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

Measuring elapsed time with time()

The time() function returns the current calendar time as a time_t value (seconds since the Unix epoch). Record the value before and after the code segment and compute the difference with difftime to obtain the elapsed time in whole seconds.

#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 duration = difftime(end, start);
    printf("Elapsed time: %.2f seconds
", duration);
    return 0;
}

Higher‑resolution timing with clock()

clock()

returns the processor time consumed by the program as a clock_t. Dividing the difference by the macro CLOCKS_PER_SEC converts the value to seconds, typically giving millisecond‑level precision.

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

Microsecond precision using gettimeofday()

The POSIX gettimeofday() function fills a struct timeval with seconds and microseconds since the epoch. By subtracting the start and end structures you can achieve microsecond‑level timing.

#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 duration = (end.tv_sec - start.tv_sec) + (end.tv_usec - start.tv_usec) / 1e6;
    printf("Elapsed time: %.6f seconds
", duration);
    return 0;
}

Key considerations

time() : returns whole‑second granularity; low precision but simple for coarse measurements.

clock() : measures CPU time; precision depends on CLOCKS_PER_SEC and the operating system.

gettimeofday() : provides wall‑clock time with microsecond resolution; may be affected by system load or clock adjustments (e.g., NTP).

Actual timing results will vary with hardware, compiler optimizations, and OS scheduling; the numbers shown in the examples are illustrative.