Unfortunately some text encodings (UTF-16 among them) use nuls for codepoints other than U+00. In fact UTF-16 will use nuls for every character before U+100, in other words all of ASCII and Latin-1. Therefore you can't just support _all_ text encodings for filenames on these OSes, unless the OS provides a second syscall for it (this is what Windows did since they wanted to use UTF-16LE across the board).

I've only mentioned syscalls in this, in truth it extends all through the C stdlib which everything ends up using in some way as well.

And you should be using one specified encoding for file names if you want them to be displayed correctly in all applications. It would be inconvenient if different applications stored file names in different encodings.

For the same reason, encoding should be specified in libraries documentation for all functions accepting or returning strings.