// gcc rw.c -pthread
// a.out

#include<stdio.h>
#include<pthread.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 *);
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)
{
    printf("prevoius value %d\n",mutex);
    sem_wait(&mutex);
    printf("Mutex acquired by reader %d\n",mutex);
    readercount++;
    if(readercount==1) sem_wait(&db);
    sem_post(&mutex);
    printf("Mutex returned by reader %d\n",mutex);
    printf("Reader %s is Reading\n",p);
sleep(3);
    sem_wait(&mutex);
    printf("Reader %s Completed Reading\n",p);
    readercount--;
    if(readercount==0) sem_post(&db);
    sem_post(&mutex);
}

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