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

#define MAX_READERS 5

int readers;
sem_t can_read;
sem_t can_write;
sem_t rcount;

void *writer(void *data) {
    while (1) {
        sem_wait(&can_write);
        puts("WRITING!");
        sem_post(&can_read);
    }
}

void *reader(void *data) {
    while (1) {
        sem_wait(&can_read);

        sem_wait(&rcount);
        readers++;
        if (readers < MAX_READERS) {
            sem_post(&can_read);
        }
        sem_post(&rcount);

        printf("READING! (id=%d)\n", (int) data);

        sem_wait(&rcount);
        readers--;
        if (readers == 0) {
            sem_post(&can_write);
        } else {
            sem_post(&can_read);
        }
        sem_post(&rcount);
    }
}

int main(int argc, char **argv) {
    int i;
    pthread_t thread_reader[MAX_READERS];
    pthread_t thread_writer;

    readers = 0;
    sem_init(&can_write, 0, 1);
    sem_init(&can_read, 0, 0);
    sem_init(&rcount, 0, 1);

    pthread_create(&thread_writer, NULL, writer, NULL);
    for (i = 0; i < MAX_READERS; i++) {
        pthread_create(&thread_reader[i], NULL, reader, (void*) i);
    }

    pthread_join(thread_writer, NULL);
    for (i = 0; i < MAX_READERS; i++) {
        pthread_join(thread_reader[i], NULL);
    }
    return 0;
}