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High Resolution Timer function in C/C++

From time to time I need a high resolution timer for my projects. The code below can be dropped in in any .c or .cpp file or if you like you can split it into a header and .c/.cpp file if you'd like. Please contact me via twitter for changes and improvements.

You use this by simply calling ns() in in your code. It will return the current time in nanoseconds.

Time	In nanoseconds
1 second	1.000.000.000 ns
1 milli second	1.000.000 ns
1 micro second	1.000 ns

/* ----------------------------------------------------------------------- */
/*
  Easy embeddable cross-platform high resolution timer function. For each 
  platform we select the high resolution timer. You can call the 'ns()' 
  function in your file after embedding this. 
*/
#include <stdint.h>
#if defined(__linux)
#  define HAVE_POSIX_TIMER
#  include <time.h>
#  ifdef CLOCK_MONOTONIC
#     define CLOCKID CLOCK_MONOTONIC
#  else
#     define CLOCKID CLOCK_REALTIME
#  endif
#elif defined(__APPLE__)
#  define HAVE_MACH_TIMER
#  include <mach/mach_time.h>
#elif defined(_WIN32)
#  define WIN32_LEAN_AND_MEAN
#  include <windows.h>
#endif
static uint64_t ns() {
  static uint64_t is_init = 0;
#if defined(__APPLE__)
    static mach_timebase_info_data_t info;
    if (0 == is_init) {
      mach_timebase_info(&info);
      is_init = 1;
    }
    uint64_t now;
    now = mach_absolute_time();
    now *= info.numer;
    now /= info.denom;
    return now;
#elif defined(__linux)
    static struct timespec linux_rate;
    if (0 == is_init) {
      clock_getres(CLOCKID, &linux_rate);
      is_init = 1;
    }
    uint64_t now;
    struct timespec spec;
    clock_gettime(CLOCKID, &spec);
    now = spec.tv_sec * 1.0e9 + spec.tv_nsec;
    return now;
#elif defined(_WIN32)
    static LARGE_INTEGER win_frequency;
    if (0 == is_init) {
      QueryPerformanceFrequency(&win_frequency);
      is_init = 1;
    }
    LARGE_INTEGER now;
    QueryPerformanceCounter(&now);
    return (uint64_t) ((1e9 * now.QuadPart)  / win_frequency.QuadPart);
#endif
}
/* ----------------------------------------------------------------------- */

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