条件变量使用问题

[whdugh]

在看卷2进程间通信条件变量一章，代码如下：

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

#define MAXNITEMS 10000
#define MAXNTHREADS 100

/**************global shared by threads*********/
int nitems;//read-only by produces and consumer
int buf[MAXNITEMS];

//生产者变量和互斥锁收集到一个结构中
struct{
	pthread_mutex_t mutex;
	int nput;  //next index to store
	int nval;  //next value to store
}put = {PTHREAD_MUTEX_INITIALIZER};


//
struct{
	pthread_mutex_t mutex;
	pthread_cond_t cond;
	int nready;//number ready for consumer
}nready = {
	PTHREAD_MUTEX_INITIALIZER,PTHREAD_COND_INITIALIZER
};

int mymin(int a,int b)
{
	return a>b?b:a;
}
void *produce(void *arg)
{
	for (;;)
	{
		pthread_mutex_lock(&put.mutex);

		//这里要加判断是否大于最大items数
		if (put.nput > nitems)
		{
			pthread_mutex_unlock(&put.mutex);
			//printf("超过最大items[%d]\n",nitems);
			return NULL;
		}

		put.nput++;
		put.nval++;
		buf[put.nput] = put.nval;
		pthread_mutex_unlock(&put.mutex);

		pthread_mutex_lock(&nready.mutex);
		if (nready.nready == 0)  //
		{
			printf("thread produce signal 11\n");
			pthread_cond_signal(&nready.cond);//条件满足唤醒consumer
			printf("thread produce signal 22\n\n");
		}

		//if (buf[put.nput] >= 0)
		{
			nready.nready++; //什么时候加1???
		}

		pthread_mutex_unlock(&nready.mutex);

		sleep(1);
	}
}

void *consumer(void *arg)
{
	//for (;;)
	int i;
	for (i = 0; i < nitems; ++ i)
	{
	
	{
		pthread_mutex_lock(&nready.mutex);
		//if (nready.nready <= 0)
		while(nready.nready == 0) //
		{
			printf("thread consumer wait 111!\n");
			pthread_cond_wait(&nready.cond,&nready.mutex);//等待条件满足
			printf("thread consumer wait 22!\n\n");
		}
		printf("nready.nready = %d\n",nready.nready);
		nready.nready--;
		pthread_mutex_unlock(&nready.mutex);
		


		sleep(1);

	}
	}
	return NULL;
}

int main(int argc,char *argv[])
{
	pthread_t tid_produces[MAXNTHREADS],tid_consumer;
	int i;
	int nthreads;
	int count[MAXNTHREADS];


	if (argc !=3)
	{
		printf("usage:produces<#items><#threads>");
		exit(-1);
	}

	nitems = mymin(atoi(argv[1]),MAXNITEMS);
	nthreads = mymin(atoi(argv[2]),MAXNTHREADS);


	for (i = 0 ; i < nthreads;++i)
	{
		pthread_create(&tid_produces[i],NULL,produce,NULL);
	}
	pthread_create(&tid_consumer,NULL,consumer,NULL);

	//等待线程被执行
	for (i = 0 ; i < nthreads;++i)
	{
		pthread_join(tid_produces[i],NULL);
	}
	pthread_join(tid_consumer,NULL);

	return 0;
}

上面的源码和书上一致，程序运行发现，如果线程produce先执行的话，if条件满足调用pthread_cond_signal，nready++，然后consume线程等不到while条件满足，直接nready--，然后线程produce这边又调用signal，这样是不是有问题啊

[huawang99]

你上面的程序是这样的逻辑： t1: produce线程，先发送signal，然后nready++变为1，最后进入sleep（1）； t2. consume线程启动后，发现nready为1，直接跳过while循环，执行nready--变为0，然后进入sleep（1）； t3： produce线程，sleep（1）结束，重复t1步骤 t4: consume线程，重复t2步骤。