commit 92b47a321e14f98c524f6e67e7ecabad5afa7886
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Nov 23 17:17:09 2020

    libstdc++: Add configure checks for semaphores
    
    This moves the checks for POSIX semaphores to configure time. As well as
    requiring <semaphore.h> and SEM_VALUE_MAX, we also require the
    sem_timedwait function. That was only optional in POSIX 2001 (and is
    absent on Darwin).
    
    libstdc++-v3/ChangeLog:
    
            * acinclude.m4 (GLIBCXX_CHECK_GTHREADS): Check for
            * config.h.in: Regenerate.
            * configure: Regenerate.
            * include/bits/semaphore_base.h (_GLIBCXX_HAVE_POSIX_SEMAPHORE):
            Check autoconf macro instead of defining it here.

diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index 486347b34d94..a4a0bb840181 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/libstdc++-v3/acinclude.m4
@@ -4089,6 +4089,43 @@ AC_DEFUN([GLIBCXX_CHECK_GTHREADS], [
     fi
   fi
 
+  AC_CHECK_HEADER(semaphore.h, [
+    AC_MSG_CHECKING([for POSIX Semaphores and sem_timedwait])
+    AC_TRY_COMPILE([
+	#include <unistd.h>
+	#include <semaphore.h>
+	#include <limits.h>
+      ],
+      [
+	#if !defined _POSIX_TIMEOUTS || _POSIX_TIMEOUTS <= 0
+	# error "POSIX Timeouts option not supported"
+	#elif !defined _POSIX_SEMAPHORES || _POSIX_SEMAPHORES <= 0
+	# error "POSIX Semaphores option not supported"
+	#else
+	#if defined SEM_VALUE_MAX
+	constexpr int sem_value_max = SEM_VALUE_MAX;
+	#elif defined _POSIX_SEM_VALUE_MAX
+	constexpr int sem_value_max = _POSIX_SEM_VALUE_MAX;
+	#else
+	# error "SEM_VALUE_MAX not available"
+	#endif
+	sem_t sem;
+	sem_init(&sem, 0, sem_value_max);
+	struct timespec ts = { 0 };
+	sem_timedwait(&sem, &ts);
+	#endif
+      ],
+      [ac_have_posix_semaphore=yes],
+      [ac_have_posix_semaphore=no])],
+      [ac_have_posix_semaphore=no])
+
+  if test $ac_have_posix_semaphore = yes ; then
+    AC_DEFINE(_GLIBCXX_HAVE_POSIX_SEMAPHORE,
+	      1,
+	      [Define to 1 if POSIX Semaphores with sem_timedwait are available in <semaphore.h>.])
+  fi
+  AC_MSG_RESULT([$ac_have_posix_semaphore])
+
   CXXFLAGS="$ac_save_CXXFLAGS"
   AC_LANG_RESTORE
 ])
diff --git a/libstdc++-v3/include/bits/semaphore_base.h b/libstdc++-v3/include/bits/semaphore_base.h
index 5e29d3783fe1..0692f95f24f2 100644
--- a/libstdc++-v3/include/bits/semaphore_base.h
+++ b/libstdc++-v3/include/bits/semaphore_base.h
@@ -39,11 +39,9 @@
 
 #include <ext/numeric_traits.h>
 
-#if __has_include(<semaphore.h>)
+#ifdef _GLIBCXX_HAVE_POSIX_SEMAPHORE
+# include <limits.h>
 # include <semaphore.h>
-# if defined SEM_VALUE_MAX || _POSIX_SEM_VALUE_MAX
-#  define _GLIBCXX_HAVE_POSIX_SEMAPHORE 1
-# endif
 #endif
 
 #include <chrono>