> Why would one would like to truncate an existing file? To overwrite it. It's a very common use case.

Yeah, but we have plain "w" for that already.

> What I have never found any use for personally is opening a file in read-write mode, while also truncating it, which is the full extent of w+ behaviour. I cannot think of many reasons to have an output stream to also function as an input.

Exactly my point. If I am about to fully overwrite the file content, why would I want to re-read them without even an intermediate fclose()? I know that content already, I've just wrote it out! The only fringe use cases I can think of are really the edge cases of some general scenarios that would normally use "r+" (e.g. databases) but could occasionally go "okay, let's drop everything and start ab initio" — but those can just call ftrunc() in those circumstances. Or just open the file twice with different modes, the filesystem being what it is (global shared mutable state), you'll see your changes through.