Naive Timing of C Code.

timing_0.1.c

#include <sys/time.h>

// ...
{
    struct timeval t1, t2, diff;
    double interval;
    int i;

    gettimeofday(t1, NULL);
    for ( i = 0 ; i < 10000 ; i++ ){
        my_time_critical_function();
    }
    gettimeofday(t2, NULL);
    
    timersub(&t2, &t1, &diff);
    interval = diff.tv_sec + (diff.tv_usec / 1000000.0 ) ;

    printf("It took %f seconds\n", interval);
}
// ...

If you're on linux you should look into using
clock_gettime(CLOCK_MONOTONIC, &tv);
it has a better resolution than gettimeofday!

@oneamtu: I'm writing a blogpost and embedding this. It's one step in the evolution of a timing solution, and in the end, I end up using Glib's Timers which are cross-platform and have the maximum accuracy on the platform (as I understand). I'll be mentioning clock_gettime() in the post though! Cheers for mentioning it!