一个简单的互斥量与条件变量例子

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
//互斥变量和条件变量静态初始化
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)
{
   // sleep(30);
    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);
}

/*
pthread_t pthread_self(void);
函数作用：获得线程自身的ID。
pthread_t的类型为unsigned long int，
所以在打印的时候要使用%lu方式，否则将产生奇怪的结果
*/
void *thread1(void *junk)
{
    for(i=1;i<=6;i++)
    {
       pthread_mutex_lock(&mutex);/*锁住互斥量*/
    {
        printf("Thread1: lock ------%lu\n", pthread_self());
       }
       if(i%3==0)
    {
         printf("Thread1:signal1------%lu\n", pthread_self());
            pthread_cond_signal(&cond);/*条件改变，发送信号，通知t_b进程*/
         printf("Thread1:signal2------%lu\n", pthread_self());
         sleep(1);
    }
       pthread_mutex_unlock(&mutex);/*解锁互斥量*/
    printf("Thread1: unlock ------%lu\n", pthread_self());
    sleep(1);
    }
}

void *thread2(void *junk)
{
    while(i<6)
    {
        pthread_mutex_lock(&mutex);
     printf("Thread2: lock ------%lu\n", pthread_self());
     if(i%3!=0)
     {
         printf("Thread2: wait1------%lu\n", pthread_self());
            pthread_cond_wait(&cond,&mutex);/*解锁mutex，并等待cond改变*/
         printf("Thread2: wait2------%lu\n", pthread_self());
      }
        pthread_mutex_unlock(&mutex);
        printf("Thread2: unlock ------%lu\n", pthread_self());
     sleep(1);
     }
}

/*
[ztteng@ztteng 7.7_8]$ ./pthreadcond
Thread2: lock ------3068291952
Thread2: wait1------3068291952
Thread1: lock ------3078781808
Thread1: unlock ------3078781808
Thread1: lock ------3078781808
Thread1: unlock ------3078781808
Thread1: lock ------3078781808
Thread1:signal1------3078781808
Thread1:signal2------3078781808
Thread1: unlock ------3078781808
Thread2: wait2------3068291952
Thread2: unlock ------3068291952
Thread1: lock ------3078781808
Thread1: unlock ------3078781808
Thread2: lock ------3068291952
Thread2: wait1------3068291952
Thread1: lock ------3078781808
Thread1: unlock ------3078781808
Thread1: lock ------3078781808
Thread1:signal1------3078781808
Thread1:signal2------3078781808
Thread1: unlock ------3078781808
Thread2: wait2------3068291952
Thread2: unlock ------3068291952
*/

 　　

 

这里有几个问题；（1）一个是为什么是thread2先执行（2）假若thread1先执行；并发送了signal信号，这时thread2还没有起来会怎么样