Getting the Time of Day Stamp

Example A-3 : Program to Exercise gettimeofday()

#include <sys/time.h>
#include <stdio.h>
#define LOOPS 1000
/*
 * This function loops on gettimeofday() until the returned time
 * changes by more than 1 microsecond, then reports the difference.
 * 
 * We look for a change of >1 usec because in some older systems
 * (apparently not in 6.2) IRIX, in order to ensure that gettimeofday()
 * never returns the same value twice while not actually updating the
 * timer, just adds 1 usec on each call until a normal dispatching
 * tick occurs. In 6.2 systems, IRIX actually recalculates the timer
 * on each call.
 * 
 * The function also updates a maximum loop-count value.
 */
long getTODdiff(int * pMaxLoops)
{
    long first, second;
    int nloops = 0;
    struct timeval tod;
    struct timezone tz;
    gettimeofday(&tod, &tz);
    first = tod.tv_usec;
    do
    {
        gettimeofday(&tod, &tz);
        second = tod.tv_usec;
        ++nloops;
    } while (first == (second-nloops));
    if (first > second)
        second += 1000000;
    if (pMaxLoops)
        if (nloops > *pMaxLoops)
            *pMaxLoops = nloops;
    return second - first;
}
int main(int argc, char *argv[])
{
    int j, limit;
    int maxLoops = 0;
    long sample, sum, min, max;
    double mean;
    
    limit = LOOPS;
    if (argc > 1)
        limit = atoi(argv[1]);
    /* get past the first call, which is likely to be short */
    sum = getTODdiff(NULL);
    
    /* exercise gettimeofday a few times */
    for (j=0, sum=0, min=999999, max=0; j< limit; j++)
    {
        sample = getTODdiff(&maxLoops);
        sum += sample;
        if (sample > max) max = sample;
        if (sample < min) min = sample;
    }
    mean = sum/LOOPS;
    printf("gettimeofday() increments: min=%ld, mean=%g, max=%ld\n",
                                           min,      mean,    max);
    if (maxLoops > 1)
        printf("Max number of loops %d\n", maxLoops);
    else
        printf("Honest timer update in this system.\n");
    return 0;
}