本文共 1501 字，大约阅读时间需要 5 分钟。

深入理解linux 47页里面介绍父进程通过wait4/waitpid等方法监控子进程的退出状态，子进程进入僵死状态时，父进程通过waitX就会促使父进程释放子进程的资源，促使子进程从僵死状态进入终止状态。

我们通过2.6.23版本内核代码分析该场景。

long do_wait(pid_t pid, int options, struct siginfo __user *infop, int __user *stat_addr, struct rusage __user *ru)	DECLARE_WAITQUEUE(wait, current);	struct task_struct *tsk;		add_wait_queue(&current->signal->wait_chldexit,&wait);//子进程exit可以唤醒父进程repeat:	/*如果父进程没有收到子进程的结束信号，则将在schedule函数执行后睡眠。直到子进程发出结束信号，唤醒父进程。*/	current->state = TASK_INTERRUPTIBLE;		tsk = current;//父进程被唤醒		do		struct task_struct *p;		struct list_head *_p;				list_for_each(_p,&tsk->children)//遍历子进程，根据子进程当前状态决定下一步动作			p = list_entry(_p, struct task_struct, sibling);						switch (p->state) 			case TASK_TRACED:				... ...							case TASK_STOPPED:				... ...							default:				if (p->exit_state == EXIT_DEAD)					continue;				if (p->exit_state == EXIT_ZOMBIE)					retval = wait_task_zombie(p, (options & WNOWAIT), infop, stat_addr, ru);					=>int wait_task_zombie(struct task_struct *p, int noreap, struct siginfo __user *infop, int __user *stat_addr, struct rusage __user *ru)						state = xchg(&p->exit_state, EXIT_DEAD);						if (p != NULL)							release_task(p);							/*释放子进程描述符*/							=>void release_task(struct task_struct * p)								call_rcu(&p->rcu, delayed_put_task_struct);						return retval;					break;						tsk = next_thread(tsk);				while (tsk != current);	end:	current->state = TASK_RUNNING;//父进程恢复就绪态	remove_wait_queue(&current->signal->wait_chldexit,&wait);//子进程信号已经没用了		return retval;

转载地址：http://xvlji.baihongyu.com/