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.Dd October 28, 2019
.Dt PTY 4
.Os
.Sh NAME
.Nm pty
.Nd old-style compatibility pseudo-terminal driver
.Sh SYNOPSIS
.Cd "device pty"
.Sh DESCRIPTION
The
.Nm
driver provides support for the traditional BSD naming scheme that was
used for accessing pseudo-terminals before it was replaced by
.Xr pts 4 .
This traditional naming is still used in Linux.
When the device
.Pa /dev/ptyXX
is being opened, a new terminal shall be created with the
.Xr pts 4
driver.
A device node for this terminal shall be created, which has the name
.Pa /dev/ttyXX .
.Pp
The
.Nm
driver also provides a cloning System V
.Pa /dev/ptmx
device.
.Pp
New code should not try to allocate pseudo-terminals using this
interface.
It is only provided for compatibility with older C libraries
that tried to open such devices when
.Xr posix_openpt 2
was being called,
and for running Linux binaries.
.Sh FILES
The BSD-style compatibility pseudo-terminal driver uses the following
device names:
.Bl -tag -width ".Pa /dev/pty[l-sL-S][0-9a-v]"
.It Pa /dev/pty[l-sL-S][0-9a-v]
Pseudo-terminal master devices.
.It Pa /dev/tty[l-sL-S][0-9a-v]
Pseudo-terminal slave devices.
.It Pa /dev/ptmx
Control device, returns a file descriptor to a new master
pseudo-terminal when opened.
.El
.Sh DIAGNOSTICS
None.
.Sh SEE ALSO
.Xr posix_openpt 2 ,
.Xr pts 4 ,
.Xr tty 4
.Sh HISTORY
A
pseudo-terminal driver appeared in
.Bx 4.2 .
.Sh BUGS
Unlike previous implementations, the master and slave device nodes are
destroyed when the PTY becomes unused.
A call to
.Xr stat 2
on a nonexistent master device will already cause a new master device
node to be created.
The master device can only be destroyed by opening and closing it.
.Pp
The
.Nm
driver cannot be unloaded, because it cannot determine if it is being
used.