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\n", val);
12 pthread_cond_wait(&cond, &mutex);//call pthread_cond_wait
13 }
14 printf("therad_zero_run --> val:%d, zero it and unlock\n", 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\n", 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