pthread_cond_signal pthread_cond_wait
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pthread_cond_signal
pthread_cond_signal函数的作用是发送一个信号给另外一个正在处于阻塞等待状态的线程,使其脱离阻塞状态,继续执行.如果没有线程处在阻塞等待状态,pthread_cond_signal也会成功返回。
但使用pthread_cond_signal不会有“惊群现象”产生,他最多只给一个线程发信号。假如有多个线程正在阻塞等待着这个条件变量的话,那么是根据各等待线程优先级的高低确定哪个线程接收到信号开始继续执行。如果各线程优先级相同,则根据等待时间的长短来确定哪个线程获得信号。但无论如何一个pthread_cond_signal调用最多发信一次。另外,互斥量的作用一般是用于对某个资源进行互斥性的存取,很多时候是用来保证操作是一个原子性的操作,是不可中断的。
但是pthread_cond_signal在多处理器上可能同时唤醒多个线程,当你只能让一个线程处理某个任务时,其它被唤醒的线程就需要继续 wait,而且规范要求pthread_cond_signal至少唤醒一个pthread_cond_wait上的线程,其实有些实现为了简单在单处理器上也会唤醒多个线程.
此外,某些应用,如线程池,pthread_cond_broadcast唤醒全部线程,但我们通常只需要一部分线程去做执行任务,所以其它的线程需要继续wait.pthread_cond_signal()也可能唤醒多个线程,而如果你同时只允许一个线程访问的话 强烈推荐对pthread_cond_wait() 使用while循环来做条件判断.
用法:
pthread_cond_wait() , 多线程的条件变量
条件变量是利用线程间共享的全局变量进行同步的一种机制,主要包括两个动作:一个线程等待"条件变量的条件成立"而挂起;另一个线程使"条件成立"(给出条件成立信号)。为了防止竞争,条件变量的使用总是和一个互斥锁结合在一起。pthread_cond_wait() 用于阻塞当前线程,等待别的线程使用pthread_cond_signal()或pthread_cond_broadcast来唤醒它。 pthread_cond_wait() 必须与pthread_mutex 配套使用。pthread_cond_wait()函数一进入wait状态就会自动release mutex。当其他线程通过pthread_cond_signal()或pthread_cond_broadcast,把该线程唤醒,使pthread_cond_wait()通过(返回)时,该线程又自动获得该mutex。
1. 创建和注销
Consider two shared variables x and y, protected by the mutex mut, and a condition vari-
int x,y;pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER;pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
pthread_mutex_lock(&mut);while (x <= y) {pthread_cond_wait(&cond, &mut);}/* operate on x and y */pthread_mutex_unlock(&mut);
pthread_mutex_lock(&mut);/* modify x and y */if (x > y) pthread_cond_broadcast(&cond);pthread_mutex_unlock(&mut);
pthread_cond_signal函数与条件变量的典型应用就是用来实现producer/consumer模型。
#include <stdio.h>#include <unistd.h>#include <pthread.h>#include <stdlib.h> #define BUFFER_SIZE 8 struct Products{int buffer[BUFFER_SIZE];/*保证存取操作的原子性 互斥性*/pthread_mutex_t locker;/*是否可读*/ pthread_cond_t notEmpty;/*是否可写*/ pthread_cond_t notFull;int posReadFrom;int posWriteTo;}; int BufferIsFull(struct Products* products)// 判断缓冲区是否装满{if ((products->posWriteTo + 1) % BUFFER_SIZE == products->posReadFrom){return (1);}return (0);} int BufferIsEmpty(struct Products* products)//判断缓冲区是否空{if (products->posWriteTo == products->posReadFrom){return (1);}return (0);} /*制造产品*/ void Produce(struct Products* products, int item){/*原子操作*/pthread_mutex_lock(&products->locker);/*无空间可写入*/while (BufferIsFull(products)){pthread_cond_wait(&products->notFull, &products->locker);} /*写入数据*/products->buffer[products->posWriteTo] = item;products->posWriteTo++; if (products->posWriteTo >= BUFFER_SIZE)products->posWriteTo = 0;/*发信*/pthread_cond_signal(&products->notEmpty);/*解锁*/pthread_mutex_unlock(&products->locker);} /* 消费*/int Consume(struct Products* products){int item; pthread_mutex_lock(&products->locker);/*为空时持续等待,无数据可读*/while (BufferIsEmpty(products)){pthread_cond_wait(&products->notEmpty, &products->locker);} /*提取数据*/item = products->buffer[products->posReadFrom];products->posReadFrom++;/*如果到末尾,从头读取*/if (products->posReadFrom >= BUFFER_SIZE)products->posReadFrom = 0;pthread_cond_signal(&products->notFull); pthread_mutex_unlock(&products->locker); return item;} #define END_FLAG (-1) struct Products products; void* ProducerThread(void* data){int i;for(i = 0; i < 16; ++i){printf("producer: %d\n", i);Produce(&products, i);// 加入}Produce(&products, END_FLAG);return NULL;} void* ConsumerThread(void* data){int item; while(1){item = Consume(&products);if(END_FLAG == item)break;printf("consumer: %d\n", item);}return (NULL);} int main(int argc, char* argv[]){pthread_t producer;pthread_t consumer;int result; pthread_create(&producer, NULL, &ProducerThread, NULL);pthread_create(&consumer, NULL, &ConsumerThread, NULL); pthread_join(producer, (void *)&result);pthread_join(consumer, (void *)&result); exit(EXIT_SUCCESS);}参考资料 http://baike.baidu.com/view/5725833.htm
http://www.eefocus.com/andrew_dj/blog/12-08/282740_85e58.html
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