Simple prime sieve

genprimes.c

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

int main(int argc, char** argv)
{
	if (argc != 2) {
		printf("usage: genprimes num\n");
		return 0;
	}

	int limit = atoi(argv[1]);
	if (limit < 10) {
		printf("the upper bound must be at least 10\n");
		return 0;
	}

	unsigned char *sieve = (unsigned char*)calloc(sizeof(unsigned char), limit+1);
	
	int pos;
	for (pos = 2; pos < limit; pos++) {
		if (sieve[pos] != 0) {
			continue;
		}

		printf("%d\n", pos);

		int i;
		for (i = 2*pos; i <= limit; i += pos) {
			sieve[i] = 1;
		}
	}
}