Why isn't timespec_get defined on any C compiler on my Mac?

Question

According to the C11 Standard (7.27.2.5), there is a function timespec_get specified in time.h. I have tried several compilers, including clang and several versions of gcc, which are supposed to support C11, but this function is always missing. The macro TIME_UTC is also missing.

Here is a test file mytime.c:

#include <time.h>
#include <stdio.h>
int main() {
  printf("C version: %ld\n", __STDC_VERSION__);
  fflush(stdout);
  struct timespec ts;
  timespec_get(&ts, TIME_UTC);
}

and the output using Clang:

$ cc --version
Apple LLVM version 8.0.0 (clang-800.0.42.1)
Target: x86_64-apple-darwin15.6.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

$ cc -std=c11 mytime.c
mytime.c:9:3: warning: implicit declaration of function 'timespec_get' is invalid in C99
      [-Wimplicit-function-declaration]
  timespec_get(&ts, TIME_UTC);
  ^
mytime.c:9:21: error: use of undeclared identifier 'TIME_UTC'
  timespec_get(&ts, TIME_UTC);
                    ^
1 warning and 1 error generated.

I commented out the timespec_get line just to make sure I am using C11, and I am.

I get basically the same results for gcc versions 4.8, 5, and 6.

I am using a Mac, OS 10.11.6.

Answer 1

The Mac OS X standard library does not conform to any modern version of C or POSIX. It is stuck at C99 and POSIX 2001 and has conformance problems even with respect to these.

Answer 2

MacOS 10.15 supports timespec_get. This is taken from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/time.h:

#if (__DARWIN_C_LEVEL >= __DARWIN_C_FULL) && \
        ((defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L) || \
        (defined(__cplusplus) && __cplusplus >= 201703L))
/* ISO/IEC 9899:201x 7.27.2.5 The timespec_get function */
#define TIME_UTC        1       /* time elapsed since epoch */
__API_AVAILABLE(macosx(10.15), ios(13.0), tvos(13.0), watchos(6.0))
int timespec_get(struct timespec *ts, int base);
#endif

You need to be building with C11 or C++17 or newer when time.h is included.

I have not done any timing or other investigation into whether gettimeofday or timespec_get is better for this purpose on Mac 10.15, only that timespec_get becomes available.