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Most Unix systems have historically used init as the system process to which orphans are reparented, but in modern DragonFly BSD, FreeBSD, and Linux systems, an orphan process may be reparented to a "subreaper" process instead of init. [1] [2] A process can be orphaned unintentionally, such as when the parent process terminates or crashes.
In Unix-like operating systems, every process except process 0 (the swapper) is created when another process executes the fork() system call. The process that invoked fork is the parent process and the newly created process is the child process. Every process (except process 0) has one parent process, but can have many child processes.
Version 7 Unix: /etc listing, showing init and rc Version 7 Unix: contents of an /etc/rc Bourne shell script. 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
If a child process receives a signal, a waiting parent will then continue execution leaving an orphan process behind. [ citation needed ] Hence it is sometimes needed to check the argument set by wait, waitpid or waitid and, in the case that WIFSIGNALED is true, wait for the child process again to deallocate resources.
However, the process's entry in the process table remains. The parent can read the child's exit status by executing the wait system call , whereupon the zombie is removed. The wait call may be executed in sequential code, but it is commonly executed in a handler for the SIGCHLD signal , which the parent receives whenever a child has died.
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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.
Such an orphan process becomes a child of a special root process, which then waits for the child process to terminate. Likewise, a similar strategy is used to deal with a zombie process, which is a child process that has terminated but whose exit status is ignored by its parent process. Such a process becomes the child of a special parent ...