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：

 1 #include <stdlib.h>
 2 #include <stdio.h>
 3 #include <unistd.h>
 4 #include <pthread.h>
 5 #include <errno.h>
 6 
 7 int a = 100;
 8 int b = 200;
 9 
10 pthread_mutex_t lock;
11 
12 void * threadA()
13 {
14     pthread_mutex_lock(&lock);
15     printf("thread A got lock!\n");
16     a -= 50;
17     sleep(3);        //如果不加锁，threadB输出会是50和200
18     b += 50;        //加锁后会sleep 3秒后，并为b加上50 threadB才能打印
19     pthread_mutex_unlock(&lock);
20     printf("thread A released the lock!\n");
21     a -= 50;
22 }
23 
24 void * threadC()
25 {    
26     sleep(1);
27     while(pthread_mutex_trylock(&lock) == EBUSY) //轮询直到获得锁
28     {
29         printf("thread C is trying to get lock!\n");
30         usleep(100000);
31     }
32     printf("thread C got the lock!\n");
33     a = 1000;
34     b = 2000;
35     pthread_mutex_unlock(&lock);
36     printf("thread C released the lock!\n");
37     
38 }
39 
40 void * threadB()
41 {
42     sleep(2);                //让threadA能先执行
43     pthread_mutex_lock(&lock);
44     printf("thread B got the lock! a=%d b=%d\n", a, b);
45     pthread_mutex_unlock(&lock);
46     printf("thread B released the lock!\n", a, b);
47 }
48 
49 int main()
50 {
51     pthread_t tida, tidb, tidc;
52     pthread_mutex_init(&lock, NULL);
53     pthread_create(&tida, NULL, threadA, NULL);
54     pthread_create(&tidb, NULL, threadB, NULL);
55     pthread_create(&tidc, NULL, threadC, NULL);
56     pthread_join(tida, NULL);
57     pthread_join(tidb, NULL);
58     pthread_join(tidc, NULL);
59     return 0;
60 }