Some info regarding _multiprocessing:

Modules/_multiprocessing/semaphore.c is currently conditionally included in setup.py (HAVE_SEM_OPEN && !POSIX_SEMAPHORES_NOT_ENABLED), but always included in Modules/Setup.

Here's some historical bpo's (more or less) relevant to Modules/_multiprocessing/semaphore.c:

- bpo-7272
- bpo-5545
- bpo-3770


Commit 99c48a8d315fe55c8e1e8eac2e01cd930cb89686 message says:
    Author: Hye-Shik Chang <hyeshik@gmail.com>
    Date:   Sat Jun 28 01:04:31 2008 +0000

    Give information for compililation of _multiprocessing.SemLock on FreeBSD:
    
    FreeBSD's P1003.1b semaphore support is highly experimental and
    it's disabled by default.  Even if a user loads the experimental
    kernel module manually, _multiprocessing doesn't work correctly due
    to several known incompatibilities around sem_unlink and sem_getvalue,
    yet.


Excerpt from the FreeBSD 9.0 release notes[^1], released four years later (2012-01-12):

    kern.features.sysv_sem.                   System V semaphores support
    kern.features.p1003_1b_mqueue.            POSIX P1003.1B message queues support
    kern.features.p1003_1b_semaphores.        POSIX P1003.1B semaphores support
    kern.features.kposix_priority_scheduling  POSIX P1003.1B real-time extensions


I haven't checked OpenBSD or NetBSD yet.


Other relevant commits:

  - c4920e86ef7511b4e858028e870b1811437a71d0
  - 40a6164afa79f6b97e7e40e0f35f6081fde437c2



[^1]: https://www.freebsd.org/releases/9.0R/relnotes-detailed/