Linux environment programming-wait (zombie process recycling)

Earlier we introduced what is a zombie process, today we will take a look at how to recycle the PCB left by the process

It’s still the old rules. Check out the video.

Seeking attention

 

#include<sys/types.h>

#include<sys/wait.h>

pid_t wait (int * status);

wait () will temporarily stop the execution of the current process until a signal comes or the child process ends. If the child process has ended when wait () is called , wait () will immediately return the child process end status value. The end status value of the child process will be returned by the parameter status , and the process identification code of the child process will be returned soon. If you don't care about the end status value, the parameter status can be set to NULL . Please refer to waitpid () for the end status value of the child process . If the execution is successful, it returns the child process identification code (PID) , if an error occurs, it returns -1. The reason for the failure is stored in errno .

 

Summarize the three functions of this function

1. The parent process calls it and blocks and waits for the end of the child process
2. Reclaim the remaining resources of the child process
3. Get the status of the end of the child process

 Take a look at some macros for judging the return status [I translated it into Chinese with Youdao, so that everyone can understand]

1.WIFEXITED (stat_val)

If it returns status for a normally terminated child process, it is calculated as a non-zero value.

2.WEXITSTATUS (stat_val)

If the value of WIFEXITED(stat_val) is non-zero, then this macro will be calculated as the 8-bit low-level value of the status parameter

The value passed by the child process to _exit() or exit(), or the value returned by the child process from main().

3.WIFSIGNALED (stat_val)

If the child process terminated due to receiving a signal returns status, it is calculated as a non-zero value

Not captured (see <signal.h>).

4.WTERMSIG (stat_val)

If the value of wifsignal (stat_val) is non-zero, the macro will calculate the number of signals that cause ter-

The minimum value of the child process.

5.WIFSTOPPED (stat_val)

If the status is returned for the currently stopped child process, it is calculated as a non-zero value.

6.WSTOPSIG (stat_val)

If the value of WIFSTOPPED(stat_val) is non-zero, then this macro will count the number of signals that generate child elements

The process stops.

7.WIFCONTINUED (stat_val)

If the status is returned from the child process continued after the job control is stopped, it is calculated as a non-zero value.

 

Code Demo

#include <iostream>
#include <unistd.h>
#include <sys/wait.h>

using namespace std;

int 
main(int argc, char*argv[])
{
	pid_t pid, wpid;

	pid = fork();

	if(pid == 0){
		cout << "hello world!" << endl;
		sleep(60);
	}else if(pid > 0){
		printf("I am parent, ID: %u\n", getpid());
		int status;
		printf("wait son die....\n");
		if((wpid = wait(&status)) == -1){			// block wait son progress die
			perror("wait error");
			exit(1);								// clear the PCB of son
		}											// get reason about son's die

		if(WIFEXITED(status)!=0 && WIFSIGNALED(status)==0){ 
			printf("son progress normal exit\n");
			printf("son progress exit status : %d \n", WEXITSTATUS(status));
		}else{// 0   
			printf("son expretion exit,signal ID:%d \n",  WTERMSIG(status));
		}
		
		printf("I am parent, ID: %u, wait son ID: %u \n", getpid(), wpid);
		
	}else{
		perror("fork error");
		exit(1);
	}
	
	return 0;
}

effect: