Editing Parent process (section)

== Orphan processes ==
{{Main|Orphan process}}
[[Orphan process]]es are an opposite situation to zombie processes, referring to the case in which a parent process terminates before its child processes, which are said to become "orphaned". Unlike the asynchronous child-to-parent notification that happens when a child process terminates (via the {{Mono|SIGCHLD}} signal), child processes are not notified immediately when their parent finishes. Instead, the system simply redefines the "parent PID" field in the child process's data to be the process that is the "ancestor" of every other process in the system, whose PID generally has the value of 1 (one), and whose name is traditionally "init" (except in the Linux kernel 3.4 and above [more info below]). Thus, it was said that init "adopts" every orphan process on the system.<ref>{{cite web |title=init comes first |url=https://www.tldp.org/LDP/sag/html/init-process.html}}</ref><ref>{{cite web |title=An overview of Linux processes |url=https://www.ibm.com/developerworks/community/blogs/58e72888-6340-46ac-b488-d31aa4058e9c/entry/an_overview_of_linux_processes21?lang=en |publisher=IBM |date=26 March 2012}}</ref>

A somewhat common assumption by programmers new to UNIX was that the child processes of a terminating process will be adopted by this process's immediate parent process (hence those child processes' "grandparent"). Such assumption was incorrect{{snd}} unless, of course, that "grandparent" was the init itself.

After Linux kernel 3.4 this is no longer true, in fact processes can issue the [http://man7.org/linux/man-pages/man2/prctl.2.html prctl()] system call with the PR_SET_CHILD_SUBREAPER option, and as a result they, not process #1, will become the parent of any of their orphaned descendant processes. This is the way of working of modern service managers and daemon supervision utilities including systemd, upstart, and the nosh service manager.

This is an abstract of the manual page, reporting that:
<blockquote>
A subreaper fulfills the role of init(1) for its descendant processes. When a process becomes orphaned (i.e., its immediate parent terminates) then that process will be reparented to the nearest still living ancestor subreaper. Subsequently, calls to getppid() in the orphaned process will now return the PID of the subreaper process, and when the orphan terminates, it is the subreaper process that will
 receive a SIGCHLD signal and will be able to wait(2) on the process to discover its termination status.<ref>{{cite web |title=Linux Programmer's Manual |url=http://man7.org/linux/man-pages/man2/prctl.2.html}}</ref>
</blockquote>