Child process

A child process in computing is a process created by another process (the parent process).

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 Unix based systems such as Linux, 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]

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.^[2][5]

See also

• Exit
• pstree, for UNIX to find the child process (pstree PID, where PID is the process id of the process).^[6]

References

1. ↑ signal(7): overview of signals – Linux Conventions and Miscellany Manual
2. ↑ 2.0 2.1 wait(2): wait for process to change state – Linux System Calls Manual
3. ↑ http://www.win.tue.nl/~aeb/linux/lk/lk-5.html#ss5.5
4. ↑ http://fuse4bsd.creo.hu/localcgi/man-cgi.cgi?signal+3
5. ↑ sigaction(3): examine and change a signal action – Linux Library Functions Manual
6. ↑ pstree(1): print process trees – Linux User Commands Manual

This article is based on material taken from 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.