/*
    ChibiOS/RT - Copyright (C) 2006,2007,2008,2009,2010 Giovanni Di Sirio.

    This file is part of ChibiOS/RT.

    ChibiOS/RT is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 3 of the License, or
    (at your option) any later version.

    ChibiOS/RT is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

/**
 * @page article_lifecycle Threads Lifecycle
 * In ChibiOS/RT threads are divided in two categories:
 * - <b>Static threads</b>. The memory used for static threads is allocated at
 *   compile time so static threads are always there, there is no management
 *   to be done.
 * - <b>Dynamic threads</b>. Dynamic threads are allocated at runtime from one
 *   of the available allocators (see @ref heaps, @ref pools).
 * .
 * Dynamic threads create the problem of who is responsible of releasing
 * their memory because a thread cannot dispose its own memory.<br>
 * This is handled in ChibiOS/RT through the mechanism of "thread references",
 * When the @p CH_USE_DYNAMIC option is enabled the threads become objects
 * with a reference counter. The memory of a thread, if dynamic, is freed
 * when the last reference to the thread is released while the thread is in
 * its @p THD_STATE_FINAL state.<br>
 * The following diagram explains the mechanism:
 * @dot
  digraph example {
    rankdir="LR";
    node [shape=circle, fontname=Helvetica, fontsize=8, fixedsize="true", width="0.75", height="0.75"];
    edge [fontname=Helvetica, fontsize=8];

    init [label="No thread", style="bold"];
    alive [label="Alive"];
    final [label="Terminated"];
    detached [label="Detached", style="bold"];

    init -> alive [label="chThdCreateX()"];
    alive -> alive [label="chThdAddRef()"];
    alive -> alive [label="chThdRelease()\n[ref > 0]"];
    alive -> detached [label="chThdRelease()\n[ref == 0]"];
    alive -> init [label="chThdWait()\n[ref == 0]"];
    alive -> final [label="chThdExit()\nreturn"];
    final -> final [label="chThdAddRef()"];
    final -> final [label="chThdRelease()\nchThdWait()\n[ref > 0]"];
    final -> init [label="chThdRelease()\nchThdWait()\n[ref == 0]"];
  }
 * @enddot
 * <br>
 * As you can see the easiest way to ensure that the memory is released is
 * to make another thread perform a @p chThdWait() on the dynamic thread.<br>
 * If all the references to the threads are released while the thread is
 * still alive then the thread goes in a "detached" state and its memory
 * cannot be recovered unless there is a dedicated task in the system that
 * scans the threads through the @ref registry subsystem, scanning the registry
 * has the side effect to release the zombies (detached and then terminated
 * threads).
 */