为什么计算函数运行时使用time.h返回0秒

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

void function_to_measure() {
    // 函数实现
}

int main() {
    clock_t start, end;
    double cpu_time_used;

    start = clock();
    function_to_measure();
    end = clock();

    cpu_time_used = ((double) (end - start)) / CLOCKS_PER_SEC;
    printf("Time used: %f seconds\n", cpu_time_used);

    return 0;
}

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

void function_to_measure() {
    // 函数实现
}

int main() {
    struct timeval start, end;
    double time_use;

    gettimeofday(&start, NULL);
    function_to_measure();
    gettimeofday(&end, NULL);

    time_use = (end.tv_sec - start.tv_sec) * 1000000 + (end.tv_usec - start.tv_usec);
    time_use /= 1000000; // 转换为秒
    printf("Time used: %f seconds\n", time_use);

    return 0;
}