• UNIX process relationship and job control
– Groups, Sessions, Foreground and background processes
– Our program can now have many processes, when Ctrl-C is typed, which process should receive SIGINT?

– A process group is a collection of (related) processes. Each group has a process group ID.
– Each group has a group leader who pid = pgid
– To get the group ID of a process: pid_t getpgrp(void)
– A signal can be sent to the whole group of processes.

• Process groups:
– A process may joint an existing group, create a new group.
int setpgid(pid_t, pid, pid_t, pgid)
• A process can set group ID of itself or its children
• _POSIX_JOB_CONTROL must be defined
• Most shells with job control create new group for each line of command (job).

• Sessions
– A session is one or more process groups
– proc1 | proc2 & proc3 | proc4 | proc5 results in a session with three groups, see ‘ps –j’
Login shell proc1 proc2 proc3 proc4 proc5
– A login shell is a session in general.

• A session can have a single controlling terminal
– Terminal device for a terminal login
– Pseudo-terminal device for a network login
– The I/O devices somewhat link to the window and keyboard.
• The session leader that establishes the connection to the control terminal is called the controlling process .

• Only one I/O device for all processes (and process groups) in a session. Which process should get the input from the keyboard?
• Foreground and background process
– One foreground group
– Many background groups
– Input
• Only foreground group
• Terminals’ interrupt signals are only sent to the processes in the foreground group.
– Output
• Typically shared

• Sessions
– To establish a new session: pid setsid(void);
• Process become the session leader
• Process become a new group leader of a new group
• Process has no controlling terminal (break up the old one)
– Each shell is a session. When a shell is created, a terminal must be setup.
• Fails if the caller is a group leader.
– A process can open file /dev/tty to talk to the controlling terminal regardless how standard IO are redirected.
• A way to by pass I/O redirection, see example1.c

• How to make a group foreground and background?
– So that the terminal device driver knows where to send the terminal input and the terminalgenerated signals.
pid_t tcgetpgrp(int filedes);
Return the process group ID of the foreground process group associated with fieldes.
int tcsetpgrp(int filedes, pid_t pgrpid);
– If the process has control terminal, set the foreground process group ID to pgrpid.
– Pgrpid must be group ID in the same session.

• Job control
– Allows start multiple jobs from a single terminal and control which job can access the terminal.
– Foreground jobs can access terminal
– Background jobs may not:
• When a backgound job try to read, SIGTTIN signal is sent
• A background job must be able to output to the terminal
(options may be set by the stty command)
– See control.c for an example of switching terminal among groups.

• Orphaned process group:
– Parent of every member is either in the orphaned group or is not a member of the group’s session.
– Happens when a process forks a child and then dies.
• The child becomes a member of the orphaned group.
• Can have problems: the child may transform from a foreground process to a background process automatically.
– Remember in the control.c program, foreground group is set in both the parent and the child.
– Can be in an inconsistent state.
– If any IO is involved, strange things may happen.

• How to make sure that a shell program handles terminal I/O and signals correctly
– Create a new group for each job
• Both parent and child do setpgid
– For foreground job:
• After fork, shell set tcsetpgrp to give foreground jobs control over terminal
• Shell waits for all foreground processes in the foreground job to finish. After that, shell set tcsetpgrp to itself and print the prompt.
– For background job:
• Create a separate group so that processes in background jobs do not have access to terminal.

• What do tcgetpgrp and tcsetpgrp do?
• How are foreground and background processes different?
• How can a shell make sure that the signal is only sent to the foreground processes?