first-reader.c

// Preference is given to the reader which could possibily strave the writer process.
#include <pthread.h>
#include <semaphore.h>
#include <stdio.h>
#include <sys/ipc.h>
#include <sys/shm.h>


sem_t wrt;
pthread_mutex_t mutex;
int cnt = 1;
int numreader = 0;

key_t key;
int shmid;

void *writer(void *wno)
{
    sem_wait(&wrt);

    key = ftok("shmfile",65);

    // shmget returns an identifier in shmid
    shmid = shmget(key,1024,0666|IPC_CREAT);

    // shmat to attach to shared memory
    int *num = (int*) shmat(shmid,(void*)0,0);

    cnt++;
    *num = cnt;

    printf("Data written in memory: %d\n",cnt);

    //detach from shared memory
    shmdt(num);

    sem_post(&wrt);

}
void *reader(void *rno)
{

    pthread_mutex_lock(&mutex);     // reader acquires the lock before modifying numreader
    numreader++;
    if(numreader == 1) {
        sem_wait(&wrt); // When the reader is reading the data the writer should be blocked.
    }
    pthread_mutex_unlock(&mutex);

    key = ftok("shmfile",65);

    // shmget returns an identifier in shmid
    shmid = shmget(key,1024,0666|IPC_CREAT);

    // shmat to attach to shared memory
    int* num = (int*) shmat(shmid,(void*)0,0);

    printf("Data read from memory: %d\n",cnt);

    //detach from shared memory
    shmdt(num);

    pthread_mutex_lock(&mutex);     // reader acquires the lock before modifying numreader
    numreader--;
    if(numreader == 0) {
        sem_post(&wrt);         // When the number of readers in the reader function becomes zero, the blocked writer is woken up
    }
    pthread_mutex_unlock(&mutex);
}

int main()
{

    pthread_t read[100],write[100];
    pthread_mutex_init(&mutex, NULL);
    sem_init(&wrt,0,1);

    int rnum, wnum;
    int a[10] = {1,2,3,4,5,6,7,8,9,10}; // numbers for Readers and Writers.

    printf("Enter the number of readers : ");
    scanf("%d", &rnum);
    printf("\nEnter the number of writers : ");
    scanf("%d", &wnum);
    printf("\n");

    for(int i = 0; i < rnum; i++) {
        pthread_create(&read[i], NULL, (void *)reader, (void *)&a[i]);
    }
    for(int i = 0; i < wnum; i++) {
        pthread_create(&write[i], NULL, (void *)writer, (void *)&a[i]);
    }

    for(int i = 0; i < rnum; i++) {
        pthread_join(read[i], NULL);
    }
    for(int i = 0; i < wnum; i++) {
        pthread_join(write[i], NULL);
    }

    pthread_mutex_destroy(&mutex);
    sem_destroy(&wrt);

    shmctl(shmid,IPC_RMID,NULL);

    return 0;

}