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.\" Copyright 2022 Oxide Computer Company
.Dd February 5, 2022
.Dt PTM 4D
.Os
.Sh NAME
.Nm ptm ,
.Nm pts
.Nd STREAMS pseudo-terminal manager and subsidiary drivers
.Sh SYNOPSIS
.Pa /dev/ptmx
.Pp
.Pa /dev/pts/*
.Sh DESCRIPTION
The pseudo-terminal subsystem simulates a terminal connection, where the
manager side represents the terminal and the subsidiary represents the user
process's special device end point.
The manager device is set up as a cloned device where its major device number
is the major for the clone device and its minor device number is the major for
the
.Nm ptm
driver; see
.Dv CLONE_DEV
in
.Xr ddi_create_minor_node 9F .
.Pp
There are no nodes in the file system for manager devices.
The manager pseudo driver is opened using the
.Xr open 2
system call with
.Pa /dev/ptmx
as the device parameter.
The clone open finds the next available minor device for the
.Nm ptm
major device.
.Pp
A manager device is only available if it and its corresponding subsidiary
device are not already open.
Only one open is allowed on a manager device.
Multiple opens are allowed on the subsidiary device.
.Pp
When the manager device is opened, the corresponding subsidiary device is
automatically locked out.
No user may open the subsidiary device until its permissions are adjusted and
the device is unlocked by calling the functions
.Xr grantpt 3C
and
.Xr unlockpt 3C .
The user can then invoke the
.Xr open 2
system call with the device name returned by the
.Xr ptsname 3C
function.
.Pp
After both the manager and subsidiary have been opened, the user has two file
descriptors which are the end points of a full duplex connection composed of
two streams which are automatically connected at the manager and subsidiary
drivers.
The user may then push modules onto either side of the stream pair.
Unless compiled in XPG4v2 mode
.Po
see
.Sx "XPG4v2 MODE"
.Pc ,
the consumer needs to push the
.Xr ptem 4M
and
.Xr ldterm 4M
modules onto the subsidiary device to get terminal semantics.
.Pp
The manager and subsidiary drivers pass all messages to their adjacent queues.
Only the
.Dv M_FLUSH
needs some processing.
Because the read queue of one side is connected to the write queue of the
other, the
.Dv FLUSHR
flag is changed to the
.Dv FLUSHW
flag and vice versa.
.Pp
When the manager device is closed, an
.Dv M_HANGUP
message is sent to the subsidiary device which will render the device unusable.
The process on the subsidiary side gets an
.Er EIO
error when attempting to write on that stream, but it will be able to read
any data remaining on the stream head read queue.
When all the data has been read,
.Xr read 2
returns
.Sy 0
indicating that the stream can no longer be used.
.Pp
On the last close of the subsidiary device, a 0-length message is sent to the
manager device.
When the application on the manager side issues a
.Xr read 2
or
.Xr getmsg 2
and
.Sy 0
is returned, the user of the manager device decides whether to issue a
.Xr close 2
that dismantles the entire pseudo-terminal.
If the manager device is not closed, the pseudo-terminal will be available to
another user to open the subsidiary device.
.Pp
Since 0-length messages are used to indicate that the process on the
subsidiary side has closed, and should be interpreted that way by the process
on the manager side, applications on the subsidiary side should not write
0-length messages.
Unless the application is compiled in XPG4v2 mode
.Po
see
.Sx "XPG4v2 MODE"
.Pc ,
then any 0-length messages written to the subsidiary device will be discarded
by the
.Xr ptem 4M
module.
.Pp
If
.Dv O_NONBLOCK
or
.Dv O_NDELAY
is set on the manager side:
.Bl -bullet
.It
Read on the manager side returns
.Sy -1
with
.Va errno
set to
.Er EAGAIN
if no data is available
.It
Write returns
.Sy -1
with
.Va errno
set to
.Er EAGAIN
if there is internal flow control
.El
.Pp
Standard STREAMS system calls can access pseudo-terminal devices.
The subsidiary devices support the
.Dv O_NDELAY
and
.Dv O_NONBLOCK
flags.
.Sh XPG4v2 MODE
.Em XPG4v2
requires that subsidiary pseudo-terminal devices provide the process with an
interface that is identical to the terminal interface, without needing to
explicitly push any modules to achieve this.
It also requires that 0-length messages written on the subsidiary device will
be propagated to the manager device.
.Pp
Experience has shown that most software does not expect subsidiary
pseudo-terminal devices to operate in this manner.
This XPG4v2-compliant behaviour is only enabled in XPG4v2/SUS
.Po
see
.Xr standards 7
.Pc
mode.
.Sh IOCTLS
The manager driver provides several ioctls to support the
.Xr grantpt 3C ,
.Xr unlockpt 3C ,
and
.Xr ptsname 3C
functions:
.Bl -tag -width Ds
.It Dv ISPTM
Determines whether the file descriptor is that of an open manager device.
On success, it returns the value
.Sy 0 .
.It Dv UNLKPT
Unlocks the manager and subsidiary devices.
It returns
.Sy 0
on success.
On failure,
.Vt errno
is set to
.Vt EINVAL
indicating that the manager device is not open.
.El
.Sh FILES
.Bl -tag -width Pa
.It Pa /dev/ptmx
Pseudo-terminal manager clone device.
.It Pa /dev/pts/N
Pseudo-terminal subsidiary devices, where
.Sy N
is a non-negative integer.
Located via calls to
.Xr ptsname 3C .
.El
.Sh EXAMPLES
.Sy Example 1
Opening the manager and subsidiary device for a pseudo-terminal.
.Bd -literal -offset Ds
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <stropts.h>
#include <fcntl.h>
#include <err.h>
\&...
int fdm, fds;
char *subsidiaryname;
\&...
/*
 * NOTE: Portable applications should use posix_openpt(3C) here:
 */
if ((fdm = open("/dev/ptmx", O_RDWR | O_NOCTTY)) < 0) {
        err(1, "open manager");
}
if (grantpt(fdm) != 0 || unlockpt(fdm) != 0 ||
    (subsidiaryname = ptsname(fdm)) == NULL) {
        close(fdm);
        err(1, "locate subsidiary");
}
if ((fds = open(subsidiaryname, O_RDWR | O_NOCTTY)) < 0) {
        close(fdm);
        err(1, "open subsidiary");
}
if (ioctl(fds, I_PUSH, "ptem") != 0 ||
    ioctl(fds, I_PUSH, "ldterm") != 0) {
        close(fds);
        close(fdm);
        err(1, "push modules");
}
.Ed
.Sh SEE ALSO
.Xr close 2 ,
.Xr getmsg 2 ,
.Xr open 2 ,
.Xr read 2 ,
.Xr grantpt 3C ,
.Xr posix_openpt 3C ,
.Xr ptsname 3C ,
.Xr unlockpt 3C ,
.Xr ldterm 4M ,
.Xr pckt 4M ,
.Xr ptem 4M ,
.Xr standards 7 ,
.Xr ddi_create_minor_node 9F