How can I find the calculation time in nanosecond in C?

Question

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

int main()
{
    int i,j,vertex=100;
    double cal;
    clock_t start, end;
    start = clock();

    for(i=0; i<vertex; i++)
    {
        for(j=0; j<vertex; j++)
        {
            i=i+j;
        }
    }

    for(j=0; j<vertex; j++)
    {
        for(i=0; i<vertex; i++)
        {
            i=i+j;
        }
    }
    end = clock();
    cal = (end-start)/CLOCKS_PER_SEC;

    printf("%.20lf",cal);
    return 0;
}

Answer 1

Source: https://stackoverflow.com/questions/12722904/how-to-use-struct-timeval-to-get-the-execution-time

Corrected program below:

#include <stdio.h>
#include <time.h>
#include <sys/time.h> // for struct timeval

int main()
{
    int i,j,vertex=100;
    double cal;
    clock_t start, end;
    struct timeval tvalBefore, tvalAfter;
    gettimeofday (&tvalBefore, NULL);
    start = clock();

    for(i=0; i<vertex; i++)
    {
        for(j=0; j<vertex; j++)
        {
            i=i+j;
        }
    }

    for(j=0; j<vertex; j++)
    {
        for(i=0; i<vertex; i++)
        {
            i=i+j;
        }
    }
    end = clock();
    //sleep(1);
    gettimeofday (&tvalAfter, NULL);
    cal = (end-start)/CLOCKS_PER_SEC;

    printf("%.20lf",cal);
    printf("\nTime in microseconds: %ld microseconds\n",
            ((tvalAfter.tv_sec - tvalBefore.tv_sec)*1000000L
           +tvalAfter.tv_usec) - tvalBefore.tv_usec
          ); // Added semicolon
    return 0;
}

Please refer man pages (if on linux ) or google the function calls & you will get enough data t o make sense of them.

C++ has a nano second clock, not sure about it in C.Some third party libs can give nano second precision, but even achieving computing times of micro seconds is a tremendous effort .