[Linux] Process waiting

Process waiting

The parent process waits for the child process to launch, gets its exit return value, and releases the child process's resources

Purpose: avoid zombie processes

Header file: #include <sys/wait.h>

1、 pid_t wait(int *status);

Block and wait for any child process to exit, obtain the exit return value through status, and release the resource
**Return value: **If successful, return the pid of the exited child process; if an error occurs, return -1

2、pid_t waitpid(pid_t pid, int *status, int options);

You can wait for any child process to exit, or you can wait for the specified child process to exit.
pid_t pid: equal to -1, means waiting for any child process to exit; greater than 0, means waiting for the specified child process to exit. It
can be blocking waiting for the child process to exit, or it can be non- Block waiting for the child process to exit
int options: set to 0, perform blocking waiting; set to WNOHANG , perform non-blocking waiting
Return value:

If successful, return the child process pid;
if no child process exits, return 0 (in non-blocking state, it will wait forever in blocking state);
if an error occurs, return -1

Note: If there is a child process that has exited, wait/waitpid will process it directly without waiting

Blocking and non-blocking

Blocking: In order to complete a function, initiate a call, if the function completion conditions are not met, wait forever.
Non-blocking: In order to complete a function, initiate a call, if the function completion conditions are not met, immediately report an error and return

Code example:

   #include<stdio.h>
   #include<stdlib.h>
   #include<unistd.h>
   int main()
   {
    
    
     int pid=fork();
     if(pid<0)
     {
    
    
       //出错
      perror("fork error");
      return -1;
    }
    else if(pid==0)
    {
    
    	//子进程
      sleep(5);//休眠5秒
     return 99;//main中的return--退出子进程，退出返回值为99
    }
    else
    {
    
    
      //父进程
      while(1)sleep(1);//保持休眠
    }
                                                                                                                      
    return 0;
  
  }

The child process sleeps for 5 seconds and then exits and enters a zombie state. Memory leaks will occur. Now
modify the code and add a memory waiting operation

   #include<stdlib.h>
   #include<unistd.h>
   #include<sys/wait.h>
   int main()
   {
    
    
     int pid=fork();
     if(pid<0)
     {
    
    
      //出错
      perror("fork error");
      return -1;
    }
    else if(pid==0)
    {
    
    
      //子进程
      sleep(5);
      return 99;//main中的return--退出子进程，退出返回值为99
    }
    //int ret=waitpid(-1,NULL,0);   //与21行等价，用哪个都行
    int ret=wait(NULL);//等待任意子进程退出
 
    printf("progress:%d exit\n",ret);                                                                                 
      //父进程
      while(1)sleep(1);//死循环
 
  
    return 0;
  }

Print information: The child process with pid=6035 exited

The result shows that the child process exits directly and no longer exists

Let's look at the code of waitpid's non-blocking state:

   #include<stdio.h>                                                                                                   
   #include<stdlib.h>
   #include<unistd.h>
   #include<sys/wait.h>
   int main()
   {
    
    
     int pid=fork();
     if(pid<0)
     {
    
    
      //出错
      perror("fork error");
      return -1;
    }
    else if(pid==0)
    {
    
    
      //子进程
      sleep(5);
      return 99;//main中的return--退出子进程，退出返回值为99
    }
    
   // int ret=wait(NULL);
   // printf("progress:%d exit\n",ret);
    int ret; 
    while((ret=waitpid(-1,NULL,WNOHANG))==0)
      {
    
    
        //非阻塞状态下，没有子进程退出，返回0
        printf("当前子进程还没有退出\n");
        sleep(1);
      }
      if(ret<0)
      {
    
    
        perror("waitpid error:");
      }
      printf("progress:%d exit\n",ret);
   //父进程
      while(1)
      {
    
    
        sleep(1);//死循环
      }                                                                                                               
    return 0;
    
  }