互斥量mutex的简单使用(实例讲解)

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!\n");
    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!\n");
    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!\n");
        usleep(100000);
    }
    printf("thread c got the lock!\n");
    a = 1000;
    b = 2000;
    pthread_mutex_unlock(&lock);
    printf("thread c released the lock!\n");

}

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