---
 man3/ttyname.3 | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/man3/ttyname.3 b/man3/ttyname.3
index 0be50c62a..f184b4505 100644
--- a/man3/ttyname.3
+++ b/man3/ttyname.3

@@ -75,7 +75,7 @@ was too small to allow storing the pathname.
 .\" glibc commit 15e9a4f378c8607c2ae1aa465436af4321db0e23
 .B ENODEV
 File descriptor refers to a slave pseudoterminal device
-but the corresponding pathname could not be found.
+but the corresponding pathname could not be found (see NOTES).
 .SH ATTRIBUTES
 For an explanation of the terms used in this section, see
 .BR attributes (7).

@@ -93,6 +93,22 @@ T}	Thread safety	MT-Safe
 .TE
 .SH CONFORMING TO
 POSIX.1-2001, POSIX.1-2008, 4.2BSD.
+.SH NOTES
+Processes keeping a file descriptor to a
+.BR pts (4)
+device open when switching to
+.BR mount_namespaces (7)
+that use a different
+.IR /dev/ptmx
+instance may still accidently find that a device path of the same name
+for that file descriptor exists. However, this device path refers to a
+different device and thus cannot not be used to modify the device that
+the file descriptor refers to. Calling
+.BR ttyname (7)
+or
+.BR ttyname_r (7)
+on the file descriptor in such a mount namespace will cause these
+functions to return NULL and set errno to ENODEV.
 .SH SEE ALSO
 .BR tty (1),
 .BR fstat (2),