分叉具有多线程C的进程

如何解决分叉具有多线程C的进程

我正在遵循有关pthread和fork的指南，但是对于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",(char*)arg);

 return NULL;  
}
int main() {
 pthread_t tid1,tid2;
 pthread_create(&tid1,NULL,sleepnprint,"New Thread One");
 pthread_create(&tid2,"New Thread Two");
 
 child = fork();
 printf("%d:%s\n","fork()ing complete");
 sleep(3);
   
 printf("%d:%s\n","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. 在网站上说子进程只有一个线程，这是否意味着子进程不会进入sleepnprint函数？
3. 为什么在8970:fork()ing complete之前打印8973:fork()ing complete 8970:New Thread Two starting up...？线程和进程的顺序是随机的吗？

解决方法

主线程（main（）函数）使用pthread_create()创建两个线程，但由于它们是线程，因此它们是同一进程的一部分（getpid（）为主线程和辅助线程返回8970）。如果需要任务标识符，请在线程入口点调用gettid()（您将分别获得8971和8972）。

然后，父分支，父进程和子进程都在main（）函数中继续。它们分别显示其pid：8970和8973。

当分叉多线程进程时，子进程中仅“调用”线程被重现（父进程的其他线程未分叉：子进程是单线程的，直到在其一侧创建新线程为止）。因此，在您的示例中，子进程编号8973没有在父进程（编号8970）中创建两个线程。

是的，所有线程和进程并行运行（以任何顺序）。

为说明上述内容，这是程序的增强版本：

#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",(char*)arg);

  return NULL;  
}

int main() {

  pthread_t tid1,tid2;
  pthread_create(&tid1,NULL,sleepnprint,"New Thread One");
  pthread_create(&tid2,"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","Child process finished");

    exit(0);
  }

  // Father process

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

  printf("%d:%s\n","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