Initial import

ChibiOS_2.0.2/docs/src/lifecycle.dox

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+/*
+    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/>.
+
+                                      ---
+
+    A special exception to the GPL can be applied should you wish to distribute
+    a combined work that includes ChibiOS/RT, without being obliged to provide
+    the source code for any proprietary components. See the file exception.txt
+    for full details of how and when the exception can be applied.
+*/
+
+/**
+ * @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).
+ */