integer, floating point and Altivec instructions are supported.

- Atomic instruction sequences are not properly supported, in the sense that their atomicity is not preserved. This will affect any use of synchronization via memory shared between processes. They will appear to work, but fail sporadically.

- If your program does its own memory management, rather than using malloc/new/free/delete, it should still work, but Valgrind's error checking won't be so effective. If you describe your program's memory management scheme using "client requests" (see The Client Request mechanism), Memcheck can do better. Nevertheless, using malloc/new and free/delete is still the best approach.

- Valgrind's signal simulation is not as robust as it could be. Basic POSIX-compliant sigaction and sigprocmask functionality is supplied, but it's conceivable that things could go badly awry if you do weird things with signals. Workaround: don't. Programs that do non-POSIX signal tricks are in any case inherently unportable, so should be avoided if possible.

- Machine instructions, and system calls, have been implemented on demand. So it's possible, although unlikely, that a program will fall over with a message to that effect. If this happens, please report ALL the details printed out, so we can try and implement the missing feature.

- Memory consumption of your program is majorly increased whilst running under Valgrind. This is due to the large amount of administrative information maintained behind the scenes. Another cause is that Valgrind dynamically translates the original executable. Translated, instrumented code is 12-18 times larger than the original so you can easily end up with 50+ MB of translations when running (eg) a web browser.

- Valgrind can handle dynamically-generated code just fine. If you regenerate code over the top of old code (ie. at the same memory addresses), if the code is on the stack Valgrind will realise the code has changed, and work correctly. This is necessary to handle the trampolines GCC uses to implemented nested functions. If you regenerate code somewhere other than the stack, you will need to use the `--smc-check=all` flag, and Valgrind will run more slowly than normal.

- As of version 3.0.0, Valgrind has the following limitations in its implementation of x86/AMD64 floating point relative to the IEEE754 standard.

  Precision: There is no support for 80 bit arithmetic. Internally, Valgrind represents all such "long double" numbers in 64 bits, and so there may be some differences in results. Whether or not this is critical remains to be seen. Note, the x86/amd64 fldt/fstpt instructions (read/write 80-bit numbers) are correctly simulated, using conversions to/from 64 bits, so that in-memory images of 80-bit numbers look correct if anyone wants to see.

  The impression observed from many FP regression tests is that the accuracy differences aren't significant. Generally speaking, if a program relies on 80-bit precision, there may be difficulties porting it to non x86/amd64 platforms which only support 64-bit FP precision. Even on x86/amd64, the program may get different results depending on whether it is compiled to use SSE2 instructions (64-bits only), or x87 instructions (80-bit). The net effect is to make FP programs behave as if they had been run on a machine with 64-bit IEEE floats, for example PowerPC. On amd64 FP arithmetic is done by default on SSE2, so amd64 looks more like PowerPC than x86 from an FP perspective, and there are far fewer noticable accuracy differences than with x86.

  Rounding: Valgrind does observe the 4 IEEE-mandated rounding modes (to nearest, to +infinity, to -infinity, to zero) for the following conversions: float to integer, integer to float where there is a possibility of loss of precision, and float-to-float rounding. For all other FP operations, only the IEEE default mode (round to nearest) is supported.

  Numeric exceptions in FP code: IEEE754 defines five types of numeric exception that can happen: invalid operation (sqrt of negative number, etc), division by zero, overflow, underflow, inexact (loss of precision).

  For each exception, two courses of action are defined by 754: either (1) a user-defined exception handler may be called, or (2) a default action is defined, which "fixes things up" and allows the computation to proceed without throwing an exception.

  Currently Valgrind only supports the default fixup actions. Again, feedback on the importance of exception support would be appreciated.

  When Valgrind detects that the program is trying to exceed any of these limitations (setting exception handlers, rounding mode, or precision control), it can print a message giving a traceback of where this has happened, and continue execution. This behaviour used to be the default, but the messages are annoying and so showing them is now optional. Use `--show-emwarns=yes` to see them.

  The above limitations define precisely the IEEE754 'default' behaviour: default fixup on all exceptions, round-to-nearest operations, and 64-bit precision.

- As of version 3.0.0, Valgrind has the following limitations in its implementation of x86/AMD64 SSE2 FP arithmetic.

  Essentially the same: no exceptions, and limited observance of rounding mode. Also, SSE2 has control bits which make it treat denormalised numbers as zero (DAZ) and a related action, flush denormals to zero (FTZ). Both of these cause SSE2 arithmetic to be less accurate than IEEE requires. Valgrind detects, ignores, and can warn about, attempts to enable either mode.

- As of version 3.1.0, Valgrind has the following limitations in its implementation of PPC32 FP arithmetic, both scalar and Altivec.

  Scalar: essentially as with x86/AMD64: no exceptions, and limited observance of rounding mode. For Altivec, FP arithmetic is done in IEEE/Java mode, which is more accurate than the Linux default setting. "More accurate" means that denormals are handled properly, rather than simply being flushed to zero.

Programs which are known not to work are:

- emacs starts up but immediately concludes it is out of memory and aborts. It may be that Memcheck does not provide a good enough emulation of the `mallinfo` function. Emacs works fine if you build it to use the standard malloc/free routines.

## 2.13. An Example Run

This is the log for a run of a small program using Memcheck The program is in fact correct, and the reported error is as the result of a potentially serious code generation bug in GNU g++ (snapshot 20010527).

    sewardj@phoenix:~/newmat10$
    ~/Valgrind-6/valgrind -v ./bogon
    ==25832== Valgrind 0.10, a memory error detector for x86 RedHat 7.1.
    ==25832== Copyright (C) 2000-2001, and GNU GPL'd, by Julian Seward.
    ==25832== Startup, with flags:
    ==25832== --suppressions=/home/sewardj/Valgrind/redhat71.supp
    ==25832== reading syms from /lib/ld-linux.so.2
    ==25832== reading syms from /lib/libc.so.6
    ==25832== reading syms from /mnt/pima/jrs/Inst/lib/libgcc_s.so.0
    ==25832== reading syms from /lib/libm.so.6
    ==25832== reading syms from /mnt/pima/jrs/Inst/lib/libstdc++.so.3
    ==25832== reading syms from /home/sewardj/Valgrind/valgrind.so
    ==25832== reading syms from /proc/self/exe
    ==25832==
    ==25832== Invalid read of size 4
    ==25832==    at 0x8048724: _ZN10BandMatrix6ReSizeEiii (bogon.cpp:45)
    ==25832==    by 0x80487AF: main (bogon.cpp:66)
    ==25832==  Address 0xBFFFF74C is not stack'd, malloc'd or free'd
    ==25832==
    ==25832== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
    ==25832== malloc/free: in use at exit: 0 bytes in 0 blocks.
    ==25832== malloc/free: 0 allocs, 0 frees, 0 bytes allocated.
    ==25832== For a detailed leak analysis, rerun with: --leak-check=yes
    ==25832==
    ==25832== exiting, did 1881 basic blocks, 0 misses.
    ==25832== 223 translations, 3626 bytes in, 56801 bytes out.

The GCC folks fixed this about a week before gcc-3.0 shipped.

## 2.14. Warning Messages You Might See

Most of these only appear if you run in verbose mode (enabled by `-v`):

- `More than 100 errors detected. Subsequent errors will still be recorded, but in less detail than before.`

  After 100 different errors have been shown, Valgrind becomes more conservative about collecting them. It then requires only the program counters in the top two stack frames to match when deciding whether or not two errors are really the same one. Prior to this point, the PCs in the top four frames are required to match. This hack has the effect of slowing down the appearance of new errors after the first 100. The 100 constant can be changed by recompiling Valgrind.

- `More than 1000 errors detected. I'm not reporting any more. Final error counts may be inaccurate. Go fix your program!`

  After 1000 different errors have been detected, Valgrind ignores any more. It seems unlikely that collecting even more different ones would be of practical help to anybody, and it avoids the danger that Valgrind spends more and more of its time comparing new errors against an ever-growing collection. As above, the 1000 number is a compile-time constant.

- `Warning: client switching stacks?`

  Valgrind spotted such a large change in the stack pointer, `%esp`, that it guesses the client is switching to a different stack. At this point it makes a kludgey guess where the base of the new stack is, and sets memory permissions accordingly. You may get many bogus error messages following this, if Valgrind guesses wrong. At the moment "large change" is defined as a change of more that 2000000 in the value of the `%esp` (stack pointer) register.

- `Warning: client attempted to close Valgrind's logfile fd <number>`

  Valgrind doesn't allow the client to close the logfile, because you'd never see any diagnostic information after that point. If you see this message, you may want to use the `--log-fd=<number>` option to specify a different logfile file-descriptor number.

- `Warning: noted but unhandled ioctl <number>`

  Valgrind observed a call to one of the vast family of `ioctl` system calls, but did not modify its memory status info (because I have not yet got round to it). The call will still have gone through, but you may get spurious errors after this as a result of the non-update of the memory info.

- `Warning: set address range perms: large range <number>`

  Diagnostic message, mostly for benefit of the valgrind developers, to do with memory permissions.