互斥量mutex的简单使用

几个重要的函数：

#include <pthread.h>

int pthread_mutex_init(pthread_mutex_t *restrict mutex, const pthread_mutex_t *restrict attr);    //初始化mutex

int pthread_mutex_destroy(pthread_mutex_t *mutex);  //如果mutex是动态分配的，则释放内存前调用此函数。

int pthread_mutex_lock(pthread_mutex_t *mutex);    //加锁

int pthread_mutex_trylock(pthread_mutex_t *mutex);  //若已有其他线程占用锁，则返回EBUSY，否则返回0，不阻塞。

int pthread_mutex_unlock(pthread_mutex_t *mutex);   //解锁

例程：

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

int a = 100;
int b = 200;

pthread_mutex_t lock;

void * threadA()
{
    pthread_mutex_lock(&lock);
    printf("thread A got lock!
");
    a -= 50;
    sleep(3);        //如果不加锁，threadB输出会是50和200
    b += 50;        //加锁后会sleep 3秒后，并为b加上50 threadB才能打印
    pthread_mutex_unlock(&lock);
    printf("thread A released the lock!
");
    a -= 50;
}

void * threadC()
{    
    sleep(1);
    while(pthread_mutex_trylock(&lock) == EBUSY) //轮询直到获得锁
    {
        printf("thread C is trying to get lock!
");
        usleep(100000);
    }
    printf("thread C got the lock!
");
    a = 1000;
    b = 2000;
    pthread_mutex_unlock(&lock);
    printf("thread C released the lock!
");
    
}

void * threadB()
{
    sleep(2);                //让threadA能先执行
    pthread_mutex_lock(&lock);
    printf("thread B got the lock! a=%d b=%d
", a, b);
    pthread_mutex_unlock(&lock);
    printf("thread B released the lock!
", a, b);
}

int main()
{
    pthread_t tida, tidb, tidc;
    pthread_mutex_init(&lock, NULL);
    pthread_create(&tida, NULL, threadA, NULL);
    pthread_create(&tidb, NULL, threadB, NULL);
    pthread_create(&tidc, NULL, threadC, NULL);
    pthread_join(tida, NULL);
    pthread_join(tidb, NULL);
    pthread_join(tidc, NULL);
    return 0;
}