READER WRITER SEMAPHORE

 

// READER WRITER SEMAPHORE

#include <stdio.h>

#include <pthread.h>

#include <semaphore.h>

#include <unistd.h>

int shared_data = 0;

int read_count = 0;

sem_t wrt;                

pthread_mutex_t mutex;     

void *writer(void *writer_id) {

int id = *(int *)writer_id;

sem_wait(&wrt);     

shared_data += 1;

 printf("Writer %d modified shared_data to %d\n", id, shared_data);

sem_post(&wrt);     

return NULL; }

 void *reader(void *reader_id) {

int id = *(int *)reader_id;

pthread_mutex_lock(&mutex); 

read_count++;               

if (read_count == 1)        

sem_wait(&wrt);

 pthread_mutex_unlock(&mutex);  

printf("Reader %d: read shared_data as %d\n", id, shared_data);

pthread_mutex_lock(&mutex); 

read_count--;               

if (read_count == 0)        

sem_post(&wrt);

 pthread_mutex_unlock(&mutex);

return NULL; }

int main() {

pthread_t read[5], write[3]; 

int reader_ids[5] = {1, 2, 3, 4, 5};

int writer_ids[3] = {1, 2, 3};

sem_init(&wrt, 0, 1);         

pthread_mutex_init(&mutex, NULL);

for (int i = 0; i < 3; i++) {

pthread_create(&write[i], NULL, writer, (void *)&writer_ids[i]); }

for (int i = 0; i < 5; i++) {

pthread_create(&read[i], NULL, reader, (void *)&reader_ids[i]); }

for (int i = 0; i < 3; i++) {

pthread_join(write[i], NULL); }

for (int i = 0; i < 5; i++) {

pthread_join(read[i], NULL); }

sem_destroy(&wrt);

pthread_mutex_destroy(&mutex);

return 0;

}