线程间通信 条件变量

surpassgood

#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,thread2,(void *)NULL);/*创建进程t_a*/
    pthread_create(&t_b,NULL,thread1,(void *)NULL); /*创建进程t_b*/
    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) 打印i %3!=0 的数
{
    for(i=1;i<=9;i++)
    {
        pthread_mutex_lock(&mutex);/*锁住互斥量*/
        if(i%3==0)
             pthread_cond_signal(&cond);/*条件改变，发送信号，通知t_b进程*/
        else
             printf("thead1:%d/n",i);
        pthread_mutex_unlock(&mutex);/*解锁互斥量*/

sleep(1);
}
}

void *thread2(void *junk)  打印能 i%3==0的数
{
    while(i<9)
    {
        pthread_mutex_lock(&mutex);

if(i%3!=0)
            pthread_cond_wait(&cond,&mutex);/*等待*/
        printf("thread2:%d/n",i);
        pthread_mutex_unlock(&mutex);

sleep(1);
}
}
结果很奇怪
thread2 : 1
thread2 : 2
thread1 : 3
thread1 : 3
thread2 : 4
thread2 : 5
thread1 : 6
thread1 : 6
thread2 : 7
thread2 : 8
thread1 : 9
怎么thread1打印了两次，求解释

myworkstation

你的i变量在两个线程中同步访问而i变量又没上锁数据不同步了。使用i之前先 pthread_mutex_lock(&mutex);