[Bug libstdc++/114838] New: __gthread_cond_t et. al. used unconditionally by std_mutex.h

[Bug libstdc++/114838] New: __gthread_cond_t et. al. used unconditionally by std_mutex.h

[pexu@gcc-bugzilla.mail.kapsi.fi]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114838

            Bug ID: 114838
           Summary: __gthread_cond_t et. al. used unconditionally by
                    std_mutex.h
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pexu@gcc-bugzilla.mail.kapsi.fi
  Target Milestone: ---

Hi.

libstdc++/include/bits/std_mutex.h, which is included by <mutex>, currently
uses internal condition variable types __gthread_cond_t et. al.
unconditionally.  

However, these types might be unavailable.

For an example, libgcc/config/i386/gthr-win32.h (i.e. Windows targets) only
defines these (and __GTHREAD_HAS_COND) if _WIN32_WINNT >= 0x0600. 
Unfortunately, it is possible, that _WIN32_WINNT is set by default to a lower
value.  Such is the case with gdb and it uses 0x0501 by default.

As of writing, both 0x0501 (Windows XP, EOL 2014) and 0x0600 (Windows Vista,
EOL 2017) have been out of support for a number of years.

This issue is also present on GCC 13.

[Bug libstdc++/114838] __gthread_cond_t et. al. used unconditionally by std_mutex.h

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114838

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It's guarded with _GLIBCXX_HAS_GTHREADS which is defined by configure when
__GTHREADS_CXX0X is defined by <gthr.h>, which for gthr-win32.h means:

#if _WIN32_WINNT >= 0x0600
#define __GTHREAD_HAS_COND 1
#define __GTHREADS_CXX0X 1
#endif

So libstdc++ assumes that __GTHREADS_CXX0X is fixed at build-time, not
something that changes from one compilation to the next.