How to Measure Execution Time in C: time(), clock() and gettimeofday()

Measuring elapsed seconds with time()

Include #include <stdio.h> and #include <time.h>. Record the start time with time_t start = time(NULL);, execute the workload, record the end time, compute the difference with difftime, and print the result.

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

int main() {
    time_t start = time(NULL);
    for (long i = 0; i < 1000000000L; ++i) ;
    time_t end = time(NULL);
    double secs = difftime(end, start);
    printf("Elapsed: %.2f seconds
", secs);
    return 0;
}

Typical output on a modern CPU is around 3 seconds.

Higher‑resolution timing with clock()

Use clock_t start = clock(); and clock_t end = clock();. Convert the tick difference to seconds by dividing by CLOCKS_PER_SEC.

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

int main() {
    clock_t start = clock();
    for (long i = 0; i < 1000000000L; ++i) ;
    clock_t end = clock();
    double secs = (double)(end - start) / CLOCKS_PER_SEC;
    printf("Elapsed: %.6f seconds
", secs);
    return 0;
}

CLOCKS_PER_SEC

is defined in time.h and its exact value varies by platform.

Microsecond‑level timing with gettimeofday()

Include #include <sys/time.h>. Declare two struct timeval variables, capture timestamps with gettimeofday(&start, NULL) and gettimeofday(&end, NULL), then compute elapsed time as seconds plus microseconds divided by 1 e6.

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

int main() {
    struct timeval start, end;
    gettimeofday(&start, NULL);
    for (long i = 0; i < 1000000000L; ++i) ;
    gettimeofday(&end, NULL);
    double secs = (end.tv_sec - start.tv_sec) +
                  (end.tv_usec - start.tv_usec) / 1e6;
    printf("Elapsed: %.6f seconds
", secs);
    return 0;
}

Typical output shows about 2.5 seconds, demonstrating finer granularity than time() or clock().

Key considerations

time() returns whole‑second resolution; suitable only for coarse measurements.

clock() measures CPU time with higher resolution but is limited by CLOCKS_PER_SEC and may not reflect wall‑clock time on multi‑core systems.

gettimeofday() provides microsecond resolution and is appropriate for high‑precision benchmarks, though it is deprecated on some platforms in favor of clock_gettime.

Actual timings depend on hardware, compiler optimizations, and system load.