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<book id="FAQ" xreflabel="Valgrind FAQ">
<bookinfo>
<title>Valgrind FAQ</title>
<releaseinfo>&rel-type; &rel-version; &rel-date;</releaseinfo>
<copyright>
<year>&vg-lifespan;</year>
<holder><ulink url="&vg-devs-url;">Valgrind Developers</ulink></holder>
</copyright>
<legalnotice>
<para>Email: <ulink url="mailto:&vg-vemail;">&vg-vemail;</ulink></para>
</legalnotice>
</bookinfo>
<article id="faq">
<title>Valgrind Frequently Asked Questions</title>
<!-- FAQ starts here -->
<qandaset>
<!-- Background -->
<qandadiv id="faq.background" xreflabel="Background">
<title>Background</title>
<qandaentry id="faq.pronounce">
<question id="q-pronounce">
<para>How do you pronounce "Valgrind"?</para>
</question>
<answer id="a-pronounce">
<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 id="q-whence">
<para>Where does the name "Valgrind" come from?</para>
</question>
<answer id="a-whence">
<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>
</qandadiv>
<!-- Compiling, Installing and Configuring -->
<qandadiv id="faq.installing" xreflabel="Compiling, installing and configuring">
<title>Compiling, installing and configuring</title>
<qandaentry id="faq.make_dies">
<question id="q-make_dies">
<para>When building Valgrind, 'make' dies partway with
an assertion failure, something like this:</para>
<screen>
% make: expand.c:489: allocated_variable_append:
Assertion 'current_variable_set_list->next != 0' failed.
</screen>
</question>
<answer id="a-make_dies">
<para>It's probably a bug in 'make'. Some, but not all, instances of
version 3.79.1 have this bug, see
<ulink url="http://www.mail-archive.com/bug-make@gnu.org/msg01658.html">this</ulink>.
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>
<qandaentry id="faq.glibc_devel">
<question>
<para>When building Valgrind, 'make' fails with this:</para>
<screen>
/usr/bin/ld: cannot find -lc
collect2: ld returned 1 exit status
</screen>
</question>
<answer>
<para>You need to install the glibc-static-devel package.</para>
</answer>
</qandaentry>
</qandadiv>
<!-- Valgrind aborts unexpectedly -->
<qandadiv id="faq.abort" xreflabel="Valgrind aborts unexpectedly">
<title>Valgrind aborts unexpectedly</title>
<qandaentry id="faq.exit_errors">
<question id="q-exit_errors">
<para>Programs run OK on Valgrind, but at exit produce a bunch of
errors involving <literal>__libc_freeres</literal> and then die
with a segmentation fault.</para>
</question>
<answer id="a-exit_errors">
<para>When the program exits, Valgrind runs the procedure
<function>__libc_freeres</function> 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>The 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
<option>--run-libc-freeres=no</option> 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.</para>
</answer>
</qandaentry>
<qandaentry id="faq.bugdeath">
<question id="q-bugdeath">
<para>My (buggy) program dies like this:</para>
<screen>valgrind: m_mallocfree.c:248 (get_bszB_as_is): Assertion 'bszB_lo == bszB_hi' failed.</screen>
<para>or like this:</para>
<screen>valgrind: m_mallocfree.c:442 (mk_inuse_bszB): Assertion 'bszB != 0' failed.</screen>
<para>or otherwise aborts or crashes in m_mallocfree.c.</para>
</question>
<answer id="a-bugdeath">
<para>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.</para>
</answer>
</qandaentry>
<qandaentry id="faq.msgdeath">
<question id="q-msgdeath">
<para>My program dies, printing a message like this along the
way:</para>
<screen>vex x86->IR: unhandled instruction bytes: 0x66 0xF 0x2E 0x5</screen>
</question>
<answer id="a-msgdeath">
<para>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.</para>
<para>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.</para>
</answer>
</qandaentry>
<qandaentry id="faq.java">
<question id="q-java">
<para>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?</para>
</question>
<answer id="a-java">
<para>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
<option>--smc-check=all</option> in this case. Valgrind will run
much more slowly, but should detect the use of the out-of-date
code.</para>
<para>Alternatively, if you have the source code to the JIT compiler
you can insert calls to the
<computeroutput>VALGRIND_DISCARD_TRANSLATIONS</computeroutput>
client request to mark out-of-date code, saving you from using
<option>--smc-check=all</option>.</para>
<para>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.</para>
<para>If your Java programs do not run under Valgrind, even with
<option>--smc-check=all</option>, please file a bug report and
hopefully we'll be able to fix the problem.</para>
</answer>
</qandaentry>
</qandadiv>
<!-- Valgrind behaves unexpectedly -->
<qandadiv id="faq.unexpected" xreflabel="Valgrind behaves unexpectedly">
<title>Valgrind behaves unexpectedly</title>
<qandaentry id="faq.reports">
<question id="q-reports">
<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 id="a-reports">
<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 was
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++/faq/index.html#4_4_leak">
http://gcc.gnu.org/onlinedocs/libstdc++/faq/index.html#4_4_leak</ulink>
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.</para>
</answer>
</qandaentry>
<qandaentry id="faq.unhelpful">
<question id="q-unhelpful">
<para>The stack traces given by Memcheck (or another tool) aren't
helpful. How can I improve them?</para>
</question>
<answer id="a-unhelpful">
<para>If they're not long enough, use <option>--num-callers</option>
to make them longer.</para>
<para>If they're not detailed enough, make sure you are compiling
with <option>-g</option> 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 <function>dlclose</function> on these shared objects.</para>
<para>Also, <option>-fomit-frame-pointer</option> and
<option>-fstack-check</option> can make stack traces worse.</para>
<para>Some example sub-traces:</para>
<itemizedlist>
<listitem>
<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>
</listitem>
<listitem>
<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>
</listitem>
<listitem>
<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>
</listitem>
<listitem>
<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>
</listitem>
<listitem>
<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>
</listitem>
</itemizedlist>
</answer>
</qandaentry>
<qandaentry id="faq.aliases">
<question id="q-aliases">
<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 id="a-aliases">
<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 <function>bcmp</function> instead of
<function>memcmp</function>, <function>index</function> instead of
<function>strchr</function>, and <function>rindex</function> instead of
<function>strrchr</function>.</para>
</answer>
</qandaentry>
<qandaentry id="faq.crashes">
<question id="q-crashes">
<para>My program crashes normally, but doesn't under Valgrind, or vice
versa. What's happening?</para>
</question>
<answer id="a-crashes">
<para>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.</para>
<para>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.</para>
<para>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.</para>.
</answer>
</qandaentry>
<qandaentry id="faq.hiddenbug">
<question id="q-hiddenbug">
<para> Memcheck doesn't report any errors and I know my program has
errors.</para>
</question>
<answer id="a-hiddenbug">
<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
<option>--trace-children=yes</option> 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 option <option>--log-socket=127.0.0.1:12345</option> (if you want
logging output sent to port <literal>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 for more details.</para>
<para>Second, if your program is statically linked, most Valgrind
tools will only work well if they are able to replace certain
functions, such as <function>malloc</function>, with their own
versions. By default, statically linked <function>malloc
functions</function> are not replaced. A key indicator of this is
if Memcheck says:
<programlisting>
All heap blocks were freed -- no leaks are possible
</programlisting>
when you know your program calls <function>malloc</function>. The
workaround is to use the option
<option>--soname-synonyms=somalloc=NONE</option>
or to avoid statically linking your program.</para>
<para>There will also be no replacement if you use an alternative
<function>malloc library</function> such as tcmalloc, jemalloc,
... In such a case, the
option <option>--soname-synonyms=somalloc=zzzz</option> (where
zzzz is the soname of the alternative malloc library) will allow
Valgrind to replace the functions.</para>
</answer>
</qandaentry>
<qandaentry id="faq.overruns">
<question id="q-overruns">
<para>Why doesn't Memcheck find the array overruns in this
program?</para>
<programlisting>
int static[5];
int main(void)
{
int stack[5];
static[5] = 0;
stack [5] = 0;
return 0;
}
</programlisting>
</question>
<answer id="a-overruns">
<para>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.</para>
<para>However, the experimental tool SGcheck can detect errors like
this. Run Valgrind with the <option>--tool=exp-sgcheck</option> option
to try it, but be aware that it is not as robust as Memcheck.</para>
</answer>
</qandaentry>
</qandadiv>
<!-- Miscellaneous -->
<qandadiv id="faq.misc" xreflabel="Miscellaneous">
<title>Miscellaneous</title>
<qandaentry id="faq.writesupp">
<question id="q-writesupp">
<para>I tried writing a suppression but it didn't work. Can you
write my suppression for me?</para>
</question>
<answer id="a-writesupp">
<para>Yes! Use the <option>--gen-suppressions=yes</option> 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
mangled (that is, not demangled).</para>
</answer>
</qandaentry>
<qandaentry id="faq.deflost">
<question id="q-deflost">
<para>With Memcheck's memory leak detector, what's the
difference between "definitely lost", "indirectly lost", "possibly
lost", "still reachable", and "suppressed"?</para>
</question>
<answer id="a-deflost">
<para>The details are in the Memcheck section of the user manual.</para>
<para>In short:</para>
<itemizedlist>
<listitem>
<para>"definitely lost" means your program is leaking memory --
fix those leaks!</para>
</listitem>
<listitem>
<para>"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.
</para>
</listitem>
<listitem>
<para>"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
<option>--show-possibly-lost=no</option> if you don't want to see
these reports.</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
<option>--show-reachable=yes</option> 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>
<qandaentry id="faq.undeferrors">
<question id="q-undeferrors">
<para>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?</para>
</question>
<answer id="a-undeferrors">
<para>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
<option>--track-origins=yes</option> option. It will run slower than
usual, but will give you extra information about the origin of
uninitialised values.</para>
<para>Or if you want to do it the old fashioned way, you can use the
client request
<computeroutput>VALGRIND_CHECK_VALUE_IS_DEFINED</computeroutput> 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.</para>
<para>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.</para>
</answer>
</qandaentry>
<qandaentry id="faq.attach">
<question id="q-attach">
<para>Is it possible to attach Valgrind to a program that is already
running?</para>
</question>
<answer id="a-attach">
<para>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.</para>
<para>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
<computeroutput>callgrind_control</computeroutput> program for details.
</para>
</answer>
</qandaentry>
</qandadiv>
<!-- Further Assistance -->
<qandadiv id="faq.help" xreflabel="How To Get Further Assistance">
<title>How To Get Further Assistance</title>
<!-- WARNING: this file should not xref other parts of the docs, because it
is built standalone as FAQ.txt. That's why we link to, for example, the
online copy of the manual. -->
<qandaentry id="e-help">
<!-- <question><para/></question> -->
<answer id="a-help">
<para>Read the appropriate section(s) of the
<ulink url="&vg-docs-url;">Valgrind Documentation</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>If you think an answer in this FAQ is incomplete or inaccurate, please
e-mail <ulink url="mailto:&vg-vemail;">&vg-vemail;</ulink>.</para>
<para>If you have tried all of these things and are still
stuck, you can try mailing the
<ulink url="&vg-lists-url;">valgrind-users mailing list</ulink>.
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.</para>
</answer>
</qandaentry>
</qandadiv>
<!-- FAQ ends here -->
</qandaset>
<!-- template
<qandadiv id="faq.installing" xreflabel="Installing">
<title>Installing</title>
<qandaentry id="faq.problem">
<question id="q-problem">
<para></para>
</question>
<answer id="a-problem">
<para></para>
</answer>
</qandaentry>
</qandadiv>
-->
</article>
</book>