reader_writer.c

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


void *reader(void *); 
void *writer(void *);

int readcount=0,writecount=0,sh_var=5,bsize[5];
sem_t x,y,z,rsem,wsem;
pthread_t r[3],w[2];


void *reader(void *i) {
    printf("\n\n reader- %d is reading", i); 
    sem_wait(&z);
    sem_wait(&rsem);
    sem_wait(&x);
    readcount++;
    if(readcount==1)
        sem_wait(&wsem);
    sem_post(&x);
    sem_post(&rsem);
    sem_post(&z);
    printf("\nupdated value : %d ", sh_var); 
    sem_wait(&x);
    readcount--; 
    if(readcount==0)
        sem_post(&wsem); 
    sem_post(&x);
}


void *writer(void *i) {
    printf("\n\n writer- %d is writing", i); 
    sem_wait(&y);
    writecount++;
    if(writecount==1)
        sem_wait(&rsem);
    sem_post(&y);
    sem_wait(&wsem);
    sh_var=sh_var+5; 
    sem_post(&wsem); 
    sem_wait(&y); 
    writecount--; 
    if(writecount==0)
        sem_post(&rsem); 
    sem_post(&y);
}


int main() {
    sem_init(&x,0,1);
    sem_init(&wsem,0,1); 
    sem_init(&y,0,1); 
    sem_init(&z,0,1); 
    sem_init(&rsem,0,1);
    pthread_create(&r[0],NULL,(void *)reader,(void *)0); 
    pthread_create(&w[0],NULL,(void *)writer,(void *)0); 
    pthread_create(&r[1],NULL,(void *)reader,(void *)1); 
    pthread_create(&r[2],NULL,(void *)reader,(void *)2); 
    pthread_create(&r[3],NULL,(void *)reader,(void *)3); 
    pthread_create(&w[1],NULL,(void *)writer,(void *)3); 
    pthread_create(&r[4],NULL,(void *)reader,(void *)4);
    pthread_join(r[0],NULL); 
    pthread_join(w[0],NULL); 
    pthread_join(r[1],NULL); 
    pthread_join(r[2],NULL); 
    pthread_join(r[3],NULL); 
    pthread_join(w[1],NULL); 
    pthread_join(r[4],NULL);
    return(0); 
}