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.