This question already has an answer here:
Why is Linux not POSIX-compliant? I've seen in lots of places that it's not (Wikipedia Page) but there's been no real explanation to back this up.
Is there something the C library and/or tools could do to get around this (i.e. no modifications to the kernel itself)? What needs to be done?
The supposed duplicate is asking which Linux distribution is POSIX-compliant; this is asking why Linux isn't POSIX-compliant. I'm asking for specific details (i.e. some function or command isn't compliant), not the reasons the specific distributions don't (try to) get certified.
This comment from @PhilipCouling (thanks!) explains it well:
Compliance and certification are different subjects. The answers point to cost of (re)certification which is irrelevant to the subject of (non)compliance.