条件变量pthread_cond_t怎么用

最近看《UNIX环境高级编程》多线程同步，看到他举例说条件变量pthread_cond_t怎么用，愣是没有看懂，只好在网上找了份代码，跑了跑，才弄明白

 #include <pthread.h>
 #include <stdio.h>
 #include <stdlib.h>
 pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;/*初始化互斥锁*/
 pthread_cond_t cond = PTHREAD_COND_INITIALIZER;/*初始化条件变量*/
 void *thread1(void *);
 void *thread2(void *);
 int i=1;
 int main(void)
 {
 pthread_t t_a;
 pthread_t t_b;
 pthread_create(&t_a,NULL,thread1,(void *)NULL);/*创建进程t_a*/
 pthread_create(&t_b,NULL,thread2,(void *)NULL); /*创建进程t_b*/
 pthread_join(t_a, NULL);/*等待进程t_a结束*/
 pthread_join(t_b, NULL);/*等待进程t_b结束*/
 pthread_mutex_destroy(&mutex);
 pthread_cond_destroy(&cond);
 exit(0);
 }
 void *thread1(void *junk)
 {
 for(i=1;i<=6;i++)
 {
 pthread_mutex_lock(&mutex);/*锁住互斥量*/
 printf("thread1: lock %d/n", __LINE__);
 if(i%3==0){
 printf("thread1:signal 1 %d/n", __LINE__);
 pthread_cond_signal(&cond);/*条件改变，发送信号，通知t_b进程*/
 printf("thread1:signal 2 %d/n", __LINE__);
 sleep(1);
 }
 pthread_mutex_unlock(&mutex);/*解锁互斥量*/
 printf("thread1: unlock %d/n/n", __LINE__);
 sleep(1);
 }
 }
 void *thread2(void *junk)
 {
 while(i<6)
 {
 pthread_mutex_lock(&mutex);
 printf("thread2: lock %d/n", __LINE__);
 if(i%3!=0){
 printf("thread2: wait 1 %d/n", __LINE__);
 pthread_cond_wait(&cond,&mutex);/*解锁mutex，并等待cond改变*/
 printf("thread2: wait 2 %d/n", __LINE__);
 }
 pthread_mutex_unlock(&mutex);
 printf("thread2: unlock %d/n/n", __LINE__);
 sleep(1);
 }
 }

 #include <pthread.h>
 #include <stdio.h>
 #include <stdlib.h>
 pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;/*初始化互斥锁*/
 pthread_cond_t cond = PTHREAD_COND_INITIALIZER;/*初始化条件变量*/
 void *thread1(void *);
 void *thread2(void *);
 int i=1;
 int main(void)
 {
     pthread_t t_a;
     pthread_t t_b;
     pthread_create(&t_a,NULL,thread1,(void *)NULL);/*创建进程t_a*/
     pthread_create(&t_b,NULL,thread2,(void *)NULL); /*创建进程t_b*/
     pthread_join(t_a, NULL);/*等待进程t_a结束*/
     pthread_join(t_b, NULL);/*等待进程t_b结束*/
     pthread_mutex_destroy(&mutex);
     pthread_cond_destroy(&cond);
     exit(0);
 }
 void *thread1(void *junk)
 {
     for(i=1;i<=6;i++)
     {
         pthread_mutex_lock(&mutex);/*锁住互斥量*/
         printf("thread1: lock %d/n", __LINE__);
         if(i%3==0){
             printf("thread1:signal 1  %d/n", __LINE__);
             pthread_cond_signal(&cond);/*条件改变，发送信号，通知t_b进程*/
             printf("thread1:signal 2  %d/n", __LINE__);
             sleep(1);
         }
         pthread_mutex_unlock(&mutex);/*解锁互斥量*/
         printf("thread1: unlock %d/n/n", __LINE__);
         sleep(1);
     }
 }
 void *thread2(void *junk)
 {
     while(i<6)
     {
         pthread_mutex_lock(&mutex);
         printf("thread2: lock %d/n", __LINE__);
         if(i%3!=0){
             printf("thread2: wait 1  %d/n", __LINE__);
             pthread_cond_wait(&cond,&mutex);/*解锁mutex，并等待cond改变*/
             printf("thread2: wait 2  %d/n", __LINE__);
         }
         pthread_mutex_unlock(&mutex);
         printf("thread2: unlock %d/n/n", __LINE__);
         sleep(1);
     }
 }

编译：

[X61@horizon threads]$ gcc thread_cond.c -lpthread -o tcd

以下是程序运行结果：

[X61@horizon threads]$ ./tcd 
thread1: lock 30
thread1: unlock 40

thread2: lock 52
thread2: wait 1 55
thread1: lock 30
thread1: unlock 40

thread1: lock 30
thread1:signal 1 33
thread1:signal 2 35
thread1: unlock 40

thread2: wait 2 57
thread2: unlock 61

thread1: lock 30
thread1: unlock 40

thread2: lock 52
thread2: wait 1 55
thread1: lock 30
thread1: unlock 40

thread1: lock 30
thread1:signal 1 33
thread1:signal 2 35
thread1: unlock 40

thread2: wait 2 57
thread2: unlock 61

这里的两个关键函数就在pthread_cond_wait和pthread_cond_signal函数。

本例中：

线程一先执行，获得mutex锁，打印，然后释放mutex锁，然后阻塞自己1秒。

线程二此时和线程一应该是并发的执行 ，这里是一个要点，为什么说是线程此时是并发的执行，因为此时不做任何干涉的话，是没有办法确定是线程一先获得执行还是线程二先获得执行，到底那个线程先获得执行，取决于操作系统的调度，想刻意的让线程2先执行，可以让线程2一出来，先sleep一秒。
这里并发执行的情况是，线程一先进入循环，然后获得锁，此时估计线程二执行,阻塞在
pthread_mutex_lock(&mutex);
这行语句中，直到线程1释放mutex锁
pthread_mutex_unlock(&mutex);/*解锁互斥量*/
然后线程二得已执行，获取metux锁，满足if条件，到pthread_cond_wait (&cond,&mutex);/*等待*/
这里的线程二阻塞，不仅仅是等待cond变量发生改变，同时释放mutex锁 ，因为当时看书没有注意，所以这里卡了很久。
mutex锁释放后，线程1终于获得了mutex锁，得已继续运行，当线程1的if（i%3==0）的条件满足后，通过pthread_cond_signal发送信号，告诉等待cond的变量的线程（这个情景中是线程二），cond条件变量已经发生了改变。

不过此时线程二并没有立即得到运行 ，因为线程二还在等待mutex锁的释放，所以线程一继续往下走，直到线程一释放mutex锁，线程二才能停止等待，打印语句，然后往下走通过pthread_mutex_unlock(&mutex)释放mutex锁，进入下一个循环。