How to measure time in milliseconds using ANSI C

Returns up to nanoseconds, rounded to the resolution of the implementation.

Looks like an ANSI ripoff from POSIX' clock_gettime.

Example: a printf is done every 100ms on Ubuntu 15.10:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

static long get_nanos(void) {
    struct timespec ts;
    timespec_get(&ts, TIME_UTC);
    return (long)ts.tv_sec * 1000000000L + ts.tv_nsec;
}

int main(void) {
    long nanos;
    long last_nanos;
    long start;
    nanos = get_nanos();
    last_nanos = nanos;
    start = nanos;
    while (1) {
        nanos = get_nanos();
        if (nanos - last_nanos > 100000000L) {
            printf("current nanos: %ld\n", nanos - start);
            last_nanos = nanos;
        }
    }
    return EXIT_SUCCESS;
}

The C11 N1570 standard draft 7.27.2.5 "The timespec_get function says":

If base is TIME_UTC, the tv_sec member is set to the number of seconds since an implementation defined epoch, truncated to a whole value and the tv_nsec member is set to the integral number of nanoseconds, rounded to the resolution of the system clock. (321)

321) Although a struct timespec object describes times with nanosecond resolution, the available resolution is system dependent and may even be greater than 1 second.

C++11 also got std::chrono::high_resolution_clock: C++ Cross-Platform High-Resolution Timer

timespec_get 함수에 해당하는 TIME_UTC 에 대한 상수는 다음과 같이 정의되어 있다. (사실 별 쓸모는 없지만 왠지 궁금해서 적어놓는다.)

glibc 2.21 implementation

Can be found under sysdeps/posix/timespec_get.c as:

int
timespec_get (struct timespec *ts, int base)
{
  switch (base)
    {
    case TIME_UTC:
      if (__clock_gettime (CLOCK_REALTIME, ts) < 0)
        return 0;
      break;

    default:
      return 0;
    }

  return base;
}

so clearly:

• only TIME_UTC is currently supported

• it forwards to __clock_gettime (CLOCK_REALTIME, ts), which is a POSIX API: http://pubs.opengroup.org/onlinepubs/9699919799/functions/clock_getres.html

  Linux x86-64 has a clock_gettime system call.

  Note that this is not a fail-proof micro-benchmarking method because:

  • man clock_gettime says that this measure may have discontinuities if you change some system time setting while your program runs. This should be a rare event of course, and you might be able to ignore it.

  • this measures wall time, so if the scheduler decides to forget about your task, it will appear to run for longer.

  For those reasons getrusage() might be a better better POSIX benchmarking tool, despite it's lower microsecond maximum precision.

  More information at: Measure time in Linux - time vs clock vs getrusage vs clock_gettime vs gettimeofday vs timespec_get?