+<strong><em>json_object_get_string(json_object_object_get(afb_req_json(req), name))</em></strong></p></blockquote>
+
+<a name="Initialisation.of.the.plugin.and.declaration.of.verbs"></a>
+<h2>Initialisation of the plugin and declaration of verbs</h2>
+
+<p>To be active, the verbs of the plugin should be declared to
+afb-daemon. And even more, the plugin itself must be recorded.</p>
+
+<p>The mechanism for doing this is very simple: when afb-need starts,
+it loads the plugins that are listed in its argument or configuration.</p>
+
+<p>Loading a plugin follows the following steps:</p>
+
+<ol>
+<li><p>It loads the plugin using <em>dlopen</em>.</p></li>
+<li><p>It searchs for the symbol named <strong>pluginAfbV1Register</strong> using <em>dlsym</em>.
+This symbol is assumed to be the exported initialisation function of the plugin.</p></li>
+<li><p>It build an interface object for the plugin.</p></li>
+<li><p>It calls the found function <strong>pluginAfbV1Register</strong> and pass it the pointer
+to its interface.</p></li>
+<li><p>The function <strong>pluginAfbV1Register</strong> setup the plugin, initialize it.</p></li>
+<li><p>The function <strong>pluginAfbV1Register</strong> returns the pointer to a structure
+that describes the plugin: its version, its name (prefix or API name), and the
+list of its verbs.</p></li>
+<li><p>Afb-daemon checks that the returned version and name can be managed.
+If it can manage it, the plugin and its verbs are recorded and can be used
+when afb-daemon finishes it initialisation.</p></li>
+</ol>
+
+
+<p>Here is the listing of the function <strong>pluginAfbV1Register</strong> of the plugin
+<em>tic-tac-toe</em>:</p>
+
+<pre><code>/*
+ * activation function for registering the plugin called by afb-daemon
+ */
+const struct AFB_plugin *pluginAfbV1Register(const struct AFB_interface *itf)
+{
+ afbitf = itf; // records the interface for accessing afb-daemon
+ return &plugin_description; // returns the description of the plugin
+}
+</code></pre>
+
+<p>This is a very small function because the <em>tic-tac-toe</em> plugin doesn’t have initialisation step.
+It merely record the daemon’s interface and returns its descritption.</p>
+
+<p>The variable <strong>afbitf</strong> is a variable global to the plugin. It records the
+interface to afb-daemon and is used for logging and pushing events.
+Here is its declaration:</p>
+
+<pre><code>/*
+ * the interface to afb-daemon
+ */
+const struct AFB_interface *afbitf;
+</code></pre>
+
+<p>The description of the plugin is defined as below.</p>
+
+<pre><code>/*
+ * array of the verbs exported to afb-daemon
+ */
+static const struct AFB_verb_desc_v1 plugin_verbs[] = {
+ /* VERB'S NAME SESSION MANAGEMENT FUNCTION TO CALL SHORT DESCRIPTION */
+ { .name= "new", .session= AFB_SESSION_NONE, .callback= new, .info= "Starts a new game" },
+ { .name= "play", .session= AFB_SESSION_NONE, .callback= play, .info= "Tells the server to play" },
+ { .name= "move", .session= AFB_SESSION_NONE, .callback= move, .info= "Tells the client move" },
+ { .name= "board", .session= AFB_SESSION_NONE, .callback= board, .info= "Get the current board" },
+ { .name= "level", .session= AFB_SESSION_NONE, .callback= level, .info= "Set the server level" },
+ { .name= "join", .session= AFB_SESSION_CHECK,.callback= join, .info= "Join a board" },
+ { .name= "undo", .session= AFB_SESSION_NONE, .callback= undo, .info= "Undo the last move" },
+ { .name= "wait", .session= AFB_SESSION_NONE, .callback= wait, .info= "Wait for a change" },
+ { .name= NULL } /* marker for end of the array */
+};
+
+/*
+ * description of the plugin for afb-daemon
+ */
+static const struct AFB_plugin plugin_description =
+{
+ /* description conforms to VERSION 1 */
+ .type= AFB_PLUGIN_VERSION_1,
+ .v1= { /* fills the v1 field of the union when AFB_PLUGIN_VERSION_1 */
+ .prefix= "tictactoe", /* the API name (or plugin name or prefix) */
+ .info= "Sample tac-tac-toe game", /* short description of of the plugin */
+ .verbs = plugin_verbs /* the array describing the verbs of the API */
+ }
+};
+</code></pre>
+
+<p>The structure <strong>plugin_description</strong> describes the plugin.
+It declares the type and version of the plugin, its name, a description
+and a list of its verbs.</p>
+
+<p>The list of verbs is an array of structures describing the verbs and terminated by a marker:
+a verb whose name is NULL.</p>
+
+<p>The description of the verbs for this version is made of 4 fields:</p>
+
+<ul>
+<li><p>the name of the verbs,</p></li>
+<li><p>the session management flags,</p></li>
+<li><p>the implementation function to be call for the verb,</p></li>
+<li><p>a short description.</p></li>
+</ul>
+
+
+<p>The structure describing verbs is defined as follows:</p>
+
+<pre><code>/*
+ * Description of one verb of the API provided by the plugin
+ * This enumeration is valid for plugins of type 1
+ */
+struct AFB_verb_desc_v1
+{
+ const char *name; /* name of the verb */
+ enum AFB_session_v1 session; /* authorisation and session requirements of the verb */
+ void (*callback)(struct afb_req req); /* callback function implementing the verb */
+ const char *info; /* textual description of the verb */
+};
+</code></pre>
+
+<p>For technical reasons, the enumeration <strong>enum AFB_session_v1</strong> is not exactly an
+enumeration but the wrapper of constant definitions that can be mixed using bitwise or
+(the C operator |).</p>
+
+<p>The constants that can bit mixed are:</p>
+
+<table>
+<thead>
+<tr>
+<th>Constant name </th>
+<th> Meaning</th>
+</tr>
+</thead>
+<tbody>
+<tr>
+<td><strong>AFB_SESSION_CREATE</strong> </td>
+<td> Equals to AFB_SESSION_LOA_EQ_0|AFB_SESSION_RENEW</td>
+</tr>
+<tr>
+<td><strong>AFB_SESSION_CLOSE</strong> </td>
+<td> Closes the session after the reply and set the LOA to 0</td>
+</tr>
+<tr>
+<td><strong>AFB_SESSION_RENEW</strong> </td>
+<td> Refreshes the token of authentification</td>
+</tr>
+<tr>
+<td><strong>AFB_SESSION_CHECK</strong> </td>
+<td> Just requires the token authentification</td>
+</tr>
+<tr>
+<td><strong>AFB_SESSION_LOA_LE_0</strong> </td>
+<td> Requires the current LOA to be lesser then or equal to 0</td>
+</tr>
+<tr>
+<td><strong>AFB_SESSION_LOA_LE_1</strong> </td>
+<td> Requires the current LOA to be lesser then or equal to 1</td>
+</tr>
+<tr>
+<td><strong>AFB_SESSION_LOA_LE_2</strong> </td>
+<td> Requires the current LOA to be lesser then or equal to 2</td>
+</tr>
+<tr>
+<td><strong>AFB_SESSION_LOA_LE_3</strong> </td>
+<td> Requires the current LOA to be lesser then or equal to 3</td>
+</tr>
+<tr>
+<td><strong>AFB_SESSION_LOA_GE_0</strong> </td>
+<td> Requires the current LOA to be greater then or equal to 0</td>
+</tr>
+<tr>
+<td><strong>AFB_SESSION_LOA_GE_1</strong> </td>
+<td> Requires the current LOA to be greater then or equal to 1</td>
+</tr>
+<tr>
+<td><strong>AFB_SESSION_LOA_GE_2</strong> </td>
+<td> Requires the current LOA to be greater then or equal to 2</td>
+</tr>
+<tr>
+<td><strong>AFB_SESSION_LOA_GE_3</strong> </td>
+<td> Requires the current LOA to be greater then or equal to 3</td>
+</tr>
+<tr>
+<td><strong>AFB_SESSION_LOA_EQ_0</strong> </td>
+<td> Requires the current LOA to be equal to 0</td>
+</tr>
+<tr>
+<td><strong>AFB_SESSION_LOA_EQ_1</strong> </td>
+<td> Requires the current LOA to be equal to 1</td>
+</tr>
+<tr>
+<td><strong>AFB_SESSION_LOA_EQ_2</strong> </td>
+<td> Requires the current LOA to be equal to 2</td>
+</tr>
+<tr>
+<td><strong>AFB_SESSION_LOA_EQ_3</strong> </td>
+<td> Requires the current LOA to be equal to 3</td>
+</tr>
+</tbody>
+</table>
+
+
+<p>If any of this flags is set, afb-daemon requires the token authentification
+as if the flag <strong>AFB_SESSION_CHECK</strong> had been set.</p>
+
+<p>The special value <strong>AFB_SESSION_NONE</strong> is zero and can be used to avoid any check.</p>
+
+<blockquote><p>Note that <strong>AFB_SESSION_CREATE</strong> and <strong>AFB_SESSION_CLOSE</strong> might be removed in later versions.</p></blockquote>
+
+<a name="Sending.messages.to.the.log.system"></a>
+<h2>Sending messages to the log system</h2>
+
+<p>Afb-daemon provides 4 levels of verbosity and 5 verbs for logging messages.</p>
+
+<p>The verbosity is managed. Options allow the change the verbosity of afb-daemon
+and the verbosity of the plugins can be set plugin by plugin.</p>
+
+<p>The verbs for logging messages are defined as macros that test the
+verbosity level and that call the real logging function only if the
+message must be output. This avoid evaluation of arguments of the
+formatting messages if the message must not be output.</p>
+
+<a name="Verbs.for.logging.messages"></a>
+<h3>Verbs for logging messages</h3>
+
+<p>The 5 logging verbs are:</p>
+
+<table>
+<thead>
+<tr>
+<th>Macro </th>
+<th style="text-align:center;"> Verbosity </th>
+<th> Meaning </th>
+<th style="text-align:center;"> syslog level</th>
+</tr>
+</thead>
+<tbody>
+<tr>
+<td>ERROR </td>
+<td style="text-align:center;"> 0 </td>
+<td> Error conditions </td>
+<td style="text-align:center;"> 3</td>
+</tr>
+<tr>
+<td>WARNING </td>
+<td style="text-align:center;"> 1 </td>
+<td> Warning conditions </td>
+<td style="text-align:center;"> 4</td>
+</tr>
+<tr>
+<td>NOTICE </td>
+<td style="text-align:center;"> 1 </td>
+<td> Normal but significant condition </td>
+<td style="text-align:center;"> 5</td>
+</tr>
+<tr>
+<td>INFO </td>
+<td style="text-align:center;"> 2 </td>
+<td> Informational </td>
+<td style="text-align:center;"> 6</td>
+</tr>
+<tr>
+<td>DEBUG </td>
+<td style="text-align:center;"> 3 </td>
+<td> Debug-level messages </td>
+<td style="text-align:center;"> 7</td>
+</tr>
+</tbody>
+</table>
+
+
+<p>You can note that the 2 verbs <strong>WARNING</strong> and <strong>INFO</strong> have the same level
+of verbosity. But they don’t have the same <em>syslog level</em>. It means that
+they are output with a different level on the logging system.</p>
+
+<p>All of these verbs have the same signature:</p>
+
+<pre><code>void ERROR(const struct AFB_interface *afbitf, const char *message, ...);
+</code></pre>
+
+<p>The first argument <strong>afbitf</strong> is the interface to afb daemon that the
+plugin received at its initialisation when <strong>pluginAfbV1Register</strong> was called.</p>
+
+<p>The second argument <strong>message</strong> is a formatting string compatible with printf/sprintf.</p>
+
+<p>The remaining arguments are arguments of the formating message like for printf.</p>
+
+<a name="Managing.verbosity"></a>
+<h3>Managing verbosity</h3>
+
+<p>Depending on the level of verbosity, the messages are output or not.
+The following table explains what messages will be output depending
+ont the verbosity level.</p>
+
+<table>
+<thead>
+<tr>
+<th style="text-align:center;">Level of verbosity </th>
+<th> Outputed macro</th>
+</tr>
+</thead>
+<tbody>
+<tr>
+<td style="text-align:center;">0 </td>
+<td> ERROR</td>
+</tr>
+<tr>
+<td style="text-align:center;">1 </td>
+<td> ERROR + WARNING + NOTICE</td>
+</tr>
+<tr>
+<td style="text-align:center;">2 </td>
+<td> ERROR + WARNING + NOTICE + INFO</td>
+</tr>
+<tr>
+<td style="text-align:center;">3 </td>
+<td> ERROR + WARNING + NOTICE + INFO + DEBUG</td>
+</tr>
+</tbody>
+</table>
+
+
+<a name="Output.format.and.destination"></a>
+<h3>Output format and destination</h3>
+
+<p>The syslog level is used for forging a prefix to the message.
+The prefixes are:</p>
+
+<table>
+<thead>
+<tr>
+<th style="text-align:center;">syslog level </th>
+<th> prefix</th>
+</tr>
+</thead>
+<tbody>
+<tr>
+<td style="text-align:center;">0 </td>
+<td> <0> EMERGENCY</td>
+</tr>
+<tr>
+<td style="text-align:center;">1 </td>
+<td> <1> ALERT</td>
+</tr>
+<tr>
+<td style="text-align:center;">2 </td>
+<td> <2> CRITICAL</td>
+</tr>
+<tr>
+<td style="text-align:center;">3 </td>
+<td> <3> ERROR</td>
+</tr>
+<tr>
+<td style="text-align:center;">4 </td>
+<td> <4> WARNING</td>
+</tr>
+<tr>
+<td style="text-align:center;">5 </td>
+<td> <5> NOTICE</td>
+</tr>
+<tr>
+<td style="text-align:center;">6 </td>
+<td> <6> INFO</td>
+</tr>
+<tr>
+<td style="text-align:center;">7 </td>
+<td> <7> DEBUG</td>
+</tr>
+</tbody>
+</table>
+
+
+<p>The message is issued to the standard error.
+The final destination of the message depends on how the systemd service
+was configured through the variable <strong>StandardError</strong>: It can be
+journal, syslog or kmsg. (See man sd-daemon).</p>
+
+<a name="Sending.events"></a>
+<h2>Sending events</h2>
+
+<a name="Writing.an.asynchronous.verb.implementation"></a>
+<h2>Writing an asynchronous verb implementation</h2>
+
+<a name="How.to.build.a.plugin"></a>
+<h2>How to build a plugin</h2>
+
+<p>Afb-daemon provides a <em>pkg-config</em> configuration file that can be
+queried by the name <strong>afb-daemon</strong>.
+This configuration file provides data that should be used
+for compiling plugins. Examples:</p>
+
+<pre><code>$ pkg-config --cflags afb-daemon
+$ pkg-config --libs afb-daemon
+</code></pre>
+
+<a name="Example.for.cmake.meta.build.system"></a>
+<h3>Example for cmake meta build system</h3>
+
+<p>This example is the extract for building the plugin <em>afm-main</em> using <em>CMAKE</em>.</p>
+
+<pre><code>pkg_check_modules(afb afb-daemon)
+if(afb_FOUND)
+ message(STATUS "Creation afm-main-plugin for AFB-DAEMON")
+ add_library(afm-main-plugin MODULE afm-main-plugin.c)
+ target_compile_options(afm-main-plugin PRIVATE ${afb_CFLAGS})
+ target_include_directories(afm-main-plugin PRIVATE ${afb_INCLUDE_DIRS})
+ target_link_libraries(afm-main-plugin utils ${afb_LIBRARIES})
+ set_target_properties(afm-main-plugin PROPERTIES
+ PREFIX ""
+ LINK_FLAGS "-Wl,--version-script=${CMAKE_CURRENT_SOURCE_DIR}/afm-main-plugin.export-map"
+ )
+ install(TARGETS afm-main-plugin LIBRARY DESTINATION ${plugin_dir})
+else()
+ message(STATUS "Not creating the plugin for AFB-DAEMON")
+endif()
+</code></pre>
+
+<p>Let now describe some of these lines.</p>
+
+<pre><code>pkg_check_modules(afb afb-daemon)
+</code></pre>