Calculating Function time in nanoseconds in C code

Question

I need to know how can I calculate the time of a function in C code in nanoseconds.

I tried to repeat the function until consume some microseconds. Are there any other functions in time.h that can be used to calculate the time in nanoseconds?

Answer 1

You are never going to get nanosecond accuracy. Think about what you are asking: on a 1 GHz CPU 1 nanosecond is a clock cycle. No matter what you attempt to call, you will never get that kind of accuracy, you are better off sticking to microseconds. A similar question with many examples is here: C++ Cross-Platform High-Resolution Timer.

For c only: on windows you want to use the QueryPerformanceCounter. And here is more on QPC. Here is a related question on how to use QueryPerformanceCounter.

Answer 2

Regardless of how you approach this or what type of system/OS you are using, you are getting an approximate answer at best, with considerable variance due to the nature of the problem.

Second, you need a system that supports this kind of call. It's pretty easy if you're using QNX Neutrino:

http://www.qnx.com/developers/docs/6.3.0SP3/neutrino/lib_ref/c/clock_gettime.html

/*
 * This program calculates the time required to
 * execute the program specified as its first argument.
 * The time is printed in seconds, on standard out.
 */
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <time.h>

#define BILLION  1000000000L;

int main( int argc, char** argv )
  {
    struct timespec start, stop;
    double accum;

    if( clock_gettime( CLOCK_REALTIME, &start) == -1 ) {
      perror( "clock gettime" );
      return EXIT_FAILURE;
    }

    system( argv[1] );

    if( clock_gettime( CLOCK_REALTIME, &stop) == -1 ) {
      perror( "clock gettime" );
      return EXIT_FAILURE;
    }

    accum = ( stop.tv_sec - start.tv_sec )
             + (double)( stop.tv_nsec - start.tv_nsec )
               / (double)BILLION;
    printf( "%lf\n", accum );
    return EXIT_SUCCESS;
  }

Answer 3

The clock function in standard C is not useful for this. It usually has horrible resolution and it's inconsistent (between platforms) whether it measures elapsed wall time or cpu time consumed. You should use the POSIX-standard clock_gettime function (which has nanosecond resolution and lets you specify which clock you want to measure against) and emulate it with whatever system-specific clock operations are available on platforms that lack the POSIX function.