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生产者消费者问题（英语：Producer-consumer problem），也称有限缓冲问题（英语：Bounded-buffer problem），是一个多线程同步问题的经典案例。该问题描述了两个共享固定大小缓冲区的线程——即所谓的“生产者”和“消费者”——在实际运行时会发生的问题。生产者的主要作用是生成一定量的数据放到缓冲区中，然后重复此过程。与此同时，消费者也在缓冲区消耗这些数据。该问题的关键就是要保证生产者不会在缓冲区满时加入数据，消费者也不会在缓冲区中空时消耗数据
一种实现如下：
#include <stdio.h> #include <unistd.h> #include <pthread.h> #include <string.h> #define MAX 5 //缓冲区的的大小 pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER; pthread_cond_t cond = PTHREAD_COND_INITIALIZER; typedef struct{ char buffer[MAX]; int count; }Buffer; Buffer share = {"", 0}; char ch = ‘A‘; void *producer(void *arg) { printf("Producer : starting \n"); while(ch != ‘K‘) { pthread_mutex_lock(&mutex); if(share.count != MAX) { share.buffer[share.count++] = ch++; printf("Producer: put char[%c]\n", ch-1); if(share.count == MAX) { printf("Producer: signaling full\n"); pthread_cond_signal(&cond);//若果缓存满了发送信号 } } pthread_mutex_unlock(&mutex); } sleep(1); printf("Produce: Exiting \n"); pthread_exit(NULL); } void *consumer(void *junk) { int i; printf("Consumer : starting\n"); while (ch != ‘K‘) { pthread_mutex_lock(&mutex); printf("\t Consumer : Waiting\n"); while(share.count != MAX){ pthread_cond_wait(&cond, &mutex); //条件不成立释放锁. printf("Consumer wating for FULL signal\n"); } printf("Consumer : getting buffer :: "); for(i = 0; share.buffer[i] && share.count;++i, share.count--) putchar(share.buffer[i]); putchar(‘\n‘); pthread_mutex_unlock(&mutex); } } int main() { pthread_t read, write; pthread_create(&read, NULL, (void *) consumer, NULL); pthread_create(&write, NULL, (void *)producer, NULL); pthread_join(read, NULL); pthread_join(write, NULL); return 0; }
修改consumer的代码：
void *consumer(void *junk) { int i; printf("Consumer : starting\n"); while (ch != ‘K‘) { pthread_mutex_lock(&mutex); printf("\t Consumer : Waiting\n"); //while(share.count != MAX){ pthread_cond_wait(&cond, &mutex); //条件不成立释放锁. printf("Consumer wating for FULL signal\n"); //} printf("Consumer : getting buffer :: "); for(i = 0; share.buffer[i] && share.count;++i, share.count--) putchar(share.buffer[i]); putchar(‘\n‘); pthread_mutex_unlock(&mutex); } }
编译运行会有两种结果:
一种是：
Consumer : starting Consumer : Waiting Producer : starting Producer: put char[A] Producer: put char[B] Producer: put char[C] Producer: put char[D] Producer: put char[E] Producer: signaling full Consumer wating for FULL signal Consumer : getting buffer :: ABCDE Consumer : Waiting Producer: put char[F] Producer: put char[G] Producer: put char[H] Producer: put char[I] Producer: put char[J] Producer: signaling full Consumer wating for FULL signal Consumer : getting buffer :: FGHIJ Produce: Exiting
另一种是：
Producer : starting Producer: put char[A] Producer: put char[B] Producer: put char[C] Producer: put char[D] Producer: put char[E] Producer: signaling full Consumer : starting Consumer : Waiting
可以看出来第二种是先执行了生产者，生产者填充满buffer之后，发送条件消息，但是此时consumer还没有执行，也并没有等待条件
然后生产者释放锁，接着消费者获取锁，然后等待条件，由于缓冲已经满，
if(share.count == MAX)
生产者不会进入发送消息的代码,所以消费者一直等待条件．
而上面一种的结果是因为，消费这先执行了，进入等待条件，所以没有这个问题，但是问题在于我们不能保证pthread_cond_wait()一定是先与pthread_cond_signal()执行的．
但是通过加while()代码，就不会出现等待条件变量的问题，可以直接执行下面的代码．
这个生产者消费者的解决方案是，同时只有一个生产者和消费者，是1:1的关系，生产生产完成后再让消费者过来取，消费者和生产者在各自的活动期间，对方都在等待，这种效率比较低．
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原文地址：http://www.cnblogs.com/biglucky/p/4643465.html
