Pour http://linuxfr.org/forums/programmationc/posts/comment-effectuer-une-tache-le-plus-rapidement-possible-threads-fork

prime_sum.c

/*
 * % gcc -O3 -std=c99 -o sum sum.c; ./sum
 * sum:142913828922,count:148933,cpu:0.00
 */

#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <time.h>

#define LAST_NR 2000000
#define PRIME 0
#define NOT_PRIME 1

int main()
{
    clock_t before = clock(), after;
    float spent;
    char status[LAST_NR];
    uint64_t sq = 1 + (uint64_t) sqrt(LAST_NR), count = 0, ret;
    uint64_t sum = 0;

    memset(status, PRIME, LAST_NR);

    for (uint64_t i = 2; i <= sq; i++)
        if (status[i] == PRIME) {
            sum += i;
            count++;

            for (uint64_t j = 2 * i; j <= LAST_NR; j += i)
                status[j] = NOT_PRIME;
        }

    for (uint64_t i = sq + 1; i <= LAST_NR; i++)
        if (status[i] == PRIME) {
            sum += i;
            count++;
        }

    after = clock();
    spent = (float) (after - before) / (float) CLOCKS_PER_SEC;

    ret = printf("sum:%li,count:%li,cpu:%.2f\n", sum, count, spent);
    return (ret > 0) ? EXIT_SUCCESS : EXIT_FAILURE;
}