wait系统调用解析

一、在介绍wait系统调用之前，先看一下僵尸进程：

系统在调用exit()后，进程还没有马上消失掉，还残留一些信息：PID、退出状态。僵尸进程对系统毫无作用！

 

 

//下面的程序中，由于子进程已经结束，而父进程在休眠中，无法对子进程收集＝>出现僵尸进程60秒。

#include<sys/types.h>
#include<unistd.h>
#include<stdio.h>
#include<stdlib.h>
int main(){
    pid_t pid;
    pid = fork();
    if(pid < 0) //如果出错
        printf("Err occurred!/n");
    else if(pid == 0) //only 子进程
        exit(0);
    else            //only 父进程
        sleep(60);
   
    //收集僵尸进程
    wait(NULL);       
}

 

===================

explore@ubuntu:~/Documents/Linux C Programming/Process/wait$ gcc zombie.c
explore@ubuntu:~/Documents/Linux C Programming/Process/wait$ ./a.out&
[1] 3318
explore@ubuntu:~/Documents/Linux C Programming/Process/wait$
======

explore@ubuntu:~/Documents/Linux C Programming/Process/wait$ ps -ax
...

 3318 pts/0    S      0:00 ./a.out
 3319 pts/0    Z      0:00 [a.out] <defunct>
 3324 pts/0    R+     0:00 ps -ax

 

 

========================================================

二、wait()具体应用:

 

=====================＆status = NULL ==========================

//父进程的wait调用一直在等待，僵尸进程的出现

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

int main(){
    pid_t pid_c, pid_p;
    pid_c = fork();
    if(pid_c < 0){
        printf("ERR occurred!/n");
    }else if(pid_c == 0){  //子进程
        printf("This is Child Process with PID %d./n", getpid());
        sleep(10);
    }else{ //父进程
        pid_p = wait(NULL);
        printf("Parent Process catch a Zombie Process with PID %d./n", pid_p);
    }
   
    exit(0);
}

 

======

explore@ubuntu:~/Documents/Linux C Programming/Process/wait$ gcc wait.c
explore@ubuntu:~/Documents/Linux C Programming/Process/wait$ ./a.out
This is Child Process with PID 3605.
Parent Process catch a Zombie Process with PID 3605.
explore@ubuntu:~/Documents/Linux C Programming/Process/wait$ time ./a.out
This is Child Process with PID 3607.
Parent Process catch a Zombie Process with PID 3607.

real    0m10.003s
user    0m0.000s
sys    0m0.000s
explore@ubuntu:~/Documents/Linux C Programming/Process/wait$

 

 

========================status非NULL===========================

/*

关于wait(int *status)的参数有两个宏来帮助收集僵尸进程的信息。

1.WIFEXITED(status)判断被收集进程是否为正常退出，返回值＝0（非正常退出）；返回值＝非0（正常退出）

2.WEXITSTATUS(status)用来提取子进程的返回值，如果是exit(4)退出，则WEIXTSTATUS(status)的返回值为4。

*/

 

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

int main(){
    int status;
    pid_t pid_c, pid_p;
    pid_c = fork();
    if(pid_c < 0){
        printf("ERR occurred!/n");
    }else if(pid_c == 0){  //子进程
        printf("This is Child Process with PID %d./n", getpid());
        exit(3);
    }else{ //父进程
        pid_p = wait(&status);
        if(WIFEXITED(status)){ //WIFEXITED返回非0值
            printf("The Child Process %d exit normally./n", pid_c);   
            printf("The return Code is %d./n", WEXITSTATUS(status));       
        }else{        //WIFEXITED返回0
            printf("The Child Process %d exit abnormally./n", pid_c);
        }
    }
   
    exit(0);
}
=====编译运行============

explore@ubuntu:~/Documents/Linux C Programming/Process/wait$ gcc wait_status.c
explore@ubuntu:~/Documents/Linux C Programming/Process/wait$ ./a.out
This is Child Process with PID 3640.
The Child Process 3640 exit normally.
The return Code is 3.