用多线程 C 分叉一个进程

Question

我正在关注有关pthread 和fork 的指南，但我对以下代码的工作方式感到困惑。 以下代码来自https://github.com/angrave/SystemProgramming/wiki/Pthreads%2C-Part-2%3A-Usage-in-Practice

#include <pthread.h>
#include <stdio.h>
#include <unistd.h>

static pid_t child = -2;

void *sleepnprint(void *arg) {
 printf("%d:%s starting up...\n", getpid(), (char *) arg);

 while (child == -2) {sleep(1);} /* Later we will use condition variables */

 printf("%d:%s finishing...\n",getpid(), (char*)arg);

 return NULL;  
}
int main() {
 pthread_t tid1, tid2;
 pthread_create(&tid1,NULL, sleepnprint, "New Thread One");
 pthread_create(&tid2,NULL, sleepnprint, "New Thread Two");
 
 child = fork();
 printf("%d:%s\n",getpid(), "fork()ing complete");
 sleep(3);
   
 printf("%d:%s\n",getpid(), "Main thread finished");
 
 pthread_exit(NULL);
 return 0; /* Never executes */
}

8970:New Thread One starting up...
8970:fork()ing complete
8973:fork()ing complete
8970:New Thread Two starting up...
8970:New Thread Two finishing...
8970:New Thread One finishing...
8970:Main thread finished
8973:Main thread finished

1. 8970是父进程吗？
2. 网站上说子进程只有单线程，是不是说子进程没有去sleepnprint函数？
3. 为什么8970:fork()ing complete 8973:fork()ing complete 在8970:New Thread Two starting up... 之前打印？线程和进程的顺序是随机的吗？

Answer 1

主线程（main() 函数）使用pthread_create() 创建两个线程，但由于它们是线程，它们属于同一进程（getpid() 为主线程和辅助线程返回 8970）。如果你想要任务标识符，在线程入口点调用gettid()（你会分别得到8971和8972）。

然后，父进程分叉，父进程和子进程都在 main() 函数中继续。他们分别显示他们的pid：8970和8973。

当一个多线程进程fork时，只有调用线程在子进程中“再现”（父进程的其他线程不被fork：子进程是单线程的，直到它在它身边创建新线程)。因此，在您的示例中，子进程编号 8973 没有在父进程（编号 8970）中创建的两个线程。

是的，所有线程和进程都并行运行（以任何顺序）。

为了说明前面的内容，这里是您的程序的略微增强版本：

#define _GNU_SOURCE  // To get gettid()
#include <pthread.h>
#include <sys/types.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>

static volatile pid_t child = -2;

void *sleepnprint(void *arg)
{
  printf("Process %d, Task %d:%s starting up...\n", getpid(), gettid(), (char *) arg);

  // This is not the best way to synchronize threads but this works here:
  // once the main thread returns from fork(), child = pid of child process
  // (i.e. != -2)
  while (child == -2) {sleep(1);} /* Later we will use condition variables */

  printf("Process %d, Task %d:%s finishing...\n", getpid(), gettid(), (char*)arg);

  return NULL;  
}

int main() {

  pthread_t tid1, tid2;
  pthread_create(&tid1,NULL, sleepnprint, "New Thread One");
  pthread_create(&tid2,NULL, sleepnprint, "New Thread Two");
 
  child = fork();

  // In father process: child = child process pid
  // In child process: child = 0

  if (child == 0) {

    // This is the child process

    printf("%d:%s\n",getpid(), "Child process finished");

    exit(0);
  }

  // Father process

  printf("%d:%s\n",getpid(), "fork()ing complete");
  sleep(3);

  printf("%d:%s\n",getpid(), "Main thread finished");

  pthread_exit(NULL);
  return 0; /* Never executes */
}

编译和执行：

$ gcc example.c -l pthread
$ ./a.out
Process 6141, Task 6142:New Thread One starting up...
Process 6141, Task 6142:New Thread One finishing...
Process 6141, Task 6143:New Thread Two starting up...
Process 6141, Task 6143:New Thread Two finishing...
6144:Child process finished
6141:fork()ing complete
6141:Main thread finished