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 <pthread.h>

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 <pthread.h>

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 <pthread.h>

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 <pthread.h>
#include <time.h>

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 <pthread.h>

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 <pthread.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>

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 thread3
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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