A child process (CP) in computing is a

process A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic. Things called a process include: Business and management * Business process, activities that produce a specific s ...

created by another process (the

parent process In computing, a parent process is a process that has created one or more child processes. Unix-like systems In Unix-like operating systems, every process except (the swapper) is created when another process executes the fork() system call. T ...

). This technique pertains to multitasking operating systems, and is sometimes called a subprocess or traditionally a subtask. There are two major procedures for creating a child process: the fork system call (preferred in

Unix-like A Unix-like (sometimes referred to as UN*X, *nix or *NIX) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Uni ...

systems and the

POSIX The Portable Operating System Interface (POSIX; ) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines application programming interfaces (APIs), along with comm ...

standard) and the

spawn Spawn or spawning may refer to: * Spawning, the eggs and sperm of aquatic animals Arts, entertainment and media * Spawn (character), a fictional character in the comic series of the same name and in the associated franchise ** ''Spawn: Armageddon' ...

(preferred in the modern (NT) kernel of

Microsoft Windows Windows is a Product lining, product line of Proprietary software, proprietary graphical user interface, graphical operating systems developed and marketed by Microsoft. It is grouped into families and subfamilies that cater to particular sec ...

, as well as in some historical operating systems).

History

Child processes date to the late 1960s, with an early form in later revisions of the Multiprogramming with a Fixed number of Tasks Version II (MFT-II) form of the IBM

OS/360 OS/360, officially known as IBM System/360 Operating System, is a discontinued batch processing operating system developed by IBM for their then-new System/360 mainframe computer, announced in 1964; it was influenced by the earlier IBSYS/IBJOB a ...

operating system, which introduced ''sub-tasking'' (see task). The current form in Unix draws on

Multics Multics ("MULTiplexed Information and Computing Service") is an influential early time-sharing operating system based on the concept of a single-level memory.Dennis M. Ritchie, "The Evolution of the Unix Time-sharing System", Communications of t ...

(1969), while the Windows NT form draws on

OpenVMS OpenVMS, often referred to as just VMS, is a multi-user, multiprocessing and virtual memory-based operating system. It is designed to support time-sharing, batch processing, transaction processing and workstation applications. Customers using Op ...

(1978), from

RSX-11 RSX-11 is a discontinued family of multi-user real-time operating systems for PDP-11 computers created by Digital Equipment Corporation. In widespread use through the late 1970s and early 1980s, RSX-11 was influential in the development of later ...

(1972).

Children created by fork

A child process inherits most of its attributes, such as

file descriptor In Unix and Unix-like computer operating systems, a file descriptor (FD, less frequently fildes) is a process-unique identifier (handle) for a file or other input/output resource, such as a pipe or network socket. File descriptors typically h ...

s, from its parent. In

Unix Unix (, ; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...

, a child process is typically created as a copy of the parent, using the

fork In cutlery or kitchenware, a fork (from 'pitchfork') is a utensil, now usually made of metal, whose long handle terminates in a head that branches into several narrow and often slightly curved tines with which one can spear foods either to h ...

system call. The child process can then overlay itself with a different program (using ) as required. Each process may create many child processes but will have at most one parent process; if a process does not have a parent this usually indicates that it was created directly by the

kernel Kernel may refer to: Computing * Kernel (operating system), the central component of most operating systems * Kernel (image processing), a matrix used for image convolution * Compute kernel, in GPGPU programming * Kernel method, in machine learnin ...

. In some systems, including

Linux Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...

-based systems, the very first process (called

init In Unix-based computer operating systems, init (short for ''initialization'') is the first process started during booting of the operating system. Init is a daemon process that continues running until the system is shut down. It is the direc ...

) is started by the kernel at

booting In computing, booting is the process of starting a computer as initiated via Computer hardware, hardware such as a physical button on the computer or by a software command. After it is switched on, a computer's central processing unit (CPU) h ...

time and never terminates (see

Linux startup process The Linux booting process involves multiple stages and is in many ways similar to the BSD and other Unix-style boot processes, from which it derives. Although the Linux booting process depends very much on the computer architecture, those architect ...

); other parentless processes may be launched to carry out various

daemon A demon is a malevolent supernatural being, evil spirit or fiend in religion, occultism, literature, fiction, mythology and folklore. Demon, daemon or dæmon may also refer to: Entertainment Fictional entities * Daemon (G.I. Joe), a character ...

tasks in

userspace A modern computer operating system usually uses virtual memory to provide separate address spaces or regions of a single address space, called user space and kernel space. This separation primarily provides memory protection and hardware prote ...

. Another way for a process to end up without a parent is if its parent dies, leaving an

orphan process An orphan process is a computer process whose parent process has finished or terminated, though it remains running itself. Unix-like In a Unix-like operating system any orphaned process will be immediately adopted by an implementation-defined s ...

; but in this case it will shortly be adopted by ''init''. The SIGCHLD

signal A signal is both the process and the result of transmission of data over some media accomplished by embedding some variation. Signals are important in multiple subject fields including signal processing, information theory and biology. In ...

is sent to the parent of a child process when it

exit Exit(s) may refer to: Architecture and engineering * Door * Portal (architecture), an opening in the walls of a structure * Emergency exit * Overwing exit, a type of emergency exit on an airplane * Exit ramp, a feature of a road interchange A ...

s, is interrupted, or resumes after being interrupted. By default the signal is simply ignored.

Children created by spawn

End of life

When a child process terminates, some information is returned to the parent process. When a child process terminates before the parent has called wait, the kernel retains some information about the process, such as its

exit status In computing, the exit status (also exit code or exit value) of a terminated process is an integer number that is made available to its parent process (or caller). In DOS, this may be referred to as an errorlevel. When computer programs ar ...

, to enable its parent to call ''wait'' later. Because the child is still consuming system resources but not executing it is known as a

zombie process On Unix and Unix-like computer operating systems, a zombie process or defunct process is a process that has completed execution (via the exit system call) but still has an entry in the process table: it is a process in the " terminated stat ...

. The ''wait'' system call is commonly invoked in the SIGCHLD handler.

POSIX.1-2001 The Portable Operating System Interface (POSIX; ) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines application programming interfaces (APIs), along with comma ...

allows a parent process to elect for the kernel to automatically reap child processes that terminate by explicitly setting the disposition of SIGCHLD to SIG_IGN (although ignore is the default, automatic reaping only occurs if the disposition is set to ignore explicitly), or by setting the SA_NOCLDWAIT flag for the SIGCHLD signal. Linux 2.6 kernels adhere to this behavior, and FreeBSD supports both of these methods since version 5.0. However, because of historical differences between

System V Unix System V (pronounced: "System Five") is one of the first commercial versions of the Unix operating system. It was originally developed by AT&T and first released in 1983. Four major versions of System V were released, numbered 1, 2, 3, an ...

and

BSD The Berkeley Software Distribution (BSD), also known as Berkeley Unix or BSD Unix, is a discontinued Unix operating system developed and distributed by the Computer Systems Research Group (CSRG) at the University of California, Berkeley, beginni ...

behaviors with regard to ignoring SIGCHLD, calling ''wait'' remains the most portable paradigm for cleaning up after forked child processes.

References

External links