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

/* the number of threads */
int num_threads;

/* whose turn it is */
volatile int turn = 0;

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

    /* busy wait until it is our turn */
    while (turn != id)
        ;

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

    /* next thread's turn */
    turn++;

    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]);
    }

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

    /* 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);
}