Comment by dzaima

I'm of course not saying that C has a single universal ABI. But for any single platform (OS+ISA) where it is possible and meaningful to have shared libraries in the first place, there's a pretty clear single ABI to follow, so the distinction is, for practical purposes, completely meaningless. (ok windows does have some mess, but that's windows for you, it'll achieve a mess in anything)

Still have no clue what you mean by threading; sure, threads exist, even officially so in C11, but still just in completely no way whatsoever affect ABI any more than any other part of the standard library, i.e. "as stable as the stdlib impl is".