[ale] old school: pty and tty

[cfowler]

I can give you a slight overview.  But really the only way you can
experience the difference is to program for both.  Otherwise this is
just incomprehensible stuff.

tty:

Real tty.  I guess you can call this a serial port, virtual terminal,
etc.  These are devices that have some form of hardware attached to the
other end.  In the ttyS0 it is usually a 16C550 UART. For tty[0-?]. It
is a driver for video memory, keyboard and mouse.  On the serial side of
things you can modify the UART registers.  You do that with an ioctl()
command or the stty command which implements ioctl().  You can change
parameters such as speed, char size, bits, signal status, etc...  You
can modify the physical attributes of the hardware.


pty:

Pseudo tty.  This is not a reall tty.  It is really a pipe with some
extra features.  It provides some of the POSIX services that a real tty
provide.  Like the ability of becoming a controlling tty, different read
modes.  Character attributes, etc.

This device is really used when it comes to telnet, ssh, and other
network programs that provide a tty to the user.  In the past a user
wanted to communicate with a Unix box and login just as they are on a
serial terminal.  But instead of over the network.  So UNIX programmers
had to come up with a way to provide serial terminal like functions
without the hardware.

Slave Vs Master:

In pseudo tty's you have a slave and a master.  When you log into a
Linux box remotely and issue the tty command, the system will display
the slave.  As far as you are concerned, your STDIN, STDOUT, and STDERR
are all connected to the slave like /dev/ttyp0.  Signals and other
things coming down the line behave as if you were on a serial port.  

CTRL-C.  When you  type your CTRL-C on you keyboard, the app does not
see such a sequence.  The pty driver sees the CTRL-C then it signals
SIGINT to the process that is control of that slave.  This is where
controlling tty comes into play. 

The master:

telnet really reads and writes to 2 file descriptors.  It will read from
the master (ptyp0) and write to the socket.  It will read from the
socker and write to the master (ptyp0).  The OS then passes that data
through preprocessing for control chars, etc then it send the data to
ttyp0 the slave.  That is how the relationship works.  

As you can see the only thing that makes a tty real is hardware on the
other end. And what makes a pseudo-tty is software.

I have a POSIX.1 Spec guide which goes into detail on the requirments. 
Or you can write some code.


On Fri, 2003-01-31 at 15:16, John Wells wrote:
> Anyone know any good links to explanations of the differences between pty
> and tty, or a good overview of each?
> 
> Thanks,
> John
> 
> 
> _______________________________________________
> Ale mailing list
> Ale at ale.org
> http://www.ale.org/mailman/listinfo/ale


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