Process Relationships

1. Process Relationships • all processes have parent processes • parent’s 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 exec’ing 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 exec’ing 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 exec’er • process group ID’s 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 ’80’s • 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.