A defunct (or zombie) process is one whose exit status has yet to be reaped by its parent. The exit status is reaped via the wait(2), waitid(2), or waitpid(2) system call. In the normal course of system operation, zombies may occur, but are typically short-lived. This may happen if a parent exits without having reaped the exit status of some or all of its children. In that case, those children are reparented to PID 1. See init(1M), which periodically reaps such processes.

An irresponsible parent process may not exit for a very long time and thus leave zombies on the system. Since the operating system destroys nearly all components of a process before it becomes defunct, such defunct processes do not normally impact system operation. However, they do consume a small amount of system memory.

preap forces the parent of the process specified by pid to waitid(2) for pid, if pid represents a defunct process.

preap will attempt to prevent the administrator from unwisely reaping a child process which might soon be reaped by the parent, if:

• The process is a child of init(1m).
• The parent process is stopped and might wait on the child when it is again allowed to run.
• The process has been defunct for less than one minute.

The following option is supported:

-F

Forces the parent to reap the child, overriding safety checks.

The following operand is supported:

pid

Process ID list.

The following exit values are returned by preap, which prints the exit status of each target process reaped:

0

Successfully operation.

non-zero

Failure, such as no such process, permission denied, or invalid option.

See attributes(5) for descriptions of the following attributes:

proc(1), init(1M), wait(2), waitid(2), waitpid(2), proc(4), attributes(5)

preap should be applied sparingly and only in situations in which the administrator or developer has confirmed that defunct processes will not be reaped by the parent process. Otherwise, applying preap may damage the parent process in unpredictable ways.