如何使用freopen重定向输出？

FILE *freopen(const char *filename, const char *mode, FILE *stream);

#include <stdio.h>

int main() {
    // 将标准输出重定向到 "output.txt"
    freopen("output.txt", "w", stdout);

    printf("Hello, World!\n");

    // 关闭文件流
    fclose(stdout);

    return 0;
}

#include <stdio.h>

int main() {
    // 将标准错误重定向到 "error.txt"
    freopen("error.txt", "w", stderr);

    fprintf(stderr, "This is an error message.\n");

    // 关闭文件流
    fclose(stderr);

    return 0;
}

#include <stdio.h>

int main() {
    // 将标准输入重定向自 "input.txt"
    freopen("input.txt", "r", stdin);

    int num;
    scanf("%d", &num);

    printf("Read number: %d\n", num);

    // 关闭文件流
    fclose(stdin);

    return 0;
}

FILE *fp = freopen("output.txt", "w", stdout);
if (fp == NULL) {
    perror("Error opening file");
    return 1;
}

if (freopen("output.txt", "w", stdout) == NULL) {
    perror("Error opening file");
    return 1;
}

printf("Hello, World!\n");

fclose(stdout);