mutex互斥量小例程

例如int *a  int *b  分别指向两块内存，上面的值分别初始化为（200， 100） 线程A执行这样的一个操作：将*a的值减少50，*b的值增加50.

 线程B执行：打印出（a 跟 b 指向的内存的值的和）。

      如果串行运行：A: *a -= 50; *b += 50;  B: printf("%d\n", *a + *b);  

      如果并发执行，则有可能会出现一下调度：*a -= 50; printf("%d\n", *a + *b); *b += 50;

      因此我们可以引入互斥量，在对共享数据读写时进行锁操作，实现对内存的访问以互斥的形式进行。

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

int a = 200;
int b = 100;
pthread_mutex_t lock;

void* ThreadA(void*argc)
{
    printf("pthreadA is running\n");
    pthread_mutex_lock(&lock); //锁
    printf("pthreadA locked...\n");
    a -= 50;
    sleep(5); //执行到一半 使用sleep 放弃cpu调度
    b += 50;
    pthread_mutex_unlock(&lock);
    printf("pthreadA unlocked...\n");
    printf("pthreadA exit\n");
}

void* ThreadB(void*argc)
{
    sleep(1); //放弃CPU调度 目的先让A线程运行。
    printf("pthreadB is running\n");
    //pthread_mutex_unlock(&lock);
    //printf("pthreadB unlocked...\n");
    pthread_mutex_lock(&lock);
    printf("pthreadB locked...\n");
    printf("%d\n", a + b);
    pthread_mutex_unlock(&lock);
    printf("pthreadB unlocked...\n");
    printf("pthreadB exit\n");
}

int main()
{
    pthread_t tida, tidb;
    pthread_mutex_init(&lock, NULL);
    pthread_create(&tida, NULL, ThreadA, NULL);
    pthread_create(&tidb, NULL, ThreadB, NULL);
    pthread_join(tida, NULL);
    pthread_join(tidb, NULL);
    exit(0);
}

运行结果：
[vibe@localhost code]$ ./a
pthreadA is running
pthreadA locked...
pthreadB is running
pthreadA unlocked...
pthreadA exit
pthreadB locked...
300
pthreadB unlocked...
pthreadB exit