/* Example program with pthreads.
 *
 * Compute the sum of the first N square natural numbers, with
 * each square being computed by a different pthread.
 */

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

static void *
thread_body(void *opaque)
{
    int *intp = (int *)opaque;

    /* Compute the square in-loco. */
    *intp = (*intp) * (*intp);

    /* Return the input pointer back through the return value. */
    pthread_exit(intp);
    /* Execution never arrives here. */
}

int
main(int argc, char **argv)
{
    pthread_t *tids; /* array of thread identifiers */
    int *squares; /* array of integers */
    int n = 5; /* default number of threads is 5 */
    int sum = 0;
    int i;

    if (argc > 1) {
        /* User provided some command-line arguments, we need
         * to validate them. */
        int m = atoi(argv[1]);

        if (m > 0 && m <= 10000) { /* valid */
            n = m;
        } else { /* invalid */
            printf("Input argument is not a valid number of threads "
                   "in [1..10000]\n");
            return -1;
        }
    }

    /* Allocate the two arrays. */
    tids = malloc(n * sizeof(tids[0]));
    if (tids == NULL) {
        printf("Error: failed to allocate tids\n");
        return -1;
    }

    squares = malloc(n * sizeof(squares[0]));
    if (squares == NULL) {
        free(tids);
        printf("Error: failed to allocate squares\n");
        return -1;
    }

    /* Create a pool of threads. */
    for (i = 0; i < n; i ++) {
        int ret;

        squares[i] = i+1; /* the natural number, pthread input */
        ret = pthread_create(&tids[i], NULL, thread_body, &squares[i]);
        if (ret) {
            printf("Error: pthread_create() failed [err=%d]\n", ret);
        }
    }

    /* Wait for the thread to finish their job. */
    for (i = 0; i < n; i ++) {
        int *retptr;
        int ret = pthread_join(tids[i], (void **)&retptr);

        if (ret) {
            printf("Error: pthread_join() failed [err=%d]\n", ret);
        } else {
            printf("Thread #%d returned %d\n", i + 1, *retptr);
        }
    }

    /* Collect threads output and compute the sum of squares. */
    for (i = 0; i < n; i++) {
        sum += squares[i];
    }

    free(squares);
    free(tids);

    printf("The first %d squares sum up to %d\n", n, sum);

    return 0;
}