ptm.7 (2010 09)

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ptm(7) ptm(7)

NAME

ptm - STREAMS master pty (pseudo-terminal) driver

SYNOPSIS

#include <sys/stropts.h>

#include <sys/ptyio.h>

#include <sys/strtio.h>

int open("/dev/ptmx", O_RDWR);

DESCRIPTION

A pseudo-terminal (pty) consists of a tightly-coupled pair of character devices, called the master device

and slave device. The pty master and slave device drivers work together to simulate a terminal connec-

tion where the master provides a connection to the pseudo terminal server process and the slave provides

a terminal device special ﬁle access for the terminal application processes, as depicted below:

----------------

| pty functions |

Application <--> |----------------| <--> Server

Processes | Slave | Master | Process

| (pts) | (ptm) |

----------------

The slave driver,

pts with ptem (STREAMS pty emulation module) and ldterm (STREAMS line discip-

line module) pushed on top (not shown for simplicity), provides a terminal interface as described in ter-

mio(7). Whereas devices that provide the terminal interface described in termio (7) have a hardware dev-

ice behind them; in contrast, the slave device has another process manipulating it through the master

side of the pty. Data written on the master device is given to the slave device as input and data written

on the slave device is presented as input on the master device.

In order to use the STREAMS pty subsystem, a node for the master pty driver

/dev/ptmx and N

number of slave pty devices must be installed (see pts (7) for details on slave pty). There are no nodes in

the ﬁle system for each individual master device. Rather, the master driver is set up as a STREAMS

clone driver (see clone (7)) with its major device number set to the major for the clone driver and its minor

device number set to the major for the

ptm driver. The master driver is opened using the

open() sys-

tem call with

/dev/ptmx as the device ﬁle parameter. The clone open ﬁnds the next available minor

number for the master device. The master device is available only if it and its corresponding slave device

are not already opened. Only one open is allowed on a master device whereas multiple open are allowed

on the slave device. When the master device is opened, the corresponding slave device is automatically

locked out (see pts (7) on how to unlock the slave and obtain the slave device name). After both the mas-

ter and slave have been opened, the user has two ﬁle descriptors which represent the end points of a full

duplex connection composed of two streams. These two streams are automatically connected by the mas-

ter and slave devices when they are opened. The user may then push the necessary modules on the mas-

ter and slave streams (e.g., ptem and ldterm, on pts for terminal semantics, and pckt on

ptm for

Packet Mode feature).

The master and slave drivers pass all STREAMS messages to their adjacent drivers. Only the

M_FLUSH

message needs some special processing because the read queue of the master is connected to the write

queue of the slave and vice versa. Hence, the FLUSHR ﬂag is changed to FLUSHW ﬂag and vice versa

whenever a M_FLUSH message travels across the master−slave link. When the master device is closed,

an M_HANGUP message is sent to the corresponding slave device which will render that slave device

unusable. The process on the slave side gets the errno [ENXIO] when attempting a write() system call

on the slave device but it will be able to read any data remaining on the slave stream. Finally, when all

the data have been read, the read() system call will return 0 (zero) indicating that the slave can no

longer be used. On the last close of the slave device, a zero-length M_DATA message is sent to the

corresponding master device. When the application on the master side issues a read() or getmsg()

system calls and a 0 is returned. The user of the master device decides whether to close the master dev-

ice ﬁle which will dismantle the streams on the master side. If the master device remains opened, the

corresponding slave device can be opened and used again by another user.

Unlike the slave device, the master device does not act like a terminal. If

O_NDELAY or O_NONBLOCK is

set, a read on the master device returns -1 with errno set to [EAGAIN] if no data is available, and a write

returns − 1 with errno set to [EAGAIN] if there is internal ﬂow control on the stream.

The master

ptm driver supports the following ioctl() requests:

HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1

Summary of content (2 pages)

PAGE 1
ptm(7) ptm(7) NAME ptm - STREAMS master pty (pseudo-terminal) driver SYNOPSIS #include #include #include int open("/dev/ptmx", O_RDWR); DESCRIPTION A pseudo-terminal (pty) consists of a tightly-coupled pair of character devices, called the master device and slave device.
PAGE 2
ptm(7) ISPTM ptm(7) Determines whether the file descriptor is that of an open master device. On success, it returns the major and minor number (type dev_t) of the master device which can be used to determine the name of the corresponding slave device. On failure, it returns −1 with errno set to [EINVAL]. ISPTM on HP-UX can return valid device number with negative value. For example, with major number of the STREAMS pty master being 0x9c, ICPTM will return 0x9C000000 which is a negative number.