Ironically, you just answered the "difficult" interview question in a few lines of casual commenting.

If you're not allowed to use atoi and itoa, it's still not all that difficult to do in C. Incrementing ASCII characters and handling carries is really trivial.

I'd be kind of worried if I was hiring a C developer who didn't know the basic things you described above. I'm not sure why you think those are esoteric concepts that we shouldn't expect developers to know.