There are various (albeit notionally optional) parts of POSIX that are either not implemented at all or implemented in surprising ways on BSD derived systems. F

There are various (albeit notionally optional) parts of POSIX that are either not implemented at all or implemented in surprising ways on BSD derived systems. For example, macOS does not implement sem_init() at all, while OpenBSD sem_init(3) man page stilll says that "This is perhaps a stretch of the intention of POSIX, but is compliant, with the caveat that sem_init() always reports a permissions error when an attempt to create a shared semaphore is made." (that wording was at one time present in most BSDs, but both current NetBSD and FreeBSD seem to not have that limitation).