编写一个程序，开启3个 线程，这3个线程的ID分别为ABC，每个线程将自己的ID在屏幕上打印10遍，要求输出结果必须按ABC的顺序显示，如ABCABC……依次递推；

#include <stdio.h>
#include <pthread.h>
pthread_cond_t cond; 	//条件变量
pthread_mutex_t mutex; 	//互斥锁
int flag = 0; 	//标志,0:打印 A, 1:打印 B, 2:打印 C
int count = 0; 

void* A(void* arg)
{
	while(1)
	{
		if(count == 10)     //打印10次后退出进程
		{
			pthread_exit(NULL);
		}
		pthread_mutex_lock(&mutex);//上锁
		if(flag == 0) 	//flag为0打印A
		{
			printf("A");
			flag = 1; 	//打印完后,flag置1，准备打印B
		}else 	//flag不为0，线程休眠
		{
			pthread_cond_wait(&cond,&mutex);
		}
		pthread_mutex_unlock(&mutex); 	//解锁
		pthread_cond_signal(&cond); 	//唤醒
	}
}

void* B(void* arg)
{
	while(1)
	{
		if(count == 10)     //打印10次后退出进程
		{
			pthread_exit(NULL);
		}
		pthread_mutex_lock(&mutex);//上锁
		if(flag == 1) 	//flag为1打印B
		{
			printf("B");
			flag = 2; 	//打印完后,flag置2，准备打印C
		}else 	//flag不为1，线程休眠
		{
			pthread_cond_wait(&cond,&mutex);
		}
		pthread_mutex_unlock(&mutex); 	//解锁
		pthread_cond_signal(&cond); 	//唤醒
	}
}

void* C(void* arg)
{
	while(1)
	{
		if(count == 10)     //打印10次后退出进程
		{
			printf("\n");
			pthread_exit(NULL);
		}
		pthread_mutex_lock(&mutex);//上锁
		if(flag == 2) 	//flag为2打印C
		{
			printf("C");
			flag = 0; 	//打印完后,flag置0，准备打印A
			count++;     //ABC都打印了一遍
		}else 	//flag不为2，线程休眠
		{
			pthread_cond_wait(&cond,&mutex);
		}
		pthread_mutex_unlock(&mutex); 	//解锁
		pthread_cond_signal(&cond); 	//唤醒
	}
}

int main(int argc, const char *argv[])
{
	//创建条件变量
	if(0 != pthread_cond_init(&cond,NULL))
	{
		perror("pthread_cond_init");
		return -1;
	}
	//创建互斥锁
	if(0 != pthread_mutex_init(&mutex,NULL))
	{
		perror("pthread_mutex_init");
		return -1;
	}
	//打开三个线程
	pthread_t pth1,pth2, pth3;
	if(0 != pthread_create(&pth1, NULL, A, NULL))
	{
		perror("pthread_create");
		return -1;
	}
	if(0 != pthread_create(&pth2, NULL, B, NULL))
	{
		perror("pthread_create");
		return -1;
	}
	if(0 != pthread_create(&pth3, NULL, C, NULL))
	{
		perror("pthread_create");
		return -1;
	}
	//阻塞，等待回收进程资源
	pthread_join(pth1, NULL);
	pthread_join(pth2, NULL);
	pthread_join(pth3, NULL);
	//销毁互斥锁
	pthread_mutex_destroy(&mutex);
	//销毁条件变量
	pthread_cond_destroy(&cond);
	return 0;
}

 结果：

ubuntu@ubuntu:day6$ ./a.out 
ABCABCABCABCABCABCABCABCABCABC