進程之wait/waitpid使用

上個博客講了如何使用fork簡單的創建一個新的進程，本篇文章將講下如何避免殭屍進程的產生，殭屍進程的產生就是因爲子進程退出時沒有父進程替它"收屍"即沒有獲取子進程的狀態信息，一般我們可以使用wait或者waitpid函數來進行處理

下面的代碼示例演示了子進程如何成爲殭屍進程的過程

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


void main(void)
{
	pid_t pid;

	if (0 > (pid = fork()))
	{
		printf("fork error\n");
		return;
	}
	else if (0 == pid)
	{
		printf("I am child process pid = %ld\n", (long)getpid());
		sleep(1);
		printf("child process exit...\n");
		return;
	}
	else
	{
		printf("I am parent process pid = %ld\n", (long)getpid());
		while (1)
		{
			sleep(1);
		}
	}
	return;
}

運行結果：4160的子進程4161成爲了Z進程即殭屍進程

mcchen@mcchen-virtual-machine:/home/samba/share/mywork/test/wait$ sudo gcc -o wait wait.c 
mcchen@mcchen-virtual-machine:/home/samba/share/mywork/test/wait$ ./wait 
I am parent process pid = 4160
I am child process pid = 4161
child process exit...

mcchen@mcchen-virtual-machine:/home/samba/share/mywork/test/wait$ ps
   PID TTY          TIME CMD
  3736 pts/4    00:00:00 bash
  4160 pts/4    00:00:00 wait
  4161 pts/4    00:00:00 wait <defunct>


mcchen@mcchen-virtual-machine:/proc/4160$ cd ../4161
mcchen@mcchen-virtual-machine:/proc/4161$ cat status 
Name:	wait
State:	Z (zombie)
Tgid:	4161
Ngid:	0
Pid:	4161
PPid:	4160
TracerPid:	0
Uid:	1000	1000	1000	1000
Gid:	1000	1000	1000	1000

下面將使用wait函數來獲得子進程的狀態

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

void main(void)
{
	pid_t pid;
	int statloc;

	if (0 > (pid = fork()))
	{
		printf("fork error\n");
		return;
	}
	else if (0 == pid)
	{
		printf("I am child process pid = %ld\n", (long)getpid());
		sleep(1);
		printf("child process exit...\n");
		return;
	}
	else
	{
		printf("I am parent process pid = %ld\n", (long)getpid());
		pid = wait(&statloc);
		printf("pid = %ld has exit\n", (long)pid);
		while(1)
		{
			sleep(1);
		}
	}
	return;
}

下面的結果顯示，子進程11633正常退出，並且狀態信息被父進程11632捕捉，這時就不會成爲殭屍進程

mcchen@mcchen-virtual-machine:/home/samba/share/mywork/test/wait$
I am parent process pid = 11632
I am child process pid = 11633
child process exit...
pid = 11633 has exit

mcchen@mcchen-virtual-machine:/home/samba/share/mywork/test/wait$ ps
   PID TTY          TIME CMD
  3736 pts/4    00:00:00 bash
 11632 pts/4    00:00:00 wait
 11638 pts/4    00:00:00 ps

下面的實例演示了和wait相關的三個獲取終止狀態的函數

WIFEXITED(status) ：若爲正常終止子進程返回的狀態，則爲真，使用WEXITSTATUS(status)來獲取狀態

WIFSIGNALED(status) ：若爲異常終止子進程返回的狀態，則爲真，對於這種情況，可執行WTERMSIG(status)獲取使子進程終止的信號編號

WIFSTOPPED(status) ：若爲當前暫停子進程的返回的狀態，則爲真，對於這種情況，可執行WSTOPSIG(status)獲取使子進程暫停的信號編號

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <stdlib.h>
void main(void)
{
	pid_t pid;
	int statloc;

	if (0 > (pid = fork()))
	{
		printf("fork error\n");
		return;
	}
	else if (0 == pid)
	{
		printf("I am child process pid = %ld\n", (long)getpid());
		sleep(1);
		printf("child process exit...\n");
		return; //直接正常返回,或者使用exit(1) 
		//abort(); //異常返回,或者其它進程發信號終止該子進程
	}
	else
	{
		printf("I am parent process pid = %ld\n", (long)getpid());
		pid = wait(&statloc);
		if (WIFEXITED(statloc))
		{
			printf("normal termination, exit status = %d\n", WEXITSTATUS(statloc));
		}
		else if (WIFSIGNALED(statloc))
		{
			printf("abnormal termination, signal number = %d\n", statloc);
		}
		else if (WIFSTOPPED(statloc))
		{
			printf("child stopped, signal number = %d\n", WSTOPSIG(statloc));
		}
		printf("pid = %ld has exit\n", (long)pid);
		while(1)
		{
			sleep(1);
		}
	}
	return;
}

正常return返回的和發送信號導致異常返回的結果不一樣，如下

//正常使用return 或者exit返回的結果
mcchen@mcchen-virtual-machine:/home/samba/share/mywork/test/wait$ 
I am parent process pid = 12570
I am child process pid = 12571
child process exit...
normal termination, exit status = 0
pid = 12571 has exit

//使用abort終止進程函數，或者其它進程發送信號終止的結果
mcchen@mcchen-virtual-machine:/home/samba/share/mywork/test/wait$ 
I am parent process pid = 12580
I am child process pid = 12581
child process exit...
abnormal termination, signal number = 134
pid = 12581 has exit

上面講的可以使用wait等待子進程的狀態信息避免殭屍進程的產生，還有另一種方式就是父進程在子進程退出前先退出，這樣的話因爲子進程沒有了父進程，系統就講init進程作爲子進程的父進程，而init永遠不會退出，同時會在子進程退出後爲其收屍，所以子進程便不會成爲殭屍進程，下面將演示父進程先退出，init成爲父進程

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <stdlib.h>
void main(void)
{
	pid_t pid;
	int statloc;
	int i = 0;

	if (0 > (pid = fork()))
	{
		printf("fork error\n");
		return;
	}
	else if (0 == pid)
	{
		printf("I am child process pid = %ld\n", (long)getpid());
		while (i++ < 5)
		{
			sleep(1); //讓父進程先退出
		}
		printf("child process exit...\n");
		return; //直接正常返回,或者使用exit(1) 
	}
	else
	{
		printf("I am parent process pid = %ld\n", (long)getpid());
		printf("parent process exit...\n");
		return;
	}
	return;
}

結果是父進程先退出，init頂替作爲子進程的父進程

mcchen@mcchen-virtual-machine:/home/samba/share/mywork/test/wait$ ./wait &
[1] 13017
mcchen@mcchen-virtual-machine:/home/samba/share/mywork/test/wait$ 
I am parent process pid = 13017
parent process exit...           //父進程先退出
I am child process pid = 13018
child process exit...            //子進程後退出
[1]+  Exit 23                 ./wait
mcchen@mcchen-virtual-machine:/home/samba/share/mywork/test/wait$ ps
   PID TTY          TIME CMD
 11735 pts/4    00:00:01 bash
 13018 pts/4    00:00:00 wait
 13019 pts/4    00:00:00 ps
mcchen@mcchen-virtual-machine:/home/samba/share/mywork/test/wait$ cd /proc/13018
mcchen@mcchen-virtual-machine:/proc/13018$ cat status 
Name:	wait_1
Umask:	0002
State:	S (sleeping)
Tgid:	13018
Ngid:	0
Pid:	13018
PPid:	1     //init成爲父進程

以上就是關於避免殭屍進程產生的方式，以爲wait函數相關的使用，後面會繼續介紹進程相關的其它函數使用方法，感謝您的閱讀，如有不對的地方，歡迎指出謝謝。