readerwriter.c

#include<stdio.h> 
#include<semaphore.h> 
sem_t mutex; 
sem_t db; 
int readercount=0; 
pthread_t reader1,reader2,writer1,writer2; 
void *reader(void *); 
void *writer(void *); 
void main() 
{ 
sem_init(&mutex,0,1); 
sem_init(&db,0,1); 
while(1) 
{ 
pthread_create(&reader1,NULL,reader,"1"); 
pthread_create(&reader2,NULL,reader,"2"); 
pthread_create(&writer1,NULL,writer,"1"); 
pthread_create(&writer2,NULL,writer,"2"); 
} 
} 
void *reader(void *p) 
{ 
int temp; 
sem_getvalue(&mutex,&temp); 
printf("in reader : %s prevoius value %d\n",(char *)p,temp); 
sem_wait(&mutex); 
sem_getvalue(&mutex,&temp); 
printf("Mutex acquired by reader %d\n",temp); 
readercount++; 
if(readercount==1) sem_wait(&db); 
sem_post(&mutex); 
sem_getvalue(&mutex,&temp); 
printf("Mutex returned by reader %d\n",temp); 
printf("Reader %s is Reading\n",(char *)p); 
sleep(3); 
sem_wait(&mutex); 
printf("Reader %s Completed Reading\n",(char *)p); 
readercount--; 
if(readercount==0) sem_post(&db); 
sem_post(&mutex); 
} 

 
void *writer(void *p) 
{ 
printf("Writer : %s is Waiting\n",(char *)p); 
sem_wait(&db); 
printf("Writer %s is writing\n",(char *)p); 
sem_post(&db); 
sleep(2); 
}