vdso_time.c

#define _POSIX_C_SOURCE 200809L
#include <stdio.h>
#include <time.h>
#include <sys/time.h>
#include <unistd.h>
#include <string.h>

static long diff_ns(struct timespec a, struct timespec b)
{
    long s = b.tv_sec - a.tv_sec;
    long n = b.tv_nsec - a.tv_nsec;
    return s * 1000000000L + n;
}

int main(void)
{
    struct timespec t1, t2;
    clock_gettime(CLOCK_MONOTONIC, &t1);
    for (int i = 0; i < 1000000; i++) {
        struct timespec tmp;
        clock_gettime(CLOCK_MONOTONIC, &tmp);
    }
    clock_gettime(CLOCK_MONOTONIC, &t2);
    long mono_ns = diff_ns(t1, t2) / 1000000;
    struct timeval v1, v2;
    gettimeofday(&v1, NULL);
    for (int i = 0; i < 1000000; i++) {
        struct timeval tmp;
        gettimeofday(&tmp, NULL);
    }
    gettimeofday(&v2, NULL);
    long gtod_ns = (v2.tv_sec - v1.tv_sec) * 1000000L + (v2.tv_usec - v1.tv_usec);
    printf("CLOCK_MONOTONIC: %ld ms per 1e6 calls\n", mono_ns);
    printf("gettimeofday:    %ld ms per 1e6 calls\n", gtod_ns);
#if defined(__linux__)
    FILE *f = fopen("/proc/self/maps", "r");
    if (f) {
        char line[512];
        printf("vDSO mappings:\n");
        while (fgets(line, sizeof line, f)) {
            if (strstr(line, "vdso")) {
                fputs(line, stdout);
            }
        }
        fclose(f);
    }
#endif
    return 0;
}