读锁优先的实现

1、问题：在操作系统内对线程是写锁优先，如何实现读锁优先呢？

分析：的利用互斥量和条件变量来进行实现，由于已经对操作系统的写锁优先进行了实现，那么对其原理应该已经掌握了，直接代码实现。

代码如下：

(1)、utili.h

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

(2)、pthread_rwlock.h

#ifndef _PTHREAD_RWLOCK_H
#define _PTHREAD_RWLOCK_H

#include"utili.h"

enum {EINVAL, EBUSY};

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   >0
}my_pthread_rwlock_t;

#define RW_MAGIC 0x2016911

#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;

void reader_clean_up(void *arg);
void writer_clean_up(void *arg);

int my_pthread_rwlock_init(my_pthread_rwlock_t *rw, my_pthread_rwlockattr_t *at);
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);

#endif

(3)、pthread_rwlock_init.c

#include"pthread_rwlock.h"

int my_pthread_rwlock_init(my_pthread_rwlock_t *rw, my_pthread_rwlockattr_t *at){
    int result;
    if(at != NULL)
        return EINVAL;

    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_destroy(&rw->rw_condreaders);
err2:
    pthread_mutex_destroy(&rw->rw_mutex);
err1:
    return result;
}

(4)、pthread_rwlock_wrlock.c

#include"pthread_rwlock.h"

int my_pthread_rwlock_wrlock(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;

    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;
}

(5)、pthread_rwlock_rdlock.c

#include"pthread_rwlock.h"

int my_pthread_rwlock_rdlock(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;

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

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

(6)、pthread_rwlock_trywrlock.c

#include"utili.h"
#include"pthread_rwlock.h"

int 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){ 
        result = EBUSY;
   }else{
        rw->rw_refcount = -1; 
   }   

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

(7)、pthread_rwlock_tryrdlock.c

#include"utili.h"
#include"pthread_rwlock.h"

int 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;
}

(8)、pthread_rwlock_unlock.c

#include"pthread_rwlock.h"

int my_pthread_rwlock_unlock(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_refcount--;
    }else if(rw->rw_refcount == -1){
        rw->rw_refcount = 0;
    }else{
        printf("refcount 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);
    }   
    //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;
}

(9)、pthread_rwlock_destroy.c

#include"pthread_rwlock.h"

int my_pthread_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;
}

(10)、test.c

#include"utili.h"
#include"pthread_rwlock.h"

my_pthread_rwlock_t rwlock = MY_PTHREAD_RWLOCK_INITIALIZER;

void* fun1(void *arg){
    my_pthread_rwlock_wrlock(&rwlock);
    printf("This is fun1. wrlock\n");
    sleep(5);
    printf("fun1 wake up.\n");
    my_pthread_rwlock_unlock(&rwlock);
}
void* fun2(void *arg){
    my_pthread_rwlock_rdlock(&rwlock);
    printf("This is fun2.rdlock.\n");
    my_pthread_rwlock_unlock(&rwlock);
}
void* fun3(void *arg){
    my_pthread_rwlock_wrlock(&rwlock);
    printf("This is fun3,wrlock.\n");
    my_pthread_rwlock_unlock(&rwlock);
}

int main(){
    pthread_t tid1, tid2[5],tid3[5];
    int i;
    pthread_create(&tid1, NULL, fun1, NULL);
    sleep(1);
    for(i=0; i<5; ++i)
    {
        pthread_create(&tid2[i], NULL, fun2, NULL);
    }

    for(i=0; i<5; ++i)
    {
        pthread_create(&tid3[i], NULL, fun3, NULL);
    }
    pthread_join(tid1, NULL);
    for(i=0; i<5; ++i)
    {
        pthread_join(tid2[i], NULL);
        pthread_join(tid3[i], NULL);
    }

    return 0;
}

(11)、Makefile

OBJ=test.o pthread_rwlock_rdlock.o pthread_rwlock_unlock.o pthread_rwlock_wrlock.o pthread_rwlock_init
.o
SRC=test.c pthread_rwlock_rdlock.c pthread_rwlock_unlock.c pthread_rwlock_wrlock.c pthread_rwlock_init
.c
test:$(OBJ)
    gcc -o test $(OBJ) -lpthread
test.o:test.c
    gcc -o test.o -c test.c -g
pthread_rwlock_rdlock.o:pthread_rwlock_rdlock.c
    gcc -o pthread_rwlock_rdlock.o -c pthread_rwlock_rdlock.c -g
pthread_rwlock_unlock.o:pthread_rwlock_unlock.c
    gcc -o pthread_rwlock_unlock.o -c pthread_rwlock_unlock.c -g
pthread_rwlock_wrlock.o:pthread_rwlock_wrlock.c
    gcc -o pthread_rwlock_wrlock.o -c pthread_rwlock_wrlock.c
pthread_rwlock_init.o:pthread_rwlock_init.c
    gcc -o pthread_rwlock_init.o -c pthread_rwlock_init.c
.PHONY:clean
clean:
    rm *.o test

运行结果

此时已经实现了读锁优先(只要对写锁优先熟悉，稍作更改就可以了)。