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The pthread_mutex_clocklock function is available in glibc since the 2.30 release. If this function is available in the C library it can be used to fix PR libstdc++/78237 by supporting steady_clock properly with timed_mutex. This means that code using timed_mutex::try_lock_for or timed_mutex::wait_until with steady_clock is no longer subject to timing out early or potentially waiting for much longer if the system clock is warped at an inopportune moment. If pthread_mutex_clocklock is available then steady_clock is deemed to be the "best" clock available which means that it is used for the relative try_lock_for calls and absolute try_lock_until calls using steady_clock and user-defined clocks. Calls explicitly using system_clock (aka high_resolution_clock) continue to use CLOCK_REALTIME via __gthread_cond_timedwait. If pthread_mutex_clocklock is not available then system_clock is deemed to be the "best" clock available which means that the previous suboptimal behaviour remains. 2019-12-02 Mike Crowe <mac@mcrowe.com> PR libstdc++/78237 Add full steady_clock support to timed_mutex * acinclude.m4 (GLIBCXX_CHECK_PTHREAD_MUTEX_CLOCKLOCK): Define to detect presence of pthread_mutex_clocklock function. * config.h.in: Regenerate. * configure: Regenerate. * configure.ac: Call GLIBCXX_CHECK_PTHREAD_MUTEX_CLOCKLOCK. * include/std/mutex (__timed_mutex_impl): Remove unnecessary __clock_t. (__timed_mutex_impl::_M_try_lock_for): Use best clock to turn relative timeout into absolute timeout. (__timed_mutex_impl::_M_try_lock_until): Keep existing implementation for system_clock. Add new implementation for steady_clock that calls _M_clocklock. Modify overload for user-defined clock to use a relative wait so that it automatically uses the best clock. [_GLIBCXX_USE_PTHREAD_MUTEX_CLOCKLOCK] (timed_mutex::_M_clocklock): New member function. (recursive_timed_mutex::_M_clocklock): Likewise. From-SVN: r278901
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