Continued from the previous article: linux_thread lock mutex (mutex)_thread synchronization_deadlock phenomenon_pthread_mutex_lock function_pthread_mutex_unlock function_deadlock phenomenon
Today I will share with you another lock in linux: a read-write lock, which is similar to a mutex lock, but a read-write lock allows higher parallelism. Its characteristics are: write exclusive, read shared. It is mainly to share some application functions of read-write locks, and start serving:
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Table of contents
1. Read-write lock status:
A read-write lock has three states:
1. Locked state in read mode (read lock)
2. Locked state in write mode (write lock)
3. Unlocked state
2. Read-write lock features:
1、读写锁是“写模式加锁”时， 解锁前，所有对该锁加锁的线程都会被阻塞。
2、读写锁是“读模式加锁”时， 如果线程以读模式对其加锁会成功；如果线程以写模式加锁会阻塞。
3、读写锁是“读模式加锁”时， 既有试图以写模式加锁的线程，也有试图以读模式加锁的线程。那么读写锁会阻塞随后的读模式锁请求。优先满足写模式锁。读锁、写锁并行阻塞，写锁优先级高
Read-write locks are also called shared-exclusive locks. When a reader-writer lock is locked in read mode, it is locked in shared mode; when it is locked in write mode, it is locked in exclusive mode. Write exclusive, read shared.
Read-write locks are very suitable for situations where the number of reads to the data structure is much greater than that of writes.
3. Main application functions:
The pthread_rwlock_t type is used to define a read-write lock variable.
pthread_rwlock_t rwlock;
All the return values of this series of functions, when the return value is not 0, can print error information through the strerror function.
3.1.pthread_rwlock_init function
Function:
Initialize a read-write lock.
Header file:
#include <pthread.h>
Function prototype:
int pthread_rwlock_init(pthread_rwlock_t *restrict rwlock, const pthread_rwlockattr_t *restrict attr);
Function parameters:
rwlock: Read-write lock that needs to be initialized.
attr: Read-write lock attribute, usually use the default attribute, just pass NULL.
Return value:
success: return 0;
failure: return non-zero.
3.2.pthread_rwlock_destroy function
Function:
Destroy a read-write lock.
Header file:
#include <pthread.h>
Function prototype:
int pthread_rwlock_destroy(pthread_rwlock_t *rwlock);
Function parameters:
rwlock: Read-write lock that needs to be initialized.
Return value:
success: return 0;
failure: return non-zero.
3.3.pthread_rwlock_rdlock function
Function:
Request a read-write lock in read mode. (Often abbreviated as: request read lock)
Header file:
#include <pthread.h>
Function prototype:
int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
Function parameters:
rwlock: The read-write lock that needs to be requested.
Return value:
success: return 0;
failure: return non-zero.
3.4.pthread_rwlock_wrlock function
Function:
Request a read-write lock in writing mode. (Often abbreviated as: request write lock)
Header file:
#include <pthread.h>
Function prototype:
int pthread_rwlock_wrlock(pthread_rwlock_t *rwlock);
Function parameters:
rwlock: The read-write lock that needs to be requested.
Return value:
success: return 0;
failure: return non-zero.
3.5.pthread_rwlock_unlock function
Function function:
Unlock
header file:
#include <pthread.h>
Function prototype:
int pthread_rwlock_unlock(pthread_rwlock_t *rwlock);
Function parameters:
rwlock: the lock to be unlocked.
Return value:
success: return 0;
failure: return non-zero.
3.6.pthread_rwlock_tryrdlock function
Function:
non-blocking request for read-write lock in read mode (non-blocking request for read-write lock)
header file:
#include <pthread.h>
function prototype:
int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock);
function parameter:
rwlock: read and write that needs to be requested Lock.
Return value:
success: return 0;
failure: return non-zero.
3.7.pthread_rwlock_trywrlock function
Function function:
Non-blocking request for read-write lock in writing mode (non-blocking request for write lock)
Header file:
#include <pthread.h>
Function prototype:
int pthread_rwlock_trywrlock(pthread_rwlock_t *rwlock);
Function parameters:
rwlock: read and write that needs to be requested Lock.
Return value:
success: return 0;
failure: return non-zero.
4. Example of read-write lock
See the following example, where multiple threads read and write the same global data at the same time.
//3个线程不定时 "写" 全局资源，5个线程不定时 "读" 同一全局资源
#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
int counter; //全局变量
pthread_rwlock_t rwlock;//读写锁
//写线程 回调函数
void *th_write(void *arg)
{
int t;
int i = (int)arg;
while (1) {
t = counter;
usleep(1000);//睡眠1000微妙
pthread_rwlock_wrlock(&rwlock);//加写锁
printf("=======write %d: %lu: counter=%d ++counter=%d\n", i, pthread_self(), t, ++counter);
pthread_rwlock_unlock(&rwlock);//解锁
usleep(5000);//睡眠5000微妙
}
return NULL;
}
//读线程回调函数
void *th_read(void *arg)
{
int i = (int)arg;
while (1)
{
pthread_rwlock_rdlock(&rwlock);//加读锁
printf("----------------------------read %d: %lu: %d\n", i, pthread_self(), counter);
pthread_rwlock_unlock(&rwlock);//解锁
usleep(900);//睡眠900微妙
}
return NULL;
}
//主函数
int main(void)
{
int i;
pthread_t tid[8];
pthread_rwlock_init(&rwlock, NULL);//初始化读写锁
//创建3个写线程
for (i = 0; i < 3; i++)
pthread_create(&tid[i], NULL, th_write, (void *)i);
//创建5个读线程
for (i = 0; i < 5; i++)
pthread_create(&tid[i+3], NULL, th_read, (void *)i);
//等待线程退出
for (i = 0; i < 8; i++)
pthread_join(tid[i], NULL);
//释放读写锁
pthread_rwlock_destroy(&rwlock); //释放读写琐
return 0;
}
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