#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);        //如果不加鎖，threadB輸出會是50和200
    b += 50;        //加鎖后會sleep 3秒后，并為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) //輪詢直到獲得鎖
    {
        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);                //讓threadA能先執行
    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;
}