HP-UX Reference (11i v1 00/12) - 5 Miscellaneous Topics, 7 Device (Special) Files, 9 General Information, Index (vol 9)

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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 connection

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

minal 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 device

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 sys-

tem for each individualmaster 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() system 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 dev-

ice. 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 master 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 master and slave devices

when they are opened. The user may then push the necessary modules on the master 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 device 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:

Section 7−−92 − 1 − HP-UX Release 11i: December 2000

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