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

/* the number of threads */
int num_threads;

/* a semaphore for each thread */
sem_t* semaphores;

/* the function called for each thread */
void* worker(void* idp) {
    /* get our thread id */
    int id = * (int*) idp;

    /* wait for our semaphore to become unlocked */
    sem_wait(&semaphores[id]);

    /* do our work */
    printf("Thread %d says hello!\n", id);

    /* unlock the semaphore belonging to the next thread (unless last) */
    if (id < (num_threads - 1)) {
        sem_post(&semaphores[id + 1]);
    }

    pthread_exit(NULL);
}

int main (int argc, char** argv) {
    /* get the number of threads */
    if (argc < 2) {
        printf("Pass the number of threads to use!\n");
        pthread_exit(0);
    } else {
        num_threads = atoi(argv[1]);
    }

    sem_t s[num_threads];
    semaphores = s;

    /* initialize the semaphores to 0 */
    int i;
    for (i = 0; i < num_threads; i++) {
        sem_init(&semaphores[i], 0, 0);
    }

    /* unlock thread 1 */
    sem_post(&semaphores[0]);

    /* an array of threads */
    pthread_t threads[num_threads];
    int ids[num_threads];

    /* spawn all threads */
    for (i = 0; i < num_threads; i++) {
        ids[i] = i;
        pthread_create(&threads[i], NULL, worker, &ids[i]);
    }

    /* join all threads */
    for (i = 0; i < num_threads; i++) {
        pthread_join(threads[i], NULL);
    }

    pthread_exit(0);
}