# Valgrind FAQ

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&vg-lifespan; Valgrind Developers

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## Valgrind Frequently Asked Questions

Background

How do you pronounce "Valgrind"?

The "Val" as in the world "value". The "grind" is pronounced with a short 'i' -- ie. "grinned" (rhymes with "tinned") rather than "grined" (rhymes with "find").

Don't feel bad: almost everyone gets it wrong at first.

Where does the name "Valgrind" come from?

From Nordic mythology. Originally (before release) the project was named Heimdall, after the watchman of the Nordic gods. He could "see a hundred miles by day or night, hear the grass growing, see the wool growing on a sheep's back", etc. This would have been a great name, but it was already taken by a security package "Heimdal".

Keeping with the Nordic theme, Valgrind was chosen. Valgrind is the name of the main entrance to Valhalla (the Hall of the Chosen Slain in Asgard). Over this entrance there resides a wolf and over it there is the head of a boar and on it perches a huge eagle, whose eyes can see to the far regions of the nine worlds. Only those judged worthy by the guardians are allowed to pass through Valgrind. All others are refused entrance.

It's not short for "value grinder", although that's not a bad guess.

Compiling, installing and configuring

When building Valgrind, 'make' dies partway with an assertion failure, something like this:

    % make: expand.c:489: allocated_variable_append:
            Assertion 'current_variable_set_list->next != 0' failed.

It's probably a bug in 'make'. Some, but not all, instances of version 3.79.1 have this bug, see this. Try upgrading to a more recent version of 'make'. Alternatively, we have heard that unsetting the CFLAGS environment variable avoids the problem.

When building Valgrind, 'make' fails with this:

    /usr/bin/ld: cannot find -lc
    collect2: ld returned 1 exit status

You need to install the glibc-static-devel package.

Valgrind aborts unexpectedly

Programs run OK on Valgrind, but at exit produce a bunch of errors involving `__libc_freeres` and then die with a segmentation fault.

When the program exits, Valgrind runs the procedure `__libc_freeres` in glibc. This is a hook for memory debuggers, so they can ask glibc to free up any memory it has used. Doing that is needed to ensure that Valgrind doesn't incorrectly report space leaks in glibc.

The problem is that running `__libc_freeres` in older glibc versions causes this crash.

Workaround for 1.1.X and later versions of Valgrind: use the `--run-libc-freeres=no` option. You may then get space leak reports for glibc allocations (please don't report these to the glibc people, since they are not real leaks), but at least the program runs.

My (buggy) program dies like this:

    valgrind: m_mallocfree.c:248 (get_bszB_as_is): Assertion 'bszB_lo == bszB_hi' failed.

or like this:

    valgrind: m_mallocfree.c:442 (mk_inuse_bszB): Assertion 'bszB != 0' failed.

or otherwise aborts or crashes in m_mallocfree.c.

If Memcheck (the memory checker) shows any invalid reads, invalid writes or invalid frees in your program, the above may happen. Reason is that your program may trash Valgrind's low-level memory manager, which then dies with the above assertion, or something similar. The cure is to fix your program so that it doesn't do any illegal memory accesses. The above failure will hopefully go away after that.

My program dies, printing a message like this along the way:

    vex x86->IR: unhandled instruction bytes: 0x66 0xF 0x2E 0x5

One possibility is that your program has a bug and erroneously jumps to a non-code address, in which case you'll get a SIGILL signal. Memcheck may issue a warning just before this happens, but it might not if the jump happens to land in addressable memory.

Another possibility is that Valgrind does not handle the instruction. If you are using an older Valgrind, a newer version might handle the instruction. However, all instruction sets have some obscure, rarely used instructions. Also, on amd64 there are an almost limitless number of combinations of redundant instruction prefixes, many of them undocumented but accepted by CPUs. So Valgrind will still have decoding failures from time to time. If this happens, please file a bug report.

I tried running a Java program (or another program that uses a just-in-time compiler) under Valgrind but something went wrong. Does Valgrind handle such programs?

Valgrind can handle dynamically generated code, so long as none of the generated code is later overwritten by other generated code. If this happens, though, things will go wrong as Valgrind will continue running its translations of the old code (this is true on x86 and amd64, on PowerPC there are explicit cache flush instructions which Valgrind detects and honours). You should try running with `--smc-check=all` in this case. Valgrind will run much more slowly, but should detect the use of the out-of-date code.

Alternatively, if you have the source code to the JIT compiler you can insert calls to the `VALGRIND_DISCARD_TRANSLATIONS` client request to mark out-of-date code, saving you from using `--smc-check=all`.

Apart from this, in theory Valgrind can run any Java program just fine, even those that use JNI and are partially implemented in other languages like C and C++. In practice, Java implementations tend to do nasty things that most programs do not, and Valgrind sometimes falls over these corner cases.

If your Java programs do not run under Valgrind, even with `--smc-check=all`, please file a bug report and hopefully we'll be able to fix the problem.

Valgrind behaves unexpectedly

My program uses the C++ STL and string classes. Valgrind reports 'still reachable' memory leaks involving these classes at the exit of the program, but there should be none.

First of all: relax, it's probably not a bug, but a feature. Many implementations of the C++ standard libraries use their own memory pool allocators. Memory for quite a number of destructed objects is not immediately freed and given back to the OS, but kept in the pool(s) for later re-use. The fact that the pools are not freed at the exit of the program cause Valgrind to report this memory as still reachable. The behaviour not to free pools at the exit could be called a bug of the library though.

Using GCC, you can force the STL to use malloc and to free memory as soon as possible by globally disabling memory caching. Beware! Doing so will probably slow down your program, sometimes drastically.

- With GCC 2.91, 2.95, 3.0 and 3.1, compile all source using the STL with `-D__USE_MALLOC`. Beware! This was removed from GCC starting with version 3.3.

- With GCC 3.2.2 and later, you should export the environment variable `GLIBCPP_FORCE_NEW` before running your program.

- With GCC 3.4 and later, that variable has changed name to `GLIBCXX_FORCE_NEW`.

There are other ways to disable memory pooling: using the `malloc_alloc` template with your objects (not portable, but should work for GCC) or even writing your own memory allocators. But all this goes beyond the scope of this FAQ. Start by reading http://gcc.gnu.org/onlinedocs/libstdc++/faq/index.html#4_4_leak if you absolutely want to do that. But beware: allocators belong to the more messy parts of the STL and people went to great lengths to make the STL portable across platforms. Chances are good that your solution will work on your platform, but not on others.

The stack traces given by Memcheck (or another tool) aren't helpful. How can I improve them?

If they're not long enough, use `--num-callers` to make them longer.

If they're not detailed enough, make sure you are compiling with `-g` to add debug information. And don't strip symbol tables (programs should be unstripped unless you run 'strip' on them; some libraries ship stripped).

Also, for leak reports involving shared objects, if the shared object is unloaded before the program terminates, Valgrind will discard the debug information and the error message will be full of `???` entries. The workaround here is to avoid calling `dlclose` on these shared objects.

Also, `-fomit-frame-pointer` and `-fstack-check` can make stack traces worse.

Some example sub-traces:

- With debug information and unstripped (best):

      Invalid write of size 1
         at 0x80483BF: really (malloc1.c:20)
         by 0x8048370: main (malloc1.c:9)

- With no debug information, unstripped:

      Invalid write of size 1
         at 0x80483BF: really (in /auto/homes/njn25/grind/head5/a.out)
         by 0x8048370: main (in /auto/homes/njn25/grind/head5/a.out)

- With no debug information, stripped:

      Invalid write of size 1
         at 0x80483BF: (within /auto/homes/njn25/grind/head5/a.out)
         by 0x8048370: (within /auto/homes/njn25/grind/head5/a.out)
         by 0x42015703: __libc_start_main (in /lib/tls/libc-2.3.2.so)
         by 0x80482CC: (within /auto/homes/njn25/grind/head5/a.out)

- With debug information and -fomit-frame-pointer:

      Invalid write of size 1
         at 0x80483C4: really (malloc1.c:20)
         by 0x42015703: __libc_start_main (in /lib/tls/libc-2.3.2.so)
         by 0x80482CC: ??? (start.S:81)

- A leak error message involving an unloaded shared object:

      84 bytes in 1 blocks are possibly lost in loss record 488 of 713
         at 0x1B9036DA: operator new(unsigned) (vg_replace_malloc.c:132)
         by 0x1DB63EEB: ???
         by 0x1DB4B800: ???
         by 0x1D65E007: ???
         by 0x8049EE6: main (main.cpp:24)

The stack traces given by Memcheck (or another tool) seem to have the wrong function name in them. What's happening?

Occasionally Valgrind stack traces get the wrong function names. This is caused by glibc using aliases to effectively give one function two names. Most of the time Valgrind chooses a suitable name, but very occasionally it gets it wrong. Examples we know of are printing `bcmp` instead of `memcmp`, `index` instead of `strchr`, and `rindex` instead of `strrchr`.

My program crashes normally, but doesn't under Valgrind, or vice versa. What's happening?

When a program runs under Valgrind, its environment is slightly different to when it runs natively. For example, the memory layout is different, and the way that threads are scheduled is different.

Most of the time this doesn't make any difference, but it can, particularly if your program is buggy. For example, if your program crashes because it erroneously accesses memory that is unaddressable, it's possible that this memory will not be unaddressable when run under Valgrind. Alternatively, if your program has data races, these may not manifest under Valgrind.

There isn't anything you can do to change this, it's just the nature of the way Valgrind works that it cannot exactly replicate a native execution environment. In the case where your program crashes due to a memory error when run natively but not when run under Valgrind, in most cases Memcheck should identify the bad memory operation.

.

Memcheck doesn't report any errors and I know my program has errors.

There are two possible causes of this.

First, by default, Valgrind only traces the top-level process. So if your program spawns children, they won't be traced by Valgrind by default. Also, if your program is started by a shell script, Perl script, or something similar, Valgrind will trace the shell, or the Perl interpreter, or equivalent.

To trace child processes, use the `--trace-children=yes` option.

If you are tracing large trees of processes, it can be less disruptive to have the output sent over the network. Give Valgrind the option `--log-socket=127.0.0.1:12345` (if you want logging output sent to port `12345` on `localhost`). You can use the valgrind-listener program to listen on that port:

    valgrind-listener 12345

Obviously you have to start the listener process first. See the manual for more details.

Second, if your program is statically linked, most Valgrind tools won't work as well, because they won't be able to replace certain functions, such as `malloc`, with their own versions. A key indicator of this is if Memcheck says:

    All heap blocks were freed -- no leaks are possible

when you know your program calls `malloc`. The workaround is to avoid statically linking your program.

Why doesn't Memcheck find the array overruns in this program?

    int static[5];

    int main(void)
    {
      int stack[5];

      static[5] = 0;
      stack [5] = 0;

      return 0;
    }

Unfortunately, Memcheck doesn't do bounds checking on global or stack arrays. We'd like to, but it's just not possible to do in a reasonable way that fits with how Memcheck works. Sorry.

However, the experimental tool SGcheck can detect errors like this. Run Valgrind with the `--tool=exp-sgcheck` option to try it, but be aware that it is not as robust as Memcheck.

Miscellaneous

I tried writing a suppression but it didn't work. Can you write my suppression for me?

Yes! Use the `--gen-suppressions=yes` feature to spit out suppressions automatically for you. You can then edit them if you like, eg. combining similar automatically generated suppressions using wildcards like `'*'`.

If you really want to write suppressions by hand, read the manual carefully. Note particularly that C++ function names must be mangled (that is, not demangled).

With Memcheck's memory leak detector, what's the difference between "definitely lost", "indirectly lost", "possibly lost", "still reachable", and "suppressed"?

The details are in the Memcheck section of the user manual.

In short:

- "definitely lost" means your program is leaking memory -- fix those leaks!

- "indirectly lost" means your program is leaking memory in a pointer-based structure. (E.g. if the root node of a binary tree is "definitely lost", all the children will be "indirectly lost".) If you fix the "definitely lost" leaks, the "indirectly lost" leaks should go away.

- "possibly lost" means your program is leaking memory, unless you're doing unusual things with pointers that could cause them to point into the middle of an allocated block; see the user manual for some possible causes. Use `--show-possibly-lost=no` if you don't want to see these reports.

- "still reachable" means your program is probably ok -- it didn't free some memory it could have. This is quite common and often reasonable. Don't use `--show-reachable=yes` if you don't want to see these reports.

- "suppressed" means that a leak error has been suppressed. There are some suppressions in the default suppression files. You can ignore suppressed errors.

Memcheck's uninitialised value errors are hard to track down, because they are often reported some time after they are caused. Could Memcheck record a trail of operations to better link the cause to the effect? Or maybe just eagerly report any copies of uninitialised memory values?

Prior to version 3.4.0, the answer was "we don't know how to do it without huge performance penalties". As of 3.4.0, try using the `--track-origins=yes` option. It will run slower than usual, but will give you extra information about the origin of uninitialised values.

Or if you want to do it the old fashioned way, you can use the client request `VALGRIND_CHECK_VALUE_IS_DEFINED` to help track these errors down -- work backwards from the point where the uninitialised error occurs, checking suspect values until you find the cause. This requires editing, compiling and re-running your program multiple times, which is a pain, but still easier than debugging the problem without Memcheck's help.

As for eager reporting of copies of uninitialised memory values, this has been suggested multiple times. Unfortunately, almost all programs legitimately copy uninitialised memory values around (because compilers pad structs to preserve alignment) and eager checking leads to hundreds of false positives. Therefore Memcheck does not support eager checking at this time.

Is it possible to attach Valgrind to a program that is already running?

No. The environment that Valgrind provides for running programs is significantly different to that for normal programs, e.g. due to different layout of memory. Therefore Valgrind has to have full control from the very start.

It is possible to achieve something like this by running your program without any instrumentation (which involves a slow-down of about 5x, less than that of most tools), and then adding instrumentation once you get to a point of interest. Support for this must be provided by the tool, however, and Callgrind is the only tool that currently has such support. See the instructions on the `callgrind_control` program for details.

How To Get Further Assistance

Read the appropriate section(s) of the Valgrind Documentation.

Search the valgrind-users mailing list archives, using the group name `gmane.comp.debugging.valgrind`.

If you think an answer in this FAQ is incomplete or inaccurate, please e-mail &vg-vemail;.

If you have tried all of these things and are still stuck, you can try mailing the valgrind-users mailing list. Note that an email has a better change of being answered usefully if it is clearly written. Also remember that, despite the fact that most of the community are very helpful and responsive to emailed questions, you are probably requesting help from unpaid volunteers, so you have no guarantee of receiving an answer.