Calculating elapsed time in a C program in milliseconds

I want to calculate the time in milliseconds taken by the execution of some part of my program. I've been looking online, but there's not much info on this topic. Any of you know how to do this?

Best way to answer is with an example:

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

/* Return 1 if the difference is negative, otherwise 0.  */
int timeval_subtract(struct timeval *result, struct timeval *t2, struct timeval *t1)
{
    long int diff = (t2->tv_usec + 1000000 * t2->tv_sec) - (t1->tv_usec + 1000000 * t1->tv_sec);
    result->tv_sec = diff / 1000000;
    result->tv_usec = diff % 1000000;

    return (diff<0);
}

void timeval_print(struct timeval *tv)
{
    char buffer[30];
    time_t curtime;

    printf("%ld.%06ld", tv->tv_sec, tv->tv_usec);
    curtime = tv->tv_sec;
    strftime(buffer, 30, "%m-%d-%Y  %T", localtime(&curtime));
    printf(" = %s.%06ld\n", buffer, tv->tv_usec);
}

int main()
{
    struct timeval tvBegin, tvEnd, tvDiff;

    // begin
    gettimeofday(&tvBegin, NULL);
    timeval_print(&tvBegin);

    // lengthy operation
    int i,j;
    for(i=0;i<999999L;++i) {
        j=sqrt(i);
    }

    //end
    gettimeofday(&tvEnd, NULL);
    timeval_print(&tvEnd);

    // diff
    timeval_subtract(&tvDiff, &tvEnd, &tvBegin);
    printf("%ld.%06ld\n", tvDiff.tv_sec, tvDiff.tv_usec);

    return 0;
}

Another option ( at least on some UNIX ) is clock_gettime and related functions. These allow access to various realtime clocks and you can select one of the higher resolution ones and throw away the resolution you don't need.

The gettimeofday function returns the time with microsecond precision (if the platform can support that, of course):

The gettimeofday() function shall obtain the current time, expressed as seconds and microseconds since the Epoch, and store it in the timeval structure pointed to by tp. The resolution of the system clock is unspecified.

C libraries have a function to let you get the system time. You can calculate elapsed time after you capture the start and stop times.

The function is called gettimeofday() and you can look at the man page to find out what to include and how to use it.