[fauigerzigerk]

>File processing and string processing are not the same

The UTF-8 spec doesn't make that distinction as far as I know. There's a simple fact: A valid UTF-8 byte sequence can contain nul characters. So you can't naively use C string handling functions on it. And as someone else has correctly pointed out, the same is true for ASCII.

I'm just pointing out a potential pitfall and a source of security issues. Some might assume that after validating UTF-8 text input, you could just dump it in a C string and process it using C's string functions. But that's not the case.