Linux如何避免僵死进程

叁歲伎倆 2022-07-16 01:07 197阅读 0赞

/* OS: CentOS
     * gcc avoid_zombie.c -o avoid_zombie
    */
    #include <stdio.h>
    #include <stdlib.h>
    #include <sys/wait.h>
    #include <unistd.h>
    #include <errno.h>
    #include <sys/types.h>
    
    int main(int argc, char *argv[])
    {
    	int ret=0;
    	pid_t pid;
    	
    	//signal(SIGCHLD, SIG_IGN); //可以避免子进程成为僵尸进程
    	
    	pid = fork(); // pid=子进程的进程号
    	if( pid < 0 )
    	{
    		perror("fail to fork");
    		exit(1);
    	}	
    	else if( pid == 0 ) //child process(pid)
    	{
    		int pid2;
    		pid2 = fork();
    		if( pid2 < 0 ) {
    			perror("fail to fork");	
    			exit(1);
    		}
    		else if( pid2 == 0 ) { // grandson process(ppid == pid), do some job.
    			printf("grandson process begin, pid=%d, ppid=%d\n", getpid(),getppid());
    			sleep(10);
    			// do some job... 要做的工作放在孙进程执行
    			printf("grandson process exit, pid=%d, ppid=%d\n", getpid(),getppid());
    			exit(0);
    		}
    		else {	// child process(pid), exit.
    			sleep(5);
    			printf("child process exit, pid=%d, ppid=%d\n", getpid(),getppid());
    			exit(2);
    		}
    		
    	}
    	else
    	{
    		sleep(20);	
    		//5秒后当子进程退出，这个时间用ps ax | grep avoid_zombie 查看，会有一个“Z+”的僵尸进程
    		//10秒后当孙进程退出，这个时间用ps ax | grep avoid_zombie 查看，
    		//还是只有一个“Z+”的僵尸进程(子进程)，此时孙进程已成为孤儿进程，孙进程由init进程回收，
    		//孙进程不会成为僵尸进程
    		
    		//(1)若主进程没有设置signal(SIGCHLD, SIG_IGN)，则必须调用wait/waitpid回收子进程.
    		//(2)若主进程设置了signal(SIGCHLD, SIG_IGN)，子进程不会变为僵尸进程，
    		//	 所以主进程无需调用waitpid回收子进程，调用waipid会返回失败。
    		//参考man waitpid的notes部分:
    		//if the disposition of SIGCHLD is set to SIG_IGN  or  the
           	//SA_NOCLDWAIT  flag  is set for SIGCHLD (see sigaction(2)), then children that termi-
           	//nate do not become zombies and a call to wait() or waitpid() will  block  until  all
           	//children  have  terminated,  and  then fail with errno set to ECHILD.
    		
    		int status=0;
    		do {
    			ret = waitpid(pid, &status, WUNTRACED | WCONTINUED);
    			if (ret == -1) {
    			   perror("waitpid");
    			   exit(EXIT_FAILURE);
    			}
    			
    			if (WIFEXITED(status)) {
    			   printf("child[%d] exited, status=%d\n", ret, WEXITSTATUS(status));
    			} else if (WIFSIGNALED(status)) {
    			   printf("killed by signal %d\n", WTERMSIG(status));
    			} else if (WIFSTOPPED(status)) {
    			   printf("stopped by signal %d\n", WSTOPSIG(status));
    			} else if (WIFCONTINUED(status)) {
    			   printf("continued\n");
    			}
           } while (!WIFEXITED(status) && !WIFSIGNALED(status));
           
    		sleep(10);	
    	}
    	
    	return EXIT_SUCCESS;
    	
    }

僵尸进程的避免方法：

方法一：

忽略SIGCLD信号：signal(SIGCHLD, SIG\_IGN)，系统将不产生僵死进程。  
方法二：

两次深度fork()，子进程退出，让孙子进程成为孤儿进程，从而init进程将负责清除这个孤儿进程。