Process group
Encyclopedia
In POSIX
-conformant operating system
s, a process group denotes a collection of one or more process
es. Process groups are used to control the distribution of signal
s. A signal directed to a process group is delivered individually to all of the processes that are members of the group.
Process groups are themselves grouped into sessions. 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. Processes are not permitted to join process groups that are not in the same session as they themselves are.
New process images created by a call to a function of the
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 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
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.)
Where a textual user interface is being used on a Unix-like system, sessions are used to implement login session
s. 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.
of the process that is (or was) the process group leader. Process groups need not necessarily have leaders, although they always begin with one. Sessions are identified by the process group ID of the session leader. POSIX prohibits the change of the process group ID of a session leader.
The system call
The system call
POSIX
POSIX , an acronym for "Portable Operating System Interface", is a family of standards specified by the IEEE for maintaining compatibility between operating systems...
-conformant operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s, a process group denotes a collection of one or more process
Process (computing)
In computing, a process is an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system , a process may be made up of multiple threads of execution that execute instructions concurrently.A computer program is a...
es. Process groups are used to control the distribution of signal
Signal (computing)
A signal is a limited form of inter-process communication used in Unix, Unix-like, and other POSIX-compliant operating systems. Essentially it is an asynchronous notification sent to a process in order to notify it of an event that occurred. When a signal is sent to a process, the operating system...
s. A signal directed to a process group is delivered individually to all of the processes that are members of the group.
Process groups are themselves grouped into sessions. 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. Processes are not permitted to join process groups that are not in the same session as they themselves are.
New process images created by a call to a function of the
execExec (operating system)The exec collection of functions of Unix-like operating systems cause the running process to be completely replaced by the program passed as an argument to the function...
family inherit the process group membership and the session membership of the old process image.Applications
The distribution of signals to process groups forms the basis of job control employed by shellShell (computing)
A shell is a piece of software that provides an interface for users of an operating system which provides access to the services of a kernel. However, the term is also applied very loosely to applications and may include any software that is "built around" a particular component, such as web...
programs. The tty
Computer terminal
A computer terminal is an electronic or electromechanical hardware device that is used for entering data into, and displaying data from, a computer or a computing system...
device driver incorporates a notion of a foreground process group, to which it sends the SIGTSTP
SIGTSTP
SIGTSTP is a signal in a Unix computer system that tells a program to stop temporarily. On POSIX-compliant platforms, SIGTSTP is the signal sent to a process by its controlling terminal when the user requests that the process be suspended. The symbolic constant for SIGTSTP is defined in the header...
, SIGQUIT
SIGQUIT
On POSIX-compliant platforms, SIGQUIT is the signal sent to a process by its controlling terminal when the user requests that the process perform a core dump. The symbolic constant for SIGQUIT is defined in the header file signal.h; on the vast majority of systems it is signal #3.SIGQUIT can...
, and SIGINT
SIGINT (POSIX)
On POSIX-compliant platforms, SIGINT is the signal sent to a process by its controlling terminal when a user wishes to interrupt the process. In source code, SIGINT is a symbolic constant defined in the header file signal.h...
signals generated by keyboard interrupts
Keyboard controller (computing)
In computing, a keyboard controller is a device which interfaces a keyboard to a computer. Its main function is to inform the computer when a key is pressed or released...
. It also sends the SIGTTIN
SIGTTIN
On POSIX-compliant platforms, SIGTTIN is the signal sent to a process when it attempts to read from the tty while in the background. The symbolic constant for SIGTTIN is defined in the header file signal.h. Symbolic signal names are used because signal numbers can vary across...
and SIGTTOU
SIGTTOU
On POSIX-compliant platforms, SIGTTOU is the signal sent to a process when it attempts to write to the tty while in the background. The symbolic constant for SIGTTOU is defined in the header file signal.h. Symbolic signal names are used because signal numbers can vary across...
signals to any processes that attempt to read from (and, if appropriate flags are set for the terminal device, write to) the terminal and that are not in the foreground process group. The shell, in turn, partitions the command pipelines
Pipeline (Unix)
In Unix-like computer operating systems , a pipeline is the original software pipeline: a set of processes chained by their standard streams, so that the output of each process feeds directly as input to the next one. Each connection is implemented by an anonymous pipe...
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
forkFork (operating system)In computing, when a process forks, it creates a copy of itself. More generally, a fork in a multithreading environment means that a thread of execution is duplicated, creating a child thread from the parent thread....
s a new child process for a command pipeline, both the parent shell process and the child processChild process
A child process in computing is a process created by another process .A child process inherits most of its attributes, such as open files, from its parent. In UNIX, a child process is in fact created as a copy of the parent...
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
Race condition
A race condition or race hazard is a flaw in an electronic system or process whereby the output or result of the process is unexpectedly and critically dependent on the sequence or timing of other events...
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.)
Where a textual user interface is being used on a Unix-like system, sessions are used to implement login session
Login session
In computing, a login session is the period of activity between a user logging in and logging out of a system.On Unix and Unix-like operating systems, a login session takes one of two main forms:...
s. 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
Graphical user interface
In computing, a graphical user interface is a type of user interface that allows users to interact with electronic devices with images rather than text commands. GUIs can be used in computers, hand-held devices such as MP3 players, portable media players or gaming devices, household appliances and...
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.
Details
The system callSystem call
In computing, a system call is how a program requests a service from an operating system's kernel. This may include hardware related services , creating and executing new processes, and communicating with integral kernel services...
setsid is used to create a new session containing a single (new) process group, with the current process as both the session leader and the process group leader of that single process group. Process groups are identified by a positive integer, the process group ID, which is the process identifierProcess identifier
In computing, the process identifier is a number used by most operating system kernels to uniquely identify a process...
of the process that is (or was) the process group leader. Process groups need not necessarily have leaders, although they always begin with one. Sessions are identified by the process group ID of the session leader. POSIX prohibits the change of the process group ID of a session leader.
The system call
setpgid is used to set the process group ID of a process, thereby either joining the process to an existing process group, or creating a new process group within the session of the process with the process becoming the process group leader of the newly created group. POSIX 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). It thereby guarantees that processes may not accidentally become process group leaders.The system call
killKill (Unix)In computing, kill is a command that is used in several popular operating systems to send signals to running processes, for example to request the termination of this process.-Unix and Unix-like:...
is capable of directing signals either to individual processes or to process groups.