Process group

In a POSIX-conformant operating system, a process group denotes a collection of one or more processes. Among other things, a process group is used to control the distribution of a signal; when a signal is directed to a process group, the signal is delivered to each process that is a member of the group.

Similarly, a session denotes a collection of one or more process groups. A process group is not permitted to migrate from one session to another, and a process may not create a process group that belongs to another session; furthermore, a process is not permitted to join a process group that is a member of another session—that is, a process is not permitted to migrate from one session to another.

When a process replaces its image with a new image (by calling one of the exec functions), the new image is subjected to the same process group (and thus session) membership as the old image.

The distribution of signals to process groups forms the basis of job control employed by shell programs. The tty device driver incorporates a notion of a foreground process group, to which it sends signals generated by keyboard interrupts, notably SIGINT ("interrupt", Control+C), SIGTSTP ("terminal stop", Control+Z), and SIGQUIT ("quit", Control+\). It also sends the SIGTTIN and SIGTTOU signals to any processes that attempt to read from or write to the terminal and that are not in the foreground process group. The shell, in turn, partitions the command pipelines that it creates into process groups, and controls what process group is the foreground process group of its controlling terminal, thus determining what processes (and thus what command pipelines) may perform I/O to and from the terminal at any given time.

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