Linux互斥量读写锁

一、互斥量

1.临界资源

        同一时刻只允许一个进程/线程访问的共享资源（比如文件、外设打印机）

2.临界区

        访问临界资源的代码

3.互斥机制

        mutex互斥锁，用来避免临界资源的访问冲突，访问临界资源前申请互斥锁，访问完释放锁

 形象点的说法 好比有一个公共卫生间，进去使用的人会给门上锁，使用完会开锁

4.创建互斥锁 

/*动态创建*/
pthread_mutex_t mutex
pthread_mutex_t_init(pthread_mutex_t *mutex,互斥锁属性 给NULL默认即可)
/*静态创建*/
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER

5.销毁互斥锁

pthread_mutex_destory(pthread_mutex_t *mutex)

Linux中，互斥锁不占任何资源，所以销毁锁不是必须的，可利用其返回值查询锁状态，锁定时返回EBUSY

6.申请互斥锁（P操作） 

pthread_mutex_lock(pthread_mutex_t *mutex) //无锁时阻塞等待
pthread_mutex_trylock(pthread_mutex_t *mutex) //无锁返回EBUSY

7.释放互斥锁（V操作）

pthread_mutex_unlock(pthread_mutex_t *mutex) 

#include <pthread.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;FILE *fp;
void *func2(void *arg){pthread_detach(pthread_self());printf("This func2 thread\n");char str[]="I write func2 line\n";char c;int i=0;while(1){pthread_mutex_lock(&mutex);while(i<strlen(str)){c = str[i];fputc(c,fp);usleep(1);i++;}pthread_mutex_unlock(&mutex);i=0;usleep(1);}pthread_exit("func2 exit");}void *func(void *arg){pthread_detach(pthread_self());printf("This is func1 thread\n");char str[]="You read func1 thread\n";char c;int i=0;while(1){pthread_mutex_lock(&mutex);while(i<strlen(str)){c = str[i];fputc(c,fp);i++;usleep(1);}pthread_mutex_unlock(&mutex);i=0;usleep(1);}pthread_exit("func1 exit");
}int main(){pthread_t tid,tid2;void *retv;int i;fp = fopen("1.txt","a+");if(fp==NULL){perror("fopen");return 0;}pthread_create(&tid,NULL,func,NULL);pthread_create(&tid2,NULL,func2,NULL);while(1){    sleep(1);} }

二、读写锁

与互斥锁的区别是：

• 互斥锁对所有线程一视同仁，同一时刻只允许一个线程访问临界资源

• 读写锁区分读者和写者，同一时刻只允许一个写者访问临界资源，而读者允许多个同时访问，更具体地：

  • 写锁状态时，其他写锁、读锁都被阻塞；

  • 读锁状态时，读锁不阻塞，写锁阻塞，但在写锁阻塞之后申请的读锁要阻塞等待写锁（否则重要的内容一直写不进去）

pthread_rwlock_t rwlock //定义读写锁
/*申请写锁*/
pthread_rwlock_wrlock(pthread_rwlock_t *rwlock)
pthread_rwlock_trywrlock(pthread_rwlock_t *rwlock)
/*申请读锁*/
pthread_rwlock_rdlock(pthread_rwlock_t *rwlock)
pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock)
/*释放锁*/
pthread_rwlock_unlock(pthread_rwlock_t *rwlock)
/*销毁读写锁*/
pthread_rwlock_destory(pthread_rwlock_t *rwlock)

#include <pthread.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>pthread_rwlock_t rwlock;FILE *fp;
void * read_func(void *arg){pthread_detach(pthread_self());printf("read thread\n");char buf[32]={0};while(1){//rewind(fp);pthread_rwlock_rdlock(&rwlock);while(fgets(buf,32,fp)!=NULL){printf("%d,rd=%s\n",(int)arg,buf);usleep(1000);}pthread_rwlock_unlock(&rwlock);sleep(1);}}void *func2(void *arg){pthread_detach(pthread_self());printf("This func2 thread\n");char str[]="I write func2 line\n";char c;int i=0;while(1){pthread_rwlock_wrlock(&rwlock);while(i<strlen(str)){c = str[i];fputc(c,fp);usleep(1);i++;}pthread_rwlock_unlock(&rwlock);i=0;usleep(1);}pthread_exit("func2 exit");}void *func(void *arg){pthread_detach(pthread_self());printf("This is func1 thread\n");char str[]="You read func1 thread\n";char c;int i=0;while(1){pthread_rwlock_wrlock(&rwlock);while(i<strlen(str)){c = str[i];fputc(c,fp);i++;usleep(1);}pthread_rwlock_unlock(&rwlock);i=0;usleep(1);}pthread_exit("func1 exit");
}int main(){pthread_t tid1,tid2,tid3,tid4;void *retv;int i;fp = fopen("1.txt","a+");if(fp==NULL){perror("fopen");return 0;}pthread_rwlock_init(&rwlock,NULL);pthread_create(&tid1,NULL,read_func,1);pthread_create(&tid2,NULL,read_func,2);pthread_create(&tid3,NULL,func,NULL);pthread_create(&tid4,NULL,func2,NULL);while(1){    sleep(1);} }

死锁的避免

概念：有两把锁以上时，多个线程各自申请到锁时，紧接着申请其他线程占用着的锁，它们都会处于阻塞等待，形成【死锁】

避免方法：

• 锁越少越好，一把锁无需考虑死锁问题

• 调整锁的顺序

  • 一种笨策略是，某个线程如果想要申请多个锁，那么可以等其释放完，其他线程再申请

  • 另一种笨策略是，各个线程按同样的顺序申请多个锁