[glib/glib-2-30] Revert "Simplify checks for CLOCK_MONOTONIC"

[Matthias Clasen <matthiasc src gnome org>]

commit ac386f54badf0343efac2becb2c99dad6d27f40d
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed Oct 19 17:46:11 2011 -0400

    Revert "Simplify checks for CLOCK_MONOTONIC"
    
    This reverts commit 7f66b04bfd21ef97492a68d0e26274b5dc76ad8f.

 configure.ac |   15 ++++++++++
 glib/gmain.c |   82 ++++++++++++++++++++++++++++++++++-----------------------
 2 files changed, 64 insertions(+), 33 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 3535a1d..5d1e6aa 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2512,6 +2512,21 @@ AC_CHECK_FUNCS(clock_gettime, [], [
 ])
 AC_SUBST(GLIB_RT_LIBS)
 
+AC_CACHE_CHECK(for monotonic clocks,
+    glib_cv_monotonic_clock,AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
+#include <time.h>
+#include <unistd.h>
+int main() {
+#if !(defined(_POSIX_MONOTONIC_CLOCK) && _POSIX_MONOTONIC_CLOCK >= 0 && defined(CLOCK_MONOTONIC))
+        #error No monotonic clock
+#endif
+	return 0;
+}
+]])],glib_cv_monotonic_clock=yes,glib_cv_monotonic_clock=no))
+if test "$glib_cv_monotonic_clock" = "yes"; then
+  AC_DEFINE(HAVE_MONOTONIC_CLOCK,1,[Have a monotonic clock])
+fi
+
 
 dnl ************************
 dnl *** g_atomic_* tests ***
diff --git a/glib/gmain.c b/glib/gmain.c
index cb5bed3..6662a09 100644
--- a/glib/gmain.c
+++ b/glib/gmain.c
@@ -2041,50 +2041,66 @@ g_get_monotonic_time (void)
 {
 #ifdef HAVE_CLOCK_GETTIME
   /* librt clock_gettime() is our first choice */
-  struct timespec ts;
-
-#ifdef CLOCK_MONOTONIC
-  clock_gettime (CLOCK_MONOTONIC, &ts);
+  {
+#ifdef HAVE_MONOTONIC_CLOCK
+    static volatile gsize clockid = 0;
 #else
-  clock_gettime (CLOCK_REALTIME, &ts);
+    static clockid_t clockid = CLOCK_REALTIME;
 #endif
+    struct timespec ts;
 
-  /* In theory monotonic time can have any epoch.
-   *
-   * glib presently assumes the following:
-   *
-   *   1) The epoch comes some time after the birth of Jesus of Nazareth, but
-   *      not more than 10000 years later.
-   *
-   *   2) The current time also falls sometime within this range.
-   *
-   * These two reasonable assumptions leave us with a maximum deviation from
-   * the epoch of 10000 years, or 315569520000000000 seconds.
-   *
-   * If we restrict ourselves to this range then the number of microseconds
-   * will always fit well inside the constraints of a int64 (by a factor of
-   * about 29).
-   *
-   * If you actually hit the following assertion, probably you should file a
-   * bug against your operating system for being excessively silly.
-   **/
-  g_assert (G_GINT64_CONSTANT (-315569520000000000) < ts.tv_sec &&
-            ts.tv_sec < G_GINT64_CONSTANT (315569520000000000));
-
-  return (((gint64) ts.tv_sec) * 1000000) + (ts.tv_nsec / 1000);
-
-#else /* !HAVE_CLOCK_GETTIME */
+#ifdef HAVE_MONOTONIC_CLOCK
+    if (g_once_init_enter (&clockid))
+      {
+	clockid_t best_clockid;
 
+	if (sysconf (_SC_MONOTONIC_CLOCK) >= 0)
+	  best_clockid = CLOCK_MONOTONIC;
+	else
+	  best_clockid = CLOCK_REALTIME;
+	g_once_init_leave (&clockid, (gsize)best_clockid);
+      }
+#endif
+
+    clock_gettime (clockid, &ts);
+
+    /* In theory monotonic time can have any epoch.
+     *
+     * glib presently assumes the following:
+     *
+     *   1) The epoch comes some time after the birth of Jesus of Nazareth, but
+     *      not more than 10000 years later.
+     *
+     *   2) The current time also falls sometime within this range.
+     *
+     * These two reasonable assumptions leave us with a maximum deviation from
+     * the epoch of 10000 years, or 315569520000000000 seconds.
+     *
+     * If we restrict ourselves to this range then the number of microseconds
+     * will always fit well inside the constraints of a int64 (by a factor of
+     * about 29).
+     *
+     * If you actually hit the following assertion, probably you should file a
+     * bug against your operating system for being excessively silly.
+     **/
+    g_assert (G_GINT64_CONSTANT (-315569520000000000) < ts.tv_sec &&
+              ts.tv_sec < G_GINT64_CONSTANT (315569520000000000));
+
+    return (((gint64) ts.tv_sec) * 1000000) + (ts.tv_nsec / 1000);
+  }
+#else
   /* It may look like we are discarding accuracy on Windows (since its
    * current time is expressed in 100s of nanoseconds) but according to
    * many sources, the time is only updated 64 times per second, so
    * microsecond accuracy is more than enough.
    */
-  GTimeVal tv;
+  {
+    GTimeVal tv;
 
-  g_get_current_time (&tv);
+    g_get_current_time (&tv);
 
-  return (((gint64) tv.tv_sec) * 1000000) + tv.tv_usec;
+    return (((gint64) tv.tv_sec) * 1000000) + tv.tv_usec;
+  }
 #endif
 }