[Patch] [libc++] Don't use <sys/time.h>/gettimeofday() when possible

[Ed Schouten]

Hi there,

I am currently doing some experiments to see how easy/hard it is to
get libc++ to work in an environment that only implements a 'sane'
subset of POSIX.

Looking at time functions that are provided by POSIX, it seems that
<sys/time.h> is slowly being deprecated in favour of <time.h>.
clock_gettime() and friends are part of the base definitions, whereas
gettimeofday() is only part of the optional X/Open System Interfaces.

libc++'s src/chrono.cpp already uses clock_gettime() on all platforms
for CLOCK_MONOTONIC, except on OS X, where clock_gettime() is not
available. I would really love to see us do the same for the real-time
clock, where we just use CLOCK_REALTIME, instead of using
gettimeofday().

Attached is a patch that adds exactly this. Would anyone mind if I
were to push this patch into the tree one of these days?

Link: http://80386.nl/pub/20141007-libcxx-gettimeofday.txt

Thanks,
-- 
Ed Schouten <ed at 80386.nl>
-------------- next part --------------
Index: include/chrono
===================================================================
--- include/chrono	(revision 219076)
+++ include/chrono	(working copy)
@@ -915,7 +915,7 @@
 class _LIBCPP_TYPE_VIS system_clock
 {
 public:
-    typedef microseconds                     duration;
+    typedef nanoseconds                      duration;
     typedef duration::rep                    rep;
     typedef duration::period                 period;
     typedef chrono::time_point<system_clock> time_point;
Index: src/chrono.cpp
===================================================================
--- src/chrono.cpp	(revision 219076)
+++ src/chrono.cpp	(working copy)
@@ -8,13 +8,13 @@
 //===----------------------------------------------------------------------===//
 
 #include "chrono"
-#include <sys/time.h>        //for gettimeofday and timeval
 #ifdef __APPLE__
+#include <sys/time.h>        // for gettimeofday and timeval
 #include <mach/mach_time.h>  // mach_absolute_time, mach_timebase_info_data_t
 #else  /* !__APPLE__ */
 #include <cerrno>  // errno
 #include <system_error>  // __throw_system_error
-#include <time.h>  // clock_gettime, CLOCK_MONOTONIC
+#include <time.h>  // clock_gettime, CLOCK_MONOTONIC and CLOCK_REALTIME
 #endif  // __APPLE__
 
 _LIBCPP_BEGIN_NAMESPACE_STD
@@ -29,9 +29,16 @@
 system_clock::time_point
 system_clock::now() _NOEXCEPT
 {
+#ifdef __APPLE__
     timeval tv;
     gettimeofday(&tv, 0);
-    return time_point(seconds(tv.tv_sec) + microseconds(tv.tv_usec));
+    return time_point(seconds(tv.tv_sec) + nanoseconds(tv.tv_usec * 1000));
+#else  // __APPLE__
+    struct timespec tp;
+    if (0 != clock_gettime(CLOCK_REALTIME, &tp))
+        __throw_system_error(errno, "clock_gettime(CLOCK_REALTIME) failed");
+    return time_point(seconds(tp.tv_sec) + nanoseconds(tp.tv_nsec));
+#endif  // __APPLE__
 }
 
 time_t