pthread_cond

条件变量 pthread_cond， 另外一种线程间的同步机制。普通的 mutex 只允许一个线程进入临界区，就是拿到mutex这把锁的线程，而cond 允许多个线程同时进入临界区，由它来控制，在某些条件成立的时候，来唤醒其中一个等待着的线程，或者是唤醒所有等待着的线程。

int pthread_cond_wait(pthread_cond_t* cond, pthread_mutex_t* mutex);
int pthread_cond_timewait(pthread_cond_t* cond, pthread_mutex_t* mutex, const struct timespec* tout)

传递给pthrad_cond_wait 的互斥量 mutex 对条件进行保护，调用者把锁住的互斥量传递给pthread_cond_wait 函数，函数把调用线程放到等待条件的线程列表里面，然后对互斥量解锁，当pthread_cond_wait 返回的时候，互斥量再次被锁住。函数 pthread_cond_timewait 与 pthread_cond_wait 差不多，只不过是多了一个超时时间的限制。
两个函数调用成功返回的时候，需要重新检查条件，因为其他线程可能更改了条件。

int pthread_cond_signal(pthread_cond_t* cond);
int pthread_cond_broadcast(pthread_cond_t* cond);

pthread_cond_signal 函数将唤醒等待该条件的某个线程，pthread_cond_broadcast 将唤醒等待改条件的所有线程。

下面的例子很简单的使用了 cond 。 使用cond 我们可以比较高效的写出一个 线程池。

#include <pthread.h>
#include <unistd.h>
#include <stdio.h>
pthread_mutex_t mutex;
pthread_cond_t cond;
int val = 0;
void *thread_zero_run(void *arg){
	while(1){
		pthread_mutex_lock(&mutex); // lock the mutex
		while(val <= 2){ //condition.  use while for double check
			printf("thread_zero_run --> val:%d, wait for wake up\n", val);
			pthread_cond_wait(&cond, &mutex);//call pthread_cond_wait
		}
	printf("therad_zero_run --> val:%d, zero it and unlock\n", val);
	val = 0; //do  sth
	pthread_mutex_unlock(&mutex); //unlock the mutex
	}
   pthread_exit((void*)0);
}
void *thread_add_run(void *arg){
	while(1){
	pthread_mutex_lock(&mutex); //lock the mutex
 	++val; //do sth
 	pthread_mutex_unlock(&mutex); //unlock the mutex
 	pthread_cond_signal(&cond); //wake up a therad whick is waiting for the cond
	printf("after add val:%d and wake up one zero thread for check\n", val);
	sleep(1);
	}
   pthread_exit((void*)0);
}
int main(){
	pthread_t t_add, t_zero;
   pthread_cond_init(&cond, NULL);
   if(pthread_create(&t_add, NULL, thread_add_run, NULL)){
     return 0;
   }
   if(pthread_create(&t_zero, NULL, thread_zero_run, NULL)){
     return 0;
   }
   pthread_join(t_add, NULL);
   pthread_join(t_zero, NULL);
   return 0;
}

输出：

after add val:1 and wake up one zero thread for check
thread_zero_run --> val:1, wait for wake up
after add val:2 and wake up one zero thread for check
thread_zero_run --> val:2, wait for wake up
after add val:3 and wake up one zero thread for check
therad_zero_run --> val:3, zero it and unlock
thread_zero_run --> val:0, wait for wake up
after add val:1 and wake up one zero thread for check
thread_zero_run --> val:1, wait for wake up
after add val:2 and wake up one zero thread for check
thread_zero_run --> val:2, wait for wake up
after add val:3 and wake up one zero thread for check
therad_zero_run --> val:3, zero it and unlock
thread_zero_run --> val:0, wait for wake up
after add val:1 and wake up one zero thread for check
thread_zero_run --> val:1, wait for wake up
after add val:2 and wake up one zero thread for check
thread_zero_run --> val:2, wait for wake up
after add val:3 and wake up one zero thread for check
therad_zero_run --> val:3, zero it and unlock
thread_zero_run --> val:0, wait for wake up

转自：http://www.cppblog.com/MemoryGarden/archive/2011/06/08/148241.html