@afxgroup
Wondering what you are using as posix reference, AFAIK there are three of various degrees of accuracy.
1) vbcc has a posix link library geared towards c99 not complete in any way.
2) GCC has got glibc (already geared heavily towards linux/hurd)
3) The Open Group
The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group.
not geared to any specific hardware or OS.
The Open Group Posix StandardProbably vbcc's is a likely candidate, they seem to progress with it.