Linux和OS X上的C语言中的ns精度单调时钟

/* 
This is based on the snippet current_utc_time.c from:
https://gist.github.com/jbenet/1087739

On OS X,compile with: gcc get_monotonic_time.c
   Linux,compile with: gcc get_monotonic_time.c -lrt
*/

#include <time.h>
#include <sys/time.h>
#include <stdio.h>

#ifdef __MACH__
#include <mach/clock.h>
#include <mach/mach.h>
#endif

// Use clock_gettime in linux,clock_get_time in OS X.
void get_monotonic_time(struct timespec *ts){
#ifdef __MACH__
  clock_serv_t cclock;
  mach_timespec_t mts;
  host_get_clock_service(mach_host_self(),SYSTEM_CLOCK,&cclock);
  clock_get_time(cclock,&mts);
  mach_port_deallocate(mach_task_self(),cclock);
  ts->tv_sec = mts.tv_sec;
  ts->tv_nsec = mts.tv_nsec;
#else
  clock_gettime(CLOCK_MONOTONIC,ts);
#endif
}

double get_elapsed_time(struct timespec *before,struct timespec *after){
  double deltat_s  = after->tv_sec - before->tv_sec;
  double deltat_ns = after->tv_nsec - before->tv_nsec;
  return deltat_s + deltat_ns*1e-9;
}

int main(){

  // Do something and time how long it takes.
  struct timespec before,after;
  get_monotonic_time(&before);
  double sum=0.;
  unsigned u;
  for(u=1; u<100000000; u++)
    sum += 1./u/u;
  get_monotonic_time(&after);
  printf("sum = %en",sum);
  printf("deltaT = %e sn",get_elapsed_time(&before,&after));

}