PTM(4D) Devices PTM(4D)


ptm, pts - STREAMS pseudo-terminal manager and subsidiary drivers





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 ptm driver; see
CLONE_DEV in ddi_create_minor_node(9F).

There are no nodes in the file system for manager devices. The manager
pseudo driver is opened using the open(2) system call with /dev/ptmx as the
device parameter. The clone open finds the next available minor device for
the ptm major device.

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.

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 grantpt(3C) and unlockpt(3C). The user can then invoke the
open(2) system call with the device name returned by the ptsname(3C)

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 (see XPG4v2 MODE), the
consumer needs to push the ptem(4M) and ldterm(4M) modules onto the
subsidiary device to get terminal semantics.

The manager and subsidiary drivers pass all messages to their adjacent
queues. Only the M_FLUSH needs some processing. Because the read queue of
one side is connected to the write queue of the other, the FLUSHR flag is
changed to the FLUSHW flag and vice versa.

When the manager device is closed, an M_HANGUP message is sent to the
subsidiary device which will render the device unusable. The process on
the subsidiary side gets an 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, read(2) returns 0 indicating
that the stream can no longer be used.

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
read(2) or getmsg(2) and 0 is returned, the user of the manager device
decides whether to issue a 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.

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
(see XPG4v2 MODE), then any 0-length messages written to the subsidiary
device will be discarded by the ptem(4M) module.

If O_NONBLOCK or O_NDELAY is set on the manager side:

+o Read on the manager side returns -1 with errno set to EAGAIN if no data
is available

+o Write returns -1 with errno set to EAGAIN if there is internal flow

Standard STREAMS system calls can access pseudo-terminal devices. The
subsidiary devices support the O_NDELAY and O_NONBLOCK flags.

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.

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 (see standards(7)) mode.


The manager driver provides several ioctls to support the grantpt(3C),
unlockpt(3C), and ptsname(3C) functions:

ISPTM Determines whether the file descriptor is that of an open manager
device. On success, it returns the value 0.

UNLKPT Unlocks the manager and subsidiary devices. It returns 0 on
success. On failure, errno is set to EINVAL indicating that the
manager device is not open.


/dev/ptmx Pseudo-terminal manager clone device.

/dev/pts/N Pseudo-terminal subsidiary devices, where
N is a non-negative integer. Located via
calls to ptsname(3C).


Example 1 Opening the manager and subsidiary device for a pseudo-terminal.

#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) {
err(1, "locate subsidiary");
if ((fds = open(subsidiaryname, O_RDWR | O_NOCTTY)) < 0) {
err(1, "open subsidiary");
if (ioctl(fds, I_PUSH, "ptem") != 0 ||
ioctl(fds, I_PUSH, "ldterm") != 0) {
err(1, "push modules");


close(2), getmsg(2), open(2), read(2), grantpt(3C), posix_openpt(3C),
ptsname(3C), unlockpt(3C), ldterm(4M), pckt(4M), ptem(4M), standards(7),

illumos February 5, 2022 illumos