atomic-sum.c

// compile with: gcc atomic-sum.c -lpthread
#define __STDC_FORMAT_MACROS
#include <pthread.h>
#include <stdio.h>
#include <time.h>
#include <unistd.h>
#include <inttypes.h>

uint64_t sum = 0;
static int n_threads = 4;
static int n_perthr = 1000000;

void *worker(void *arg)
{
	int i;
	uint64_t s = 0;
	for (i = 1; i <= n_perthr; i++)
		__sync_fetch_and_add(&sum, 1);
		// sum++; // Without any kind of lock, it will produce random outputs
	return NULL;
}

int main(int argc, char *argv[])
{
	int i;
	pthread_t threads[n_threads];
	for (i = 0; i < n_threads; i++)
		pthread_create(&threads[i], NULL, worker, NULL);
	for (i = 0; i < n_threads; i++)
		pthread_join(threads[i], NULL);

	uint64_t max = n_threads*n_perthr;
	printf("Sum is %"PRIu64", should be %d*%d = %"PRIu64"\n",
			sum, n_threads, n_perthr, max);
	return 0;
}