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

#define THREADS 32
#define START 0
#define END 100000

/* a global variable */
unsigned long sum = 0;

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

    /* calculate the start and end points by evenly dividing the range */
    unsigned long start = ((END - START) / THREADS) * id;
    unsigned long end = start + ((END - START) / THREADS) - 1;

    /* the last thread needs to do all remaining ones */
    if (id == (THREADS - 1)) {
        end = END;
    }

    /* do the calculation */
    unsigned long i;
    for (i = start; i <= end; i++) {
        sum += i;
    }

    return NULL;
}

int main() {
    /* an array of threads */
    pthread_t threads[THREADS];
    int ids[THREADS];
    int i;

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

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

    /* now all results are in */
    printf("Final answer = %lu.\n", sum);
    pthread_exit(NULL);
}