Final version including:

1. All past blocking points are resolved.

2. Atomicity,
+ Implementation and rebuild Semlock are atomic (protected via the dedicated semaphore),
+ Release, get_value and is_zero methods are atomic too (protected via the mutex of each semaphore),
+ Acquire function is nearly-atomic.

3. Shared memory,
+ The shared memory and its associated semphore have unique names based on
    the main process ID.
+ a new counter  was added to count pending thread
    when blocking acquire operations happen.
+ When a Semlock is destroyed, the shared memory and its dediocted semaphore are destroyed too
    if there is no more opened Semclock.

Author: YvesDup

Lib/multiprocessing/spawn.py

@@ -201,6 +201,12 @@ def get_preparation_data(name):
                 main_path = os.path.join(process.ORIGINAL_DIR, main_path)
             d['init_main_from_path'] = os.path.normpath(main_path)
 
+    # see gh-125828: workaround on MacOS to emulate `get_value` on Semaphore.
+    if sys.platform == 'darwin':
+        import _multiprocessing
+        d['_macosx_sharedmem_name'] = _multiprocessing._MACOSX_SHAREDMEM_NAME
+        d['_macosx_shmlock_name']     = _multiprocessing._MACOSX_SHMLOCK_NAME
+
     return d
 
 #
@@ -245,6 +251,18 @@ def prepare(data):
     elif 'init_main_from_path' in data:
         _fixup_main_from_path(data['init_main_from_path'])
 
+    # see gh-125828: workaround on MacOS to emulate `get_value` on Semaphore.
+    if sys.platform == 'darwin' and '_macosx_sharedmem_name' in data:
+        import _multiprocessing
+        _multiprocessing._set_shm_names(
+            data['_macosx_sharedmem_name'],
+            data['_macosx_shmlock_name']
+        )
+        # Update module attributes so grandchild processes also get
+        # the correct names when get_preparation_data() is called.
+        _multiprocessing._MACOSX_SHAREDMEM_NAME = data['_macosx_sharedmem_name']
+        _multiprocessing._MACOSX_SHMLOCK_NAME     = data['_macosx_shmlock_name']
+
 # Multiprocessing module helpers to fix up the main module in
 # spawned subprocesses
 def _fixup_main_from_name(mod_name):

Misc/NEWS.d/next/macOS/2025-03-06-18-52-05.gh-issue-125828.JkMjD2.rst

@@ -1,4 +1,4 @@
-Fix the not implemented ``get_value`` for :class:`multiprocessing.Semaphore` on MacOSX
-by adding a dedicated workaround  in ``_multiprocessing.SemLock``.
-All changes are located in the ``semaphore.c`` file of `multiprocessing` module.
-
+Fix the not implemented ``get_value`` for :class:`multiprocessing.Semaphore` on MacOSX
+by adding a dedicated workaround  in ``_multiprocessing.SemLock``.
+All changes are located in the ``semaphore.c`` file of `multiprocessing` module.
+