一个简单的线程竟态实例

写这个东西完全是初级的东西，给不理解的人看的，理解了请pass...

1.源代码：

#include <stdio.h>
#include <pthread.h>


int critical_var =0;

pthread_mutex_t mutex_c= PTHREAD_MUTEX_INITIALIZER;

void* printf_fn(void *data)
{

        int critical_var_bak=pthread_self();
        while(1){
//pthread_mutex_lock(&mutex_c);
                critical_var = critical_var_bak;
                sleep(1);
                if(critical_var != critical_var_bak){
                        printf("found ...\n");
                        printf("tid:%d critical:%d\n",critical_var_bak,critical_var);
                }
//pthread_mutex_unlock(&mutex_c);
        }

return (void*)NULL;

}


int main()
{

        pthread_t tid1,tid2;
        pthread_create(&tid1,NULL,printf_fn,NULL);
        pthread_create(&tid2,NULL,printf_fn,NULL);


        pthread_join(tid1,NULL);
        pthread_join(tid2,NULL);

        return 0;

}

2.编译运行

gcc pthread_test.c -o pthread_test -g -lpthread

打印：
found ...
tid:-1328888064 critical:-1337280768
found ...
tid:-1337280768 critical:-1328888064
found ...
tid:-1328888064 critical:-1337280768
found ...
tid:-1337280768 critical:-1328888064
found ...
tid:-1328888064 critical:-1337280768
found ...
tid:-1337280768 critical:-1328888064

很明显的可以看出来 critial的变量在线程切换的时候改变了状态。

3. 如果打开互斥锁(//pthread_mutex_lock(&mutex_c);)，就完全可以避免了此类问题。改一下 printf_fn

void* printf_fn(void *data)
{

        int critical_var_bak=pthread_self();
        while(1){
pthread_mutex_lock(&mutex_c);
                critical_var = critical_var_bak;
                sleep(1);
                if(critical_var != critical_var_bak){
                        printf("found ...\n");
                        printf("tid:%d critical:%d\n",critical_var_bak,critical_var);
                }
pthread_mutex_unlock(&mutex_c);
        }

return (void*)NULL;

}

4.重新编译运行

5.结果没有critical的打印了。