[immibis]

I'm not sure why you are blaming POSIX! The role of POSIX is to write down what is already common practice in almost all POSIX-like systems. It doesn't usually specify new behaviour.

[GTP]

I always assumed it was the other way around: a system follows POSIX to be POSIX-compliant.

[immibis]

It's both.

There are POSIX-specific standards (like pthreads) but they come about after other people invent threads and don't agree on an API.