linux多线程之读写锁

基本概念：

读写锁也叫做共享互斥锁。

当读写锁是写加锁状态时，在这个锁被解锁之前，所有试图对这个锁加锁的线程都会被阻塞。当读写锁在读加锁状态，所有试图以读模式对它进行加锁的线程都可以得到访问权。

与互斥量相比，读写锁在使用之前必须初始化，在释放它们底层的内存之前必须销毁。

一、锁的初始化与销毁

PTHREAD_RWLOCK_DESTROY(P)  POSIX Programmer's Manual PTHREAD_RWLOCK_DESTROY(P)

NAME
       pthread_rwlock_destroy,  pthread_rwlock_init - destroy and initialize a
       read-write lock object

SYNOPSIS
       #include 

       int pthread_rwlock_destroy(pthread_rwlock_t *rwlock);
       int pthread_rwlock_init(pthread_rwlock_t *restrict rwlock,
              const pthread_rwlockattr_t *restrict attr);

两个函数的返回值：若成功，返回0；否则，返回错误编号

二、读锁

PTHREAD_RWLOCK_RDLOCK(P)   POSIX Programmer's Manual  PTHREAD_RWLOCK_RDLOCK(P)

NAME
       pthread_rwlock_rdlock,  pthread_rwlock_tryrdlock  -  lock  a read-write
       lock object for reading

SYNOPSIS
       #include 

       int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
       int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock);

两个函数的返回值：若成功，返回0；否则，返回错误编号
pthread_rwlock_tryrdlock函数可以获取锁时，返回0.否则，返回错误EBUSY

三、写锁

PTHREAD_RWLOCK_TRYWRLOCK(P)POSIX Programmer's ManuaPTHREAD_RWLOCK_TRYWRLOCK(P)

NAME
       pthread_rwlock_trywrlock,  pthread_rwlock_wrlock  -  lock  a read-write
       lock object for writing

SYNOPSIS
       #include 

       int pthread_rwlock_trywrlock(pthread_rwlock_t *rwlock);
       int pthread_rwlock_wrlock(pthread_rwlock_t *rwlock);

两个函数的返回值：若成功，返回0；否则，返回错误编号

pthread_rwlock_trywrlock函数可以获取锁时，返回0.否则，返回错误EBUSY

四、带有超时的读写锁

PTHREAD_RWLOCK_TIMEDWRLOCK(POSIX Programmer's ManPTHREAD_RWLOCK_TIMEDWRLOCK(P)

NAME
       pthread_rwlock_timedrdlock - lock a read-write lock for reading
       pthread_rwlock_timedwrlock - lock a read-write lock for writing

SYNOPSIS
       #include 
       #include 

       int pthread_rwlock_timedrdlock(pthread_rwlock_t *restrict rwlock,
              const struct timespec *restrict abs_timeout);
       int pthread_rwlock_timedwrlock(pthread_rwlock_t *restrict rwlock,
              const struct timespec *restrict abs_timeout);

两个函数的返回值：若成功，返回0；否则，返回错误编号
如果它们不能获取锁，那么超时到期时，这两个函数将返回ETIMEDOUT

五、解锁

PTHREAD_RWLOCK_UNLOCK(P)   POSIX Programmer's Manual  PTHREAD_RWLOCK_UNLOCK(P)

NAME
       pthread_rwlock_unlock - unlock a read-write lock object

SYNOPSIS
       #include 

       int pthread_rwlock_unlock(pthread_rwlock_t *rwlock);

返回值：若成功，返回0；否则，返回错误编号

读写锁属性：

读写锁支持的唯一属性是进程共享属性。它与互斥量的进程共享属性是相同的，这里不展开讨论。

例子：gcc pthread_rwlock.c -pthread

#include 
#include 
#include 
#include 
#include 

static int num = 0;
static int count = 100000;
static pthread_rwlock_t rwlock;

void Perror(const char *s)
{
    perror(s);
    exit(EXIT_FAILURE);
}

void* fun2(void *arg)
{
    pthread_t thread_id = pthread_self();
    printf("the thread2 id is %ld\n", (long)thread_id);
    int i = 1;
    for (; i<=count; ++i) {
        pthread_rwlock_wrlock(&rwlock);
        num += 1;
        pthread_rwlock_unlock(&rwlock);
    }
}

void* fun3(void *arg)
{
    pthread_t thread_id = pthread_self();
    printf("the thread3 id is %ld\n", (long)thread_id);
    int i = 1;
    for (; i<=count; ++i) {
        pthread_rwlock_wrlock(&rwlock);
        num += 1;
        pthread_rwlock_unlock(&rwlock);
    }
}

int main()
{
    int err;
    pthread_t thread1;
    pthread_t thread2;
    pthread_t thread3;

    // init
    pthread_rwlock_init(&rwlock, NULL);

    thread1 = pthread_self();
    printf("the thread1 id is %ld\n", (long)thread1);

    // Create thread
    err = pthread_create(&thread2, NULL, fun2, NULL);
    if (err != 0) {
        Perror("can't create thread2\n");
    }
    err = pthread_create(&thread3, NULL, fun3, NULL);
    if (err != 0) {
        Perror("can't create thread3\n");
    }

    // detach thread
    err = pthread_detach(thread2);
    if (err != 0) {
        Perror("can't detach thread2\n");
    }
    err = pthread_detach(thread3);
    if (err != 0) {
        Perror("can't detach thread3\n");
    }

    int i = 1;
    for (; i<=count; ++i) {
        pthread_rwlock_rdlock(&rwlock);
        int temp = num;
        pthread_rwlock_unlock(&rwlock);
    }

    sleep(10);
    printf("The num is %d\n", num);
    
    pthread_rwlock_destroy(&rwlock);
    return 0;
}

参考：《unix环境高级编程》·第三版

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