diff --git a/Lib/multiprocessing/process.py b/Lib/multiprocessing/process.py
index c2fc581b1f4..cd52736a70f 100644
--- a/Lib/multiprocessing/process.py
+++ b/Lib/multiprocessing/process.py
@@ -301,10 +301,16 @@ class _MainProcess(BaseProcess):
         self._parent_pid = None
         self._popen = None
         self._config = {'authkey': AuthenticationString(os.urandom(32)),
-                        'semprefix': 'mp'}
+                        'semprefix': '/mp'}
         # Note that some versions of FreeBSD only allow named
-        # semaphores to have names of up to 14 characters.  Therfore
+        # semaphores to have names of up to 14 characters.  Therefore
         # we choose a short prefix.
+        #
+        # On MacOSX in a sandbox it may be necessary to use a
+        # different prefix -- see #19478.
+        #
+        # Everything in self._config will be inherited by descendant
+        # processes.
 
 
 _current_process = _MainProcess()
diff --git a/Lib/multiprocessing/synchronize.py b/Lib/multiprocessing/synchronize.py
index 82c30a27a5e..9d8e2824772 100644
--- a/Lib/multiprocessing/synchronize.py
+++ b/Lib/multiprocessing/synchronize.py
@@ -115,8 +115,8 @@ class SemLock(object):
 
     @staticmethod
     def _make_name():
-        return '/%s-%s' % (process.current_process()._config['semprefix'],
-                           next(SemLock._rand))
+        return '%s-%s' % (process.current_process()._config['semprefix'],
+                          next(SemLock._rand))
 
 #
 # Semaphore