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

#define THREAD_POOL_SIZE 2

void *do_work(void *args) {
    sleep(2);
    int value = *((int*)args);
    int sq = value * value;
    printf("%d\n", sq);
    return NULL;
}

int main(void) {
    //you need to setup four async operations and wait for them to be done before processing further operations.
    pthread_t thread_pool[THREAD_POOL_SIZE];
    int proc_items[] = { 1, 8, 17, 12, 13, 14, 19, 22, 31, 56, 43, 77 };

    int items_left = THREAD_POOL_SIZE;
    for (int i=0; i<12; ++i) {
        int idx = i % THREAD_POOL_SIZE;
        pthread_create(&thread_pool[idx], NULL, do_work, (void*)&proc_items[i]);
        items_left--;

        if (items_left == 0) {
            void *retval;
            pthread_join(thread_pool[(i - (THREAD_POOL_SIZE - 1)) % THREAD_POOL_SIZE], &retval);
            items_left++;
        }
    }

    return EXIT_SUCCESS;
}