Process Relationships

1
Process Relationships

• all processes have parent processes
• parents are notified of child termination
• can obtain exit status
• all processes belong to process groups
• all process groups belong to sessions
• POSIX .1 introduced sessions concept

2
Terminal Logins

• classic UNIX access provided by dumb terminals
• monitor, keyboard and local line or modem
  connection
• contemporary access uses the terminal metaphor
• pseudo-terminals provided for network access

3
4.3 BSD Terminal Logins

• terminal logins involve opening terminal or
  pseudo-terminal devices and execing the login
  program
• login program takes user ID, prompts for
  password, and then establishes parameters of the
  working session
• last step is for login to exec our login shell

4
SVR4 Terminal Logins

• SVR4 logins are handled as per BSD, with the
  addition of ttymon logins.
• ttymon logins have a different call structure,
  but function as per BSD logins from the exec of
  the login program forward

5
Network Logins

• network logins use the same terminal metaphor
  that physical terminal logins employ
• from the call to login forward, local terminal
  logins and network logins are essentially similar

6
BSD and SVR4 Network Logins

• network logins take place via a TCP/IP connection
  request, at which point a specific server
  establishes a two-way communication and opens a
  pseudo-terminal
• telnet telnetd handles terminal access
• ssh sshd handles terminal access
• both BSD and SVR4 use the same basic logic for
  terminal access, though the call structures vary

7
Common login Features

• all logins evaluate user data from password,
  group, etc. and establish working environments
• all logins complete by execing a shell process
  (login shell), and finish when that shell exits

8
Process Groups

• all processes belong to a process group
• processes default to the process group of their
  parent or execer
• process group IDs are positive integers
• process groups may have a leader
• PGID PID for leader
• processes can join other groups in same session
• must not be group leader

9
PGID functions

• two function used to manipulate process groups
• getpgrp() fetches process group of caller
• setpgid() places a process in an existing group
  or creates a new one
• can be done for calling process or children

10
Sessions

• collection of one or more process groups
• processes are usually grouped via a pipeline
• created by calling setsid()

11
setsid() function

• setsid() does three things
• caller made session leader of new session
• caller made group leader of new group
• process is made to have no controlling terminal,
  if one existed prior to the call
• caller must not be a process group leader

12
Controlling Terminal

• each session can have only one controlling
  terminal, and a terminal controls at most one
  session
• the session leader connected to the controlling
  terminal is the controlling process
• session leaders allocate controlling terminals
  for their sessions via ioctl calls
• process groups in a session with a controlling
  terminal are either foreground or background
• one foreground group
• terminal signals go to this group
• one or more background groups

13
tcgetpgrp() and tcsetpgrp() Functions

• tcgetpgrp() returns foreground group associated
  with a particular file descriptor
• tcsetpgrp() sets the foreground process group
• can be used by process with a controlling
  terminal to change the foreground group to one
  from another group within the same session

14
Job Control

• BSD feature from early 80s
• controls multiple foreground and background
  processes
• shells with job control can assign processes to
  foreground or background
• foreground processes receive terminal input,
  background do not
• standard terminal signals are sent to the
  foreground group

15
Job Controll Shell Execution of Programs

• programs executed within job control shells are
  assigned to the foreground group by default
• if assigned to the background group, programs
  will stop if need to read from terminal
• output to terminal may be allowed or disallowed
• user will be notified at next prompt
• background process can be moved to foreground

16
Orphaned Process Groups

• a process group is orphaned when no parent
  processes exist outside the group but in the
  session.
• terminal signals are not sent to the processes in
  orphaned groups
• any stopped process in an orphaned group is
  automatically sent the the SIGHUP and SIGCONT
  signals when the group is orphaned.