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***.
181 Dependencies when compiling
182 ---------------------------
184 Afb-daemon provides a configuration file for *pkg-config*.
187 pkg-config --cflags afb-daemon
189 will print the flags to use for compiling, like this:
191 $ pkg-config --cflags afb-daemon
192 -I/opt/local/include -I/usr/include/json-c
194 For linking, you should use
196 $ pkg-config --libs afb-daemon
199 As you see, afb-daemon automatically includes dependency to json-c.
200 This is done through the **Requires** keyword of pkg-config
201 because almost all plugin will use **json-c**.
203 If this behaviour is a problem, let us know.
205 Internally, afb-daemon uses **libsystemd** for its event loop
206 and for its binding to D-Bus.
207 Plugins developpers are encouraged to also use this library.
208 But it is a matter of choice.
209 Thus there is no dependency to **libsystemd**.
211 > Afb-daemon provides no library for plugins.
212 > The functions that the plugin need to have are given
213 > to the plugin at runtime through pointer using read-only
216 Header files to include
217 -----------------------
219 The plugin *tictactoe* has the following lines for its includes:
224 #include <json-c/json.h>
225 #include <afb/afb-plugin.h>
227 The header *afb/afb-plugin.h* includes all the features that a plugin
228 needs except two foreign header that must be included by the plugin
231 - *json-c/json.h*: this header must be include to handle json objects;
232 - *systemd/sd-event.h*: this must be include to access the main loop;
233 - *systemd/sd-bus.h*: this may be include to use dbus connections.
235 The *tictactoe* plugin does not use systemd features so it is not included.
237 When including *afb/afb-plugin.h*, the macro **_GNU_SOURCE** must be
243 The designer of a plugin must defines names for its plugin
244 (or its API) and for the verbs of its API. He also
245 must defines names for arguments given by name.
247 While forging names, the designer should take into account
248 the rules for making valid names and some rules that make
249 the names easy to use across plaforms.
251 The names and strings used ALL are UTF-8 encoded.
253 ### Names for API (plugin)
255 The names of the API are checked.
256 All characters are authorised except:
258 - the control characters (\u0000 .. \u001f)
259 - the characters of the set { ' ', '"', '#', '%', '&',
260 '\'', '/', '?', '`', '\x7f' }
262 In other words the set of forbidden characters is
263 { \u0000..\u0020, \u0022, \u0023, \u0025..\u0027,
264 \u002f, \u003f, \u0060, \u007f }.
266 Afb-daemon make no distinction between lower case
267 and upper case when searching for an API by its name.
271 The names of the verbs are not checked.
273 However, the validity rules for verb's names are the
274 same as for API's names except that the dot (.) character
277 Afb-daemon make no distinction between lower case
278 and upper case when searching for an API by its name.
280 ### Names for arguments
282 The names for arguments are not restricted and can be
285 The arguments are searched with the case sensitive
286 string comparison. Thus the names "index" and "Index"
289 ### Forging names widely available
291 The key names of javascript object can be almost
292 anything using the arrayed notation:
296 That is not the case with the dot notation:
300 Using the dot notation, the key must be a valid javascript
303 For this reason, the chosen names should better be
304 valid javascript identifier.
306 It is also a good practice, even for arguments, to not
307 rely on the case sensitivity and to avoid the use of
308 names different only by the case.
310 Writing a synchronous verb implementation
311 -----------------------------------------
313 The verb **tictactoe/board** is a synchronous implementation.
319 static void board(struct afb_req req)
322 struct json_object *description;
324 /* retrieves the context for the session */
325 board = board_of_req(req);
326 INFO(afbitf, "method 'board' called for boardid %d", board->id);
328 /* describe the board */
329 description = describe(board);
331 /* send the board's description */
332 afb_req_success(req, description, NULL);
335 This examples show many aspects of writing a synchronous
336 verb implementation. Let summarize it:
338 1. The function **board_of_req** retrieves the context stored
339 for the plugin: the board.
341 2. The macro **INFO** sends a message of kind *INFO*
342 to the logging system. The global variable named **afbitf**
343 used represents the interface to afb-daemon.
345 3. The function **describe** creates a json_object representing
348 4. The function **afb_req_success** sends the reply, attaching to
349 it the object *description*.
351 ### The incoming request
353 For any implementation, the request is received by a structure of type
356 > Note that this is a PLAIN structure, not a pointer to a structure.
358 The definition of **struct afb_req** is:
361 * Describes the request by plugins from afb-daemon
364 const struct afb_req_itf *itf; /* the interfacing functions */
365 void *closure; /* the closure for functions */
368 It contains two pointers: one, *itf*, points to the functions needed
369 to handle the internal request represented by the second pointer, *closure*.
371 > The structure must never be used directly.
372 > Insted, use the intended functions provided
373 > by afb-daemon and described here.
375 *req* is used to get arguments of the request, to send
376 answer, to store session data.
378 This object and its interface is defined and documented
379 in the file names *afb/afb-req-itf.h*
381 The above example uses 2 times the request object *req*.
383 The first time, it is used for retrieving the board attached to
384 the session of the request.
386 The second time, it is used to send the reply: an object that
387 describes the current board.
389 ### Associating a context to the session
391 When the plugin *tic-tac-toe* receives a request, it musts regain
392 the board that describes the game associated to the session.
394 For a plugin, having data associated to a session is a common case.
395 This data is called the context of the plugin for the session.
396 For the plugin *tic-tac-toe*, the context is the board.
398 The requests *afb_req* offer four functions for
399 storing and retrieving the context associated to the session.
403 - **afb_req_context_get**:
404 retrieves the context data stored for the plugin.
406 - **afb_req_context_set**:
407 store the context data of the plugin.
409 - **afb_req_context**:
410 retrieves the context data of the plugin,
411 if needed, creates the context and store it.
413 - **afb_req_context_clear**:
414 reset the stored data.
416 The plugin *tictactoe* use a convenient function to retrieve
417 its context: the board. This function is *board_of_req*:
420 * retrieves the board of the request
422 static inline struct board *board_of_req(struct afb_req req)
424 return afb_req_context(req, (void*)get_new_board, (void*)release_board);
427 The function **afb_req_context** ensure an existing context
428 for the session of the request.
429 Its two last arguments are functions. Here, the casts are required
430 to avoid a warning when compiling.
432 Here is the definition of the function **afb_req_context**
435 * Gets the pointer stored by the plugin for the session of 'req'.
436 * If the stored pointer is NULL, indicating that no pointer was
437 * already stored, afb_req_context creates a new context by calling
438 * the function 'create_context' and stores it with the freeing function
441 static inline void *afb_req_context(struct afb_req req, void *(*create_context)(), void (*free_context)(void*))
443 void *result = afb_req_context_get(req);
444 if (result == NULL) {
445 result = create_context();
446 afb_req_context_set(req, result, free_context);
451 The second argument if the function that creates the context.
452 For the plugin *tic-tac-toe* it is the function **get_new_board**.
453 The function **get_new_board** creates a new board and set its
454 count of use to 1. The boards are counting their count of use
455 to free there ressources when no more used.
457 The third argument if the function that frees the context.
458 For the plugin *tic-tac-toe* it is the function **release_board**.
459 The function **release_board** decrease the the count of use of
460 the board given as argument. If the use count decrease to zero,
461 the board data are freed.
463 The definition of the other functions for dealing with contexts are:
466 * Gets the pointer stored by the plugin for the session of 'req'.
467 * When the plugin has not yet recorded a pointer, NULL is returned.
469 void *afb_req_context_get(struct afb_req req);
472 * Stores for the plugin the pointer 'context' to the session of 'req'.
473 * The function 'free_context' will be called when the session is closed
474 * or if plugin stores an other pointer.
476 void afb_req_context_set(struct afb_req req, void *context, void (*free_context)(void*));
479 * Frees the pointer stored by the plugin for the session of 'req'
480 * and sets it to NULL.
482 * Shortcut for: afb_req_context_set(req, NULL, NULL)
484 static inline void afb_req_context_clear(struct afb_req req)
486 afb_req_context_set(req, NULL, NULL);
489 ### Sending the reply to a request
491 Two kinds of replies can be made: successful replies and
494 > Sending a reply to a request must be done at most one time.
496 The two functions to send a reply of kind "success" are
497 **afb_req_success** and **afb_req_success_f**.
500 * Sends a reply of kind success to the request 'req'.
501 * The status of the reply is automatically set to "success".
502 * Its send the object 'obj' (can be NULL) with an
503 * informationnal comment 'info (can also be NULL).
505 * For conveniency, the function calls 'json_object_put' for 'obj'.
506 * Thus, in the case where 'obj' should remain available after
507 * the function returns, the function 'json_object_get' shall be used.
509 void afb_req_success(struct afb_req req, struct json_object *obj, const char *info);
512 * Same as 'afb_req_success' but the 'info' is a formatting
513 * string followed by arguments.
515 * For conveniency, the function calls 'json_object_put' for 'obj'.
516 * Thus, in the case where 'obj' should remain available after
517 * the function returns, the function 'json_object_get' shall be used.
519 void afb_req_success_f(struct afb_req req, struct json_object *obj, const char *info, ...);
521 The two functions to send a reply of kind "failure" are
522 **afb_req_fail** and **afb_req_fail_f**.
525 * Sends a reply of kind failure to the request 'req'.
526 * The status of the reply is set to 'status' and an
527 * informationnal comment 'info' (can also be NULL) can be added.
529 * Note that calling afb_req_fail("success", info) is equivalent
530 * to call afb_req_success(NULL, info). Thus even if possible it
531 * is strongly recommanded to NEVER use "success" for status.
533 * For conveniency, the function calls 'json_object_put' for 'obj'.
534 * Thus, in the case where 'obj' should remain available after
535 * the function returns, the function 'json_object_get' shall be used.
537 void afb_req_fail(struct afb_req req, const char *status, const char *info);
540 * Same as 'afb_req_fail' but the 'info' is a formatting
541 * string followed by arguments.
543 * For conveniency, the function calls 'json_object_put' for 'obj'.
544 * Thus, in the case where 'obj' should remain available after
545 * the function returns, the function 'json_object_get' shall be used.
547 void afb_req_fail_f(struct afb_req req, const char *status, const char *info, ...);
549 > For conveniency, these functions call **json_object_put** to release the object **obj**
550 > that they send. Then **obj** can not be used after calling one of these reply functions.
551 > When it is not the expected behaviour, calling the function **json_object_get** on the object **obj**
552 > before cancels the effect of **json_object_put**.
554 Getting argument of invocation
555 ------------------------------
557 Many verbs expect arguments. Afb-daemon let plugins
558 retrieve their arguments by name not by position.
560 Arguments are given by the requests either through HTTP
561 or through WebSockets.
563 For example, the verb **join** of the plugin **tic-tac-toe**
564 expects one argument: the *boardid* to join. Here is an extract:
569 static void join(struct afb_req req)
571 struct board *board, *new_board;
574 /* retrieves the context for the session */
575 board = board_of_req(req);
576 INFO(afbitf, "method 'join' called for boardid %d", board->id);
578 /* retrieves the argument */
579 id = afb_req_value(req, "boardid");
584 The function **afb_req_value** search in the request *req*
585 for an argument whose name is given. When no argument of the
586 given name was passed, **afb_req_value** returns NULL.
588 > The search is case sensitive. So the name *boardid* is not the
589 > same name than *BoardId*. But this must not be assumed so two
590 > expected names of argument should not differ only by case.
592 ### Basic functions for querying arguments
594 The function **afb_req_value** is defined as below:
597 * Gets from the request 'req' the string value of the argument of 'name'.
598 * Returns NULL if when there is no argument of 'name'.
599 * Returns the value of the argument of 'name' otherwise.
601 * Shortcut for: afb_req_get(req, name).value
603 static inline const char *afb_req_value(struct afb_req req, const char *name)
605 return afb_req_get(req, name).value;
608 It is defined as a shortcut to call the function **afb_req_get**.
609 That function is defined as below:
612 * Gets from the request 'req' the argument of 'name'.
613 * Returns a PLAIN structure of type 'struct afb_arg'.
614 * When the argument of 'name' is not found, all fields of result are set to NULL.
615 * When the argument of 'name' is found, the fields are filled,
616 * in particular, the field 'result.name' is set to 'name'.
618 * There is a special name value: the empty string.
619 * The argument of name "" is defined only if the request was made using
620 * an HTTP POST of Content-Type "application/json". In that case, the
621 * argument of name "" receives the value of the body of the HTTP request.
623 struct afb_arg afb_req_get(struct afb_req req, const char *name);
625 That function takes 2 parameters: the request and the name
626 of the argument to retrieve. It returns a PLAIN structure of
627 type **struct afb_arg**.
629 There is a special name that is defined when the request is
630 of type HTTP/POST with a Content-Type being application/json.
631 This name is **""** (the empty string). In that case, the value
632 of this argument of empty name is the string received as a body
633 of the post and is supposed to be a JSON string.
635 The definition of **struct afb_arg** is:
638 * Describes an argument (or parameter) of a request
641 const char *name; /* name of the argument or NULL if invalid */
642 const char *value; /* string representation of the value of the argument */
643 /* original filename of the argument if path != NULL */
644 const char *path; /* if not NULL, path of the received file for the argument */
645 /* when the request is finalized this file is removed */
648 The structure returns the data arguments that are known for the
649 request. This data include a field named **path**. This **path**
650 can be accessed using the function **afb_req_path** defined as
654 * Gets from the request 'req' the path for file attached to the argument of 'name'.
655 * Returns NULL if when there is no argument of 'name' or when there is no file.
656 * Returns the path of the argument of 'name' otherwise.
658 * Shortcut for: afb_req_get(req, name).path
660 static inline const char *afb_req_path(struct afb_req req, const char *name)
662 return afb_req_get(req, name).path;
665 The path is only defined for HTTP/POST requests that send file.
667 ### Arguments for received files
669 As it is explained just above, clients can send files using
672 Received files are attached to a arguments. For example, the
673 following HTTP fragment (from test/sample-post.html)
674 will send an HTTP/POST request to the method
675 **post/upload-image** with 2 arguments named *file* and
678 <h2>Sample Post File</h2>
679 <form enctype="multipart/form-data">
680 <input type="file" name="file" />
681 <input type="hidden" name="hidden" value="bollobollo" />
683 <button formmethod="POST" formaction="api/post/upload-image">Post File</button>
686 In that case, the argument named **file** has its value and its
687 path defined and not NULL.
689 The value is the name of the file as it was
690 set by the HTTP client and is generally the filename on the
693 The path is the path of the file saved on the temporary local storage
694 area of the application. This is a randomly generated and unic filename
695 not linked in any way with the original filename on the client.
697 The plugin can use the file at the given path the way that it wants:
698 read, write, remove, copy, rename...
699 But when the reply is sent and the query is terminated, the file at
700 this path is destroyed if it still exist.
702 ### Arguments as a JSON object
704 Plugins can get all the arguments as one single object.
705 This feature is provided by the function **afb_req_json**
706 that is defined as below:
709 * Gets from the request 'req' the json object hashing the arguments.
710 * The returned object must not be released using 'json_object_put'.
712 struct json_object *afb_req_json(struct afb_req req);
714 It returns a json object. This object depends on how the request was
717 - For HTTP requests, this is an object whose keys are the names of the
718 arguments and whose values are either a string for common arguments or
719 an object like { "file": "...", "path": "..." }
721 - For WebSockets requests, the returned object is the object
722 given by the client transparently transported.
724 > In fact, for Websockets requests, the function **afb_req_value**
725 > can be seen as a shortcut to
726 > ***json_object_get_string(json_object_object_get(afb_req_json(req), name))***
728 Initialisation of the plugin and declaration of verbs
729 -----------------------------------------------------
731 To be active, the verbs of the plugin should be declared to
732 afb-daemon. And even more, the plugin itself must be recorded.
734 The mechanism for doing this is very simple: when afb-need starts,
735 it loads the plugins that are listed in its argument or configuration.
737 Loading a plugin follows the following steps:
739 1. It loads the plugin using *dlopen*.
741 2. It searchs for the symbol named **pluginAfbV1Register** using *dlsym*.
742 This symbol is assumed to be the exported initialisation function of the plugin.
744 3. It build an interface object for the plugin.
746 4. It calls the found function **pluginAfbV1Register** and pass it the pointer
749 5. The function **pluginAfbV1Register** setup the plugin, initialize it.
751 6. The function **pluginAfbV1Register** returns the pointer to a structure
752 that describes the plugin: its version, its name (prefix or API name), and the
755 7. Afb-daemon checks that the returned version and name can be managed.
756 If it can manage it, the plugin and its verbs are recorded and can be used
757 when afb-daemon finishes it initialisation.
759 Here is the listing of the function **pluginAfbV1Register** of the plugin
763 * activation function for registering the plugin called by afb-daemon
765 const struct AFB_plugin *pluginAfbV1Register(const struct AFB_interface *itf)
767 afbitf = itf; // records the interface for accessing afb-daemon
768 return &plugin_description; // returns the description of the plugin
771 This is a very small function because the *tic-tac-toe* plugin doesn't have initialisation step.
772 It merely record the daemon's interface and returns its descritption.
774 The variable **afbitf** is a variable global to the plugin. It records the
775 interface to afb-daemon and is used for logging and pushing events.
776 Here is its declaration:
779 * the interface to afb-daemon
781 const struct AFB_interface *afbitf;
783 The description of the plugin is defined as below.
786 * array of the verbs exported to afb-daemon
788 static const struct AFB_verb_desc_v1 plugin_verbs[] = {
789 /* VERB'S NAME SESSION MANAGEMENT FUNCTION TO CALL SHORT DESCRIPTION */
790 { .name= "new", .session= AFB_SESSION_NONE, .callback= new, .info= "Starts a new game" },
791 { .name= "play", .session= AFB_SESSION_NONE, .callback= play, .info= "Asks the server to play" },
792 { .name= "move", .session= AFB_SESSION_NONE, .callback= move, .info= "Tells the client move" },
793 { .name= "board", .session= AFB_SESSION_NONE, .callback= board, .info= "Get the current board" },
794 { .name= "level", .session= AFB_SESSION_NONE, .callback= level, .info= "Set the server level" },
795 { .name= "join", .session= AFB_SESSION_CHECK,.callback= join, .info= "Join a board" },
796 { .name= "undo", .session= AFB_SESSION_NONE, .callback= undo, .info= "Undo the last move" },
797 { .name= "wait", .session= AFB_SESSION_NONE, .callback= wait, .info= "Wait for a change" },
798 { .name= NULL } /* marker for end of the array */
802 * description of the plugin for afb-daemon
804 static const struct AFB_plugin plugin_description =
806 /* description conforms to VERSION 1 */
807 .type= AFB_PLUGIN_VERSION_1,
808 .v1= { /* fills the v1 field of the union when AFB_PLUGIN_VERSION_1 */
809 .prefix= "tictactoe", /* the API name (or plugin name or prefix) */
810 .info= "Sample tac-tac-toe game", /* short description of of the plugin */
811 .verbs = plugin_verbs /* the array describing the verbs of the API */
815 The structure **plugin_description** describes the plugin.
816 It declares the type and version of the plugin, its name, a description
817 and a list of its verbs.
819 The list of verbs is an array of structures describing the verbs and terminated by a marker:
820 a verb whose name is NULL.
822 The description of the verbs for this version is made of 4 fields:
824 - the name of the verbs,
826 - the session management flags,
828 - the implementation function to be call for the verb,
830 - a short description.
832 The structure describing verbs is defined as follows:
835 * Description of one verb of the API provided by the plugin
836 * This enumeration is valid for plugins of type 1
838 struct AFB_verb_desc_v1
840 const char *name; /* name of the verb */
841 enum AFB_session_v1 session; /* authorisation and session requirements of the verb */
842 void (*callback)(struct afb_req req); /* callback function implementing the verb */
843 const char *info; /* textual description of the verb */
846 For technical reasons, the enumeration **enum AFB_session_v1** is not exactly an
847 enumeration but the wrapper of constant definitions that can be mixed using bitwise or
850 The constants that can bit mixed are:
852 Constant name | Meaning
853 -------------------------|-------------------------------------------------------------
854 **AFB_SESSION_CREATE** | Equals to AFB_SESSION_LOA_EQ_0|AFB_SESSION_RENEW
855 **AFB_SESSION_CLOSE** | Closes the session after the reply and set the LOA to 0
856 **AFB_SESSION_RENEW** | Refreshes the token of authentification
857 **AFB_SESSION_CHECK** | Just requires the token authentification
858 **AFB_SESSION_LOA_LE_0** | Requires the current LOA to be lesser then or equal to 0
859 **AFB_SESSION_LOA_LE_1** | Requires the current LOA to be lesser then or equal to 1
860 **AFB_SESSION_LOA_LE_2** | Requires the current LOA to be lesser then or equal to 2
861 **AFB_SESSION_LOA_LE_3** | Requires the current LOA to be lesser then or equal to 3
862 **AFB_SESSION_LOA_GE_0** | Requires the current LOA to be greater then or equal to 0
863 **AFB_SESSION_LOA_GE_1** | Requires the current LOA to be greater then or equal to 1
864 **AFB_SESSION_LOA_GE_2** | Requires the current LOA to be greater then or equal to 2
865 **AFB_SESSION_LOA_GE_3** | Requires the current LOA to be greater then or equal to 3
866 **AFB_SESSION_LOA_EQ_0** | Requires the current LOA to be equal to 0
867 **AFB_SESSION_LOA_EQ_1** | Requires the current LOA to be equal to 1
868 **AFB_SESSION_LOA_EQ_2** | Requires the current LOA to be equal to 2
869 **AFB_SESSION_LOA_EQ_3** | Requires the current LOA to be equal to 3
871 If any of this flags is set, afb-daemon requires the token authentification
872 as if the flag **AFB_SESSION_CHECK** had been set.
874 The special value **AFB_SESSION_NONE** is zero and can be used to avoid any check.
876 > Note that **AFB_SESSION_CREATE** and **AFB_SESSION_CLOSE** might be removed in later versions.
878 Sending messages to the log system
879 ----------------------------------
881 Afb-daemon provides 4 levels of verbosity and 5 verbs for logging messages.
883 The verbosity is managed. Options allow the change the verbosity of afb-daemon
884 and the verbosity of the plugins can be set plugin by plugin.
886 The verbs for logging messages are defined as macros that test the
887 verbosity level and that call the real logging function only if the
888 message must be output. This avoid evaluation of arguments of the
889 formatting messages if the message must not be output.
891 ### Verbs for logging messages
893 The 5 logging verbs are:
895 Macro | Verbosity | Meaning | syslog level
896 --------|:---------:|-----------------------------------|:-----------:
897 ERROR | 0 | Error conditions | 3
898 WARNING | 1 | Warning conditions | 4
899 NOTICE | 1 | Normal but significant condition | 5
900 INFO | 2 | Informational | 6
901 DEBUG | 3 | Debug-level messages | 7
903 You can note that the 2 verbs **WARNING** and **INFO** have the same level
904 of verbosity. But they don't have the same *syslog level*. It means that
905 they are output with a different level on the logging system.
907 All of these verbs have the same signature:
909 void ERROR(const struct AFB_interface *afbitf, const char *message, ...);
911 The first argument **afbitf** is the interface to afb daemon that the
912 plugin received at its initialisation when **pluginAfbV1Register** was called.
914 The second argument **message** is a formatting string compatible with printf/sprintf.
916 The remaining arguments are arguments of the formating message like for printf.
918 ### Managing verbosity
920 Depending on the level of verbosity, the messages are output or not.
921 The following table explains what messages will be output depending
922 ont the verbosity level.
924 Level of verbosity | Outputed macro
925 :-----------------:|--------------------------
927 1 | ERROR + WARNING + NOTICE
928 2 | ERROR + WARNING + NOTICE + INFO
929 3 | ERROR + WARNING + NOTICE + INFO + DEBUG
931 ### Output format and destination
933 The syslog level is used for forging a prefix to the message.
936 syslog level | prefix
937 :-----------:|---------------
948 The message is issued to the standard error.
949 The final destination of the message depends on how the systemd service
950 was configured through the variable **StandardError**: It can be
951 journal, syslog or kmsg. (See man sd-daemon).
956 Since version 0.5, plugins can broadcast events to any potential listener.
957 This kind of bradcast is not targeted. Event targeted will come in a future
958 version of afb-daemon.
960 The plugin *tic-tac-toe* broadcasts events when the board changes.
961 This is done in the function **changed**:
964 * signals a change of the board
966 static void changed(struct board *board, const char *reason)
969 struct json_object *description;
971 /* get the description */
972 description = describe(board);
976 afb_daemon_broadcast_event(afbitf->daemon, reason, description);
979 The description of the changed board is pushed via the daemon interface.
981 Within the plugin *tic-tac-toe*, the *reason* indicates the origin of
982 the change. For the function **afb_daemon_broadcast_event**, the second
983 parameter is the name of the broadcasted event. The third argument is the
984 object that is transmitted with the event.
986 The function **afb_daemon_broadcast_event** is defined as below:
989 * Broadcasts widely the event of 'name' with the data 'object'.
990 * 'object' can be NULL.
991 * 'daemon' MUST be the daemon given in interface when activating the plugin.
993 * For conveniency, the function calls 'json_object_put' for 'object'.
994 * Thus, in the case where 'object' should remain available after
995 * the function returns, the function 'json_object_get' shall be used.
997 void afb_daemon_broadcast_event(struct afb_daemon daemon, const char *name, struct json_object *object);
999 > Be aware, as for reply functions, the **object** is automatically released using
1000 > **json_object_put** by the function. Then call **json_object_get** before
1001 > calling **afb_daemon_broadcast_event** to keep **object** available
1002 > after the returning of the function.
1004 In fact the event name received by the listener is prefixed with
1005 the name of the plugin. So when the change occurs after a move, the
1006 reason is **move** and then the clients receive the event **tictactoe/move**.
1008 > Note that nothing is said about the case sensitivity of event names.
1009 > However, the event is always prefixed with the name that the plugin
1010 > declared, with the same case, followed with a slash /.
1011 > Thus it is safe to compare event using a case sensitive comparison.
1015 Writing an asynchronous verb implementation
1016 -------------------------------------------
1018 The *tic-tac-toe* example allows two clients or more to share the same board.
1019 This is implemented by the verb **join** that illustrated partly the how to
1022 When two or more clients are sharing a same board, one of them can wait
1023 until the state of the board changes. (This coulded also be implemented using
1024 events because an even is generated each time the board changes).
1026 In this case, the reply to the wait is sent only when the board changes.
1027 See the diagram below:
1029 CLIENT A CLIENT B TIC-TAC-TOE
1031 +--------------|----------------->| wait . . . . . . . .
1036 | +----------------->| move . . . .
1038 | |<-----------------+ success of move .
1040 |<-------------|------------------+ success of wait <
1042 Here, this is an invocation of the plugin by an other client that
1043 unblock the suspended *wait* call.
1044 But in general, this will be a timer, a hardware event, the sync with
1045 a concurrent process or thread, ...
1047 So the case is common, this is an asynchronous implementation.
1049 Here is the listing of the function **wait**:
1051 static void wait(struct afb_req req)
1053 struct board *board;
1054 struct waiter *waiter;
1056 /* retrieves the context for the session */
1057 board = board_of_req(req);
1058 INFO(afbitf, "method 'wait' called for boardid %d", board->id);
1060 /* creates the waiter and enqueues it */
1061 waiter = calloc(1, sizeof *waiter);
1063 waiter->next = board->waiters;
1064 afb_req_addref(req);
1065 board->waiters = waiter;
1068 After retrieving the board, the function adds a new waiter to the
1069 current list of waiters and returns without sending a reply.
1071 Before returning, it increases the reference count of the
1072 request **req** using the function **afb_req_addref**.
1074 > When the implentation of a verb returns without sending a reply,
1075 > it **MUST** increment the reference count of the request
1076 > using **afb_req_addref**. If it doesn't bad things can happen.
1078 Later, when the board changes, it calls the function **changed**
1079 of *tic-tac-toe* with the reason of the change.
1081 Here is the full listing of the function **changed**:
1084 * signals a change of the board
1086 static void changed(struct board *board, const char *reason)
1088 struct waiter *waiter, *next;
1089 struct json_object *description;
1091 /* get the description */
1092 description = describe(board);
1094 waiter = board->waiters;
1095 board->waiters = NULL;
1096 while (waiter != NULL) {
1097 next = waiter->next;
1098 afb_req_success(waiter->req, json_object_get(description), reason);
1099 afb_req_unref(waiter->req);
1104 afb_event_sender_push(afb_daemon_get_event_sender(afbitf->daemon), reason, description);
1107 The list of waiters is walked and a reply is sent to each waiter.
1108 After the sending the reply, the reference count of the request
1109 is decremented using **afb_req_unref** to allow its resources to be freed.
1111 > The reference count **MUST** be decremented using **afb_req_unref** because,
1112 > otherwise, there is a leak of resources.
1113 > It must be decremented **AFTER** the sending of the reply, because, otherwise,
1114 > bad things may happen.
1116 How to build a plugin
1117 ---------------------
1119 Afb-daemon provides a *pkg-config* configuration file that can be
1120 queried by the name **afb-daemon**.
1121 This configuration file provides data that should be used
1122 for compiling plugins. Examples:
1124 $ pkg-config --cflags afb-daemon
1125 $ pkg-config --libs afb-daemon
1127 ### Example for cmake meta build system
1129 This example is the extract for building the plugin *afm-main* using *CMAKE*.
1131 pkg_check_modules(afb afb-daemon)
1133 message(STATUS "Creation afm-main-plugin for AFB-DAEMON")
1134 add_library(afm-main-plugin MODULE afm-main-plugin.c)
1135 target_compile_options(afm-main-plugin PRIVATE ${afb_CFLAGS})
1136 target_include_directories(afm-main-plugin PRIVATE ${afb_INCLUDE_DIRS})
1137 target_link_libraries(afm-main-plugin utils ${afb_LIBRARIES})
1138 set_target_properties(afm-main-plugin PROPERTIES
1140 LINK_FLAGS "-Wl,--version-script=${CMAKE_CURRENT_SOURCE_DIR}/afm-main-plugin.export-map"
1142 install(TARGETS afm-main-plugin LIBRARY DESTINATION ${plugin_dir})
1144 message(STATUS "Not creating the plugin for AFB-DAEMON")
1147 Let now describe some of these lines.
1149 pkg_check_modules(afb afb-daemon)
1151 This first lines searches to the *pkg-config* configuration file for
1152 **afb-daemon**. Resulting data are stored in the following variables:
1155 ------------------|------------------------------------------------
1156 afb_FOUND | Set to 1 if afb-daemon plugin development files exist
1157 afb_LIBRARIES | Only the libraries (w/o the '-l') for compiling afb-daemon plugins
1158 afb_LIBRARY_DIRS | The paths of the libraries (w/o the '-L') for compiling afb-daemon plugins
1159 afb_LDFLAGS | All required linker flags for compiling afb-daemon plugins
1160 afb_INCLUDE_DIRS | The '-I' preprocessor flags (w/o the '-I') for compiling afb-daemon plugins
1161 afb_CFLAGS | All required cflags for compiling afb-daemon plugins
1163 If development files are found, the plugin can be added to the set of
1166 add_library(afm-main-plugin MODULE afm-main-plugin.c)
1168 This line asks to create a shared library having only the
1169 source file afm-main-plugin.c (that is compiled).
1170 The default name of the created shared object is
1171 **libafm-main-plugin.so**.
1173 set_target_properties(afm-main-plugin PROPERTIES
1175 LINK_FLAGS "-Wl,--version-script=${CMAKE_CURRENT_SOURCE_DIR}/afm-main-plugin.export-map"
1178 This lines are doing two things:
1180 1. It renames the built library from **libafm-main-plugin.so** to **afm-main-plugin.so**
1181 by removing the implicitely added prefix *lib*. This step is not mandatory
1182 at all because afb-daemon doesn't check names of files when loading it.
1183 The only convention that use afb-daemon is that extension is **.so**
1184 but this convention is used only when afb-daemon discovers plugin
1185 from a directory hierarchy.
1187 2. It applies a version script at link to only export the conventional name
1188 of the entry point: **pluginAfbV1Register**. See below. By default, the linker
1189 that creates the shared object exports all the public symbols (C functions that
1190 are not **static**).
1194 target_include_directories(afm-main-plugin PRIVATE ${afb_INCLUDE_DIRS})
1195 target_link_libraries(afm-main-plugin utils ${afb_LIBRARIES})
1197 As you can see it uses the variables computed by ***pkg_check_modules(afb afb-daemon)***
1198 to configure the compiler and the linker.
1200 ### Exporting the function pluginAfbV1Register
1202 The function **pluginAfbV1Register** must be exported. This can be achieved
1203 using a version script when linking. Here is the version script that is
1204 used for *tic-tac-toe* (plugins/samples/export.map).
1206 { global: pluginAfbV1Register; local: *; };
1208 This sample [version script](https://sourceware.org/binutils/docs-2.26/ld/VERSION.html#VERSION)
1209 exports as global the symbol *pluginAfbV1Register* and hides any
1212 This version script is added to the link options using the
1213 option **--version-script=export.map** is given directly to the
1214 linker or using th option **-Wl,--version-script=export.map**
1215 when the option is given to the C compiler.
1217 ### Building within yocto
1219 Adding a dependency to afb-daemon is enough. See below:
1221 DEPENDS += " afb-daemon "