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#include #include
(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)。