<time.h>

<time.h> provides time functions: time() for wall-clock seconds since epoch, clock() for CPU time, struct tm for broken-down calendar time, and strftime() for formatting. Use clock_gettime(CLOCK_MONOTONIC, ...) for nanosecond-resolution wall-clock timing.

Note: struct tm fields like tm_mon are 0-indexed.

This example measures wall-clock and CPU time for a compute loop.

// compile: gcc -o timeexample timeexample.c
// run: ./timeexample
// description: measure CPU and wall-clock time for a loop
 
#include <time.h>
#include <stdio.h>
 
int main() {
    struct timespec wall_start, wall_end;
    clock_t cpu_start, cpu_end;
 
    clock_gettime(CLOCK_MONOTONIC, &wall_start);
    cpu_start = clock();
 
    volatile long sum = 0;
    for (long i = 0; i < 100000000L; i++)
        sum += i;
 
    cpu_end = clock();
    clock_gettime(CLOCK_MONOTONIC, &wall_end);
 
    double wall = (wall_end.tv_sec - wall_start.tv_sec) +
                  (wall_end.tv_nsec - wall_start.tv_nsec) * 1e-9;
    double cpu = (double)(cpu_end - cpu_start) / CLOCKS_PER_SEC;
 
    printf("wall: %.3f s, cpu: %.3f s\n", wall, cpu);
    return 0;
}