Process groups, sessions, controlling terminal, and job control Process relationship: –Parent/child –Same group –Same session

Process groups, sessions, controlling terminal, and job control Process groups: –A process group is a collection of 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)

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 it self or its children _POSIX_JOB_CONTROL must be defined Most shells with job control create new group for each job (foreground or background).

Sessions –A session is one or more process groups proc1 | proc2 & proc3 | proc4 | proc5 – results in a session with three groups Login shell proc2 proc1 proc5 proc4 proc3

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, what must be done? »getty Fails if the caller if a group leader.

Sessions/groups A session can have a single controlling terminal. Usually the one that we log in. (the window and the keyboard that we can do IO). The session leader that establishes the connection to the control terminal is called the controlling process. There can be one foreground groups and any number of background groups in a session. Terminals’ interrupt signals are only sent to the processes in the foreground group. A process can open file /dev/tty to take to the controlling terminal regardless how standard IO are redirected.

How to make a group foreground and background? pid_t tcgetpgrp(int filedes); int tcsetpgrp(int filedes, pid_t 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) How shell execution programs –Shell without job control may redirect standard IO to /dev/null for background jobs

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. –If any IO is involved, strange things may happen. How should we modify the shell implementation to make sure the I/O and signals are correctly handled?

Answer: –Create a new 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.