linux-kernel - Re: Process id recycling and status of tasks

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Message-ID: <4AE165A4.70706@ladisch.de>
Date:	Fri, 23 Oct 2009 10:13:24 +0200
From:	Clemens Ladisch <clemens@...isch.de>
To:	"Leonidas ." <leonidas137@...il.com>
CC:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Process id recycling and status of tasks

Leonidas . wrote:
> 1. What would be an ideal way to check if a task is alive or dead from
> kernel space?

Get a reference to the task's pid (call get_task_pid(), or get_pid() on
the return value of task_pid()), then later check whether pid_task()
works.

(pid_alive() is not what you want because this would require that you
hold a reference to the task_struct.)

> even though a certain pid might exist, it might have got recycled by
> the time I check again, right?

The functions above do not work with PID numbers but with struct pid
which is a reference-counted object.  (The functions with "get" in their
name increase the reference count, so don't forget to put_pid() when you
no longer need it.)  See also the big comment in include/linux/pid.h.

HTH
Clemens
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