NationMaster - Encyclopedia: Process group

In POSIX-conformant operating systems, a process group denotes a collection of one or more processes. The collection is identified by a positive integer, the process group ID, which is the process identifier of the process that is (or was) the process group leader. To meet Wikipedias quality standards, this article or section may require cleanup. ... An operating system (OS) is an essential software program that manages the hardware and software resources of a computer. ... In computing, a process is a running instance of a program, including all variables and other state. ... In computing, the process identifier (normally referred to as the process ID or just PID) is a number used by some operating system kernels (such as that of UNIX or Windows NT) to uniquely identify a process. ...

Process groups are used to control the distribution of signals. The kill system call is capable of directing signals either to individual processes or to process groups. A signal directed to a process group is delivered individually to all of the processes that are members of the group. A signal is an asynchronous event transmitted between one process and another. ... Wikibooks has more about this subject: kill In Unix and Unix-like operating systems, kill is a command used to send simple messages to processes running on the system. ... In computing, a system call is the mechanism used by an application program to request service from the operating system, or more specifically, the operating system kernel. ...

The distribution of signals to process groups in turn 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 the SIGTSTP, SIGQUIT, and SIGINT signals generated by keyboard interrupts. It also sends the SIGTTIN and SIGTTOU signals to any processes that attempt to read from (and, if appropriate flags are set for the terminal device, write to) the terminal 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. In computing, a shell is a piece of software that essentially provides a kind of interface for end-users. ... TTY is a short form of several things: Short form of Teletype, in turn short form of Teletypewriter. ... On POSIX compliant platforms, SIGTSTP is the symbolic name for the signal sent to a computer program by its controlling terminal when the user requests that it be suspended. ... SIGQUIT is a symbolic signal name used on POSIX compliant platforms. ... SIGINT stands for SIGnals INTelligence, which is intelligence-gathering by interception of signals, whether by radio interception or other means. ... On POSIX compliant platforms, SIGTTIN is the symbolic name for the signal thrown by computer programs that attempt to read from the tty while in the background. ... On POSIX compliant platforms, SIGTTOU is the symbolic name for the signal thrown by computer programs that attempt to write to the tty while in the background. ...

Process groups need not necessarily have leaders, although they always begin with one. POSIX mandates that processes may not accidentally become process group leaders. To this end, it prohibits the re-use of a process ID where a process group with that identifier still exists (i.e. where the leader of a process group has exited, but other processes in the group still exist). To become a process group leader, a process explicitly invokes the setpgid() or the setsid() system call.

The setpgid() system call is a general-purpose call for creating new or joining existing process groups; to become a process group leader using it, a process has to set its own process group to the same value as its own process ID, usually with the call setpgid(0,0). When the shell forks a new child process for a command pipeline, both the parent shell process and the child process immediately attempt to make the process into the leader of the process group for the command pipeline. (They both attempt to do the same thing in order to avoid a race condition between the child becoming the process group leader, the child executing the program image of the command being executed, and the parent, or the tty device driver, attempting to send signals to the process group for job control.) A fork, when applied to computing is when a process creates a copy of itself, which then acts as a child of the original process, now called the parent. More generally, a fork in a multithreading environment means that a thread of execution is duplicated. ... A child process is a computer process created by another process (the parent process). ... A race hazard (or race condition) is a flaw in a system or process where the output exhibits unexpected critical dependence on the relative timing of events. ...

Process groups are themselves grouped into sessions. Where a textual user interface is being used on a Unix-like system, sessions are used to implement login sessions. (This concept is largely lost, and the kernel's notion of sessions largely ignored, where where a graphical user interface is being used. Graphical user interfaces, such as where the X display manager is employed, use a different mechanism for implementing 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.) Processes are not permitted to join process groups that are not in the same session as they themselves are, process groups are not permitted to migrate from one session to another, and a process may only create new process groups belonging to the same session as it itself belongs to. A typical text terminal produces input and displays output and errors A text terminal or often just terminal (sometimes text console) is a serial computer interface for text entry and display. ... In computer science, the kernel is the core piece of most operating systems. ... A graphical user interface (or GUI, often pronounced gooey) is a method of interacting with a computer through a metaphor of direct manipulation of graphical images and widgets in addition to text. ... In the X Window System, an X display manager runs as a program, allowing starting a session on an X server from the same or another computer. ...

The setsid() system call is used to create a new session. If it succeeds, it always makes a process into a process group leader, because its purpose is to create a new session containing a single (new) process group, with the current process as both the leader of the session and of that single process group.

References

Single UNIX Specification, Issue 6

The Single UNIX Specification (SUS) is the collective name of a family of standards for computer operating systems to qualify for the name Unix. The SUS is developed and maintained by the Austin Group, based on earlier work by the IEEE and The Open Group. ...

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