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diff --git a/doc/afb-plugin-writing.html b/doc/afb-plugin-writing.html new file mode 100644 index 00000000..20e25972 --- /dev/null +++ b/doc/afb-plugin-writing.html @@ -0,0 +1,1542 @@ +<html> +<head> + <link rel="stylesheet" type="text/css" href="doc.css"> + <meta charset="UTF-8"> +</head> +<body> +<a name="HOWTO.WRITE.a.PLUGIN.for.AFB-DAEMON"></a> +<h1>HOWTO WRITE a PLUGIN for AFB-DAEMON</h1> + +<pre><code>version: 1 +Date: 27 mai 2016 +Author: José Bollo +</code></pre> + +<p><ul> + <li><a href="#HOWTO.WRITE.a.PLUGIN.for.AFB-DAEMON">HOWTO WRITE a PLUGIN for AFB-DAEMON</a> + <ul> + <li><a href="#Summary">Summary</a> + <ul> + <li><a href="#Nature.of.a.plugin">Nature of a plugin</a></li> + <li><a href="#Kinds.of.plugins">Kinds of plugins</a> + <ul> + <li><a href="#Application.plugins">Application plugins</a></li> + <li><a href="#Service.plugins">Service plugins</a></li> + </ul> + </li> + <li><a href="#Live.cycle.of.a.plugin.within.afb-daemon">Live cycle of a plugin within afb-daemon</a></li> + <li><a href="#Content.of.a.plugin">Content of a plugin</a> + <ul> + <li><a href="#The.name.of.the.plugin">The name of the plugin</a></li> + <li><a href="#Names.of.verbs">Names of verbs</a></li> + <li><a href="#The.initialisation.function">The initialisation function</a></li> + <li><a href="#Functions.implementing.verbs">Functions implementing verbs</a> +</li> + </ul> + </li> + </ul> + </li> + <li><a href="#The.Tic-Tac-Toe.example">The Tic-Tac-Toe example</a></li> + <li><a href="#Dependencies.when.compiling">Dependencies when compiling</a></li> + <li><a href="#Header.files.to.include">Header files to include</a></li> + <li><a href="#Choosing.names">Choosing names</a> + <ul> + <li><a href="#Names.for.API..plugin.">Names for API (plugin)</a></li> + <li><a href="#Names.for.verbs">Names for verbs</a></li> + <li><a href="#Names.for.arguments">Names for arguments</a></li> + <li><a href="#Forging.names.widely.available">Forging names widely available</a></li> + </ul> + </li> + <li><a href="#Writing.a.synchronous.verb.implementation">Writing a synchronous verb implementation</a> + <ul> + <li><a href="#The.incoming.request">The incoming request</a></li> + <li><a href="#Associating.a.context.to.the.session">Associating a context to the session</a></li> + <li><a href="#Sending.the.reply.to.a.request">Sending the reply to a request</a></li> + </ul> + </li> + <li><a href="#Getting.argument.of.invocation">Getting argument of invocation</a> + <ul> + <li><a href="#Basic.functions.for.querying.arguments">Basic functions for querying arguments</a></li> + <li><a href="#Arguments.for.received.files">Arguments for received files</a></li> + <li><a href="#Arguments.as.a.JSON.object">Arguments as a JSON object</a></li> + </ul> + </li> + <li><a href="#Initialisation.of.the.plugin.and.declaration.of.verbs">Initialisation of the plugin and declaration of verbs</a></li> + <li><a href="#Sending.messages.to.the.log.system">Sending messages to the log system</a> + <ul> + <li><a href="#Verbs.for.logging.messages">Verbs for logging messages</a></li> + <li><a href="#Managing.verbosity">Managing verbosity</a></li> + <li><a href="#Output.format.and.destination">Output format and destination</a></li> + </ul> + </li> + <li><a href="#Sending.events">Sending events</a></li> + <li><a href="#Writing.an.asynchronous.verb.implementation">Writing an asynchronous verb implementation</a></li> + <li><a href="#How.to.build.a.plugin">How to build a plugin</a> + <ul> + <li><a href="#Example.for.cmake.meta.build.system">Example for cmake meta build system</a></li> + <li><a href="#Exporting.the.function.pluginAfbV1Register">Exporting the function pluginAfbV1Register</a></li> + <li><a href="#Building.within.yocto">Building within yocto</a></li> + </ul> + </li> + </ul> + </li> +</ul></p> + +<a name="Summary"></a> +<h2>Summary</h2> + +<p>The binder afb-daemon serves files through +the HTTP protocol and offers access to API’s through +HTTP or WebSocket protocol.</p> + +<p>The plugins are used to add API’s to afb-daemon. +This part describes how to write a plugin for afb-daemon. +Excepting this summary, this part is intended to be read +by developpers.</p> + +<p>Before going into details, through a tiny example, +a short overview plugins basis is needed.</p> + +<a name="Nature.of.a.plugin"></a> +<h3>Nature of a plugin</h3> + +<p>A plugin is a separate piece of code made of a shared library. +The plugin is loaded and activated by afb-daemon when afb-daemon +starts.</p> + +<p>Technically, a plugin is not linked to any library of afb-daemon.</p> + +<a name="Kinds.of.plugins"></a> +<h3>Kinds of plugins</h3> + +<p>There is two kinds of plugins: application plugins and service +plugins.</p> + +<a name="Application.plugins"></a> +<h4>Application plugins</h4> + +<p>Application plugins are intended to be instanciated for each +application: when an application using that plugin is started, +its binder starts a new instance of the plugin.</p> + +<p>It means that the application plugins mainly have only one +context to manage for one client.</p> + +<a name="Service.plugins"></a> +<h4>Service plugins</h4> + +<p>Service plugins are intended to be instanciated only one time +only and connected to many clients.</p> + +<p>So either it does not manage context at all or otherwise, +if it manages context, it should be able to manage one context +per client.</p> + +<p>In details, it may be useful to have service plugins at a user +level.</p> + +<a name="Live.cycle.of.a.plugin.within.afb-daemon"></a> +<h3>Live cycle of a plugin within afb-daemon</h3> + +<p>The plugins are loaded and activated when afb-daemon starts.</p> + +<p>At start, the plugin initialise itself. +If it fails to initialise then afb-daemon stops.</p> + +<p>Conversely, if it success to initialize, it must declare +a name, that must be unique, and a list of API’s verbs.</p> + +<p>When initialized, the functions implementing the API’s verbs +of the plugin are activated on call.</p> + +<p>At the end, nothing special is done by afb-daemon. +Consequently, developpers of plugins should use ‘atexit’ +or ‘on_exit’ during initialisation if they need to +perform specific actions when stopping.</p> + +<a name="Content.of.a.plugin"></a> +<h3>Content of a plugin</h3> + +<p>For afb-daemon, a plugin contains 2 different +things: names and functions.</p> + +<p>There is two kind of names: + - the name of the plugin, + - the names of the verbs.</p> + +<p>There is two kind of functions: + - the initialisation function + - functions implementing verbs</p> + +<p>Afb-daemon translates the name of the method that is +invoked to a pair of API and verb names. For example, +the method named <strong>foo/bar</strong> translated to the API +name <strong>foo</strong> and the verb name <strong>bar</strong>. +To serve it, afb-daemon search the plugin that record +the name <strong>foo</strong> and if it also recorded the verb <strong>bar</strong>, +it calls the implementation function declared for this verb.</p> + +<p>Afb-daemon make no distinction between lower case +and upper case when searching for a method. +Thus, The names <strong>TicTacToe/Board</strong> and <strong>tictactoe/borad</strong> +are equals.</p> + +<a name="The.name.of.the.plugin"></a> +<h4>The name of the plugin</h4> + +<p>The name of the plugin is also known as the name +of the API that defines the plugin.</p> + +<p>This name is also known as the prefix.</p> + +<p>The name of a plugin MUST be unique within afb-daemon.</p> + +<p>For example, when a client of afb-daemon +calls a method named <strong>foo/bar</strong>. Afb-daemon +extracts the prefix <strong>foo</strong> and the suffix <strong>bar</strong>. +<strong>foo</strong> is the API name and must match a plugin name, +the plugin that implements the verb <strong>bar</strong>.</p> + +<a name="Names.of.verbs"></a> +<h4>Names of verbs</h4> + +<p>Each plugin exposes a set of verbs that can be called +by client of afb-daemon.</p> + +<p>The name of a verb MUST be unique within a plugin.</p> + +<p>Plugins link verbs to functions that are called +when clients emit requests for that verb.</p> + +<p>For example, when a client of afb-daemon +calls a method named <strong>foo/bar</strong>.</p> + +<a name="The.initialisation.function"></a> +<h4>The initialisation function</h4> + +<p>The initialisation function serves several purposes.</p> + +<ol> +<li><p>It allows afb-daemon to check the version +of the plugin using the name of the initialisation +functions that it found. Currently, the initialisation +function is named <strong>pluginAfbV1Register</strong>. It identifies +the first version of plugins.</p></li> +<li><p>It allows the plugin to initialise itself.</p></li> +<li><p>It serves to the plugin to declare names, descriptions, +requirements and implmentations of the verbs that it exposes.</p></li> +</ol> + + +<a name="Functions.implementing.verbs"></a> +<h4>Functions implementing verbs</h4> + +<p>When a method is called, afb-daemon constructs a request +object and pass it to the implementation function for verb +within the plugin of the API.</p> + +<p>An implementation function receives a request object that +is used to get arguments of the request, to send +answer, to store session data.</p> + +<p>A plugin MUST send an answer to the request.</p> + +<p>But it is not mandatory to send the answer +before to return from the implementing function. +This behaviour is important for implementing +asynchronous actions.</p> + +<p>Implementation functions that always reply to the request +before returning are named <em>synchronous implementations</em>. +Those that don’t always reply to the request before +returning are named <em>asynchronous implementations</em>.</p> + +<p>Asynchronous implementations typically initiate an +asynchronous action and record to send the reply +on completion of this action.</p> + +<a name="The.Tic-Tac-Toe.example"></a> +<h2>The Tic-Tac-Toe example</h2> + +<p>This part explains how to write an afb-plugin. +For the sake of being practical we will use many +examples from the tic-tac-toe example. +This plugin example is in <em>plugins/samples/tic-tac-toe.c</em>.</p> + +<p>This plugin is named <strong><em>tictactoe</em></strong>.</p> + +<a name="Dependencies.when.compiling"></a> +<h2>Dependencies when compiling</h2> + +<p>Afb-daemon provides a configuration file for <em>pkg-config</em>. +Typing the command</p> + +<pre><code>pkg-config --cflags afb-daemon +</code></pre> + +<p>will print the flags to use for compiling, like this:</p> + +<pre><code>$ pkg-config --cflags afb-daemon +-I/opt/local/include -I/usr/include/json-c +</code></pre> + +<p>For linking, you should use</p> + +<pre><code>$ pkg-config --libs afb-daemon +-ljson-c +</code></pre> + +<p>As you see, afb-daemon automatically includes dependency to json-c. +This is done through the <strong>Requires</strong> keyword of pkg-config +because almost all plugin will use <strong>json-c</strong>.</p> + +<p>If this behaviour is a problem, let us know.</p> + +<p>Internally, afb-daemon uses <strong>libsystemd</strong> for its event loop +and for its binding to D-Bus. +Plugins developpers are encouraged to also use this library. +But it is a matter of choice. +Thus there is no dependency to <strong>libsystemd</strong>.</p> + +<blockquote><p>Afb-daemon provides no library for plugins. +The functions that the plugin need to have are given +to the plugin at runtime through pointer using read-only +memory.</p></blockquote> + +<a name="Header.files.to.include"></a> +<h2>Header files to include</h2> + +<p>The plugin <em>tictactoe</em> has the following lines for its includes:</p> + +<pre><code>#define _GNU_SOURCE +#include <stdio.h> +#include <string.h> +#include <json-c/json.h> +#include <afb/afb-plugin.h> +</code></pre> + +<p>The header <em>afb/afb-plugin.h</em> includes all the features that a plugin +needs except two foreign header that must be included by the plugin +if it needs it:</p> + +<ul> +<li><em>json-c/json.h</em>: this header must be include to handle json objects;</li> +<li><em>systemd/sd-event.h</em>: this must be include to access the main loop;</li> +<li><em>systemd/sd-bus.h</em>: this may be include to use dbus connections.</li> +</ul> + + +<p>The <em>tictactoe</em> plugin does not use systemd features so it is not included.</p> + +<p>When including <em>afb/afb-plugin.h</em>, the macro <strong>_GNU_SOURCE</strong> must be +defined.</p> + +<a name="Choosing.names"></a> +<h2>Choosing names</h2> + +<p>The designer of a plugin must defines names for its plugin +(or its API) and for the verbs of its API. He also +must defines names for arguments given by name.</p> + +<p>While forging names, the designer should take into account +the rules for making valid names and some rules that make +the names easy to use across plaforms.</p> + +<p>The names and strings used ALL are UTF-8 encoded.</p> + +<a name="Names.for.API..plugin."></a> +<h3>Names for API (plugin)</h3> + +<p>The names of the API are checked. +All characters are authorised except:</p> + +<ul> +<li>the control characters (\u0000 .. \u001f)</li> +<li>the characters of the set { ‘ ’, ‘“’, ‘#’, ‘%’, ‘&’, +‘’‘, ’/‘, ’?‘, ’`‘, ’\x7f' }</li> +</ul> + + +<p>In other words the set of forbidden characters is +{ \u0000..\u0020, \u0022, \u0023, \u0025..\u0027, + \u002f, \u003f, \u0060, \u007f }.</p> + +<p>Afb-daemon make no distinction between lower case +and upper case when searching for an API by its name.</p> + +<a name="Names.for.verbs"></a> +<h3>Names for verbs</h3> + +<p>The names of the verbs are not checked.</p> + +<p>However, the validity rules for verb’s names are the +same as for API’s names except that the dot (.) character +is forbidden.</p> + +<p>Afb-daemon make no distinction between lower case +and upper case when searching for an API by its name.</p> + +<a name="Names.for.arguments"></a> +<h3>Names for arguments</h3> + +<p>The names for arguments are not restricted and can be +anything.</p> + +<p>The arguments are searched with the case sensitive +string comparison. Thus the names “index” and “Index” +are not the same.</p> + +<a name="Forging.names.widely.available"></a> +<h3>Forging names widely available</h3> + +<p>The key names of javascript object can be almost +anything using the arrayed notation:</p> + +<pre><code>object[key] = value +</code></pre> + +<p>That is not the case with the dot notation:</p> + +<pre><code>object.key = value +</code></pre> + +<p>Using the dot notation, the key must be a valid javascript +identifier.</p> + +<p>For this reason, the chosen names should better be +valid javascript identifier.</p> + +<p>It is also a good practice, even for arguments, to not +rely on the case sensitivity and to avoid the use of +names different only by the case.</p> + +<a name="Writing.a.synchronous.verb.implementation"></a> +<h2>Writing a synchronous verb implementation</h2> + +<p>The verb <strong>tictactoe/board</strong> is a synchronous implementation. +Here is its listing:</p> + +<pre><code>/* + * get the board + */ +static void board(struct afb_req req) +{ + struct board *board; + struct json_object *description; + + /* retrieves the context for the session */ + board = board_of_req(req); + INFO(afbitf, "method 'board' called for boardid %d", board->id); + + /* describe the board */ + description = describe(board); + + /* send the board's description */ + afb_req_success(req, description, NULL); +} +</code></pre> + +<p>This examples show many aspects of writing a synchronous +verb implementation. Let summarize it:</p> + +<ol> +<li><p>The function <strong>board_of_req</strong> retrieves the context stored +for the plugin: the board.</p></li> +<li><p>The macro <strong>INFO</strong> sends a message of kind <em>INFO</em> +to the logging system. The global variable named <strong>afbitf</strong> +used represents the interface to afb-daemon.</p></li> +<li><p>The function <strong>describe</strong> creates a json_object representing +the board.</p></li> +<li><p>The function <strong>afb_req_success</strong> sends the reply, attaching to +it the object <em>description</em>.</p></li> +</ol> + + +<a name="The.incoming.request"></a> +<h3>The incoming request</h3> + +<p>For any implementation, the request is received by a structure of type +<strong>struct afb_req</strong>.</p> + +<blockquote><p>Note that this is a PLAIN structure, not a pointer to a structure.</p></blockquote> + +<p>The definition of <strong>struct afb_req</strong> is:</p> + +<pre><code>/* + * Describes the request by plugins from afb-daemon + */ +struct afb_req { + const struct afb_req_itf *itf; /* the interfacing functions */ + void *closure; /* the closure for functions */ +}; +</code></pre> + +<p>It contains two pointers: one, <em>itf</em>, points to the functions needed +to handle the internal request represented by the second pointer, <em>closure</em>.</p> + +<blockquote><p>The structure must never be used directly. +Insted, use the intended functions provided +by afb-daemon and described here.</p></blockquote> + +<p><em>req</em> is used to get arguments of the request, to send +answer, to store session data.</p> + +<p>This object and its interface is defined and documented +in the file names <em>afb/afb-req-itf.h</em></p> + +<p>The above example uses 2 times the request object <em>req</em>.</p> + +<p>The first time, it is used for retrieving the board attached to +the session of the request.</p> + +<p>The second time, it is used to send the reply: an object that +describes the current board.</p> + +<a name="Associating.a.context.to.the.session"></a> +<h3>Associating a context to the session</h3> + +<p>When the plugin <em>tic-tac-toe</em> receives a request, it musts regain +the board that describes the game associated to the session.</p> + +<p>For a plugin, having data associated to a session is a common case. +This data is called the context of the plugin for the session. +For the plugin <em>tic-tac-toe</em>, the context is the board.</p> + +<p>The requests <em>afb_req</em> offer four functions for +storing and retrieving the context associated to the session.</p> + +<p>These functions are:</p> + +<ul> +<li><p><strong>afb_req_context_get</strong>: +retrieves the context data stored for the plugin.</p></li> +<li><p><strong>afb_req_context_set</strong>: +store the context data of the plugin.</p></li> +<li><p><strong>afb_req_context</strong>: +retrieves the context data of the plugin, +if needed, creates the context and store it.</p></li> +<li><p><strong>afb_req_context_clear</strong>: +reset the stored data.</p></li> +</ul> + + +<p>The plugin <em>tictactoe</em> use a convenient function to retrieve +its context: the board. This function is <em>board_of_req</em>:</p> + +<pre><code>/* + * retrieves the board of the request + */ +static inline struct board *board_of_req(struct afb_req req) +{ + return afb_req_context(req, (void*)get_new_board, (void*)release_board); +} +</code></pre> + +<p>The function <strong>afb_req_context</strong> ensure an existing context +for the session of the request. +Its two last arguments are functions. Here, the casts are required +to avoid a warning when compiling.</p> + +<p>Here is the definition of the function <strong>afb_req_context</strong></p> + +<pre><code>/* + * Gets the pointer stored by the plugin for the session of 'req'. + * If the stored pointer is NULL, indicating that no pointer was + * already stored, afb_req_context creates a new context by calling + * the function 'create_context' and stores it with the freeing function + * 'free_context'. + */ +static inline void *afb_req_context(struct afb_req req, void *(*create_context)(), void (*free_context)(void*)) +{ + void *result = afb_req_context_get(req); + if (result == NULL) { + result = create_context(); + afb_req_context_set(req, result, free_context); + } + return result; +} +</code></pre> + +<p>The second argument if the function that creates the context. +For the plugin <em>tic-tac-toe</em> it is the function <strong>get_new_board</strong>. +The function <strong>get_new_board</strong> creates a new board and set its +count of use to 1. The boards are counting their count of use +to free there ressources when no more used.</p> + +<p>The third argument if the function that frees the context. +For the plugin <em>tic-tac-toe</em> it is the function <strong>release_board</strong>. +The function <strong>release_board</strong> decrease the the count of use of +the board given as argument. If the use count decrease to zero, +the board data are freed.</p> + +<p>The definition of the other functions for dealing with contexts are:</p> + +<pre><code>/* + * Gets the pointer stored by the plugin for the session of 'req'. + * When the plugin has not yet recorded a pointer, NULL is returned. + */ +void *afb_req_context_get(struct afb_req req); + +/* + * Stores for the plugin the pointer 'context' to the session of 'req'. + * The function 'free_context' will be called when the session is closed + * or if plugin stores an other pointer. + */ +void afb_req_context_set(struct afb_req req, void *context, void (*free_context)(void*)); + +/* + * Frees the pointer stored by the plugin for the session of 'req' + * and sets it to NULL. + * + * Shortcut for: afb_req_context_set(req, NULL, NULL) + */ +static inline void afb_req_context_clear(struct afb_req req) +{ + afb_req_context_set(req, NULL, NULL); +} +</code></pre> + +<a name="Sending.the.reply.to.a.request"></a> +<h3>Sending the reply to a request</h3> + +<p>Two kinds of replies can be made: successful replies and +failure replies.</p> + +<blockquote><p>Sending a reply to a request must be done at most one time.</p></blockquote> + +<p>The two functions to send a reply of kind “success” are +<strong>afb_req_success</strong> and <strong>afb_req_success_f</strong>.</p> + +<pre><code>/* + * Sends a reply of kind success to the request 'req'. + * The status of the reply is automatically set to "success". + * Its send the object 'obj' (can be NULL) with an + * informationnal comment 'info (can also be NULL). + * + * For conveniency, the function calls 'json_object_put' for 'obj'. + * Thus, in the case where 'obj' should remain available after + * the function returns, the function 'json_object_get' shall be used. + */ +void afb_req_success(struct afb_req req, struct json_object *obj, const char *info); + +/* + * Same as 'afb_req_success' but the 'info' is a formatting + * string followed by arguments. + * + * For conveniency, the function calls 'json_object_put' for 'obj'. + * Thus, in the case where 'obj' should remain available after + * the function returns, the function 'json_object_get' shall be used. + */ +void afb_req_success_f(struct afb_req req, struct json_object *obj, const char *info, ...); +</code></pre> + +<p>The two functions to send a reply of kind “failure” are +<strong>afb_req_fail</strong> and <strong>afb_req_fail_f</strong>.</p> + +<pre><code>/* + * Sends a reply of kind failure to the request 'req'. + * The status of the reply is set to 'status' and an + * informationnal comment 'info' (can also be NULL) can be added. + * + * Note that calling afb_req_fail("success", info) is equivalent + * to call afb_req_success(NULL, info). Thus even if possible it + * is strongly recommanded to NEVER use "success" for status. + * + * For conveniency, the function calls 'json_object_put' for 'obj'. + * Thus, in the case where 'obj' should remain available after + * the function returns, the function 'json_object_get' shall be used. + */ +void afb_req_fail(struct afb_req req, const char *status, const char *info); + +/* + * Same as 'afb_req_fail' but the 'info' is a formatting + * string followed by arguments. + * + * For conveniency, the function calls 'json_object_put' for 'obj'. + * Thus, in the case where 'obj' should remain available after + * the function returns, the function 'json_object_get' shall be used. + */ +void afb_req_fail_f(struct afb_req req, const char *status, const char *info, ...); +</code></pre> + +<blockquote><p>For conveniency, these functions call <strong>json_object_put</strong> to release the object <strong>obj</strong> +that they send. Then <strong>obj</strong> can not be used after calling one of these reply functions. +When it is not the expected behaviour, calling the function <strong>json_object_get</strong> on the object <strong>obj</strong> +before cancels the effect of <strong>json_object_put</strong>.</p></blockquote> + +<a name="Getting.argument.of.invocation"></a> +<h2>Getting argument of invocation</h2> + +<p>Many verbs expect arguments. Afb-daemon let plugins +retrieve their arguments by name not by position.</p> + +<p>Arguments are given by the requests either through HTTP +or through WebSockets.</p> + +<p>For example, the verb <strong>join</strong> of the plugin <strong>tic-tac-toe</strong> +expects one argument: the <em>boardid</em> to join. Here is an extract:</p> + +<pre><code>/* + * Join a board + */ +static void join(struct afb_req req) +{ + struct board *board, *new_board; + const char *id; + + /* retrieves the context for the session */ + board = board_of_req(req); + INFO(afbitf, "method 'join' called for boardid %d", board->id); + + /* retrieves the argument */ + id = afb_req_value(req, "boardid"); + if (id == NULL) + goto bad_request; + ... +</code></pre> + +<p>The function <strong>afb_req_value</strong> search in the request <em>req</em> +for an argument whose name is given. When no argument of the +given name was passed, <strong>afb_req_value</strong> returns NULL.</p> + +<blockquote><p>The search is case sensitive. So the name <em>boardid</em> is not the +same name than <em>BoardId</em>. But this must not be assumed so two +expected names of argument should not differ only by case.</p></blockquote> + +<a name="Basic.functions.for.querying.arguments"></a> +<h3>Basic functions for querying arguments</h3> + +<p>The function <strong>afb_req_value</strong> is defined as below:</p> + +<pre><code>/* + * Gets from the request 'req' the string value of the argument of 'name'. + * Returns NULL if when there is no argument of 'name'. + * Returns the value of the argument of 'name' otherwise. + * + * Shortcut for: afb_req_get(req, name).value + */ +static inline const char *afb_req_value(struct afb_req req, const char *name) +{ + return afb_req_get(req, name).value; +} +</code></pre> + +<p>It is defined as a shortcut to call the function <strong>afb_req_get</strong>. +That function is defined as below:</p> + +<pre><code>/* + * Gets from the request 'req' the argument of 'name'. + * Returns a PLAIN structure of type 'struct afb_arg'. + * When the argument of 'name' is not found, all fields of result are set to NULL. + * When the argument of 'name' is found, the fields are filled, + * in particular, the field 'result.name' is set to 'name'. + * + * There is a special name value: the empty string. + * The argument of name "" is defined only if the request was made using + * an HTTP POST of Content-Type "application/json". In that case, the + * argument of name "" receives the value of the body of the HTTP request. + */ +struct afb_arg afb_req_get(struct afb_req req, const char *name); +</code></pre> + +<p>That function takes 2 parameters: the request and the name +of the argument to retrieve. It returns a PLAIN structure of +type <strong>struct afb_arg</strong>.</p> + +<p>There is a special name that is defined when the request is +of type HTTP/POST with a Content-Type being application/json. +This name is <strong>“”</strong> (the empty string). In that case, the value +of this argument of empty name is the string received as a body +of the post and is supposed to be a JSON string.</p> + +<p>The definition of <strong>struct afb_arg</strong> is:</p> + +<pre><code>/* + * Describes an argument (or parameter) of a request + */ +struct afb_arg { + const char *name; /* name of the argument or NULL if invalid */ + const char *value; /* string representation of the value of the argument */ + /* original filename of the argument if path != NULL */ + const char *path; /* if not NULL, path of the received file for the argument */ + /* when the request is finalized this file is removed */ +}; +</code></pre> + +<p>The structure returns the data arguments that are known for the +request. This data include a field named <strong>path</strong>. This <strong>path</strong> +can be accessed using the function <strong>afb_req_path</strong> defined as +below:</p> + +<pre><code>/* + * Gets from the request 'req' the path for file attached to the argument of 'name'. + * Returns NULL if when there is no argument of 'name' or when there is no file. + * Returns the path of the argument of 'name' otherwise. + * + * Shortcut for: afb_req_get(req, name).path + */ +static inline const char *afb_req_path(struct afb_req req, const char *name) +{ + return afb_req_get(req, name).path; +} +</code></pre> + +<p>The path is only defined for HTTP/POST requests that send file.</p> + +<a name="Arguments.for.received.files"></a> +<h3>Arguments for received files</h3> + +<p>As it is explained just above, clients can send files using +HTTP/POST requests.</p> + +<p>Received files are attached to a arguments. For example, the +following HTTP fragment (from test/sample-post.html) +will send an HTTP/POST request to the method +<strong>post/upload-image</strong> with 2 arguments named <em>file</em> and +<em>hidden</em>.</p> + +<pre><code><h2>Sample Post File</h2> +<form enctype="multipart/form-data"> + <input type="file" name="file" /> + <input type="hidden" name="hidden" value="bollobollo" /> + <br> + <button formmethod="POST" formaction="api/post/upload-image">Post File</button> +</form> +</code></pre> + +<p>In that case, the argument named <strong>file</strong> has its value and its +path defined and not NULL.</p> + +<p>The value is the name of the file as it was +set by the HTTP client and is generally the filename on the +client side.</p> + +<p>The path is the path of the file saved on the temporary local storage +area of the application. This is a randomly generated and unic filename +not linked in any way with the original filename on the client.</p> + +<p>The plugin can use the file at the given path the way that it wants: +read, write, remove, copy, rename… +But when the reply is sent and the query is terminated, the file at +this path is destroyed if it still exist.</p> + +<a name="Arguments.as.a.JSON.object"></a> +<h3>Arguments as a JSON object</h3> + +<p>Plugins can get all the arguments as one single object. +This feature is provided by the function <strong>afb_req_json</strong> +that is defined as below:</p> + +<pre><code>/* + * Gets from the request 'req' the json object hashing the arguments. + * The returned object must not be released using 'json_object_put'. + */ +struct json_object *afb_req_json(struct afb_req req); +</code></pre> + +<p>It returns a json object. This object depends on how the request was +made:</p> + +<ul> +<li><p>For HTTP requests, this is an object whose keys are the names of the +arguments and whose values are either a string for common arguments or +an object like { “file”: “…”, “path”: “…” }</p></li> +<li><p>For WebSockets requests, the returned object is the object +given by the client transparently transported.</p></li> +</ul> + + +<blockquote><p>In fact, for Websockets requests, the function <strong>afb_req_value</strong> +can be seen as a shortcut to +<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= "Asks 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> + +<p>Since version 0.5, plugins can broadcast events to any potential listener. +This kind of bradcast is not targeted. Event targeted will come in a future +version of afb-daemon.</p> + +<p>The plugin <em>tic-tac-toe</em> broadcasts events when the board changes. +This is done in the function <strong>changed</strong>:</p> + +<pre><code>/* + * signals a change of the board + */ +static void changed(struct board *board, const char *reason) +{ + ... + struct json_object *description; + + /* get the description */ + description = describe(board); + + ... + + afb_daemon_broadcast_event(afbitf->daemon, reason, description); +} +</code></pre> + +<p>The description of the changed board is pushed via the daemon interface.</p> + +<p>Within the plugin <em>tic-tac-toe</em>, the <em>reason</em> indicates the origin of +the change. For the function <strong>afb_daemon_broadcast_event</strong>, the second +parameter is the name of the broadcasted event. The third argument is the +object that is transmitted with the event.</p> + +<p>The function <strong>afb_daemon_broadcast_event</strong> is defined as below:</p> + +<pre><code>/* + * Broadcasts widely the event of 'name' with the data 'object'. + * 'object' can be NULL. + * 'daemon' MUST be the daemon given in interface when activating the plugin. + * + * For conveniency, the function calls 'json_object_put' for 'object'. + * Thus, in the case where 'object' should remain available after + * the function returns, the function 'json_object_get' shall be used. + */ +void afb_daemon_broadcast_event(struct afb_daemon daemon, const char *name, struct json_object *object); +</code></pre> + +<blockquote><p>Be aware, as for reply functions, the <strong>object</strong> is automatically released using +<strong>json_object_put</strong> by the function. Then call <strong>json_object_get</strong> before +calling <strong>afb_daemon_broadcast_event</strong> to keep <strong>object</strong> available +after the returning of the function.</p></blockquote> + +<p>In fact the event name received by the listener is prefixed with +the name of the plugin. So when the change occurs after a move, the +reason is <strong>move</strong> and then the clients receive the event <strong>tictactoe/move</strong>.</p> + +<blockquote><p>Note that nothing is said about the case sensitivity of event names. +However, the event is always prefixed with the name that the plugin +declared, with the same case, followed with a slash /. +Thus it is safe to compare event using a case sensitive comparison.</p></blockquote> + +<a name="Writing.an.asynchronous.verb.implementation"></a> +<h2>Writing an asynchronous verb implementation</h2> + +<p>The <em>tic-tac-toe</em> example allows two clients or more to share the same board. +This is implemented by the verb <strong>join</strong> that illustrated partly the how to +retrieve arguments.</p> + +<p>When two or more clients are sharing a same board, one of them can wait +until the state of the board changes. (This coulded also be implemented using +events because an even is generated each time the board changes).</p> + +<p>In this case, the reply to the wait is sent only when the board changes. +See the diagram below:</p> + +<pre><code>CLIENT A CLIENT B TIC-TAC-TOE + | | | + +--------------|----------------->| wait . . . . . . . . + | | | . + : : : . + : : : . + | | | . + | +----------------->| move . . . . + | | | V . + | |<-----------------+ success of move . + | | | . + |<-------------|------------------+ success of wait < +</code></pre> + +<p>Here, this is an invocation of the plugin by an other client that +unblock the suspended <em>wait</em> call. +But in general, this will be a timer, a hardware event, the sync with +a concurrent process or thread, …</p> + +<p>So the case is common, this is an asynchronous implementation.</p> + +<p>Here is the listing of the function <strong>wait</strong>:</p> + +<pre><code>static void wait(struct afb_req req) +{ + struct board *board; + struct waiter *waiter; + + /* retrieves the context for the session */ + board = board_of_req(req); + INFO(afbitf, "method 'wait' called for boardid %d", board->id); + + /* creates the waiter and enqueues it */ + waiter = calloc(1, sizeof *waiter); + waiter->req = req; + waiter->next = board->waiters; + afb_req_addref(req); + board->waiters = waiter; +} +</code></pre> + +<p>After retrieving the board, the function adds a new waiter to the +current list of waiters and returns without sending a reply.</p> + +<p>Before returning, it increases the reference count of the +request <strong>req</strong> using the function <strong>afb_req_addref</strong>.</p> + +<blockquote><p>When the implentation of a verb returns without sending a reply, +it <strong>MUST</strong> increment the reference count of the request +using <strong>afb_req_addref</strong>. If it doesn’t bad things can happen.</p></blockquote> + +<p>Later, when the board changes, it calls the function <strong>changed</strong> +of <em>tic-tac-toe</em> with the reason of the change.</p> + +<p>Here is the full listing of the function <strong>changed</strong>:</p> + +<pre><code>/* + * signals a change of the board + */ +static void changed(struct board *board, const char *reason) +{ + struct waiter *waiter, *next; + struct json_object *description; + + /* get the description */ + description = describe(board); + + waiter = board->waiters; + board->waiters = NULL; + while (waiter != NULL) { + next = waiter->next; + afb_req_success(waiter->req, json_object_get(description), reason); + afb_req_unref(waiter->req); + free(waiter); + waiter = next; + } + + afb_event_sender_push(afb_daemon_get_event_sender(afbitf->daemon), reason, description); +} +</code></pre> + +<p>The list of waiters is walked and a reply is sent to each waiter. +After the sending the reply, the reference count of the request +is decremented using <strong>afb_req_unref</strong> to allow its resources to be freed.</p> + +<blockquote><p>The reference count <strong>MUST</strong> be decremented using <strong>afb_req_unref</strong> because, +otherwise, there is a leak of resources. +It must be decremented <strong>AFTER</strong> the sending of the reply, because, otherwise, +bad things may happen.</p></blockquote> + +<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 ${plugin_dir}) +else() + message(STATUS "Not creating the plugin for AFB-DAEMON") +endif() +</code></pre> + +<p>Let now describe some of these lines.</p> + +<pre><code>pkg_check_modules(afb afb-daemon) +</code></pre> + +<p>This first lines searches to the <em>pkg-config</em> configuration file for +<strong>afb-daemon</strong>. Resulting data are stored in the following variables:</p> + +<table> +<thead> +<tr> +<th>Variable </th> +<th> Meaning</th> +</tr> +</thead> +<tbody> +<tr> +<td>afb_FOUND </td> +<td> Set to 1 if afb-daemon plugin development files exist</td> +</tr> +<tr> +<td>afb_LIBRARIES </td> +<td> Only the libraries (w/o the ‘-l’) for compiling afb-daemon plugins</td> +</tr> +<tr> +<td>afb_LIBRARY_DIRS </td> +<td> The paths of the libraries (w/o the ‘-L’) for compiling afb-daemon plugins</td> +</tr> +<tr> +<td>afb_LDFLAGS </td> +<td> All required linker flags for compiling afb-daemon plugins</td> +</tr> +<tr> +<td>afb_INCLUDE_DIRS </td> +<td> The ‘-I’ preprocessor flags (w/o the ‘-I’) for compiling afb-daemon plugins</td> +</tr> +<tr> +<td>afb_CFLAGS </td> +<td> All required cflags for compiling afb-daemon plugins</td> +</tr> +</tbody> +</table> + + +<p>If development files are found, the plugin can be added to the set of +target to build.</p> + +<pre><code>add_library(afm-main-plugin MODULE afm-main-plugin.c) +</code></pre> + +<p>This line asks to create a shared library having only the +source file afm-main-plugin.c (that is compiled). +The default name of the created shared object is +<strong>libafm-main-plugin.so</strong>.</p> + +<pre><code>set_target_properties(afm-main-plugin PROPERTIES + PREFIX "" + LINK_FLAGS "-Wl,--version-script=${CMAKE_CURRENT_SOURCE_DIR}/afm-main-plugin.export-map" +) +</code></pre> + +<p>This lines are doing two things:</p> + +<ol> +<li><p>It renames the built library from <strong>libafm-main-plugin.so</strong> to <strong>afm-main-plugin.so</strong> +by removing the implicitely added prefix <em>lib</em>. This step is not mandatory +at all because afb-daemon doesn’t check names of files when loading it. +The only convention that use afb-daemon is that extension is <strong>.so</strong> +but this convention is used only when afb-daemon discovers plugin +from a directory hierarchy.</p></li> +<li><p>It applies a version script at link to only export the conventional name +of the entry point: <strong>pluginAfbV1Register</strong>. See below. By default, the linker +that creates the shared object exports all the public symbols (C functions that +are not <strong>static</strong>).</p></li> +</ol> + + +<p>Next line are:</p> + +<pre><code>target_include_directories(afm-main-plugin PRIVATE ${afb_INCLUDE_DIRS}) +target_link_libraries(afm-main-plugin utils ${afb_LIBRARIES}) +</code></pre> + +<p>As you can see it uses the variables computed by <strong><em>pkg_check_modules(afb afb-daemon)</em></strong> +to configure the compiler and the linker.</p> + +<a name="Exporting.the.function.pluginAfbV1Register"></a> +<h3>Exporting the function pluginAfbV1Register</h3> + +<p>The function <strong>pluginAfbV1Register</strong> must be exported. This can be achieved +using a version script when linking. Here is the version script that is +used for <em>tic-tac-toe</em> (plugins/samples/export.map).</p> + +<pre><code>{ global: pluginAfbV1Register; local: *; }; +</code></pre> + +<p>This sample <a href="https://sourceware.org/binutils/docs-2.26/ld/VERSION.html#VERSION">version script</a> +exports as global the symbol <em>pluginAfbV1Register</em> and hides any +other symbols.</p> + +<p>This version script is added to the link options using the +option <strong>–version-script=export.map</strong> is given directly to the +linker or using th option <strong>-Wl,–version-script=export.map</strong> +when the option is given to the C compiler.</p> + +<a name="Building.within.yocto"></a> +<h3>Building within yocto</h3> + +<p>Adding a dependency to afb-daemon is enough. See below:</p> + +<pre><code>DEPENDS += " afb-daemon " +</code></pre> +</body> +</html> |