PTM(7D)PTM(7D)NAMEptm - STREAMS pseudo-tty master driver
DESCRIPTION
The pseudo-tty subsystem simulates a terminal connection, where the
master side represents the terminal and the slave represents the user
process's special device end point. In order to use the pseudo-tty sub‐
system, a node for the master side driver /dev/ptmx and N number of
nodes for the slave driver must be installed. See pts(7D). The master
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. There are no nodes in the file system for master
devices. The master 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 master device is available only if it and its corresponding slave
device are not already open. When the master device is opened, the cor‐
responding slave device is automatically locked out. Only one open is
allowed on a master device. Multiple opens are allowed on the slave
device. After both the master and slave have been opened, the user has
two file descriptors which are the end points of a full duplex connec‐
tion composed of two streams which are automatically connected at the
master and slave drivers. The user may then push modules onto either
side of the stream pair.
The master and slave 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 master
device is closed an M_HANGUP message is sent to the slave device which
will render the device unusable. The process on the slave side gets the
errno EIO 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() returns 0 indicating that the stream can no
longer be used. On the last close of the slave device, a 0-length mes‐
sage is sent to the master device. When the application on the master
side issues a read() or getmsg() and 0 is returned, the user of the
master device decides whether to issue a close() that dismantles the
pseudo-terminal subsystem. If the master device is not closed, the
pseudo-tty subsystem will be available to another user to open the
slave device.
If O_NONBLOCK or O_NDELAY is set, read on the master side returns −1
with errno set to EAGAIN if no data is available, and write returns −1
with errno set to EAGAIN if there is internal flow control.
IOCTLS
The master driver supports the ISPTM and UNLKPT ioctls that are used by
the functions grantpt(3C), unlockpt(3C) and ptsname(3C). The ioctl
ISPTM determines whether the file descriptor is that of an open master
device. On success, it returns the 0. The ioctl UNLKPT unlocks the mas‐
ter and slave devices. It returns 0 on success. On failure, the errno
is set to EINVAL indicating that the master device is not open.
FILES
/dev/ptmx
master clone device
/dev/pts/M
slave devices (M = 0 -> N-1)
SEE ALSOgrantpt(3C), ptsname(3C), unlockpt(3C), pckt(7M), pts(7D)
STREAMS Programming Guide
Feb 5, 1997 PTM(7D)
