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 = ;
 int b = ;

 pthread_mutex_t lock;

 void * threadA()
 {
     pthread_mutex_lock(&lock);
     printf("thread A got lock!\n");
     a -= ;
     sleep();        //如果不加锁，threadB输出会是50和200
     b += ;        //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 -= ;
 }

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

 }

 void * threadB()
 {
     sleep();                //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 ;
 }