[Bug c++/57975] New: Core dump caused by linking with -lpthread

[Bug c++/57975] New: Core dump caused by linking with -lpthread

[michi at triodia dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57975

            Bug ID: 57975
           Summary: Core dump caused by linking with -lpthread
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: michi at triodia dot com

The following code works as expected when compiling with

    c++ -g --std=c++11 test.cpp

The program hangs until it is interrupted.

When I compile the same code with

    c++ -g --std=c++11 test.cpp -lpthread

I get a segfault in wait():

Program received signal SIGSEGV, Segmentation fault.
__pthread_mutex_unlock_usercnt (mutex=0x0, decr=0) at pthread_mutex_unlock.c:37
37    pthread_mutex_unlock.c: No such file or directory.
(gdb) bt
#0  __pthread_mutex_unlock_usercnt (mutex=0x0, decr=0) at
pthread_mutex_unlock.c:37
#1  0x00007ffff7bc8c17 in pthread_cond_wait@@GLIBC_2.3.2 ()
    at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:94
#2  0x00007ffff79698ec in
std::condition_variable::wait(std::unique_lock<std::mutex>&) ()
   from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#3  0x0000000000400a69 in main () at test.cpp:13

Here is the complete source:

#include <condition_variable>
#include <mutex>

int main(int, char**)
{
    bool predicate = false;
    std::mutex mutex;
    std::condition_variable condvar;

    std::unique_lock<decltype(mutex)> lock;
    while (!predicate)
    {
        condvar.wait(lock);
    }
}

Compiler version is

    gcc (Ubuntu/Linaro 4.7.3-1ubuntu1) 4.7.3

running on Ubuntu Raring.

libpthread is:

    libpthread.so.0 (libc6,x86-64, OS ABI: Linux 2.6.24) =>
/lib/x86_64-linux-gnu/libpthread.so.0

[Bug c++/57975] Core dump caused by linking with -lpthread

[michi at triodia dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57975

Michi Henning <michi at triodia dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|unknown                     |4.7.3

--- Comment #1 from Michi Henning <michi at triodia dot com> ---
uname reports:

Linux ubuntu 3.8.0-19-generic #30-Ubuntu SMP Wed May 1 16:35:23 UTC 2013 x86_64
x86_64 x86_64 GNU/Linux

[Bug c++/57975] Core dump caused by linking with -lpthread

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57975

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|michi at triodia dot com           |jwakely.gcc at gmail dot com

--- Comment #2 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Jon, can you help me with this?

[Bug libstdc++/57975] Core dump caused by linking with -lpthread

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57975

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-07-24
          Component|c++                         |libstdc++
     Ever confirmed|0                           |1

[Bug libstdc++/57975] Core dump caused by linking with -lpthread

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57975

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |INVALID

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It only "works" without -lpthread because the implementation of
pthread_mutex_lock() in libc.so is a no-op, so your program is a busy loop that
spins using 100% CPU, not waiting patiently on the condvar.

When you link to libpthread you get a real implementation of pthread_mutex_lock
that fails because when you call condvar.wait(lock) you fail to meet the
precondition that lock.owns_lock() is true, so it's undefined behaviour.

[Bug libstdc++/57975] Core dump caused by linking with -lpthread

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57975

--- Comment #4 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Thanks!

[Bug libstdc++/57975] Core dump caused by linking with -lpthread

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57975

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I am working (slowly) on some additional Debug Mode checks in <mutex>,
<condition_variable> etc. so at some point you'll be able to debug this with
-D_GLIBCX_DEBUG

[Bug libstdc++/57975] Core dump caused by linking with -lpthread

[michi at triodia dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57975

--- Comment #6 from Michi Henning <michi at triodia dot com> ---
How embarrassing. The joys of default constructors :-( Stupid mistake of mine.