Simple, portable, and (mostly) UB-free implementation of the Sieve of Eratosthenes in C99

primes.c

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

#define u8 uint8_t
#define u32 uint32_t

int main(void) {
    u8 *arr;
    u32 i, j, x, c;

    printf("Enter a number: ");

    if (1 != scanf("%"SCNu32, &x)) {
        puts("Could not parse integer literal.");
        return 1;
    }

    arr = malloc(x * sizeof(*arr));

    if (NULL == arr) {
        puts("Not enough memory to calculate requested range.");
        return 1;
    }

    puts("Calculating...");

    c = 0;

    arr[0] = 0;

    for (i = 1; i < x; i++) {
        arr[i] = 1;
    }

    for (i = 2; i <= x; i++) {
        if (0 == arr[i-1]) continue;

        for (j = i*2; j <= x; j += i) {
            arr[j-1] = 0;
        }

        c++; // hehe
    }

    printf("Found %"PRIu32" primes. List them all? (y/N) ", c);

    // because nobody uses [LF]CR anymore
    while ('\n' != getchar());

    if ('y' != getchar()) return 0;

    for (i = 0; i < x; i++) {
        if (arr[i]) {
            printf("%"PRIu32"\n", i+1);
        }
    }

    return 0;
}