Lecture 18 Process Relationships

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Lecture 18Process Relationships
CSCE 510 Systems Programming

• Topics
• Logins
• Process Groups and Sessions
• Controlling terminals
• Job Control
• Readings Chapter 9

October 25, 2005
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• Last Time
• x
• Today
• Terminal logins
• Network logins
• Process groups
• Process sessions
• Controlling terminals

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Terminal logins

• BSD logins
• Real user ID 0
• Effective user ID 0
• Getty calls to open the terminal
• File descriptors 0,1,2 set to the device

Process ID 1
init
Forks once per terminal
fork
init
Each child execs getty
exec
getty
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Terminal logins continued
Process ID 1
Reads /etc/ttys Forks once per
terminal Creates empty env
init

• Getty reads login_name
• Execs login

fork
init
exec
Opens /dev/ttyx File descr. 0,1,2 Reads user name
getty
exec
login
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Logins that Fail

• Getpwname to fetch passwd entry
• Call getpass to read passwd without echoing
• Call crypt to encrypt the password entered
• Check against encrypted pw_passwd from passwd
  entry of the shadow file
• Failure ?
• login calls exit(1)
• Child send signal SIGCHLD to init
• Init forks again followed by the exec of getty

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Logins that Suceed

• Chdir to home directory
• Chown on the terminal so user owns it
• Change access permissions so user can read and
  write to terminal
• Set group IDs setgid, initgroups
• Initialize Environment from pw entry
• HOME
• SHELL
• USER
• PATH
• Change user ID using setuid
• Exec shell

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After Terminal login
Process ID 1
init

• BSD logins
• Real user ID 0
• Effective user ID 0
• Getty calls to open the terminal

fork
Login shell
fd 0,1,2
Terminal Device driver
User at the terminal
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Linux and Solaris terminal logins

• Linux - Similar but look at /etc/inittab for
  information specifying the terminal devices for
  which init should start a getty process
• Solaris ttymon part of a facility to provide
  consistent administrative services
• - overall procedure is same but the parent
  of the logon process will be ttymon instead of
  init

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Network Logins Set up

• Figure 9.4

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Network Logins After Set Up

• Figure 9.5

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Process groups

• In addition to unique process ID each process
  belongs to a process group
• Each process group has a leader
• A process can join a group using the setpgid
  syscall
• In setpgid(pid_t pid, pid_t pgid)
• Children inherit the group from the parent
• A process can set the process group ID of its
  children up until they exec

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Process Sessions

• A session is one or more process groups
• Example figure 9.6
• proc1 proc2 / put in
  background /
• proc3 proc4 proc5

shell
proc1
proc2
proc3
proc4
proc5
Process group1
Process group2
Process group3
A Session (from APU2E fig 9.6)
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Setsid -

• pid_t setsid(void)
• Process becomes the session leader
• Process becomes new group leader
• Process has no controlling terminal
• System V Release 4 (SVR4) concept, eventually
  integrated into BSD
• pid_t getsid(pid_t pid) return session leaders
  group ID
• Security issues - restrictions

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Controlling Terminal

• A session can have a single controlling terminal
• This is usually the terminal or psuedo-terminal
  on which we log in
• The session leader that establishes connection is
  called the controlling process
• Process groups
• Then it has a single foreground process group
• One or more background groups
• If session has controlling terminal then it has a
  single foreground process group

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Signals from Controlling Terminal

• Certain keys cause the controlling terminal to
  send signals to all the processes in the
  foreground group
• C (at USC sometimes DELETE) sends SIGINT
• \ causes a SIGQUIT to be sent
• If a modem or network disconnect is detected the
  controlling terminal sends a SIGHUP to the
  controlling process

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Signals from Controlling Terminal

• Figure 9.7

Session
shell
proc1
proc2
proc3
proc4
proc5
Background Process group Session leader
Background Process group
Foreground Process group
Terminal input Terminal-generated
signals
SIGHUP On modem or network disconnect
Controlling terminal
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Opening the Controlling Terminal

• Sometimes a process needs to make sure it is
  talking to the controlling terminal
• man -s 3 getpass
• This function is obsolete. Do not use it!
• The getpass() function
• opens /dev/tty (the controlling terminal of the
  process),
• outputs the string prompt,
• turns off echoing,
• reads one line (the "password"),
• restores the terminal state and
• closes /dev/tty again.

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Crypt and its Problems

• Practical Unix Internet Security, 3rd Edition
  (Paperback)by Simson Garfinkel, Gene Spafford,
  Alan Schwartz

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Functions Communicating Foreground Status

• pid_t tcgetpgrp (int filedes)
• Returns PID of foreground process group
• int tcsetpgrp (int filedes, pid_t pgrpid)
• pid_t tcgetsid (int filedes)
• Returns session leaders process group

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Job Control

• Started in BSD around 1980
• on end of command puts it in background
• Z (suspend character) to send SIGTSTP to stop
  foreground process
• bg puts the job stopped into background
• jobs command allows bringing background job to
  foreground

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Support for Job Control

• Jobs control requires support at three levels
• The kernel must support job control signals
• The terminal driver in the kernel must support
  job control
• A shell must support job control

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Job Control Example

• thetisgt cat gt ooo
• 1 24839
• thetisgt jobs
• 1 Stopped cat gtooo
• thetisgt fg 1
• cat gtooo
• hello world
• D // D makes read return EOF
• thetisgt

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Summary of Job Control Features

• Figure 9.8

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Orphan3.c

• include "apue.h"
• include lterrno.hgt
• static void
• sig_hup(int signo)
• printf("SIGHUP received, pid d\n", getpid())
• static void
• pr_ids(char name)
• printf("s pid d, ppid d, pgrp d,
  tpgrp d\n",
• name, getpid(), getppid(), getpgrp(),
  tcgetpgrp(STDIN_FILENO))
• fflush(stdout)

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Orphan3.c cont.

• Int main(void)
• char c
• pid_t pid
• pr_ids("parent")
• if ((pid fork()) lt 0) err_sys("fork
  error")
• else if (pid gt 0) / parent /
• sleep(5) / sleep to let child stop itself /
• exit(0) / then parent exits /
• else / child /
• pr_ids("child")
• signal(SIGHUP, sig_hup) / setup signal
  handler /
• kill(getpid(), SIGTSTP) / stop ourself /
• pr_ids("child") / prints only if we're
  continued /
• if (read(STDIN_FILENO, c, 1) ! 1)
• printf("read error from controlling TTY,
• errno d\n", errno)
• exit(0)

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Next Time Daemon Processes

• Daemons are processes that live for a long time
• Usually start when the system is booted and pny
  terminate just before the system shutsdown
• They dont have a controlling terminal
• No SIGINTs etc generate from keyboard
• Run in background

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