11-29-2010
Quote:
Originally Posted by
pinga123
Hi ,
I have searched wiki for pseudo tty devices but it was very complex for me to understand.
A pseudo-TTY is it's a virtual device that acts like a terminal -- that is to say, acts like a serial port plus all the frills UNIX has added to them over the years with a keyboard and screen attached. You get features and side-effects like you'd expect of a real terminal: It can be configured to produce SIGINT when someone writes ctrl-C into it, you can configure it to echo its input or not, etc, etc. But it's not a real terminal -- it's a purely software thing. There's no real keyboard or screen involved anymore.
This is used for things like ssh, where you want to log into a real terminal but of course are too far away to do so. It opens a pseudo-terminal instead and just sends everything along.
Software can tell the difference between something that's a terminal and something that's not. Password inputs read only from a real TTY or a pseudo-tty for instance.
Last edited by Corona688; 11-29-2010 at 01:31 PM..
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PTS(4) Linux Programmer's Manual PTS(4)
NAME
ptmx, pts - pseudo-terminal master and slave
DESCRIPTION
The file /dev/ptmx is a character file with major number 5 and minor number 2, usually of mode 0666 and owner.group of root.root. It is
used to create a pseudo-terminal master and slave pair.
When a process opens /dev/ptmx, it gets a file descriptor for a pseudo-terminal master (PTM), and a pseudo-terminal slave (PTS) device is
created in the /dev/pts directory. Each file descriptor obtained by opening /dev/ptmx is an independent PTM with its own associated PTS,
whose path can be found by passing the descriptor to ptsname(3).
Before opening the pseudo-terminal slave, you must pass the master's file descriptor to grantpt(3) and unlockpt(3).
Once both the pseudo-terminal master and slave are open, the slave provides processes with an interface that is identical to that of a real
terminal.
Data written to the slave is presented on the master descriptor as input. Data written to the master is presented to the slave as input.
In practice, pseudo-terminals are used for implementing terminal emulators such as xterm(1), in which data read from the pseudo-terminal
master is interpreted by the application in the same way a real terminal would interpret the data, and for implementing remote-login pro-
grams such as sshd(8), in which data read from the pseudo-terminal master is sent across the network to a client program that is connected
to a terminal or terminal emulator.
Pseudo-terminals can also be used to send input to programs that normally refuse to read input from pipes (such as su(1), and passwd(1)).
FILES
/dev/ptmx, /dev/pts/*
NOTES
The Linux support for the above (known as Unix98 pty naming) is done using the devpts file system, that should be mounted on /dev/pts.
Before this Unix98 scheme, master ptys were called /dev/ptyp0, ... and slave ptys /dev/ttyp0, ... and one needed lots of preallocated
device nodes.
SEE ALSO
getpt(3), grantpt(3), ptsname(3), unlockpt(3), pty(7)
COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2002-10-09 PTS(4)