Linux使用互斥锁和条件变量实现读写锁（写优先）

（1）只要没有线程持有某个给定的读写锁用于写，那么任意数目的线程可以持有该读写锁用于读
（2）仅当没有线程持有某个读写锁用于读或用于写时，才能分配该读写锁用于写
换一种说法就是，只要没有线程在修改（写）某个给定的数据，那么任意数目的线程都可以拥有该数据的访问权（读）。仅当没有其它线程在读或者修改（写）某个给定的数据时，当前线程才可以修改（写）它。
因此，读为共享，写为独占。
本程序只实现了一种可能，即优先考虑等待着的写入者，可有其他实现方案。//my_pthread_rwlock.h

#pragma once

#include<pthread.h>
#include<stdio.h>

typedef struct
{
pthread_mutex_t rw_mutex;
pthread_cond_t rw_condreaders;
pthread_cond_t rw_condwriters;
int rw_magic;
int rw_nwaitreaders;
int rw_nwaitwriters;
int rw_refcount; // 0 >0 ==-1
}my_pthread_rwlock_t;

#define RW_MAGIC 0x20180326
#define EBUSY 22
#define MY_PTHREAD_RWLOCK_INITIALIZER {PTHREAD_MUTEX_INITIALIZER, PTHREAD_COND_INITIALIZER, PTHREAD_COND_INITIALIZER,\
RW_MAGIC,0,0,0}

typedef int my_pthread_rwlockattr_t;

int my_pthread_rwlock_init(my_pthread_rwlock_t *rw, my_pthread_rwlockattr_t *attr);
int my_pthread_rwlock_destroy(my_pthread_rwlock_t *rw);
int my_pthread_rwlock_rdlock(my_pthread_rwlock_t *rw);
int my_pthread_rwlock_wrlock(my_pthread_rwlock_t *rw);
int my_pthread_rwlock_unlock(my_pthread_rwlock_t *rw);
int my_pthread_rwlock_tryrdlock(my_pthread_rwlock_t *rw);
int my_pthread_rwlock_trywrlock(my_pthread_rwlock_t *rw);

//my_pthread_rwlock.c

#include"my_pthread_rwlock.h"
#include<pthread.h>
int my_pthread_rwlock_init(my_pthread_rwlock_t *rw, my_pthread_rwlockattr_t *attr)
{
int result;
if(attr != NULL)
return -1;
if((result = pthread_mutex_init(&rw->rw_mutex,NULL))!=0)
goto err1;
if((result = pthread_cond_init(&rw->rw_condreaders,NULL))!=0)
goto err2;
if((result = pthread_cond_init(&rw->rw_condwriters,NULL))!=0)
goto err3;
rw->rw_nwaitreaders=0;
rw->rw_nwaitwriters=0;
rw->rw_refcount=0;
rw->rw_magic=RW_MAGIC;
return 0;
err3:pthread_cond_destory(&rw->rw_condreaders);
err2:pthread_mutex_destory(&rw->rw_mutex);
err1:return result;

}
int my_pthread_rwlock_rdlock(my_pthread_rwlock_t *rw)
{
int result;
if(rw->rw_magic != RW_MAGIC)
return -1;
if((result = pthread_mutex_lock(&rw->rw_mutex)) != 0)
return result;

while(rw->rw_refcount<0 || rw->rw_nwaitwriters>0)
{
rw->rw_nwaitreaders++;
result = pthread_cond_wait(&rw->rw_condreaders, &rw->rw_mutex);
rw->rw_nwaitreaders--;
if(result != 0)
break;
}
if(result == 0)
rw->rw_refcount++;
pthread_mutex_unlock(&rw->rw_mutex);
return result;
}

int my_pthread_rwlock_wrlock(my_pthread_rwlock_t *rw)
{
int result;
if(rw->rw_magic != RW_MAGIC)
return -1;

if((result = pthread_mutex_lock(&rw->rw_mutex)) != 0)
return result;

while(rw->rw_refcount != 0)
{
rw->rw_nwaitwriters++;
result = pthread_cond_wait(&rw->rw_condwriters, &rw->rw_mutex);
rw->rw_nwaitwriters--;
if(result != 0)
break;
}
if(result == 0)
rw->rw_refcount = -1;

pthread_mutex_unlock(&rw->rw_mutex);
return result;
}

int my_pthread_rwlock_unlock(my_pthread_rwlock_t *rw)
{
int result;
if(rw->rw_magic != RW_MAGIC)
return -1;
if((result = pthread_mutex_lock(&rw->rw_mutex)) != 0)
return result;

if(rw->rw_refcount > 0)
rw->rw_refcount--;
else if(rw->rw_refcount == -1)
rw->rw_refcount = 0;
else
printf("unlock error.\n");

if(rw->rw_nwaitwriters > 0)
{
if(rw->rw_refcount == 0)
{
result = pthread_cond_signal(&rw->rw_condwriters);
}
}
else if(rw->rw_nwaitreaders > 0)
result = pthread_cond_broadcast(&rw->rw_condreaders);

pthread_mutex_unlock(&rw->rw_mutex);
return result;
}
int my_pthread_rwlock_tryrdlock(my_pthread_rwlock_t *rw)
{
int result;
if(rw->rw_magic!=RW_MAGIC)
return -1;
if((result = pthread_mutex_lock(&rw->rw_mutex))!=0)
return result;
if(rw->rw_refcount!=0||rw->rw_nwaitwriters>0)
result=EBUSY;
else
rw->rw_refcount++;
return result;
}
int my_pthread_rwlock_trywrlock(my_pthread_rwlock_t *rw)
{
int result;
if(rw->rw_magic!=RW_MAGIC)
return -1;
if((result = pthread_mutex_lock(&rw->rw_mutex))!=0)
return result;
if(rw->rw_refcount!=0)
result=EBUSY;
else
rw->rw_refcount=-1;
pthread_mutex_unlock(&rw->rw_mutex);
return result;
}
int my_pthread_rwlock_destroy(my_pthread_rwlock_t *rw)
{
if(rw->rw_magic!=RW_MAGIC)
return -1;
if(rw->rw_refcount!=0||rw->rw_nwaitreaders!=0||rw->rw_nwaitwriters!=0)
return EBUSY;
pthraed_mutex_destory(&rw->rw_mutex);
pthread_cond_destory(&rw->rw_condreaders);
pthread_cond_destory(&rw->rw_condwriters);
rw->rw_magic=0;
return 0;

}

//main.c

#include<stdio.h>
#include<unistd.h>
#include<pthread.h>
#include"my_pthread_rwlock.h"

my_pthread_rwlock_t rwlock = MY_PTHREAD_RWLOCK_INITIALIZER;

void * thread_fun1(void *arg)
{
}
void * thread_fun2(void *arg)
{
}
void * thread_fun3(void *arg)
{
}

int main()
{
pthread_t tid1, tid2, tid3;
pthread_create(&tid1, NULL, thread_fun1, NULL);
sleep(1);
pthread_create(&tid2, NULL, thread_fun2, NULL);
pthread_create(&tid3, NULL, thread_fun3, NULL);

pthread_join(tid1, NULL);
pthread_join(tid2, NULL);
pthread_join(tid3, NULL);
return 0;
}