[PATCH] [libcxx] Partially address a FIXME in steady_clock::now()

[Jon Roelofs]

Hi mclow.lists,

http://reviews.llvm.org/D4045

Files:
  libcxx/src/chrono.cpp

Index: libcxx/src/chrono.cpp
===================================================================
--- libcxx/src/chrono.cpp
+++ libcxx/src/chrono.cpp
@@ -108,11 +108,6 @@
 }
 
 #else  // __APPLE__
-// FIXME: We assume that clock_gettime(CLOCK_MONOTONIC) works on
-// non-apple systems.  Instead, we should check _POSIX_TIMERS and
-// _POSIX_MONOTONIC_CLOCK and fall back to something else if those
-// don't exist.
-
 // Warning:  If this is not truly steady, then it is non-conforming.  It is
 //  better for it to not exist and have the rest of libc++ use system_clock
 //  instead.
@@ -120,10 +115,18 @@
 steady_clock::time_point
 steady_clock::now() _NOEXCEPT
 {
+#if (defined(_POSIX_TIMERS) && _POSIX_TIMERS > 0) && \
+    (defined(_POSIX_MONOTONIC_CLOCK) && _POSIX_MONOTONIC_CLOCK > 0)
     struct timespec tp;
     if (0 != clock_gettime(CLOCK_MONOTONIC, &tp))
         __throw_system_error(errno, "clock_gettime(CLOCK_MONOTONIC) failed");
     return time_point(seconds(tp.tv_sec) + nanoseconds(tp.tv_nsec));
+#else
+#warning According to unistd.h, there is no monotonic clock on this system so \
+         we're falling back to std::system_clock (which may not be \
+         monotonic, and therefore may not be conforming).
+    return time_point(system_clock::now().time_since_epoch());
+#endif
 }
 #endif  // __APPLE__
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