code14_3.cpp

#include <pthread.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <wait.h>

pthread_mutex_t mutex;
/*子线程运行的函数，它首先获得互斥锁mutex，然后暂停5s，在释放该互斥锁*/
void *another(void *arg)
{
	printf("in child thread,lock the mutex\n");
	pthread_mutex_lock(&mutex);
	sleep(5);
	pthread_mutex_unlock(&mutex);
	return nullptr;
}

int main()
{
	pthread_mutex_init(&mutex,nullptr);
	pthread_t id;
	pthread_create(&id,NULL,another,NULL);
	/*父进程中的主线程暂停1s，以确保在执行fork操作之前
	 * 子进程已经开始运行并获得了互斥锁变量mutex*/
	sleep(1);
	int pid = fork();
	if(pid < 0)
	{
		pthread_join(id,NULL);
		pthread_mutex_destroy(&mutex);
		return 1;
	}
	else if(pid == 0)
	{
		printf("i am in the child,want to get the lock\n");
		/*子进程从父进程继承了互斥锁mutex的状态，该互斥锁处于锁住的状态，这是由父进程中的子进程
		 * 执行pthread_mutex_lock引起的因此，下面这句加锁操作会一直阻塞，尽管从逻辑上说它是不应该
		 * 阻塞的*/
		pthread_mutex_lock(&mutex);
		printf("i can not run to here.opp...\n");
		pthread_mutex_unlock(&mutex);
		exit(0);
	}
	else
		wait(NULL);
	pthread_join(id,NULL);
	pthread_mutex_destroy(&mutex);
	return 0;
}