FAQ.txt

Valgrind FAQ, version 2.1.2
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Last revised 18 July 2004
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1. Background
2. Compiling, installing and configuring
3. Valgrind aborts unexpectedly
4. Valgrind behaves unexpectedly
5. Memcheck doesn't find my bug
6. Miscellaneous


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1. Background
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1.1. 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.

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1.2. 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.


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2. Compiling, installing and configuring
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2.1. When I trying 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 www.mail-archive.com/bug-make@gnu.org/msg01658.html.  Try
upgrading to a more recent version of 'make'.  Alternatively, we have heard
that unsetting the CFLAGS environment variable avoids the problem.


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3. Valgrind aborts unexpectedly
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3.1. Programs run OK on Valgrind, but at exit produce a bunch of errors a bit
     like this

    ==20755== Invalid read of size 4
    ==20755==    at 0x40281C8A: _nl_unload_locale (loadlocale.c:238)
    ==20755==    by 0x4028179D: free_mem (findlocale.c:257)
    ==20755==    by 0x402E0962: __libc_freeres (set-freeres.c:34)
    ==20755==    by 0x40048DCC: vgPlain___libc_freeres_wrapper 
                                              (vg_clientfuncs.c:585)
    ==20755==    Address 0x40CC304C is 8 bytes inside a block of size 380 free'd
    ==20755==    at 0x400484C9: free (vg_clientfuncs.c:180)
    ==20755==    by 0x40281CBA: _nl_unload_locale (loadlocale.c:246)
    ==20755==    by 0x40281218: free_mem (setlocale.c:461)
    ==20755==    by 0x402E0962: __libc_freeres (set-freeres.c:34)

    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.

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 flag.  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.

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3.2. My (buggy) program dies like this:
      valgrind: vg_malloc2.c:442 (bszW_to_pszW): 
                Assertion `pszW >= 0' failed.

If Memcheck (the memory checker) shows any invalid reads, invalid writes
and 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 like this.  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.

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

      disInstr: unhandled instruction bytes: 0x66 0xF 0x2E 0x5

Older versions did not support some x86 instructions, particularly
SSE/SSE2 instructions.  Try a newer Valgrind;  we now support almost all
instructions.  If it still happens with newer versions, if the failing
instruction is an SSE/SSE2 instruction, you might be able to recompile
your program without it by using the flag -march to gcc.  Either way,
let us know and we'll try to fix it.

Another 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/Addrcheck may issue a warning just before this happens, but they
might not if the jump happens to land in addressable memory.


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4. Valgrind behaves unexpectedly
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4.1. My threaded server process runs unbelievably slowly on Valgrind.
     So slowly, in fact, that at first I thought it had completely
     locked up.

We are not completely sure about this, but one possibility is that
laptops with power management fool Valgrind's timekeeping mechanism,
which is (somewhat in error) based on the x86 RDTSC instruction.  A
"fix" which is claimed to work is to run some other cpu-intensive
process at the same time, so that the laptop's power-management
clock-slowing does not kick in.  We would be interested in hearing more
feedback on this.

Another possible cause is that versions prior to 1.9.6 did not support
threading on glibc 2.3.X systems well.  Hopefully the situation is much
improved with 1.9.6 and later versions.

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4.2. 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 is 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++/ext/howto.html#3 if you
absolutely want to do that. But beware:

1) there are currently changes underway for gcc which are not totally
   reflected in the docs right now ("now" == 26 Apr 03)

2) allocators belong to the more messy parts of the STL and people went
   at great lengths to make it portable across platforms. Chances are
   good that your solution will work on your platform, but not on
   others.

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4.3. 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)

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4.4. 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'.

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5. Memcheck doesn't find my bug
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5.1. I try running "valgrind --tool=memcheck my_program" and get
     Valgrind's startup message, but I don't get 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 flag
--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
documentation 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:

  No malloc'd blocks -- no leaks are possible

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

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5.2. 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 static 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.


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6. Miscellaneous
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6.1. 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_.

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

The details are in section 3.6 of the manual.

In short:

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

  - "possibly lost" means your program is probably leaking memory,
    unless you're doing funny things with pointers.

  - "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.

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(this is the end of the FAQ.)