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。

1 #include <pthread.h>
2 #include <unistd.h>
3 #include <stdio.h>
4 pthread_mutex_t mutex;
5 pthread_cond_t cond;
6 int val = 0;
7 void *thread_zero_run(void *arg){
8   while(true){
9     pthread_mutex_lock(&mutex); // lock the mutex
10     while(val <= 2){ //condition. use while for double check
11       printf("thread_zero_run --> val:%d, wait for wake up ", val);
12       pthread_cond_wait(&cond, &mutex);//call pthread_cond_wait
13     }
14     printf("therad_zero_run --> val:%d, zero it and unlock ", val);
15     val = 0; //do sth
16     pthread_mutex_unlock(&mutex); //unlock the mutex
17   }
18   pthread_exit((void*)0);
19 }
20 void *thread_add_run(void *arg){
21   while(true){
22     pthread_mutex_lock(&mutex); //lock the mutex
23     ++val; //do sth
24     pthread_mutex_unlock(&mutex); //unlock the mutex
25     pthread_cond_signal(&cond); //wake up a therad whick is waiting for the cond
26     printf("after add val:%d and wake up one zero thread for check ", val);
27     sleep(1);
28   }
29   pthread_exit((void*)0);
30 }
31 int main(){
32   pthread_t t_add, t_zero;
33   pthread_cond_init(&cond, NULL);
34   if(pthread_create(&t_add, NULL, thread_add_run, NULL)){
35     return 0;
36   }
37   if(pthread_create(&t_zero, NULL, thread_zero_run, NULL)){
38     return 0;
39   }
40   pthread_join(t_add, NULL);
41   pthread_join(t_zero, NULL);
42   return 0;
43 }
44

输出：

1 after add val:1 and wake up one zero thread for check
2 thread_zero_run --> val:1, wait for wake up
3 after add val:2 and wake up one zero thread for check
4 thread_zero_run --> val:2, wait for wake up
5 after add val:3 and wake up one zero thread for check
6 therad_zero_run --> val:3, zero it and unlock
7 thread_zero_run --> val:0, wait for wake up
8 after add val:1 and wake up one zero thread for check
9 thread_zero_run --> val:1, wait for wake up
10 after add val:2 and wake up one zero thread for check
11 thread_zero_run --> val:2, wait for wake up
12 after add val:3 and wake up one zero thread for check
13 therad_zero_run --> val:3, zero it and unlock
14 thread_zero_run --> val:0, wait for wake up
15 after add val:1 and wake up one zero thread for check
16 thread_zero_run --> val:1, wait for wake up
17 after add val:2 and wake up one zero thread for check
18 thread_zero_run --> val:2, wait for wake up
19 after add val:3 and wake up one zero thread for check
20 therad_zero_run --> val:3, zero it and unlock
21 thread_zero_run --> val:0, wait for wake up
22 ^C

补充：软件开发 , C语言 ,