[Bug libstdc++/54847] --enable-libstdcxx-time=yes non-functional on darwin

["howarth at nitro dot med.uc.edu" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54847

--- Comment #27 from Jack Howarth <howarth at nitro dot med.uc.edu> 2012-10-08 13:47:32 UTC ---
The unistd.h header on darwin12 defines...

#define _POSIX_TIMERS                   (-1)            /* [TMR] */

>From the documentation on --enable-libstdcxx-time...

dnl Check for clock_gettime, nanosleep and sched_yield, used in the
dnl implementation of 20.11.7 [time.clock], and 30.3.2 [thread.thread.this]
dnl in the C++11 standard.
dnl
dnl --enable-libstdcxx-time
dnl --enable-libstdcxx-time=yes
dnl        checks for the availability of monotonic and realtime clocks,
dnl        nanosleep and sched_yield in libc and libposix4 and, if needed,
dnl        links in the latter.
dnl --enable-libstdcxx-time=rt
dnl        also searches (and, if needed, links) librt.  Note that this is
dnl        not always desirable because, in glibc, for example, in turn it
dnl        triggers the linking of libpthread too, which activates locking,
dnl        a large overhead for single-thread programs.
dnl --enable-libstdcxx-time=no
dnl --disable-libstdcxx-time
dnl        disables the checks completely

it seems that --enable-libstdcxx-time defaults to 'no' which would seem to
imply that it is effectively --disable-libstdcxx-time. The
--enable-libstdcxx-time=yes check is too strict for darwin because it doesn't
handle the case of _POSIX_TIMER defined to -1 for testing pre-POSIX 2008
nanosleep(). What would you consider to be an acceptable change to the test
at...

      AC_MSG_CHECKING([for nanosleep])
      AC_TRY_LINK(
        [#include <unistd.h>
         #include <time.h>
        ],
        [#if _POSIX_TIMERS > 0
          timespec tp;
         #endif
          nanosleep(&tp, 0);
        ], [ac_has_nanosleep=yes], [ac_has_nanosleep=no])

      AC_MSG_RESULT($ac_has_nanosleep)

in libstdc++-v3/acinclude.m4 to solve this limitation? Again I would note that
short-circuiting this test for  _POSIX_TIMERS > 0 introduces no regressions in
the libstdc++-v3 test suite so the darwin nanosleep() appears to be sufficient.
Or is the libstdc++-v3 testsuite currently incapable of properly testing this
feature?