firstreaderwriter.c

// src: IIIT Hyderabad, Operating Systems 2011.
#include<stdio.h>
#include<sys/types.h>
#include<string.h>
#include<pthread.h>
#include<semaphore.h>

sem_t wrt,read;
int readcount=0;
char data[100] = "";

void *readThread(void *param){
	sem_wait(&read);
	readcount++;
	if(readcount==1)
		sem_wait(&wrt);
	sem_post(&read); 
	printf("Reader Thread %ld, Data read is %s\n", (long)param,data);
	sem_wait(&read);
	readcount--;
	if(readcount==0)
		sem_post(&wrt);
	sem_post(&read);
}

void *writeThread(void *param){
	sem_wait(&wrt);
	strcat(data, "hi ");
	printf("Writer thread %ld Data written: hi\n", (long)param);
	sem_post(&wrt);
}

//No reader should be waiting unless a writer has already obtained the permission to use the shared object
//Readers are given priority over writers
int main(){
	sem_init(&wrt,0,1);
	sem_init(&read,0,1);
	int i;
	pthread_t rthread[5];
	pthread_t wthread[5];

	for(i=0;i<5;i++)
	{
		pthread_create(&rthread[i],NULL,readThread,(void *)i);
	}
	for(i=0;i<5;i++)
	{
		pthread_create(&wthread[i],NULL,writeThread,(void *)i);
	}
	for(i=0;i<5;i++)
		pthread_join(rthread[i],NULL);
	for(i=0;i<5;i++)
		pthread_join(wthread[i],NULL);

	printf("Final string is %s\n",data);
	pthread_exit(NULL);
	return 0;
}