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

#define THREADS 8
#define START 0
#define END 1000000000

void* partial_sum(void* idp) {
    int id = * (int*) idp;

    int start = ((END - START) / THREADS) * id;
    int end = ((END - START) / THREADS) * (id + 1);

    // make sure last thread goes through end
    if (id == (THREADS - 1)) {
        end++;
    }

    // allocate space for the answer
    unsigned long* sum = malloc(sizeof(unsigned long));
    *sum = 0;

    // go through range, computing the sum
    for (int i = start; i < end; i++) {
        *sum = *sum + i;
    }
   
    printf("Thread %d does %d to %d.\nSum = %lu.\n", id, start, end, *sum);
    
    // end the thread and return partial sum
    pthread_exit(sum);
}

int main() {
    // our array of threads
    pthread_t threads[THREADS];

    // array of thread ids
    int ids[THREADS];

    for (int i = 0; i < THREADS; i++) {
        ids[i] = i;
        pthread_create(&threads[i], NULL, &partial_sum, &ids[i]);
    }
   
    unsigned long grand_total = 0;
    for (int i = 0; i < THREADS; i++) {
        // join thread and get return value
        unsigned long* partial;
        pthread_join(threads[i], (void**) &partial);

        grand_total = grand_total + *partial;
        
        // clean up memory used
        free(partial);
    }

    printf("Total sum = %lu.\n", grand_total);
    pthread_exit(0);
}