Linux操作系统多线程同步Mutex详细介绍

Linux操作系统多线程同步Mutex详细介绍

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1. 初始化:

在下, 线程的互斥量数据类型是pthread_mutex_t. 在使用前, 要对它进行初始化:

对于静态分配的互斥量, 可以把它设置为PTHREAD_MUTEX_INITIALIZER, 或者调用pthread_mutex_init.

对于动态分配的互斥量, 在申请内存(malloc)之后, 通过pthread_mutex_init进行初始化, 并且在释放内存(free)前需要调用pthread_mutex_destroy.

原型:

int pthread_mutex_init(pthread_mutex_t *restrict mutex, const pthread_mutexattr_t *restric attr);

int pthread_mutex_destroy(pthread_mutex_t *mutex);

头文件:

返回值: 成功则返回0, 出错则返回错误编号.

说明: 如果使用默认的属性初始化互斥量, 只需把attr设为NULL. 其他值在以后讲解.

2. 互斥操作:

对共享资源的访问, 要对互斥量进行加锁, 如果互斥量已经上了锁, 调用线程会阻塞, 直到互斥量被解锁. 在完成了对共享资源的访问后, 要对互斥量进行解锁.

首先说一下加锁函数:

头文件:

原型:

int pthread_mutex_lock(pthread_mutex_t *mutex);

int pthread_mutex_trylock(pthread_mutex_t *mutex);

返回值: 成功则返回0, 出错则返回错误编号.

说明: 具体说一下trylock函数, 这个函数是非阻塞调用模式, 也就是说, 如果互斥量没被锁住, trylock函数将把互斥量加锁, 并获得对共享资源的访问权限; 如果互斥量被锁住了, trylock函数将不会阻塞等待而直接返回EBUSY, 表示共享资源处于忙状态.

再说一下解所函数:

头文件:

原型: int pthread_mutex_unlock(pthread_mutex_t *mutex);

返回值: 成功则返回0, 出错则返回错误编号.

3. 死锁:

死锁主要发生在有多个依赖锁存在时, 会在一个线程试图以与另一个线程相反顺序锁住互斥量时发生. 如何避免死锁是使用互斥量应该格外注意的东西.

总体来讲, 有几个不成文的基本原则:

对共享资源操作前一定要获得锁.

完成操作以后一定要释放锁.

尽量短时间地占用锁.

如果有多锁, 如获得顺序是ABC连环扣, 释放顺序也应该是ABC.

线程错误返回时应该释放它所获得的锁.

示例：

#include #include #include #include #include pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;int lock_var;time_t end_time;int sum;void pthread1(void *arg);void pthread2(void *arg);void pthread3(void *arg);int main(int argc, char *argv[]){pthread_t id1,id2,id3;pthread_t mon_th_id;int ret;sum=10;end_time = time(NULL)+10;pthread_mutex_init(&mutex,NULL);ret=pthread_create(&id1,NULL,(void *)pthread1, NULL);if(ret!=0)perror("pthread cread1");ret=pthread_create(&id2,NULL,(void *)pthread2, NULL);if(ret!=0)perror("pthread cread2");ret=pthread_create(&id3,NULL,(void *)pthread3, NULL);if(ret!=0)perror("pthread cread3");pthread_join(id1,NULL);pthread_join(id2,NULL);pthread_join(id3,NULL);exit(0);}void pthread1(void *arg){int i;while(time(NULL) < end_time){if(pthread_mutex_lock(&mutex)!=0) //lock{perror("pthread_mutex_lock");}elseprintf("pthread1:pthread1 lock the variable/n");for(i=0;i<2;i++){sleep(2);lock_var++;}if(pthread_mutex_unlock(&mutex)!=0) //unlock{perror("pthread_mutex_unlock");}elseprintf("pthread1:pthread1 unlock the variable/n");sleep(1);}}void pthread2(void *arg){int nolock=0;int ret;while(time(NULL) < end_time){ret=pthread_mutex_trylock(&mutex);//try lockif(ret==EBUSY)printf("pthread2:the variable is locked by pthread1/n");else{if(ret!=0){perror("pthread_mutex_trylock");exit(1);}elseprintf("pthread2:pthread2 got lock.The variable is %d/n",lock_var);if(pthread_mutex_unlock(&mutex)!=0)//unlock{perror("pthread_mutex_unlock");}elseprintf("pthread2:pthread2 unlock the variable/n");}sleep(1);}}void pthread3(void *arg){/*int nolock=0;int ret;while(time(NULL) < end_time){ret=pthread_mutex_trylock(&mutex);if(ret==EBUSY)printf("pthread3:the variable is locked by pthread1 or 2/n");else{if(ret!=0){perror("pthread_mutex_trylock");exit(1);}elseprintf("pthread3:pthread3 got lock.The variable is %d/n",lock_var);if(pthread_mutex_unlock(&mutex)!=0){perror("pthread_mutex_unlock");}elseprintf("pthread3:pthread2 unlock the variable/n");}sleep(3);}*/}