ptm(7) ptm(7)
NAME [Toc] [Back]
ptm - STREAMS master pty (pseudo-terminal) driver
SYNOPSIS [Toc] [Back]
#include <sys/stropts.h>
#include <sys/ptyio.h>
#include <sys/strtio.h>
int open("/dev/ptmx", O_RDWR);
DESCRIPTION [Toc] [Back]
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 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 discipline module) pushed on top (not shown for
simplicity), provides a terminal interface as described in termio(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
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() 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 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 master
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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:
ISPTM 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. Therefore, it is imperative that applications
check for an explicit -1 instead of "< 0" (less than 0)
on the return value.
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ptm(7) ptm(7)
ISPTM is used by functions grantpt(), unlockpt(), and
ptsname(). User applications normally do not need to
invoke this ioctl. The format of this ioctl is:
int ioctl(master_fd, ISPTM, 0)
UNLKPT Unlocks the master and the corresponding slave devices.
On success, it returns 0. On failure, it returns -1
with errno set to [EINVAL]. UNLKPT is used by function
unlockpt(). User applications normally do not need to
invoke this ioctl. The format of this ioctl is:
int ioctl(master_fd, UNLKPT, 0)
TIOCREMOTE This ioctl puts the STREAMS pty in and out of Remote
Mode. When Remote Mode is on, input data will be
flow-controlled and passed through ldterm without any
input processing regardless of the terminal mode. When
the pty master driver receives this ioctl, it will send
an M_CTL message downstream to ldterm via ptm, pts, and
ptem. The command in the M_CTL message is set to
MC_NO_CANON or MC_DO_CANON depending whether to turn on
or off the Remote Mode. The format of this ioctl is:
int ioctl(master_fd, TIOCREMOTE, argument)
where the argument is set to 1 to turn on Remote Mode
and 0 to turn it off. Remote Mode is normally used
when doing remote line editing in a window manager, or
whenever flow-controlled input is required. Each write
to the master device produces a record boundary for the
process reading the slave devices. In normal usage, a
write of data is like the data typed as a line on the
terminal; a write of 0 (zero) bytes is like typing an
EOF (End-of-File) character.
TIOCSIGNAL This ioctl allows the master process to send a signal
to the slave process. The format of this ioctl is:
int ioctl(master_fd, TIOCSIGNAL, argument)
where the argument is the signal number as defined in
the header file <sys/signal.h>. For example the master
process can send an SIGINT signal to the slave process
by doing:
ioctl(master_fd, TIOCSIGNAL, SIGINT)
AUTHOR [Toc] [Back]
ptm was developed by HP and OSF.
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ptm(7) ptm(7)
FILES [Toc] [Back]
/dev/ptmx Streams pty master clone device
/dev/pts/N Streams pty slave devices (0 <= N < NSTRPTY), where
NSTRPTY is a kernel tunable parameter which can be
changed via SAM.
SEE ALSO [Toc] [Back]
insf(1M), getmsg(2), ioctl(2), open(2), read(2), write(2),
grantpt(3C), ptsname(3C), unlockpt(3C), clone(7), ldterm(7), pckt(7),
ptem(7), pts(7), streamio(7), termio(7).
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