Dining Philosophers

dining-philosophers.c

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

const int NUMBER_OF_PHILOSOPHERS = 5;

const int SEMAPHORE_ACROSS_THREADS = 0;
const int FORK_AVAILABLE = 1;

sem_t *forks;

void *philosopher(void *data);

int main(void)
{
    /*
     * Initialize resources.
     */
    pthread_t *philosopher_threads = (pthread_t *)malloc(NUMBER_OF_PHILOSOPHERS * sizeof(pthread_t));
    int *philosopher_thread_arguments = (int *)malloc(NUMBER_OF_PHILOSOPHERS * sizeof(int));

    forks = (sem_t *)malloc(NUMBER_OF_PHILOSOPHERS * sizeof(sem_t));

    for (int i = 0; i < NUMBER_OF_PHILOSOPHERS; ++i)
    {
        sem_init(forks + i, SEMAPHORE_ACROSS_THREADS, FORK_AVAILABLE);
    }

    /*
     * Start threads and wait for them to finish.
     */
    for (int i = 0; i < NUMBER_OF_PHILOSOPHERS; ++i)
    {
        philosopher_thread_arguments[i] = i;

        if (pthread_create(philosopher_threads + i, NULL, philosopher, (void *)(philosopher_thread_arguments + i))) 
        {
            return -1;
        }
    }

    for (int i = 0; i < NUMBER_OF_PHILOSOPHERS; ++i)
    {
        int *r;

        pthread_join(philosopher_threads[i], (void *) &r);

        printf("Philosopher #%d has exited.\n", *r);
    }

    /*
     * Free all resources.
     */
    for (int i = 0; i < NUMBER_OF_PHILOSOPHERS; ++i)
    {
        sem_destroy(forks + i);
    }

    free(forks);
    free(philosopher_thread_arguments);
    free(philosopher_threads);
    
    return 0;
}

void *philosopher(void *data)
{
    const int identifier = *(const int *)data;

    printf("Philosopher #%d is ready.\n", identifier);
    fflush(stdout);

    for (;;)
    {
        sem_t *lower_fork = forks + identifier;
        sem_t *upper_fork = forks + ((identifier + 1) % NUMBER_OF_PHILOSOPHERS);

        sem_wait(lower_fork);
        sem_wait(upper_fork);

        printf("Philosopher #%d is eating.\n", identifier);
        fflush(stdout);

        sem_post(lower_fork);
        sem_post(upper_fork);
    }

    return data;
}