gcc：CLOCK_REALTIME 未声明

Question

我试图运行我发现的 C 代码 在这个网站上

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

#define n 2048

double A[n][n];
double B[n][n];
double C[n][n];

int main() {

    //populate the matrices with random values between 0.0 and 1.0
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {

            A[i][j] = (double) rand() / (double) RAND_MAX;
            B[i][j] = (double) rand() / (double) RAND_MAX;
            C[i][j] = 0;
        }
    }

    struct timespec start, end;
    double time_spent;

    //matrix multiplication
    clock_gettime(CLOCK_REALTIME, &start);
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            for (int k = 0; k < n; k++) {
                C[i][j] += A[i][k] * B[k][j];
            }
        }
    }
    clock_gettime(CLOCK_REALTIME, &end);
    time_spent = (end.tv_sec - start.tv_sec) + (end.tv_nsec - start.tv_nsec) / 1000000000.0;
    printf("Elapsed time in seconds: %f \n", time_spent);
    return 0;
}

但是当我编译它时，gcc说：

main.c:27:19: error: 'CLOCK_REALTIME' undeclared (first use in this function)
 clock_gettime(CLOCK_REALTIME, &start);
               ^~~~~~~~~~~~~~

我使用了来自gcc-g++ rel="nofollow noreferrer">MinGW 如 本教程。

我刚刚从教程页面复制了 C 代码并使用它进行编译

gcc -O3 main.c -o matrix

（我的源文件名为 main.c）。

可能的重要信息：我使用的是 Windows 10。

编辑：编译在 Ubuntu 20.04 上运行得很好（如文章中所述）。不过，你能帮我让它在 Windows 上编译吗？

Answer 1

这是我正在开发的基准程序的内容：

typedef long long ticks_t;

static ticks_t ticks_per_second;

#if OS_Windows
static ticks_t get_timer_resolution()
{
    LARGE_INTEGER freq;
    if (!QueryPerformanceFrequency(&freq)
            || !freq.QuadPart) {
        printf("Error: cannot get Windows timer resolution.\n");
        exit(123);
    }
    return freq.QuadPart;
}

inline static ticks_t get_ticks()
{
    LARGE_INTEGER ticks;
    if (!QueryPerformanceCounter(&ticks)) {
        printf("Error: cannot get Windows timer count.\n");
        exit(123);
    }
    return ticks.QuadPart;
}
#else       // NOT WINDOWS, should be Linux / Unix
static ticks_t get_timer_resolution()
{
    return 10000000;
}

inline static ticks_t get_ticks()
{
    struct timespec t;
    clock_gettime(CLOCK_REALTIME, &t);
    return (long long)t.tv_sec * 10000000 + (t.tv_nsec + 50) / 100;
}

#endif

[...]

int main() {
    [...]
    ticks_per_second = get_timer_resolution();

然后在我需要计时的前后使用 get_ticks() ，计算差异，并使用 根据需要进行缩放ticks_per_second，例如

    nticks = nticks * 1000000 / ticks_per_second;   // convert to microsecs