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;
}