1 HOWTO WRITE a PLUGIN for AFB-DAEMON
2 ===================================
12 The binder afb-daemon serves files through
13 the HTTP protocol and offers access to API's through
14 HTTP or WebSocket protocol.
16 The plugins are used to add API's to afb-daemon.
17 This part describes how to write a plugin for afb-daemon.
18 Excepting this summary, this part is intended to be read
21 Before going into details, through a tiny example,
22 a short overview plugins basis is needed.
24 ### Nature of a plugin
26 A plugin is a separate piece of code made of a shared library.
27 The plugin is loaded and activated by afb-daemon when afb-daemon
30 Technically, a plugin is not linked to any library of afb-daemon.
34 There is two kinds of plugins: application plugins and service
37 #### Application plugins
39 Application plugins are intended to be instanciated for each
40 application: when an application using that plugin is started,
41 its binder starts a new instance of the plugin.
43 It means that the application plugins mainly have only one
44 context to manage for one client.
48 Service plugins are intended to be instanciated only one time
49 only and connected to many clients.
51 So either it does not manage context at all or otherwise,
52 if it manages context, it should be able to manage one context
55 In details, it may be useful to have service plugins at a user
58 ### Live cycle of a plugin within afb-daemon
60 The plugins are loaded and activated when afb-daemon starts.
62 At start, the plugin initialise itself.
63 If it fails to initialise then afb-daemon stops.
65 Conversely, if it success to initialize, it must declare
66 a name, that must be unique, and a list of API's verbs.
68 When initialized, the functions implementing the API's verbs
69 of the plugin are activated on call.
71 At the end, nothing special is done by afb-daemon.
72 Consequently, developpers of plugins should use 'atexit'
73 or 'on_exit' during initialisation if they need to
74 perform specific actions when stopping.
76 ### Content of a plugin
78 For afb-daemon, a plugin contains 2 different
79 things: names and functions.
81 There is two kind of names:
82 - the name of the plugin,
83 - the names of the verbs.
85 There is two kind of functions:
86 - the initialisation function
87 - functions implementing verbs
89 Afb-daemon translates the name of the method that is
90 invoked to a pair of API and verb names. For example,
91 the method named **foo/bar** translated to the API
92 name **foo** and the verb name **bar**.
93 To serve it, afb-daemon search the plugin that record
94 the name **foo** and if it also recorded the verb **bar**,
95 it calls the implementation function declared for this verb.
97 Afb-daemon make no distinction between lower case
98 and upper case when searching for a method.
99 Thus, The names **TicTacToe/Board** and **tictactoe/borad**
102 #### The name of the plugin
104 The name of the plugin is also known as the name
105 of the API that defines the plugin.
107 This name is also known as the prefix.
109 The name of a plugin MUST be unique within afb-daemon.
111 For example, when a client of afb-daemon
112 calls a method named **foo/bar**. Afb-daemon
113 extracts the prefix **foo** and the suffix **bar**.
114 **foo** is the API name and must match a plugin name,
115 the plugin that implements the verb **bar**.
119 Each plugin exposes a set of verbs that can be called
120 by client of afb-daemon.
122 The name of a verb MUST be unique within a plugin.
124 Plugins link verbs to functions that are called
125 when clients emit requests for that verb.
127 For example, when a client of afb-daemon
128 calls a method named **foo/bar**.
130 #### The initialisation function
132 The initialisation function serves several purposes.
134 1. It allows afb-daemon to check the version
135 of the plugin using the name of the initialisation
136 functions that it found. Currently, the initialisation
137 function is named **pluginAfbV1Register**. It identifies
138 the first version of plugins.
140 2. It allows the plugin to initialise itself.
142 3. It serves to the plugin to declare names, descriptions,
143 requirements and implmentations of the verbs that it exposes.
145 #### Functions implementing verbs
147 When a method is called, afb-daemon constructs a request
148 object and pass it to the implementation function for verb
149 within the plugin of the API.
151 An implementation function receives a request object that
152 is used to get arguments of the request, to send
153 answer, to store session data.
155 A plugin MUST send an answer to the request.
157 But it is not mandatory to send the answer
158 before to return from the implementing function.
159 This behaviour is important for implementing
160 asynchronous actions.
162 Implementation functions that always reply to the request
163 before returning are named *synchronous implementations*.
164 Those that don't always reply to the request before
165 returning are named *asynchronous implementations*.
167 Asynchronous implementations typically initiate an
168 asynchronous action and record to send the reply
169 on completion of this action.
171 The Tic-Tac-Toe example
172 -----------------------
174 This part explains how to write an afb-plugin.
175 For the sake of being practical we will use many
176 examples from the tic-tac-toe example.
177 This plugin example is in *plugins/samples/tic-tac-toe.c*.
179 This plugin is named ***tictactoe***.
184 The designer of a plugin must defines names for its plugin
185 (or its API) and for the verbs of its API. He also
186 must defines names for arguments given by name.
188 While forging names, the designer should take into account
189 the rules for making valid names and some rules that make
190 the names easy to use across plaforms.
192 The names and strings used ALL are UTF-8 encoded.
194 ### Names for API (plugin)
196 The names of the API are checked.
197 All characters are authorised except:
199 - the control characters (\u0000 .. \u001f)
200 - the characters of the set { ' ', '"', '#', '%', '&',
201 '\'', '/', '?', '`', '\x7f' }
203 In other words the set of forbidden characters is
204 { \u0000..\u0020, \u0022, \u0023, \u0025..\u0027,
205 \u002f, \u003f, \u0060, \u007f }.
207 Afb-daemon make no distinction between lower case
208 and upper case when searching for an API by its name.
212 The names of the verbs are not checked.
214 However, the validity rules for verb's names are the
215 same as for API's names except that the dot (.) character
218 Afb-daemon make no distinction between lower case
219 and upper case when searching for an API by its name.
221 ### Names for arguments
223 The names for arguments are not restricted and can be
226 The arguments are searched with the case sensitive
227 string comparison. Thus the names "index" and "Index"
230 ### Forging names widely available
232 The key names of javascript object can be almost
233 anything using the arrayed notation:
237 That is not the case with the dot notation:
241 Using the dot notation, the key must be a valid javascript
244 For this reason, the chosen names should better be
245 valid javascript identifier.
247 It is also a good practice, even for arguments, to not
248 rely on the case sensitivity and to avoid the use of
249 names different only by the case.
251 Options to set when compiling plugins
252 -------------------------------------
254 Afb-daemon provides a configuration file for *pkg-config*.
257 pkg-config --cflags afb-daemon
259 will print the flags to use for compiling, like this:
261 $ pkg-config --cflags afb-daemon
262 -I/opt/local/include -I/usr/include/json-c
264 For linking, you should use
266 $ pkg-config --libs afb-daemon
269 As you see, afb-daemon automatically includes dependency to json-c.
270 This is done through the **Requires** keyword of pkg-config.
272 If this behaviour is a problem, let us know.
274 Header files to include
275 -----------------------
277 The plugin *tictactoe* has the following lines for its includes:
282 #include <json-c/json.h>
283 #include <afb/afb-plugin.h>
285 The header *afb/afb-plugin.h* includes all the features that a plugin
286 needs except two foreign header that must be included by the plugin
289 - *json-c/json.h*: this header must be include to handle json objects;
290 - *systemd/sd-event.h*: this must be include to access the main loop;
291 - *systemd/sd-bus.h*: this may be include to use dbus connections.
293 The *tictactoe* plugin does not use systemd features so it is not included.
295 When including *afb/afb-plugin.h*, the macro **_GNU_SOURCE** must be
298 Writing a synchronous verb implementation
299 -----------------------------------------
301 The verb **tictactoe/board** is a synchronous implementation.
307 static void board(struct afb_req req)
310 struct json_object *description;
312 /* retrieves the context for the session */
313 board = board_of_req(req);
314 INFO(afbitf, "method 'board' called for boardid %d", board->id);
316 /* describe the board */
317 description = describe(board);
319 /* send the board's description */
320 afb_req_success(req, description, NULL);
323 This examples show many aspects of writing a synchronous
324 verb implementation. Let summarize it:
326 1. The function **board_of_req** retrieves the context stored
327 for the plugin: the board.
329 2. The macro **INFO** sends a message of kind *INFO*
330 to the logging system. The global variable named **afbitf**
331 used represents the interface to afb-daemon.
333 3. The function **describe** creates a json_object representing
336 4. The function **afb_req_success** sends the reply, attaching to
337 it the object *description*.
339 ### The incoming request
341 For any implementation, the request is received by a structure of type
344 > Note that this is a PLAIN structure, not a pointer to a structure.
346 The definition of **struct afb_req** is:
349 * Describes the request by plugins from afb-daemon
352 const struct afb_req_itf *itf; /* the interfacing functions */
353 void *closure; /* the closure for functions */
356 It contains two pointers: one, *itf*, points to the functions needed
357 to handle the internal request represented by the second pointer, *closure*.
359 > The structure must never be used directly.
360 > Insted, use the intended functions provided
361 > by afb-daemon and described here.
363 *req* is used to get arguments of the request, to send
364 answer, to store session data.
366 This object and its interface is defined and documented
367 in the file names *afb/afb-req-itf.h*
369 The above example uses 2 times the request object *req*.
371 The first time, it is used for retrieving the board attached to
372 the session of the request.
374 The second time, it is used to send the reply: an object that
375 describes the current board.
377 ### Associating a context to the session
379 When the plugin *tic-tac-toe* receives a request, it musts regain
380 the board that describes the game associated to the session.
382 For a plugin, having data associated to a session is a common case.
383 This data is called the context of the plugin for the session.
384 For the plugin *tic-tac-toe*, the context is the board.
386 The requests *afb_req* offer four functions for
387 storing and retrieving the context associated to the session.
391 - **afb_req_context_get**:
392 retrieves the context data stored for the plugin.
394 - **afb_req_context_set**:
395 store the context data of the plugin.
397 - **afb_req_context**:
398 retrieves the context data of the plugin,
399 if needed, creates the context and store it.
401 - **afb_req_context_clear**:
402 reset the stored data.
404 The plugin *tictactoe* use a convenient function to retrieve
405 its context: the board. This function is *board_of_req*:
408 * retrieves the board of the request
410 static inline struct board *board_of_req(struct afb_req req)
412 return afb_req_context(req, (void*)get_new_board, (void*)release_board);
415 The function **afb_req_context** ensure an existing context
416 for the session of the request.
417 Its two last arguments are functions. Here, the casts are required
418 to avoid a warning when compiling.
420 Here is the definition of the function **afb_req_context**
423 * Gets the pointer stored by the plugin for the session of 'req'.
424 * If the stored pointer is NULL, indicating that no pointer was
425 * already stored, afb_req_context creates a new context by calling
426 * the function 'create_context' and stores it with the freeing function
429 static inline void *afb_req_context(struct afb_req req, void *(*create_context)(), void (*free_context)(void*))
431 void *result = afb_req_context_get(req);
432 if (result == NULL) {
433 result = create_context();
434 afb_req_context_set(req, result, free_context);
439 The second argument if the function that creates the context.
440 For the plugin *tic-tac-toe* it is the function **get_new_board**.
441 The function **get_new_board** creates a new board and set its
442 count of use to 1. The boards are counting their count of use
443 to free there ressources when no more used.
445 The third argument if the function that frees the context.
446 For the plugin *tic-tac-toe* it is the function **release_board**.
447 The function **release_board** decrease the the count of use of
448 the board given as argument. If the use count decrease to zero,
449 the board data are freed.
451 The definition of the other functions for dealing with contexts are:
454 * Gets the pointer stored by the plugin for the session of 'req'.
455 * When the plugin has not yet recorded a pointer, NULL is returned.
457 void *afb_req_context_get(struct afb_req req);
460 * Stores for the plugin the pointer 'context' to the session of 'req'.
461 * The function 'free_context' will be called when the session is closed
462 * or if plugin stores an other pointer.
464 void afb_req_context_set(struct afb_req req, void *context, void (*free_context)(void*));
467 * Frees the pointer stored by the plugin for the session of 'req'
468 * and sets it to NULL.
470 * Shortcut for: afb_req_context_set(req, NULL, NULL)
472 static inline void afb_req_context_clear(struct afb_req req)
474 afb_req_context_set(req, NULL, NULL);
477 ### Sending the reply to a request
479 Two kinds of replies can be made: successful replies and
482 > Sending a reply to a request must be done at most one time.
484 The two functions to send a reply of kind "success" are
485 **afb_req_success** and **afb_req_success_f**.
488 * Sends a reply of kind success to the request 'req'.
489 * The status of the reply is automatically set to "success".
490 * Its send the object 'obj' (can be NULL) with an
491 * informationnal comment 'info (can also be NULL).
493 void afb_req_success(struct afb_req req, struct json_object *obj, const char *info);
496 * Same as 'afb_req_success' but the 'info' is a formatting
497 * string followed by arguments.
499 void afb_req_success_f(struct afb_req req, struct json_object *obj, const char *info, ...);
501 The two functions to send a reply of kind "failure" are
502 **afb_req_fail** and **afb_req_fail_f**.
505 * Sends a reply of kind failure to the request 'req'.
506 * The status of the reply is set to 'status' and an
507 * informationnal comment 'info' (can also be NULL) can be added.
509 * Note that calling afb_req_fail("success", info) is equivalent
510 * to call afb_req_success(NULL, info). Thus even if possible it
511 * is strongly recommanded to NEVER use "success" for status.
513 void afb_req_fail(struct afb_req req, const char *status, const char *info);
516 * Same as 'afb_req_fail' but the 'info' is a formatting
517 * string followed by arguments.
519 void afb_req_fail_f(struct afb_req req, const char *status, const char *info, ...);
521 Getting argument of invocation
522 ------------------------------
524 Many verbs expect arguments. Afb-daemon let plugins
525 retrieve their arguments by name not by position.
527 Arguments are given by the requests either through HTTP
528 or through WebSockets.
530 For example, the verb **join** of the plugin **tic-tac-toe**
531 expects one argument: the *boardid* to join. Here is an extract:
536 static void join(struct afb_req req)
538 struct board *board, *new_board;
541 /* retrieves the context for the session */
542 board = board_of_req(req);
543 INFO(afbitf, "method 'join' called for boardid %d", board->id);
545 /* retrieves the argument */
546 id = afb_req_value(req, "boardid");
551 The function **afb_req_value** search in the request *req*
552 for an argument whose name is given. When no argument of the
553 given name was passed, **afb_req_value** returns NULL.
555 > The search is case sensitive. So the name *boardid* is not the
556 > same name than *BoardId*. But this must not be assumed so two
557 > expected names of argument should not differ only by case.
559 ### Basic functions for querying arguments
561 The function **afb_req_value** is defined as below:
564 * Gets from the request 'req' the string value of the argument of 'name'.
565 * Returns NULL if when there is no argument of 'name'.
566 * Returns the value of the argument of 'name' otherwise.
568 * Shortcut for: afb_req_get(req, name).value
570 static inline const char *afb_req_value(struct afb_req req, const char *name)
572 return afb_req_get(req, name).value;
575 It is defined as a shortcut to call the function **afb_req_get**.
576 That function is defined as below:
579 * Gets from the request 'req' the argument of 'name'.
580 * Returns a PLAIN structure of type 'struct afb_arg'.
581 * When the argument of 'name' is not found, all fields of result are set to NULL.
582 * When the argument of 'name' is found, the fields are filled,
583 * in particular, the field 'result.name' is set to 'name'.
585 * There is a special name value: the empty string.
586 * The argument of name "" is defined only if the request was made using
587 * an HTTP POST of Content-Type "application/json". In that case, the
588 * argument of name "" receives the value of the body of the HTTP request.
590 struct afb_arg afb_req_get(struct afb_req req, const char *name);
592 That function takes 2 parameters: the request and the name
593 of the argument to retrieve. It returns a PLAIN structure of
594 type **struct afb_arg**.
596 There is a special name that is defined when the request is
597 of type HTTP/POST with a Content-Type being application/json.
598 This name is **""** (the empty string). In that case, the value
599 of this argument of empty name is the string received as a body
600 of the post and is supposed to be a JSON string.
602 The definition of **struct afb_arg** is:
605 * Describes an argument (or parameter) of a request
608 const char *name; /* name of the argument or NULL if invalid */
609 const char *value; /* string representation of the value of the argument */
610 /* original filename of the argument if path != NULL */
611 const char *path; /* if not NULL, path of the received file for the argument */
612 /* when the request is finalized this file is removed */
615 The structure returns the data arguments that are known for the
616 request. This data include a field named **path**. This **path**
617 can be accessed using the function **afb_req_path** defined as
621 * Gets from the request 'req' the path for file attached to the argument of 'name'.
622 * Returns NULL if when there is no argument of 'name' or when there is no file.
623 * Returns the path of the argument of 'name' otherwise.
625 * Shortcut for: afb_req_get(req, name).path
627 static inline const char *afb_req_path(struct afb_req req, const char *name)
629 return afb_req_get(req, name).path;
632 The path is only defined for HTTP/POST requests that send file.
634 ### Arguments for received files
636 As it is explained just above, clients can send files using
639 Received files are attached to a arguments. For example, the
640 following HTTP fragment (from test/sample-post.html)
641 will send an HTTP/POST request to the method
642 **post/upload-image** with 2 arguments named *file* and
645 <h2>Sample Post File</h2>
646 <form enctype="multipart/form-data">
647 <input type="file" name="file" />
648 <input type="hidden" name="hidden" value="bollobollo" />
650 <button formmethod="POST" formaction="api/post/upload-image">Post File</button>
653 In that case, the argument named **file** has its value and its
654 path defined and not NULL.
656 The value is the name of the file as it was
657 set by the HTTP client and is generally the filename on the
660 The path is the path of the file saved on the temporary local storage
661 area of the application. This is a randomly generated and unic filename
662 not linked in any way with the original filename on the client.
664 The plugin can use the file at the given path the way that it wants:
665 read, write, remove, copy, rename...
666 But when the reply is sent and the query is terminated, the file at
667 this path is destroyed if it still exist.
669 ### Arguments as a JSON object
671 Plugins can get all the arguments as one single object.
672 This feature is provided by the function **afb_req_json**
673 that is defined as below:
676 * Gets from the request 'req' the json object hashing the arguments.
677 * The returned object must not be released using 'json_object_put'.
679 struct json_object *afb_req_json(struct afb_req req);
681 It returns a json object. This object depends on how the request was
684 - For HTTP requests, this is an object whose keys are the names of the
685 arguments and whose values are either a string for common arguments or
686 an object like { "file": "...", "path": "..." }
688 - For WebSockets requests, the returned object is the object
689 given by the client transparently transported.
691 > In fact, for Websockets requests, the function **afb_req_value**
692 > can be seen as a shortcut to
693 > ***json_object_get_string(json_object_object_get(afb_req_json(req), name))***
695 Initialisation of the plugin and declaration of verbs
696 -----------------------------------------------------
698 To be active, the verbs of the plugin should be declared to
699 afb-daemon. And even more, the plugin itself must be recorded.
701 The mechanism for doing this is very simple: when afb-need starts,
702 it loads the plugins that are listed in its argument or configuration.
704 Loading a plugin follows the following steps:
706 1. It loads the plugin using *dlopen*.
708 2. It searchs for the symbol named **pluginAfbV1Register** using *dlsym*.
709 This symbol is assumed to be the exported initialisation function of the plugin.
711 3. It build an interface object for the plugin.
713 4. It calls the found function **pluginAfbV1Register** and pass it the pointer
716 5. The function **pluginAfbV1Register** setup the plugin, initialize it.
718 6. The function **pluginAfbV1Register** returns the pointer to a structure
719 that describes the plugin: its version, its name (prefix or API name), and the
722 7. Afb-daemon checks that the returned version and name can be managed.
723 If it can manage it, the plugin and its verbs are recorded and can be used
724 when afb-daemon finishes it initialisation.
726 Here is the listing of the function **pluginAfbV1Register** of the plugin
730 * activation function for registering the plugin called by afb-daemon
732 const struct AFB_plugin *pluginAfbV1Register(const struct AFB_interface *itf)
734 afbitf = itf; // records the interface for accessing afb-daemon
735 return &plugin_description; // returns the description of the plugin
738 This is a very small function because the *tic-tac-toe* plugin doesn't have initialisation step.
739 It merely record the daemon's interface and returns its descritption.
741 The variable **afbitf** is a variable global to the plugin. It records the
742 interface to afb-daemon and is used for logging and pushing events.
743 Here is its declaration:
746 * the interface to afb-daemon
748 const struct AFB_interface *afbitf;
750 The description of the plugin is defined as below.
753 * array of the verbs exported to afb-daemon
755 static const struct AFB_verb_desc_v1 plugin_verbs[] = {
756 /* VERB'S NAME SESSION MANAGEMENT FUNCTION TO CALL SHORT DESCRIPTION */
757 { .name= "new", .session= AFB_SESSION_NONE, .callback= new, .info= "Starts a new game" },
758 { .name= "play", .session= AFB_SESSION_NONE, .callback= play, .info= "Tells the server to play" },
759 { .name= "move", .session= AFB_SESSION_NONE, .callback= move, .info= "Tells the client move" },
760 { .name= "board", .session= AFB_SESSION_NONE, .callback= board, .info= "Get the current board" },
761 { .name= "level", .session= AFB_SESSION_NONE, .callback= level, .info= "Set the server level" },
762 { .name= "join", .session= AFB_SESSION_CHECK,.callback= join, .info= "Join a board" },
763 { .name= "undo", .session= AFB_SESSION_NONE, .callback= undo, .info= "Undo the last move" },
764 { .name= "wait", .session= AFB_SESSION_NONE, .callback= wait, .info= "Wait for a change" },
765 { .name= NULL } /* marker for end of the array */
769 * description of the plugin for afb-daemon
771 static const struct AFB_plugin plugin_description =
773 /* description conforms to VERSION 1 */
774 .type= AFB_PLUGIN_VERSION_1,
775 .v1= { /* fills the v1 field of the union when AFB_PLUGIN_VERSION_1 */
776 .prefix= "tictactoe", /* the API name (or plugin name or prefix) */
777 .info= "Sample tac-tac-toe game", /* short description of of the plugin */
778 .verbs = plugin_verbs /* the array describing the verbs of the API */
782 The structure **plugin_description** describes the plugin.
783 It declares the type and version of the plugin, its name, a description
784 and a list of its verbs.
786 The list of verbs is an array of structures describing the verbs and terminated by a marker:
787 a verb whose name is NULL.
789 The description of the verbs for this version is made of 4 fields:
791 - the name of the verbs,
793 - the session management flags,
795 - the implementation function to be call for the verb,
797 - a short description.
799 The structure describing verbs is defined as follows:
802 * Description of one verb of the API provided by the plugin
803 * This enumeration is valid for plugins of type 1
805 struct AFB_verb_desc_v1
807 const char *name; /* name of the verb */
808 enum AFB_session_v1 session; /* authorisation and session requirements of the verb */
809 void (*callback)(struct afb_req req); /* callback function implementing the verb */
810 const char *info; /* textual description of the verb */
813 For technical reasons, the enumeration **enum AFB_session_v1** is not exactly an
814 enumeration but the wrapper of constant definitions that can be mixed using bitwise or
817 The constants that can bit mixed are:
819 Constant name | Meaning
820 -------------------------|-------------------------------------------------------------
821 **AFB_SESSION_CREATE** | Equals to AFB_SESSION_LOA_EQ_0|AFB_SESSION_RENEW
822 **AFB_SESSION_CLOSE** | Closes the session after the reply and set the LOA to 0
823 **AFB_SESSION_RENEW** | Refreshes the token of authentification
824 **AFB_SESSION_CHECK** | Just requires the token authentification
825 **AFB_SESSION_LOA_LE_0** | Requires the current LOA to be lesser then or equal to 0
826 **AFB_SESSION_LOA_LE_1** | Requires the current LOA to be lesser then or equal to 1
827 **AFB_SESSION_LOA_LE_2** | Requires the current LOA to be lesser then or equal to 2
828 **AFB_SESSION_LOA_LE_3** | Requires the current LOA to be lesser then or equal to 3
829 **AFB_SESSION_LOA_GE_0** | Requires the current LOA to be greater then or equal to 0
830 **AFB_SESSION_LOA_GE_1** | Requires the current LOA to be greater then or equal to 1
831 **AFB_SESSION_LOA_GE_2** | Requires the current LOA to be greater then or equal to 2
832 **AFB_SESSION_LOA_GE_3** | Requires the current LOA to be greater then or equal to 3
833 **AFB_SESSION_LOA_EQ_0** | Requires the current LOA to be equal to 0
834 **AFB_SESSION_LOA_EQ_1** | Requires the current LOA to be equal to 1
835 **AFB_SESSION_LOA_EQ_2** | Requires the current LOA to be equal to 2
836 **AFB_SESSION_LOA_EQ_3** | Requires the current LOA to be equal to 3
838 If any of this flags is set, afb-daemon requires the token authentification
839 as if the flag **AFB_SESSION_CHECK** had been set.
841 The special value **AFB_SESSION_NONE** is zero and can be used to avoid any check.
843 > Note that **AFB_SESSION_CREATE** and **AFB_SESSION_CLOSE** might be removed in later versions.
845 Sending messages to the log system
846 ----------------------------------
848 Afb-daemon provides 4 levels of verbosity and 5 verbs for logging messages.
850 The verbosity is managed. Options allow the change the verbosity of afb-daemon
851 and the verbosity of the plugins can be set plugin by plugin.
853 The verbs for logging messages are defined as macros that test the
854 verbosity level and that call the real logging function only if the
855 message must be output. This avoid evaluation of arguments of the
856 formatting messages if the message must not be output.
858 ### Verbs for logging messages
860 The 5 logging verbs are:
862 Macro | Verbosity | Meaning | syslog level
863 --------|:---------:|-----------------------------------|:-----------:
864 ERROR | 0 | Error conditions | 3
865 WARNING | 1 | Warning conditions | 4
866 NOTICE | 1 | Normal but significant condition | 5
867 INFO | 2 | Informational | 6
868 DEBUG | 3 | Debug-level messages | 7
870 You can note that the 2 verbs **WARNING** and **INFO** have the same level
871 of verbosity. But they don't have the same *syslog level*. It means that
872 they are output with a different level on the logging system.
874 All of these verbs have the same signature:
876 void ERROR(const struct AFB_interface *afbitf, const char *message, ...);
878 The first argument **afbitf** is the interface to afb daemon that the
879 plugin received at its initialisation when **pluginAfbV1Register** was called.
881 The second argument **message** is a formatting string compatible with printf/sprintf.
883 The remaining arguments are arguments of the formating message like for printf.
885 ### Managing verbosity
887 Depending on the level of verbosity, the messages are output or not.
888 The following table explains what messages will be output depending
889 ont the verbosity level.
891 Level of verbosity | Outputed macro
892 :-----------------:|--------------------------
894 1 | ERROR + WARNING + NOTICE
895 2 | ERROR + WARNING + NOTICE + INFO
896 3 | ERROR + WARNING + NOTICE + INFO + DEBUG
898 ### Output format and destination
900 The syslog level is used for forging a prefix to the message.
903 syslog level | prefix
904 :-----------:|---------------
915 The message is issued to the standard error.
916 The final destination of the message depends on how the systemd service
917 was configured through the variable **StandardError**: It can be
918 journal, syslog or kmsg. (See man sd-daemon).
924 Writing an asynchronous verb implementation
925 -------------------------------------------
928 How to build a plugin
929 ---------------------
931 Afb-daemon provides a *pkg-config* configuration file that can be
932 queried by the name **afb-daemon**.
933 This configuration file provides data that should be used
934 for compiling plugins. Examples:
936 $ pkg-config --cflags afb-daemon
937 $ pkg-config --libs afb-daemon
939 ### Example for cmake meta build system
941 This example is the extract for building the plugin *afm-main* using *CMAKE*.
943 pkg_check_modules(afb afb-daemon)
945 message(STATUS "Creation afm-main-plugin for AFB-DAEMON")
946 add_library(afm-main-plugin MODULE afm-main-plugin.c)
947 target_compile_options(afm-main-plugin PRIVATE ${afb_CFLAGS})
948 target_include_directories(afm-main-plugin PRIVATE ${afb_INCLUDE_DIRS})
949 target_link_libraries(afm-main-plugin utils ${afb_LIBRARIES})
950 set_target_properties(afm-main-plugin PROPERTIES
952 LINK_FLAGS "-Wl,--version-script=${CMAKE_CURRENT_SOURCE_DIR}/afm-main-plugin.export-map"
954 install(TARGETS afm-main-plugin LIBRARY DESTINATION ${plugin_dir})
956 message(STATUS "Not creating the plugin for AFB-DAEMON")
959 Let now describe some of these lines.
961 pkg_check_modules(afb afb-daemon)
963 This first lines searches to the *pkg-config* configuration file for
964 **afb-daemon**. Resulting data are stored in the following variables:
967 ------------------|------------------------------------------------
968 afb_FOUND | Set to 1 if afb-daemon plugin development files exist
969 afb_LIBRARIES | Only the libraries (w/o the '-l') for compiling afb-daemon plugins
970 afb_LIBRARY_DIRS | The paths of the libraries (w/o the '-L') for compiling afb-daemon plugins
971 afb_LDFLAGS | All required linker flags for compiling afb-daemon plugins
972 afb_INCLUDE_DIRS | The '-I' preprocessor flags (w/o the '-I') for compiling afb-daemon plugins
973 afb_CFLAGS | All required cflags for compiling afb-daemon plugins
975 If development files are found, the plugin can be added to the set of
978 add_library(afm-main-plugin MODULE afm-main-plugin.c)
980 This line asks to create a shared library having only the
981 source file afm-main-plugin.c (that is compiled).
982 The default name of the created shared object is
983 **libafm-main-plugin.so**.
985 set_target_properties(afm-main-plugin PROPERTIES
987 LINK_FLAGS "-Wl,--version-script=${CMAKE_CURRENT_SOURCE_DIR}/afm-main-plugin.export-map"
990 This lines are doing two things:
992 1. It renames the built library from **libafm-main-plugin.so** to **afm-main-plugin.so**
993 by removing the implicitely added prefix *lib*. This step is not mandatory
994 at all because afb-daemon doesn't check names of files when loading it.
995 The only convention that use afb-daemon is that extension is **.so**
996 but this convention is used only when afb-daemon discovers plugin
997 from a directory hierarchy.
999 2. It applies a version script at link to only export the conventional name
1000 of the entry point: **pluginAfbV1Register**. See below. By default, the linker
1001 that creates the shared object exports all the public symbols (C functions that
1002 are not **static**).
1006 target_include_directories(afm-main-plugin PRIVATE ${afb_INCLUDE_DIRS})
1007 target_link_libraries(afm-main-plugin utils ${afb_LIBRARIES})
1009 As you can see it uses the variables computed by ***pkg_check_modules(afb afb-daemon)***
1010 to configure the compiler and the linker.
1012 ### Exporting the function pluginAfbV1Register
1014 The function **pluginAfbV1Register** must be exported. This can be achieved
1015 using a version script when linking. Here is the version script that is
1016 used for *tic-tac-toe* (plugins/samples/export.map).
1018 { global: pluginAfbV1Register; local: *; };
1020 This sample [version script](https://sourceware.org/binutils/docs-2.26/ld/VERSION.html#VERSION)
1021 exports as global the symbol *pluginAfbV1Register* and hides any
1024 This version script is added to the link options using the
1025 option **--version-script=export.map** is given directly to the
1026 linker or using th option **-Wl,--version-script=export.map**
1027 when the option is given to the C compiler.
1029 ### Building within yocto
1031 Adding a dependency to afb-daemon is enough. See below:
1033 DEPENDS += " afb-daemon "