Tty

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Tty
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What is a tty?

• tty (terminal type) is UNIXs way to define a
  terminal.
• The tty system call prints the path name of the
  user's terminal (e.g. /dev/pts/89).
• On LINUX each terminal opened (Eterm, Xterm,
  Gnome, etc.) has its own tty.

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Process Groups and tty management

• One of the areas least-understood by most UNIX
  programmers is process-group management, a topic
  that is inseparable from signal-handling.
• To understand why process-groups exist, think
  back to the world before windowing systems.

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Process Groups and tty management

• Your average user wants to run several programs
  simultaneously. Obviously you cannot have two
  processes reading from the same tty at the same
  time, they'll each get some of the characters you
  type. Likewise output should be managed.
• This has been a problem with many operating
  systems.

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Tty - Solutions

• Tenex and TOPS-20 - used process stacks. You
  could interrupt a process to run another process,
  and when the new process was finished the old
  would restart.
• While this was useful it didn't allow you to
  switch back and forth between processes without
  exiting one of them.
• Clearly there must be a better way.

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Tty SolutionsThe Berkeley approach

• The Berkeley UNIX folks came up with a different
  idea, called process groups.
• The tty has a concept of "foreground process
  group", the group of processes which is allowed
  to do input and output to the tty.
• A set of processes has a tty device to which it
  belongs, called its "controlling tty". This tty
  device is what is returned when /dev/tty is
  opened.

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• The tty watches for particular key presses (Z
  usually) and sends a signal to the foreground
  process group when it sees one. All processes in
  the group see the signal, and all stop -
  returning the control to the shell.
• At this point the shell can place any of the
  active process groups back in the foreground.
• To handle the case where a background process
  tries to read or write from the tty, the tty
  driver will send a SIGTTIN or SIGTTOU signal to
  any background process which attempts to perform
  such an operation. Under normal circumstances
  only the foreground processes can use the tty.

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Tty POSIX implementation

• The POSIX specification has provided new
  interfaces for handling process groups.
• int setpgid(int process_id, int process_group)
• Move a process into a new process group.
• int getpgid(int process_id)
• Find the process group of a process.
• int getpgrp(void)
• Find the process group of the current process.

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Tty POSIX implementation

• int tcsetpgrp(int tty, int foreground_group)
• Change the foreground process group of a tty.
• int tcgetpgrp(int tty, int foreground_group)
• Find the foreground process group of a tty.
• int kill(int -process_group, int signal_number)
• Send a signal to a process group. Note that
  process_group must be passed as a negative value.