diff --git a/Misc/NEWS.d/next/Library/2019-08-23-14-47-09.bpo-34521.Y2BYu5.rst b/Misc/NEWS.d/next/Library/2019-08-23-14-47-09.bpo-34521.Y2BYu5.rst
new file mode 100644
index 00000000000..06f42a841ea
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-08-23-14-47-09.bpo-34521.Y2BYu5.rst
@@ -0,0 +1,2 @@
+Fix file descriptors transfer in multiprocessing on FreeBSD: use
+``CMSG_SPACE()`` rather than ``CMSG_LEN()``; see :rfc:`3542`.
diff --git a/Modules/_multiprocessing/multiprocessing.c b/Modules/_multiprocessing/multiprocessing.c
index d192a074ba5..eecace887e6 100644
--- a/Modules/_multiprocessing/multiprocessing.c
+++ b/Modules/_multiprocessing/multiprocessing.c
@@ -167,7 +167,7 @@ multiprocessing_recvfd(PyObject *self, PyObject *args)
     cmsg = CMSG_FIRSTHDR(&msg);
     cmsg->cmsg_level = SOL_SOCKET;
     cmsg->cmsg_type = SCM_RIGHTS;
-    cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+    cmsg->cmsg_len = CMSG_SPACE(sizeof(int));
     msg.msg_controllen = cmsg->cmsg_len;
 
     Py_BEGIN_ALLOW_THREADS