Job Control

        The Topaz operating system provides so-called job control facilities for multiplexing a single terminal device among several jobs (groups of processes). These facilities are provided mainly for compatibility with Ultrix and its C shell (see csh(1)). In an environment with a window manager such as the Firefly, multiplexing a single terminal (i.e., window) is less important, but can still be useful. Since the C shell contains per-instance state in the form of variables and aliases, it can be useful to stop a process running under a given shell and type new commands to that same shell.

        Operating-system support for job control facilities comes in several parts: notably a way to stop and restart execution of a process, and a way to reserve a terminal device for use by one process group at a time. A command processor such as the C shell uses these facilities to grant control of a terminal to one `foreground' job at a time, and to move jobs between the `foreground' and `background'.

          To stop a process, send it a stop signal (e.g., SigTStp; see page ). To restart a stopped process, send it a continue signal (SigCont). While a process is stopped, none of its threads execute. However, intramachine remote procedure calls, including those to OS procedures, performed before the process was stopped keep executing up until the return. Normally a stop/start cycle need not and does not affect the state of a process. A process can if it desires perform `clean-up' computations before being stopped and after being restarted. To clean up beforehand, it handles SigTStp (and possibly SigTTOu and SigTTIn), performs its clean-up, and then sends itself SigStop. To clean up after being restarted, it handles SigCont.

            Stop and continue signals provide the basic mechanism for job control, but there needs to be a quick way for a user at a terminal to request that the current job be stopped as well as a mechanism to make sure that a background job doesn't interfere with the use of the terminal by the foreground job. For this reason, there is a distinguished process group associated with each terminal device. Only processes in that process group can read and write the terminal; a background process (one not in the group), will normally receive a SigTTIn or SigTTOu signal if it tries to read or write its control terminal (see Read and Write starting on page for more details). Also, the terminal has a mode such that when the user types a particular key (normally Control-Z), the terminal sends a SigTStp signal to the distinguished process group (see the IOCtl interface and tty(4)).

There are a few more details relevant to job control. A command interpreter process implementing job control needs to find out when the current foreground job stops; it uses the procedure WaitForChild (see page ) to find out when a specific child process has stopped or terminated. The command interpreter needs to reset the distinguished process group of the control terminal to itself or to a new foreground job; it uses SetDevPGRP for this (see page ).