FAQ.txt

Valgrind FAQ
Release 3.14.0 9 October 2018
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Table of Contents
1. Background
2. Compiling, installing and configuring
3. Valgrind aborts unexpectedly
4. Valgrind behaves unexpectedly
5. Miscellaneous
6. How To Get Further Assistance

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1. Background
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1.1. How do you pronounce "Valgrind"? 
The "Val" as in the word "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 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: 
<http://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. 

------------------------------------------------------------------------

2.2. 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. 

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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 
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. 

------------------------------------------------------------------------

3.2. 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. 

------------------------------------------------------------------------

3.3. 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. 

------------------------------------------------------------------------

3.4. 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. 


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4. Valgrind behaves unexpectedly
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4.1. 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: 
<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. 

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4.2. 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. If you 
use the option --keep-debuginfo=yes, then Valgrind will keep the debug 
information in order to show the stack traces, at the price of increased 
memory. An alternate workaround 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)
    
------------------------------------------------------------------------

4.3. 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. 

------------------------------------------------------------------------

4.4. 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. 

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4.5. 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 will 
only work well if they are able to replace certain functions, such as 
malloc, with their own versions. By default, statically linked malloc 
functions are not replaced. 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 use the option 
--soname-synonyms=somalloc=NONE or to avoid statically linking your 
program. 

There will also be no replacement if you use an alternative malloc 
library such as tcmalloc, jemalloc, ... In such a case, the option 
--soname-synonyms=somalloc=zzzz (where zzzz is the soname of the 
alternative malloc library) will allow Valgrind to replace the 
functions. 

------------------------------------------------------------------------

4.6. 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. 


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5. Miscellaneous
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5.1. 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 
(that is, not demangled). 

------------------------------------------------------------------------

5.2. 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. 

------------------------------------------------------------------------

5.3. 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. 

------------------------------------------------------------------------

5.4. 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. 


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6. How To Get Further Assistance
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Read the appropriate section(s) of the Valgrind Documentation: 
<http://www.valgrind.org/docs/manual/index.html>. 

Search: <http://search.gmane.org> the valgrind-users: 
<http://news.gmane.org/gmane.comp.debugging.valgrind> 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 valgrind@valgrind.org: <valgrind@valgrind.org>. 

If you have tried all of these things and are still stuck, you can try 
mailing the valgrind-users mailing list: 
<http://www.valgrind.org/support/mailing_lists.html>. 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.