[snnn]

But the reality is: most glibc functions like `dirname` could not handle non UTF-8 encodings, because some encodings (like GBK) have overlaps with ASCII, which means when you search an ASCII char(like '\') in a char array, you may accidentally hit a half of a non-English character. Therefore, people in Asia usually do not use the non UTF-8 locales.

[Karellen]

Why would you search for an ASCII char like '\', in a char array containing non-ASCII-based text, on a system with a non-ASCII-based locale?

[snnn]

Because that's how "dirname(3)" is implemented in glibc, except it searches '/' instead of '\'. Here all character encodings share the same code.