diff --git a/man7/unix.7 b/man7/unix.7
index d16cc30a8..46b1914b8 100644
--- a/man7/unix.7
+++ b/man7/unix.7
@@ -371,6 +371,19 @@ Send or receive a set of open file descriptors from another process.
 The data portion contains an integer array of the file descriptors.
 The passed file descriptors behave as though they have been created with
 .BR dup (2).
+.IP
+The kernel constant
+.BR SCM_MAX_FD
+defines a limit on the number of file descriptors in the array.
+Attempting to send an array larger than this limit cause
+.BR sendmsg (2)
+to fail with the error
+.BR EINVAL .
+.BR SCM_MAX_FD
+has the value 253
+(or 255 in kernels
+.\" commit bba14de98753cb6599a2dae0e520714b2153522d
+before 2.6.38).
 .TP
 .B SCM_CREDENTIALS
 Send or receive UNIX credentials.