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<book id="FAQ" xreflabel="Valgrind FAQ">
<bookinfo>
<title>Valgrind FAQ</title>
</bookinfo>
<chapter id="faq.background" xreflabel="Background">
<title>Background</title>
<qandaset id="qset.background">
<qandaentry id="faq.pronounce">
<question>
<para>How do you pronounce "Valgrind"?</para>
</question>
<answer>
<para>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").</para>
<para>Don't feel bad: almost everyone gets it wrong at
first.</para>
</answer>
</qandaentry>
<qandaentry id="faq.whence">
<question>
<para>Where does the name "Valgrind" come from?</para>
</question>
<answer>
<para>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".</para> <para>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.</para>
<para>It's not short for "value grinder", although that's not a
bad guess.</para>
</answer>
</qandaentry>
</qandaset>
</chapter>
<chapter id="faq.installing"
xreflabel="Compiling, installing and configuring">
<title>Compiling, installing and configuring</title>
<qandaset id="qset.installing">
<qandaentry id="faq.make_dies">
<question>
<para>When I trying building Valgrind, 'make' dies partway with
an assertion failure, something like this:
<screen>
% make: expand.c:489: allocated_variable_append:
Assertion 'current_variable_set_list->next != 0' failed.
</screen>
</para>
</question>
<answer>
<para>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.</para>
</answer>
</qandaentry>
</qandaset>
</chapter>
<chapter id="faq.abort"
xreflabel="Valgrind aborts unexpectedly">
<title>Valgrind aborts unexpectedly</title>
<qandaset id="qset.abort">
<qandaentry id="faq.exit_errors">
<question>
<para>Programs run OK on Valgrind, but at exit produce a bunch
of errors a bit like this:</para>
</question>
<answer><para>
<programlisting>
==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)
</programlisting>
and then die with a segmentation fault.</para>
<para>When the program exits, Valgrind runs the procedure
<literal>__libc_freeres()</literal> 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.</para>
<para>Problem is that running
<literal>__libc_freeres()</literal> in older glibc versions
causes this crash.</para> <para>WORKAROUND FOR 1.1.X and later
versions of Valgrind: use the
<literal>--run-libc-freeres=no</literal> 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.</para>
</answer>
</qandaentry>
<qandaentry id="faq.bugdeath">
<question>
<para>My (buggy) program dies like this:</para>
</question>
<answer>
<screen>
% valgrind: vg_malloc2.c:442 (bszW_to_pszW): Assertion 'pszW >= 0' failed.
</screen>
<para>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.</para>
</answer>
</qandaentry>
<qandaentry id="faq.msgdeath">
<question>
<para>My program dies, printing a message like this along the
way:</para>
</question>
<answer>
<screen>
% disInstr: unhandled instruction bytes: 0x66 0xF 0x2E 0x5
</screen>
<para>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
<computeroutput>-march</computeroutput> to gcc. Either way,
let us know and we'll try to fix it.</para>
<para>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.</para>
</answer>
</qandaentry>
</qandaset>
</chapter>
<chapter id="faq.unexpected"
xreflabel="Valgrind behaves unexpectedly">
<title>Valgrind behaves unexpectedly</title>
<qandaset id="qset.unexpected">
<qandaentry id="faq.slowthread">
<question>
<para>My threaded server process runs unbelievably slowly on
Valgrind. So slowly, in fact, that at first I thought it had
completely locked up.</para>
</question>
<answer>
<para>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.</para>
<para>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.</para>
</answer>
</qandaentry>
<qandaentry id="faq.reports">
<question>
<para>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.</para>
</question>
<answer>
<para>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.</para>
<para>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.</para>
<itemizedlist>
<listitem>
<para>With gcc 2.91, 2.95, 3.0 and 3.1, compile all source
using the STL with <literal>-D__USE_MALLOC</literal>. Beware!
This is removed from gcc starting with version 3.3.</para>
</listitem>
<listitem>
<para>With gcc 3.2.2 and later, you should export the environment
variable <literal>GLIBCPP_FORCE_NEW</literal> before running
your program.</para>
</listitem>
<listitem>
<para>With gcc 3.4 and later, that variable has changed name to
<literal>GLIBCXX_FORCE_NEW</literal>.</para>
</listitem>
</itemizedlist>
<para>There are other ways to disable memory pooling: using the
<literal>malloc_alloc</literal> 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 <ulink
url="http://gcc.gnu.org/onlinedocs/libstdc++/ext/howto.html#3">
http://gcc.gnu.org/onlinedocs/libstdc++/ext/howto.html#3</ulink>
if you absolutely want to do that. But beware:</para>
<orderedlist>
<listitem>
<para>there are currently changes underway for gcc which are
not totally reflected in the docs right now ("now" == 26 Apr
03)</para>
</listitem>
<listitem>
<para>allocators belong to the more messy parts of the STL
and people went to great lengths to make it portable across
platforms. Chances are good that your solution will work on
your platform, but not on others.</para>
</listitem>
</orderedlist>
</answer>
</qandaentry>
<qandaentry id="faq.unhelpful">
<question>
<para>The stack traces given by Memcheck (or another tool)
aren't helpful. How can I improve them?</para>
</question>
<answer>
<para>If they're not long enough, use
<literal>--num-callers</literal> to make them longer.</para>
<para>If they're not detailed enough, make sure you are
compiling with <literal>-g</literal> to add debug information.
And don't strip symbol tables (programs should be unstripped
unless you run 'strip' on them; some libraries ship
stripped).</para>
<para>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
<literal>???</literal> entries. The workaround here is to avoid calling
dlclose() on these shared objects.
</para>
<para>Also, <literal>-fomit-frame-pointer</literal> and
<literal>-fstack-check</literal> can make stack traces
worse.</para>
<para>Some example sub-traces:</para>
<para>With debug information and unstripped (best):</para>
<programlisting>
Invalid write of size 1
at 0x80483BF: really (malloc1.c:20)
by 0x8048370: main (malloc1.c:9)
</programlisting>
<para>With no debug information, unstripped:</para>
<programlisting>
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)
</programlisting>
<para>With no debug information, stripped:</para>
<programlisting>
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)
</programlisting>
<para>With debug information and -fomit-frame-pointer:</para>
<programlisting>
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)
</programlisting>
<para>A leak error message involving an unloaded shared object:</para>
<programlisting>
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)
</programlisting>
</answer>
</qandaentry>
<qandaentry id="faq.aliases">
<question>
<para>The stack traces given by Memcheck (or another tool) seem to
have the wrong function name in them. What's happening?</para>
</question>
<answer>
<para>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'.</para>
</answer>
</qandaentry>
</qandaset>
</chapter>
<chapter id="faq.notfound" xreflabel="Memcheck doesn't find my bug">
<title>Memcheck doesn't find my bug</title>
<qandaset id="qset.notfound">
<qandaentry id="faq.hiddenbug">
<question>
<para>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.</para>
</question>
<answer>
<para>There are two possible causes of this.</para>
<para>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.</para>
<para>To trace child processes, use the
<literal>--trace-children=yes</literal> option.</para>
<para>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
<literal>--log-socket=127.0.0.1:12345</literal> (if you want
logging output sent to <literal>port 12345</literal> on
<literal>localhost</literal>). You can use the
valgrind-listener program to listen on that port:</para>
<programlisting>
valgrind-listener 12345
</programlisting>
<para>Obviously you have to start the listener process first.
See the Manual: <ulink url="http://www.valgrind.org/docs/bookset/manual-core.out2file.html">Directing output to file</ulink> for more details.</para>
<para>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:
<programlisting>
No malloc'd blocks -- no leaks are possible
</programlisting>
when you know your program calls malloc(). The workaround is to avoid
statically linking your program.</para>
</answer>
</qandaentry>
<qandaentry id="faq.overruns">
<question>
<para>Why doesn't Memcheck find the array overruns in this program?</para>
</question>
<answer>
<programlisting>
int static[5];
int main(void)
{
int stack[5];
static[5] = 0;
stack [5] = 0;
return 0;
}
</programlisting>
<para>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.</para>
</answer>
</qandaentry>
</qandaset>
</chapter>
<chapter id="faq.misc"
xreflabel="Miscellaneous">
<title>Miscellaneous</title>
<qandaset id="qset.misc">
<qandaentry id="faq.writesupp">
<question>
<para>I tried writing a suppression but it didn't work. Can
you write my suppression for me?</para>
</question>
<answer>
<para>Yes! Use the
<computeroutput>--gen-suppressions=yes</computeroutput> 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
<literal>'*'</literal>.</para>
<para>If you really want to write suppressions by hand, read
the manual carefully. Note particularly that C++ function
names must be <literal>_mangled_</literal>.</para>
</answer>
</qandaentry>
<qandaentry id="faq.deflost">
<question>
<para>With Memcheck/Addrcheck's memory leak detector, what's
the difference between "definitely lost", "possibly lost",
"still reachable", and "suppressed"?</para>
</question>
<answer>
<para>The details are in the Manual:
<ulink url="http://www.valgrind.org/docs/bookset/mc-manual.leaks.html">Memory leak detection</ulink>.</para>
<para>In short:</para>
<itemizedlist>
<listitem>
<para>"definitely lost" means your program is leaking memory
-- fix it!</para>
</listitem>
<listitem>
<para>"possibly lost" means your program is probably leaking
memory, unless you're doing funny things with
pointers.</para>
</listitem>
<listitem>
<para>"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
<computeroutput>--show-reachable=yes</computeroutput> if you
don't want to see these reports.</para>
</listitem>
<listitem>
<para>"suppressed" means that a leak error has been
suppressed. There are some suppressions in the default
suppression files. You can ignore suppressed errors.</para>
</listitem>
</itemizedlist>
</answer>
</qandaentry>
</qandaset>
</chapter>
<!-- template
<chapter id="faq."
xreflabel="xx">
<title>xx</title>
<qandaset id="qset.">
<qandaentry id="faq.deflost">
<question>
<para></para>
</question>
<answer>
<para></para>
</answer>
</qandaentry>
</qandaset>
</chapter>
-->
<chapter id="faq.help" xreflabel="How To Get Further Assistance">
<title>How To Get Further Assistance</title>
<para>Please read all of this section before posting.</para>
<para>If you think an answer is incomplete or inaccurate, please
e-mail <ulink url="mailto:&vg-vemail;">&vg-vemail;</ulink>.</para>
<para>Read the appropriate section(s) of the Manual(s):
<ulink url="http://www.valgrind.org/docs/">Valgrind
Documentation</ulink>.</para>
<para>Read the <ulink url="http://www.valgrind.org/docs/">Distribution Documents</ulink>.</para>
<para><ulink url="http://search.gmane.org">Search</ulink> the
<ulink url="http://news.gmane.org/gmane.comp.debugging.valgrind">valgrind-users</ulink> mailing list archives, using the group name
<computeroutput>gmane.comp.debugging.valgrind</computeroutput>.</para>
<para>Only when you have tried all of these things and are still stuck,
should you post to the <ulink url="&vg-users-list;">valgrind-users
mailing list</ulink>. In which case, please read the following
carefully. Making a complete posting will greatly increase the chances
that an expert or fellow user reading it will have enough information
and motivation to reply.</para>
<para>Make sure you give full details of the problem,
including the full output of <computeroutput>valgrind
-v</computeroutput>, if applicable. Also which Linux distribution
you're using (Red Hat, Debian, etc) and its version number.</para>
<para>You are in little danger of making your posting too long
unless you include large chunks of valgrind's (unsuppressed)
output, so err on the side of giving too much information.</para>
<para>Clearly written subject lines and message bodies are appreciated,
too.</para>
<para>Finally, 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.</para>
</chapter>
</book>