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 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
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",), SIGTSTP ("terminal stop",), and SIGQUIT ("quit",).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.
When the shell
Where a textual user interface is being used on a Unix-like system, sessions are used to implement login sessions.A single process, the session leader, interacts with the controlling terminal in order to ensure that all programs are terminated when a user "hangs up" the terminal connection.(Where a session leader is absent, the processes in the terminal's foreground process group are expected to handle hangups.)
Where a graphical user interface is being used, the session concept is largely lost, and the kernel's notion of sessions largely ignored.Graphical user interfaces, such as where the X display manager is employed, use a different mechanism for implementing login sessions.
The system call
The system call
The system call