Child process

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A child process in computing is a process created by another process (the parent process). This technique pertains to multitasking operating systems, and is sometimes called a subprocess or traditionally a subtask.

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There are two major procedures for creating a child process: the fork system call (preferred in Unix-like systems and the POSIX standard) and the spawn (preferred in the modern (NT) kernel of Microsoft Windows, 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 operating system, which introduced sub-tasking (see task). The current form in Unix draws on Multics (1969), while the Windows NT form draws on OpenVMS (1978), from RSX-11 (1972).

Children created by fork

A child process inherits most of its attributes, such as file descriptors, from its parent. In Unix, a child process is typically created as a copy of the parent, using the fork system call. The child process can then overlay itself with a different program (using exec ) as required. [1]

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. In some systems, including Linux-based systems, the very first process (called init) is started by the kernel at booting time and never terminates (see Linux startup process); other parentless processes may be launched to carry out various daemon tasks in userspace. Another way for a process to end up without a parent is if its parent dies, leaving an orphan process; but in this case it will shortly be adopted by init.

The SIGCHLD signal is sent to the parent of a child process when it exits, is interrupted, or resumes after being interrupted. By default the signal is simply ignored. [2]

Children created by spawn

In computing, children created by spawn refers to child processes that are generated by a parent process using the spawn function. The term spawn is used in some programming languages and operating systems to describe the creation of a new process, similar to forking in Unix-based systems, but typically offering more flexibility in the handling of input/output, environment variables, and file descriptors.[ citation needed ] When a process spawns a child, it creates a new, independent process that runs concurrently with the parent. This process is referred to as a child process, and the original process that called the spawn function is the parent process. These child processes can perform different tasks and are commonly used in multitasking environments to enhance performance or handle separate tasks simultaneously. [3]

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, to enable its parent to call wait later. [4] Because the child is still consuming system resources but not executing it is known as a zombie process. The wait system call is commonly invoked in the SIGCHLD handler.

POSIX.1-2001 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 [5] ), 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. [6] However, because of historical differences between System V and BSD behaviors with regard to ignoring SIGCHLD, calling wait remains the most portable paradigm for cleaning up after forked child processes. [7]

See also

Related Research Articles

On Unix and Unix-like computer operating systems, a zombie process or defunct process is a process that has completed execution but still has an entry in the process table: it is a process in the "terminated state". This occurs for the child processes, where the entry is still needed to allow the parent process to read its child's exit status: once the exit status is read via the wait system call, the zombie's entry is removed from the process table and it is said to be "reaped". A child process initially becomes a zombie, only then being removed from the resource table. Under normal system operation, zombies are immediately waited on by their parent and then reaped by the system. Processes that stay zombies for a long time are usually an error and can cause a resource leak. Generally, the only kernel resource they occupy is the process table entry, their process ID. However, zombies can also hold buffers open, consuming memory. Zombies can hold handles to file descriptors, which prevents the space for those files from being available to the filesystem. This effect can be seen by a difference between du and df. While du may show a large amount of free disk space, df will show a full partition. If the zombies are not cleaned, this can fill the root partition and crash the system.

<span class="mw-page-title-main">Process (computing)</span> Particular execution of a computer program

In computing, a process is the instance of a computer program that is being executed by one or many threads. There are many different process models, some of which are light weight, but almost all processes are rooted in an operating system (OS) process which comprises the program code, assigned system resources, physical and logical access permissions, and data structures to initiate, control and coordinate execution activity. Depending on the OS, a process may be made up of multiple threads of execution that execute instructions concurrently.

A background process is a computer process that runs behind the scenes and without user intervention. Typical tasks for these processes include logging, system monitoring, scheduling, and user notification.

In computing, particularly in the context of the Unix operating system and its workalikes, fork is an operation whereby a process creates a copy of itself. It is an interface which is required for compliance with the POSIX and Single UNIX Specification standards. It is usually implemented as a C standard library wrapper to the fork, clone, or other system calls of the kernel. Fork is the primary method of process creation on Unix-like operating systems.

In computing, a parent process is a process that has created one or more child processes.

In computing, the process identifier is a number used by most operating system kernels—such as those of Unix, macOS and Windows—to uniquely identify an active process. This number may be used as a parameter in various function calls, allowing processes to be manipulated, such as adjusting the process's priority or killing it altogether.

<span class="mw-page-title-main">Daemon (computing)</span> Computer program that runs as a background process

In multitasking computer operating systems, a daemon is a computer program that runs as a background process, rather than being under the direct control of an interactive user. Traditionally, the process names of a daemon end with the letter d, for clarification that the process is in fact a daemon, and for differentiation between a daemon and a normal computer program. For example, syslogd is a daemon that implements system logging facility, and sshd is a daemon that serves incoming SSH connections.

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

An orphan process is a computer process whose parent process has finished or terminated, though it remains running itself.

Signals are standardized messages sent to a running program to trigger specific behavior, such as quitting or error handling. They are a limited form of inter-process communication (IPC), typically used in Unix, Unix-like, and other POSIX-compliant operating systems.

In computing, the exit status of a terminated process is an integer number that is made available to its parent process. In DOS, this may be referred to as an errorlevel.

File locking is a mechanism that restricts access to a computer file, or to a region of a file, by allowing only one user or process to modify or delete it at a specific time and to prevent reading of the file while it's being modified or deleted.

Unix-like operating systems identify a user by a value called a user identifier, often abbreviated to user ID or UID. The UID, along with the group identifier (GID) and other access control criteria, is used to determine which system resources a user can access. The password file maps textual user names to UIDs. UIDs are stored in the inodes of the Unix file system, running processes, tar archives, and the now-obsolete Network Information Service. In POSIX-compliant environments, the shell command id gives the current user's UID, as well as more information such as the user name, primary user group and group identifier (GID).

In computing, sigaction is a function API defined by POSIX to give the programmer access to what a program's behavior should be when receiving specific OS signals.

Fork–exec is a commonly used technique in Unix whereby an executing process spawns a new program.

In computing, exec is a functionality of an operating system that runs an executable file in the context of an already existing process, replacing the previous executable. This act is also referred to as an overlay. It is especially important in Unix-like systems, although it also exists elsewhere. As no new process is created, the process identifier (PID) does not change, but the machine code, data, heap, and stack of the process are replaced by those of the new program.

In Unix and Unix-like operating systems, job control refers to control of jobs by a shell, especially interactively, where a "job" is a shell's representation for a process group. Basic job control features are the suspending, resuming, or terminating of all processes in the job/process group; more advanced features can be performed by sending signals to the job. Job control is of particular interest in Unix due to its multiprocessing, and should be distinguished from job control generally, which is frequently applied to sequential execution.

In computer operating systems, a process may wait for another process to complete its execution. In most systems, a parent process can create an independently executing child process. The parent process may then issue a wait system call, which suspends the execution of the parent process while the child executes. When the child process terminates, it returns an exit status to the operating system, which is then returned to the waiting parent process. The parent process then resumes execution.

ptrace is a system call found in Unix and several Unix-like operating systems. By using ptrace one process can control another, enabling the controller to inspect and manipulate the internal state of its target. ptrace is used by debuggers and other code-analysis tools, mostly as aids to software development.

In Unix-like operating systems, dup and dup2 system calls create a copy of a given file descriptor. This new descriptor actually does not behave like a copy, but like an alias of the old one.

References

  1. ↑ This article is based on material taken from Child+process at the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.
  2. ↑ signal.h  – Base Definitions Reference, The Single UNIX Specification , Version 4 from The Open Group
  3. ↑ "Child process | Node.js v22.8.0 Documentation". nodejs.org. Retrieved 2024-09-15.
  4. ↑ wait(2) : wait for process to change state –  Linux Programmer's Manual – System Calls
  5. ↑ "The Linux kernel: Signals". Win.tue.nl. Retrieved 2014-04-30.
  6. ↑ Archived September 29, 2011, at the Wayback Machine
  7. ↑ sigaction(2) : examine and change a signal action –  Linux Programmer's Manual – System Calls
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