1。使用pthread_cond_wait前要先加鎖
2。pthread_cond_wait內部會解鎖，然後等待條件變量被其它線程激活
3。pthread_cond_wait被激活後會再自動加鎖
激活線程：
1。加鎖（和等待線程用同一個鎖）
2。pthread_cond_signal發送信號
3。解鎖
激活線程的上面三個操作在運行時間上都在等待線程的pthread_cond_wait函數內部。
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);
}
