Note that Valgrind does trace into the child of a fork (it would be difficult not to, since fork makes an identical copy of a process), so this option is arguably badly named. However, most children of fork calls immediately call exec anyway.

This can be useful for pruning uninteresting branches from a tree of processes being run on Valgrind. But you should be careful when using it. When Valgrind skips tracing into an executable, it doesn't just skip tracing that executable, it also skips tracing any of that executable's child processes. In other words, the flag doesn't merely cause tracing to stop at the specified executables -- it skips tracing of entire process subtrees rooted at any of the specified executables.

If the embedded gdbserver is enabled but no gdb is currently being used, the ??? command line utility can send "monitor commands" to Valgrind from a shell. The Valgrind core provides a set of ???. A tool can optionally provide tool specific monitor commands, which are documented in the tool specific chapter.

%p is replaced with the current process ID. This is very useful for program that invoke multiple processes. WARNING: If you use --trace-children=yes and your program invokes multiple processes OR your program forks without calling exec afterwards, and you don't use this specifier (or the %q specifier below), the Valgrind output from all those processes will go into one file, possibly jumbled up, and possibly incomplete.

%q{FOO} is replaced with the contents of the environment variable FOO. If the {FOO} part is malformed, it causes an abort. This specifier is rarely needed, but very useful in certain circumstances (eg. when running MPI programs). 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. If the named environment variable is not set, it causes an abort. Note that in some shells, the { and } characters may need to be escaped with a backslash.

%% is replaced with %.

If an % is followed by any other character, it causes an abort.

Less important messages will still be printed in plain text, but because the XML output and plain text output are sent to different output channels (the destination of the plain text output is still controlled by --log-fd, --log-file and --log-socket) this should not cause problems.

This option is aimed at making life easier for tools that consume Valgrind's output as input, such as GUI front ends. Currently this option works with Memcheck, Helgrind, DRD and SGcheck. The output format is specified in the file docs/internals/xml-output-protocol4.txt in the source tree for Valgrind 3.5.0 or later.

The recommended options for a GUI to pass, when requesting XML output, are: --xml=yes to enable XML output, --xml-file to send the XML output to a (presumably GUI-selected) file, --log-file to send the plain text output to a second GUI-selected file, --child-silent-after-fork=yes, and -q to restrict the plain text output to critical error messages created by Valgrind itself. For example, failure to read a specified suppressions file counts as a critical error message. In this way, for a successful run the text output file will be empty. But if it isn't empty, then it will contain important information which the GUI user should be made aware of.

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 suppression file contents dependent on the state of Valgrind's demangling machinery, and also slow down suppression matching.

The maximum value for this is 500. 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.

If this option is enabled, all stack trace entries will be shown and main-like functions will not be normalised.

For example, consider a file named /home/janedoe/blah/src/foo/bar/xyzzy.c. Specifying --fullpath-after=/home/janedoe/blah/src/ will cause Valgrind to show the name as foo/bar/xyzzy.c.

Because the string is not required to be a prefix, --fullpath-after=src/ will produce the same output. This is useful when the path contains arbitrary machine-generated characters. For example, the path /my/build/dir/C32A1B47/blah/src/foo/xyzzy can be pruned to foo/xyzzy using --fullpath-after=/blah/src/.

If you simply want to see the full path, just specify an empty string: --fullpath-after=. This isn't a special case, merely a logical consequence of the above rules.

Finally, you can use --fullpath-after multiple times. Any appearance of it causes Valgrind to switch to producing full paths and applying the above filtering rule. Each produced path is compared against all the --fullpath-after-specified strings, in the order specified. The first string to match causes the path to be truncated as described above. If none match, the full path is shown. This facilitates chopping off prefixes when the sources are drawn from a number of unrelated directories.

    ---- Print suppression ? --- [Return/N/n/Y/y/C/c] ----

The prompt's behaviour is the same as for the --db-attach option (see below).

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, by adding wildcards to function names, and by using frame-level wildcards. The wildcarding facilities are powerful yet flexible, and with a bit of careful editing, you may be able to suppress a whole family of related errors with only a few suppressions.

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.

    ---- 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.

Pressing 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.

Note: if you use GDB, more powerful debugging support is provided by the --vgdb= yes or full value. This activates Valgrind's internal gdbserver, which provides more-or-less full GDB-style control of the application: insertion of breakpoints, continuing from inside GDB, inferior function calls, and much more.

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

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.

Since <command> is likely to contain spaces, you will need to put this entire option in quotes to ensure it is correctly handled by the shell.

Mac OS X uses a deferred debug information (debuginfo) linking scheme. When object files containing debuginfo are linked into a .dylib or an executable, the debuginfo is not copied into the final file. Instead, the debuginfo must be linked manually by running dsymutil, a system-provided utility, on the executable or .dylib. The resulting combined debuginfo is placed in a directory alongside the executable or .dylib, but with the extension .dSYM.

With --dsymutil=no, Valgrind will detect cases where the .dSYM directory is either missing, or is present but does not appear to match the associated executable or .dylib, most likely because it is out of date. In these cases, Valgrind will print a warning message but take no further action.

With --dsymutil=yes, Valgrind will, in such cases, automatically run dsymutil as necessary to bring the debuginfo up to date. For all practical purposes, if you always use --dsymutil=yes, then there is never any need to run dsymutil manually or as part of your applications's build system, since Valgrind will run it as necessary.

Valgrind will not attempt to run dsymutil on any executable or library in /usr/, /bin/, /sbin/, /opt/, /sw/, /System/, /Library/ or /Applications/ since dsymutil will always fail in such situations. It fails both because the debuginfo for such pre-installed system components is not available anywhere, and also because it would require write privileges in those directories.

Be careful when using --dsymutil=yes, since it will cause pre-existing .dSYM directories to be silently deleted and re-created. Also note that dsymutil is quite slow, sometimes excessively so.

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 option 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 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 also 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 option, you may wish to consider rewriting your code to allocate on the heap rather than on the stack.

To simplify its memory management, Valgrind reserves all required space for the main thread's stack at startup. That means it needs to know the required stack size at startup.

By default, Valgrind uses the current "ulimit" value for the stack size, or 16 MB, whichever is lower. In many cases this gives a stack size in the range 8 to 16 MB, which almost never overflows for most applications.

If you need a larger total stack size, use --main-stacksize to specify it. Only set it as high as you need, since reserving far more space than you need (that is, hundreds of megabytes more than you need) constrains Valgrind's memory allocators and may reduce the total amount of memory that Valgrind can use. This is only really of significance on 32-bit machines.

On Linux, you may request a stack of size up to 2GB. Valgrind will stop with a diagnostic message if the stack cannot be allocated.

--main-stacksize only affects the stack size for the program's initial thread. It has no bearing on the size of thread stacks, as Valgrind does not allocate those.

You may need to use both --main-stacksize and --max-stackframe together. It is important to understand that --main-stacksize sets the maximum total stack size, whilst --max-stackframe specifies the largest size of any one stack frame. You will have to work out the --main-stacksize value for yourself (usually, if your applications segfaults). But Valgrind will tell you the needed --max-stackframe size, if necessary.

As discussed further in the description of --max-stackframe, a requirement for a large stack is a sign of potential portability problems. You are best advised to place all large data in heap-allocated memory.

Increasing the redzone size makes it possible to detect overruns of larger distances, but increases the amount of memory used by Valgrind. Decreasing the redzone size will reduce the memory needed by Valgrind but also reduces the chances of detecting over/underruns, so is not recommended.

Valgrind has four levels of self-modifying code detection: no detection, detect self-modifying code on the stack (which is used by GCC to implement nested functions), detect self-modifying code everywhere, and detect self-modifying code everywhere except in file-backed mappings. Note that the default option will catch the vast majority of cases. The main case it will not catch is programs such as JIT compilers that dynamically generate code and subsequently overwrite part or all of it. Running with all will slow Valgrind down noticeably. Running with none will rarely speed things up, since very little code gets put on the stack for most programs. The VALGRIND_DISCARD_TRANSLATIONS client request is an alternative to --smc-check=all that requires more programmer effort but allows Valgrind to run your program faster, by telling it precisely when translations need to be re-made.

--smc-check=all-non-file provides a cheaper but more limited version of --smc-check=all. It adds checks to any translations that do not originate from file-backed memory mappings. Typical applications that generate code, for example JITs in web browsers, generate code into anonymous mmaped areas, whereas the "fixed" code of the browser always lives in file-backed mappings. --smc-check=all-non-file takes advantage of this observation, limiting the overhead of checking to code which is likely to be JIT generated.

Some architectures (including ppc32, ppc64, ARM and MIPS) require programs which create code at runtime to flush the instruction cache in between code generation and first use. Valgrind observes and honours such instructions. Hence, on ppc32/Linux, ppc64/Linux and ARM/Linux, Valgrind always provides complete, transparent support for self-modifying code. It is only on platforms such as x86/Linux, AMD64/Linux, x86/Darwin and AMD64/Darwin that you need to use this option.

==15516== Uninitialised byte(s) found during client check request
==15516==    at 0x400633: croak (varinfo1.c:28)
==15516==    by 0x4006B2: main (varinfo1.c:55)
==15516==  Address 0x60103b is 7 bytes inside data symbol "global_i2"
==15516== 
==15516== Uninitialised byte(s) found during client check request
==15516==    at 0x400633: croak (varinfo1.c:28)
==15516==    by 0x4006BC: main (varinfo1.c:56)
==15516==  Address 0x7fefffefc is on thread 1's stack

And here are the same errors with --read-var-info=yes:

==15522== Uninitialised byte(s) found during client check request
==15522==    at 0x400633: croak (varinfo1.c:28)
==15522==    by 0x4006B2: main (varinfo1.c:55)
==15522==  Location 0x60103b is 0 bytes inside global_i2[7],
==15522==  a global variable declared at varinfo1.c:41
==15522== 
==15522== Uninitialised byte(s) found during client check request
==15522==    at 0x400633: croak (varinfo1.c:28)
==15522==    by 0x4006BC: main (varinfo1.c:56)
==15522==  Location 0x7fefffefc is 0 bytes inside local var "local"
==15522==  declared at varinfo1.c:46, in frame #1 of thread 1

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 therefore tries to run __libc_freeres at exit.

Unfortunately, in some very old versions of glibc, __libc_freeres is sufficiently buggy to cause segmentation faults. This was particularly noticeable on Red Hat 7.1. So this option 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.

Both sonamepatt and fnnamepatt can be written using the usual ? and * wildcards. For example: ":*libc.so*:foo?bar". You may use characters other than a colon to separate the two patterns. It is only important that the first character and the separator character are the same. For example, the above example could also be written "Q*libc.so*Qfoo?bar". Multiple --require-text-symbol flags are allowed, in which case shared objects that are loaded into the process will be checked against all of them.

The purpose of this is to support reliable usage of marked-up libraries. For example, suppose we have a version of GCC's libgomp.so which has been marked up with annotations to support Helgrind. It is only too easy and confusing to load the wrong, un-annotated libgomp.so into the application. So the idea is: add a text symbol in the marked-up library, for example annotated_for_helgrind_3_6, and then give the flag --require-text-symbol=:*libgomp*so*:annotated_for_helgrind_3_6 so that when libgomp.so is loaded, Valgrind scans its symbol table, and if the symbol isn't present the run is aborted, rather than continuing silently with the un-marked-up library. Note that you should put the entire flag in quotes to stop shells expanding up the * and ? wildcards.

Currently, this flexibility is only allowed for the malloc related functions, using the synonym somalloc. This synonym is usable for all tools doing standard replacement of malloc related functions (e.g. memcheck, massif, drd, helgrind, exp-dhat, exp-sgcheck).

For hardcore leak debugging, you probably want to use --leak-resolution=high together with --num-callers=40 or some such large number.

Note that the --leak-resolution setting does not affect Memcheck's ability to find leaks. It only changes how the results are presented.

When set to yes, Memcheck keeps track of the origins of all uninitialised values. Then, when an uninitialised value error is reported, Memcheck will try to show the origin of the value. An origin can be one of the following four places: a heap block, a stack allocation, a client request, or miscellaneous other sources (eg, a call to brk).

For uninitialised values originating from a heap block, Memcheck shows where the block was allocated. For uninitialised values originating from a stack allocation, Memcheck can tell you which function allocated the value, but no more than that -- typically it shows you the source location of the opening brace of the function. So you should carefully check that all of the function's local variables are initialised properly.

Performance overhead: origin tracking is expensive. It halves Memcheck's speed and increases memory use by a minimum of 100MB, and possibly more. Nevertheless it can drastically reduce the effort required to identify the root cause of uninitialised value errors, and so is often a programmer productivity win, despite running more slowly.

Accuracy: Memcheck tracks origins quite accurately. To avoid very large space and time overheads, some approximations are made. It is possible, although unlikely, that Memcheck will report an incorrect origin, or not be able to identify any origin.

Note that the combination --track-origins=yes and --undef-value-errors=no is nonsensical. Memcheck checks for and rejects this combination at startup.

When no, loads from partially invalid addresses are treated the same as loads from completely invalid addresses: an illegal-address error is issued, and the resulting bytes are marked as initialised.

Note that code that behaves in this way is in violation of the the ISO C/C++ standards, and should be considered broken. If at all possible, such code should be fixed. This option should be used only as a last resort.

This option specifies the maximum total size, in bytes, of the blocks in the queue. The default value is twenty million bytes. Increasing this increases the total amount of memory used by Memcheck but may detect invalid uses of freed blocks which would otherwise go undetected.

Setting a value of 0 means that all the blocks are re-circulated in a FIFO order.

You may also need to use this option when working with GCC 3.X or 4.X on 32-bit PowerPC Linux. This is because GCC generates code which occasionally accesses below the stack pointer, particularly for floating-point to/from integer conversions. This is in violation of the 32-bit PowerPC ELF specification, which makes no provision for locations below the stack pointer to be accessible.

Note that the resulting call graph will most probably not contain main, but will contain all the functions executed after instrumentation was enabled. Instrumentation can also programatically enabled/disabled. See the Callgrind include file callgrind.h for the macro you have to use in your source code.

For cache simulation, results will be less accurate when switching on instrumentation later in the program run, as the simulator starts with an empty cache at that moment. Switch on event collection later to cope with this error.

To only look at parts of your program, you have two possibilities:

The second option can be used if the program part you want to profile is called many times. Option 1, i.e. creating a lot of dumps is not practical here.

Collection state can be toggled at entry and exit of a given function with the option --toggle-collect. If you use this option, collection state should be disabled at the beginning. Note that the specification of --toggle-collect implicitly sets --collect-state=no.

Collection state can be toggled also by inserting the client request CALLGRIND_TOGGLE_COLLECT ; at the needed code positions.

This functionality is new in Valgrind 3.7.0, and is regarded as experimental. It is not enabled by default because its interaction with custom memory allocators is not well understood at present. User feedback is welcomed.

Collecting such information is expensive in both speed and memory, particularly for programs that do many inter-thread synchronisation events (locks, unlocks, etc). Without such information, it is more difficult to track down the root causes of races. Nonetheless, you may not need it in situations where you just want to check for the presence or absence of races, for example, when doing regression testing of a previously race-free program.

--history-level=none is the opposite extreme. It causes Helgrind not to collect any information about previous accesses. This can be dramatically faster than --history-level=full.

--history-level=approx provides a compromise between these two extremes. It causes Helgrind to show a full trace for the later access, and approximate information regarding the earlier access. This approximate information consists of two stacks, and the earlier access is guaranteed to have occurred somewhere between program points denoted by the two stacks. This is not as useful as showing the exact stack for the previous access (as --history-level=full does), but it is better than nothing, and it is almost as fast as --history-level=none.

Information about "old" conflicting accesses is stored in a cache of limited size, with LRU-style management. This is necessary because it isn't practical to store a stack trace for every single memory access made by the program. Historical information on not recently accessed locations is periodically discarded, to free up space in the cache.

This option controls the size of the cache, in terms of the number of different memory addresses for which conflicting access information is stored. If you find that Helgrind is showing race errors with only one stack instead of the expected two stacks, try increasing this value.

The minimum value is 10,000 and the maximum is 30,000,000 (thirty times the default value). Increasing the value by 1 increases Helgrind's memory requirement by very roughly 100 bytes, so the maximum value will easily eat up three extra gigabytes or so of memory.

Note that the named function will only be treated this way if it is the top entry in a stack trace, or just below another function treated this way. For example, if you have a function malloc1 that wraps malloc, and malloc2 that wraps malloc1, just specifying --alloc-fn=malloc2 will have no effect. You need to specify --alloc-fn=malloc1 as well. This is a little inconvenient, but the reason is that checking for allocation functions is slow, and it saves a lot of time if Massif can stop looking through the stack trace entries as soon as it finds one that doesn't match rather than having to continue through all the entries.

Note that C++ names are demangled. Note also that overloaded C++ names must be written in full. Single quotes may be necessary to prevent the shell from breaking them up. For example:

--alloc-fn='operator new(unsigned, std::nothrow_t const&)'

Any realloc of an ignored block will also be ignored, even if the realloc call does not occur in an ignored function. This avoids the possibility of negative heap sizes if ignored blocks are shrunk with realloc.

The rules for writing C++ function names are the same as for --alloc-fn above.