用互斥锁和条件变量实现读写锁(读优先)

不久前发过一个版本，是写优先的，当时觉得读优先跟写优先差不多就没写读优先后来想了想，还是有必要写一写的，好处是可以更为清晰的区分读优先和写优先之间实现的细小区别更能加深印象吧。

依旧头文件部分，my_pthread_rwlock.h，顺便纠正上篇中的一个错误pthread_rwlock.h——应为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 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)；

然后是主体实现部分

#include"my_pthread_rwlock.h"
#include<errno.h>

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_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_nwaitreaders >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_nwaitreaders > 0)
result = pthread_cond_broadcast(&rw->rw_condreaders);
else 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_phread_rwlock_destroy(my_pthread_rwlock_t *rw)
{
if(rw->rw_magic != RW_MAGIC)
return (EINVAL);
if(rw->rw_refcount != 0 || rw->rw_nwaitreaders != 0 || rw->rw_nwaitwriters != 0)
return (EBUSY);
pthread_mutex_destroy(&rw->rw_mutex);
pthread_cond_destroy(&rw->rw_condreaders);
pthread_cond_destroy(&rw->rw_condwriters);
rw->rw_magic = 0;
return 0;
}

my_pthread_rwlock_tryrdlock(my_pthread_rwlock_t *rw)
{
int result;
if(rw->rw_magic != RW_MAGIC)
return (EINVAL);
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++;
pthread_mutex_unlock(&rw->rw_mutex);
return result;
}

my_pthread_rwlock_trywrlock(my_pthread_rwlock_t *rw)
{
int result;
if(rw->rw_magic != RW_MAGIC)
return (EINVAL);
if((result = pthread_mutex_lock(&rw->rw_mutex)) != 0)
return result;
if(rw->rw_refcount != 0)
return (EBUSY);
else
rw->rw_refcount = -1;
pthread_mutex_unlock(&rw->rw_mutex);
return result;
}

测试程序就不发了
一生热爱，回头太难