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Linux互斥量读写锁

article 2024/12/2 22:10:24

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);
    } 

}