printwtf.c

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

#define LARGENUM 0x80000001

int main()
{
	char* buf = malloc(LARGENUM + 1);
	memset(buf, 'X', LARGENUM);
	buf[LARGENUM] = '\0';
	printf("%s", buf);
	return 0;
}

This program is ill-formed. It is not allowed to print more characters than INT_MAX in a single printf function call. See the description of the meaning of the printf return value as defined in the C language standard.