parallel_sum.c

// Compile with: gcc prog.c -lpthread
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>

const int NUM_THREADS = 5;

// typically you'd set this to whatever your real work is
const int num_digits  = 10;
const int digits[num_digits] = {1,2,3,4,5,6,7,8,9,10};

// This is the function we want to parallelize
void *Sum(void *param) {
	long threadid  = (long) param;
	long mysum     = 0;

	// divide up the work based on this thread's ID
	int chunksize = num_digits / NUM_THREADS;
	int start     = chunksize * threadid;
	int end       = start + chunksize;

	for(int i = start; i < end; i++) {
		mysum += digits[i];
	}

	printf("Thread %ld got sum: %ld\n", threadid, mysum);

	// We return our result here so the main thread can tally
	// pthreads uses void* because it doesn't know what type you want to return
	// typically you'd return a pointer to something if the work was intensive
	return (void*) mysum;
}


int main(int argc, char *argv[]) {

	// Keep a list of all the threads we're going to create
	pthread_t threads[NUM_THREADS];


	for(long t = 0; t < NUM_THREADS; t++) {
		printf("Main creating thread %ld\n", t);
		pthread_create(&threads[t], NULL, Sum, (void *)t);
	}

	long sum = 0;
	void* result;

	// Wait for our threads to all finish and get their results
	for(int t = 0; t < NUM_THREADS; t++) {
		pthread_join(threads[t], &result);
		sum += (long) result;
	}

	printf("All threads finished, sum is: %ld\n", sum);
}