waitpid()的第二个参数是什么意思？

#include<stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>

int main() {
    pid_t pid = fork();

    if (pid == 0) { // 子进程
        printf("I am the child process, my pid is %d\n", getpid());
        sleep(2);
        exit(0);
    } else if (pid > 0) { // 父进程
        int status;
        waitpid(pid, &status, 0); // 等待子进程结束
        if (WIFEXITED(status)) { // 判断子进程是否正常结束
            printf("Child process exited with status %d\n", WEXITSTATUS(status));
        } else {
            printf("Child process terminated abnormally\n");
        }
    } else { // fork失败
        perror("fork");
        exit(1);
    }

    return 0;
}