Why isn't GNU/Linux SUS v3+ compliant?

Question

I am looking for specific details as to why isn't GNU/Linux currently SUS(Single UNIX Specification) v3 or even better SUS v4 compliant? What application apis and user utilities does it miss or implement in a non-SUS compliant way?

Answer 1

To get a certification you need to pay, and it's actually really expensive, this is what BSD like and GNU/Linux operating system vendors like don't apply to it.

So there is even not a reason to check whatever or not GNU/Linux is compliant.

http://en.wikipedia.org/wiki/Single_UNIX_Specification#Non-registered_Unix-like_systems

Most of all the GNU/LInux distribution follows the Linux Standard Base, that is free of charge and recognized by almost all the Linux vendors.

http://en.wikipedia.org/wiki/Linux_Standard_Base

EDIT: As my answer is not completely correct, I add the @vonbrand comments

Linus (and people involved in the development of other parts of Linux distributions) follow the pragmatic guideline to make it as close to POSIX as is worthwhile. There are parts of POSIX (like the (in)famous STREAMS) that are ill-conceived, impossible to implement efficiently, or just codification of historic relics that should be replaced by something better

Threfore, does it makes harder to obtain a certification?

sure. POSIX mandates some interface, which Linux just won't ever have. Case closed.