diff --git a/man7/unix.7 b/man7/unix.7
index f03e7bd12..17a10d3db 100644
--- a/man7/unix.7
+++ b/man7/unix.7
@@ -381,6 +381,12 @@ Semantically, this operation is equivalent to duplicating
 .RB ( dup (2))
 a file descriptor into the file descriptor table of another process.
 .IP
+If the buffer used to receive the ancillary data containing
+file descriptors is too small (or is absent),
+then the ancillary data is truncated (or discarded)
+and the excess file descriptors are automatically closed
+in the receiving process.
+.IP
 The kernel constant
 .BR SCM_MAX_FD
 defines a limit on the number of file descriptors in the array.