Measuring elapsed time in linux for a c program

Three alternatives

1. clock()
2. gettimeofday()
3. clock_gettime()

clock_gettime() goes upto nanosecond accuracy and it supports 4 clocks.

• CLOCK_REALTIME

  System-wide realtime clock. Setting this clock requires appropriate privileges.

• CLOCK_MONOTONIC

  Clock that cannot be set and represents monotonic time since some unspecified starting point.

• CLOCK_PROCESS_CPUTIME_ID

  High-resolution per-process timer from the CPU.

• CLOCK_THREAD_CPUTIME_ID

  Thread-specific CPU-time clock.

You can use it as

#include <time.h>

struct timespec start, stop;

clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start);

/// do something

clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &stop);

double result = (stop.tv_sec - start.tv_sec) * 1e6 + (stop.tv_nsec - start.tv_nsec) / 1e3;    // in microseconds

Note: The clock() function returns CPU time for your process, not wall clock time. I believe this is what the OP was interested in. If wall clock time is desired, then gettimeofday() is a good choice as suggested by an earlier answer. clock_gettime() can do either one if your system supports it; on my linux embedded system clock_gettime() is not supported, but clock() and gettimeofday() are.

Below is the code for getting wall clock time using gettimeofday()

#include <stdio.h> // for printf()
#include <sys/time.h> // for clock_gettime()
#include <unistd.h> // for usleep()

int main() {
    struct timeval start, end;
    long secs_used,micros_used;

    gettimeofday(&start, NULL);
    usleep(1250000); // Do the stuff you want to time here
    gettimeofday(&end, NULL);

    printf("start: %d secs, %d usecs\n",start.tv_sec,start.tv_usec);
    printf("end: %d secs, %d usecs\n",end.tv_sec,end.tv_usec);

    secs_used=(end.tv_sec - start.tv_sec); //avoid overflow by subtracting first
    micros_used= ((secs_used*1000000) + end.tv_usec) - (start.tv_usec);

    printf("micros_used: %d\n",micros_used);
    return 0;
}

To start with you need to use floating point arithmetics. Any integer value divided by a larger integer value will be zero, always.

And of course you should actually do something between getting the start and end times.

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By the way, if you have access to gettimeofday it's normally preferred over clock as it has higher resolution. Or maybe clock_gettime which has even higher resolution.