互斥量mutex的简单使用(实例讲解)

#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;
}