+TIOCUCNTL
+Enable/disable a mode that allows a small number of simple user
+.I ioctl
+commands to be passed through the pseudo-terminal,
+using a protocol similar to that of TIOCPKT.
+The TIOCUCNTL and TIOCPKT modes are mutually exclusive.
+This mode is enabled from the master side of a pseudo terminal
+by specifying (by reference)
+a nonzero parameter and disabled by specifying (by reference)
+a zero parameter.
+Each subsequent
+.I read
+from the master side will return data written on the slave part of
+the pseudo terminal preceded by a zero byte,
+or a single byte reflecting a user control operation on the slave side.
+A user control command consists of a special
+.I ioctl
+operation with no data; the command is given as UIOCCMD(\fIn\fP),
+where \fIn\fP is a number in the range 1-255.
+The operation value \fIn\fP will be received as a single byte on the next
+.I read
+from the master side.
+The \fIioctl\fP UIOCCMD(0) is a no-op that may be used to probe for
+the existence of this facility.
+As with TIOCPKT mode, command operations may be detected with a
+.I select
+for exceptional conditions.
+.TP