pthread_cond_signal只能唤醒已经处于pthread_cond

pthread_cond_signal只能唤醒已经处于pthread_cond_wait的线程

也就是说，如果signal的时候没有线程在condition wait，那么本次signal就没有效果，后续的线程进入condition wait之后，无法被之前的signal唤醒。
测试代码：

 1 #include <stdio.h>
 2 #include <pthread.h>
 3 #include <unistd.h>
 4 
 5 pthread_cond_t cond;
 6 pthread_mutex_t mutex;
 7 
 8 void *test_thread(void *arg)
 9 {
10     printf("Signal main thread...\n");
11     pthread_cond_signal(&cond);
12     return NULL;
13 }
14 
15 int main()
16 {
17     pthread_t thread;
18 
19     pthread_cond_init(&cond, NULL);
20     pthread_mutex_init(&mutex, NULL);
21 
22     pthread_create(&thread, NULL, test_thread, NULL);
23 
24     sleep(5);
25     pthread_mutex_lock(&mutex);
26     pthread_cond_wait(&cond, &mutex);
27 
28     printf("Main thread signaled, quit...\n");
29     return 0;
30 }

http://www.cnblogs.com/super119/archive/2011/07/29/2120761.html