12b.c

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

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

void *writer(void *wno) {
  sem_wait(&wrt);
  cnt = cnt * 2;
  printf("Writer %d modified cnt to %d\n", (*((int *)wno)), cnt);
  sem_post(&wrt);
}
void *reader(void *rno) {

  pthread_mutex_lock(&mutex);
  numreader++;
  if (numreader == 1) 
    sem_wait(&wrt);
  pthread_mutex_unlock(&mutex);
  printf("Reader %d: read cnt as %d\n", *((int *)rno), cnt);
  pthread_mutex_lock(&mutex);
  numreader--;
  if (numreader == 0) {
    sem_post(&wrt);
  }
  pthread_mutex_unlock(&mutex);
}

int main() {

  pthread_t read[10], write[5];
  pthread_mutex_init(&mutex, NULL);
  sem_init(&wrt, 0, 1);
  int a[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

  for (int i = 0; i < 10; i++) {
    pthread_create(&read[i], NULL, (void *)reader, (void *)&a[i]);
  }
  for (int i = 0; i < 5; i++) {
    pthread_create(&write[i], NULL, (void *)writer, (void *)&a[i]);
  }

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

    pthread_join(read[i], NULL);
    pthread_join(write[i], NULL);
  }
  pthread_mutex_destroy(&mutex);
  sem_destroy(&wrt);
  return 0;
}