diff --git a/Misc/ACKS b/Misc/ACKS
index fdf89d4..edd144c 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -51,6 +51,7 @@
 Jon Anglin
 Heidi Annexstad
 Ramchandra Apte
+Arfrever Frehtes Taifersar Arahesis
 Éric Araujo
 Alicia Arlen
 Jeffrey Armstrong
diff --git a/Misc/NEWS b/Misc/NEWS
index 9f7ca1d..92c9cb6 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -27,6 +27,9 @@
 Library
 -------
 
+- Issue #21704: Fix build error for _multiprocessing when semaphores
+  are not available.  Patch by Arfrever Frehtes Taifersar Arahesis.
+
 - Fix repr(_socket.socket) on Windows 64-bit: don't fail with OverflowError
   on closed socket. repr(socket.socket) already works fine.
 
diff --git a/Modules/_multiprocessing/multiprocessing.c b/Modules/_multiprocessing/multiprocessing.c
index 1aaf360..4ae638e 100644
--- a/Modules/_multiprocessing/multiprocessing.c
+++ b/Modules/_multiprocessing/multiprocessing.c
@@ -128,7 +128,9 @@
     {"recv", multiprocessing_recv, METH_VARARGS, ""},
     {"send", multiprocessing_send, METH_VARARGS, ""},
 #endif
+#ifndef POSIX_SEMAPHORES_NOT_ENABLED
     {"sem_unlink", _PyMp_sem_unlink, METH_VARARGS, ""},
+#endif
     {NULL}
 };