Children created by spawn
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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.
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).
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).
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.
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. [1]
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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. [2] 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 [3] ), 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. [4] 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. [5]
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 always first becomes a zombie before being removed from the resource table. In most cases, 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 generally an error and cause a resource leak, but the only resource they occupy is the process table entry – process ID.
In computing, a system call is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services, creation and execution of new processes, and communication with integral kernel services such as process scheduling. System calls provide an essential interface between a process and the operating system.
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, a fork bomb is a denial-of-service attack wherein a process continually replicates itself to deplete available system resources, slowing down or crashing the system due to resource starvation.
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.
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 computing, mmap(2) is a POSIX-compliant Unix system call that maps files or devices into memory. It is a method of memory-mapped file I/O. It implements demand paging because file contents are not immediately read from disk and initially use no physical RAM at all. The actual reads from disk are performed after a specific location is accessed, in a lazy manner. After the mapping is no longer needed, the pointers must be unmapped with munmap(2). Protection information—for example, marking mapped regions as executable—can be managed using mprotect(2), and special treatment can be enforced using madvise(2).
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.
In most Unix and Unix-like operating systems, the ps program displays the currently-running processes. A related Unix utility named top provides a real-time view of the running processes.
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, kill is a command that is used in several popular operating systems to send signals to running processes.
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 command-line 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).
On POSIX-compliant platforms, SIGHUP is a signal sent to a process when its controlling terminal is closed. It was originally designed to notify the process of a serial line drop. SIGHUP is a symbolic constant defined in the header file signal.h.
In Unix-based computer operating systems, init 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 direct or indirect ancestor of all other processes and automatically adopts all orphaned processes. Init is started by the kernel during the booting process; a kernel panic will occur if the kernel is unable to start it, or it should die for any reason. Init is typically assigned process identifier 1.
Fork–exec is a commonly used technique in Unix whereby an executing process spawns a 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.