diff --git a/man7/unix.7 b/man7/unix.7
index 0850ebc1b..e4c19324d 100644
--- a/man7/unix.7
+++ b/man7/unix.7
@@ -435,6 +435,19 @@ To receive a
 message, the
 .B SO_PASSCRED
 option must be enabled on the socket.
+.PP
+If the space allocated for receiving incoming ancillary data is too small,
+then the ancillary data is truncated to the number of headers
+that will fit in the supplied buffer (or, in the case of an
+.BR SCM_RIGHTS
+file descriptor list, the list of file descriptors may be truncated).
+In the event of such truncation, the
+.BR MSG_CTRUNC
+flag will be set in the
+.I msg.msg_flags
+value returned by
+.BR recvmsg (2).
+.\"
 .SS Ioctls
 The following
 .BR ioctl (2)