不应该循环的代码(带有fork)是循环的

Question

所以我有以下C代码:

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>

int main(){
    int i = 0, n;
    n = 5;
    pid_t pid;
    printf("i=%d Right before the loop\n", i, getpid(), getppid());
    for (i = 0; i < n; i++){
        pid = fork();
        if (pid <= 0){
            printf("something happens in loop #%d. pid = %d\n", i, pid);
            break;
        }
        printf("End of loop #%d\n", i);
    }

    printf("i=%d My process ID = %d and my parent's ID = %d\n", i, getpid(), getppid());

    return 0;
}

我只有一个问题:为什么

printf("i=%d My process ID = %d and my parent's ID = %d\n", i, getpid(), getppid());

多次被执行,好像它在循环中一样？我试图通过这么多方式弄清楚,但我找不到原因.

Answer 1

原因是fork()通过创建子进程来工作,该进程是在fork()调用时开始运行的父进程的副本.所以每个子进程都运行该printf命令.

例:

这是一个不太复杂的例子:

#include <stdio.h>

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

  if (pid == 0){
    // child code    
    printf("child pid: 0\n");

  }else{
    // parent code
    printf("parent pid: %d\n", pid);
  }

  // executed by both
  printf("This text brought to you by process %d.\n", pid);
}

如果要将某些代码限制为仅由子级或父级运行,则必须执行此类操作.

在我的机器上,当我刚运行它时,它会输出:

parent pid: 12513
This text brought to you by process 12513.
child pid: 0
This text brought to you by process 0.

我的操作系统首先运行父进程,但它没有必要.