等待线程
1。使用pthread_cond_wait前要先加锁
2。pthread_cond_wait内部会解锁，然后等待条件变量被其它线程激活
3。pthread_cond_wait被激活后会再自动加锁
激活线程：
1。加锁（和等待线程用同一个锁）
2。pthread_cond_signal发送信号
3。解锁
激活线程的上面三个操作在运行时间上都在等待线程的pthread_cond_wait函数内部。
程序示例：
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
pthread_mutex_t count_lock;
pthread_cond_t count_nonzero;
unsigned count = 0;
void * decrement_count(void *arg)
{
pthread_mutex_lock (&count_lock);
printf("decrement_count get count_lock\n");
while(count==0) {
printf("decrement_count count == 0 \n");
printf("decrement_count before cond_wait \n");
pthread_cond_wait( &count_nonzero, &count_lock);
printf("decrement_count after cond_wait \n");
}
count = count -1;
pthread_mutex_unlock (&count_lock);
}
void * increment_count(void *arg){
pthread_mutex_lock(&count_lock);
printf("increment_count get count_lock\n");
if(count==0) {
printf("increment_count before cond_signal\n");
pthread_cond_signal(&count_nonzero);
printf("increment_count after cond_signal\n");
}
count=count+1;
pthread_mutex_unlock(&count_lock);
}
int main(void)
{
pthread_t tid1,tid2;
pthread_mutex_init(&count_lock,NULL);
pthread_cond_init(&count_nonzero,NULL);
pthread_create(&tid1,NULL,decrement_count,NULL);
sleep(2);
pthread_create(&tid2,NULL,increment_count,NULL);
sleep(10);
pthread_exit(0);
}