version: 1
Date: 27 mai 2016
Author: José Bollo
The binder afb-daemon serves files through the HTTP protocol and offers access to API’s through HTTP or WebSocket protocol.
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.
Before going into details, through a tiny example, a short overview plugins basis is needed.
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.
Technically, a plugin is not linked to any library of afb-daemon.
There is two kinds of plugins: application plugins and service plugins.
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.
It means that the application plugins mainly have only one context to manage for one client.
Service plugins are intended to be instanciated only one time only and connected to many clients.
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.
In details, it may be useful to have service plugins at a user level.
The plugins are loaded and activated when afb-daemon starts.
At start, the plugin initialise itself. If it fails to initialise then afb-daemon stops.
Conversely, if it success to initialize, it must declare a name, that must be unique, and a list of API’s verbs.
When initialized, the functions implementing the API’s verbs of the plugin are activated on call.
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.
For afb-daemon, a plugin contains 2 different things: names and functions.
There is two kind of names: - the name of the plugin, - the names of the verbs.
There is two kind of functions: - the initialisation function - functions implementing verbs
Afb-daemon translates the name of the method that is invoked to a pair of API and verb names. For example, the method named foo/bar translated to the API name foo and the verb name bar. To serve it, afb-daemon search the plugin that record the name foo and if it also recorded the verb bar, it calls the implementation function declared for this verb.
Afb-daemon make no distinction between lower case and upper case when searching for a method. Thus, The names TicTacToe/Board and tictactoe/borad are equals.
The name of the plugin is also known as the name of the API that defines the plugin.
This name is also known as the prefix.
The name of a plugin MUST be unique within afb-daemon.
For example, when a client of afb-daemon calls a method named foo/bar. Afb-daemon extracts the prefix foo and the suffix bar. foo is the API name and must match a plugin name, the plugin that implements the verb bar.
Each plugin exposes a set of verbs that can be called by client of afb-daemon.
The name of a verb MUST be unique within a plugin.
Plugins link verbs to functions that are called when clients emit requests for that verb.
For example, when a client of afb-daemon calls a method named foo/bar.
The initialisation function serves several purposes.
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 pluginAfbV1Register. It identifies the first version of plugins.
It allows the plugin to initialise itself.
It serves to the plugin to declare names, descriptions, requirements and implmentations of the verbs that it exposes.
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.
An implementation function receives a request object that is used to get arguments of the request, to send answer, to store session data.
A plugin MUST send an answer to the request.
But it is not mandatory to send the answer before to return from the implementing function. This behaviour is important for implementing asynchronous actions.
Implementation functions that always reply to the request before returning are named synchronous implementations. Those that don’t always reply to the request before returning are named asynchronous implementations.
Asynchronous implementations typically initiate an asynchronous action and record to send the reply on completion of this action.
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 plugins/samples/tic-tac-toe.c.
This plugin is named tictactoe.
Afb-daemon provides a configuration file for pkg-config. Typing the command
pkg-config --cflags afb-daemon
will print the flags to use for compiling, like this:
$ pkg-config --cflags afb-daemon
-I/opt/local/include -I/usr/include/json-c
For linking, you should use
$ pkg-config --libs afb-daemon
-ljson-c
As you see, afb-daemon automatically includes dependency to json-c. This is done through the Requires keyword of pkg-config because almost all plugin will use json-c.
If this behaviour is a problem, let us know.
Internally, afb-daemon uses libsystemd 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 libsystemd.
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.
The plugin tictactoe has the following lines for its includes:
#define _GNU_SOURCE
#include <stdio.h>
#include <string.h>
#include <json-c/json.h>
#include <afb/afb-plugin.h>
The header afb/afb-plugin.h includes all the features that a plugin needs except two foreign header that must be included by the plugin if it needs it:
The tictactoe plugin does not use systemd features so it is not included.
When including afb/afb-plugin.h, the macro _GNU_SOURCE must be defined.
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.
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.
The names and strings used ALL are UTF-8 encoded.
The names of the API are checked. All characters are authorised except:
In other words the set of forbidden characters is { \u0000..\u0020, \u0022, \u0023, \u0025..\u0027, \u002f, \u003f, \u0060, \u007f }.
Afb-daemon make no distinction between lower case and upper case when searching for an API by its name.
The names of the verbs are not checked.
However, the validity rules for verb’s names are the same as for API’s names except that the dot (.) character is forbidden.
Afb-daemon make no distinction between lower case and upper case when searching for an API by its name.
The names for arguments are not restricted and can be anything.
The arguments are searched with the case sensitive string comparison. Thus the names “index” and “Index” are not the same.
The key names of javascript object can be almost anything using the arrayed notation:
object[key] = value
That is not the case with the dot notation:
object.key = value
Using the dot notation, the key must be a valid javascript identifier.
For this reason, the chosen names should better be valid javascript identifier.
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.
The verb tictactoe/board is a synchronous implementation. Here is its listing:
/*
* 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);
}
This examples show many aspects of writing a synchronous verb implementation. Let summarize it:
The function board_of_req retrieves the context stored for the plugin: the board.
The macro INFO sends a message of kind INFO to the logging system. The global variable named afbitf used represents the interface to afb-daemon.
The function describe creates a json_object representing the board.
The function afb_req_success sends the reply, attaching to it the object description.
For any implementation, the request is received by a structure of type struct afb_req.
Note that this is a PLAIN structure, not a pointer to a structure.
The definition of struct afb_req is:
/*
* 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 */
};
It contains two pointers: one, itf, points to the functions needed to handle the internal request represented by the second pointer, closure.
The structure must never be used directly. Insted, use the intended functions provided by afb-daemon and described here.
req is used to get arguments of the request, to send answer, to store session data.
This object and its interface is defined and documented in the file names afb/afb-req-itf.h
The above example uses 2 times the request object req.
The first time, it is used for retrieving the board attached to the session of the request.
The second time, it is used to send the reply: an object that describes the current board.
When the plugin tic-tac-toe receives a request, it musts regain the board that describes the game associated to the session.
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 tic-tac-toe, the context is the board.
The requests afb_req offer four functions for storing and retrieving the context associated to the session.
These functions are:
afb_req_context_get: retrieves the context data stored for the plugin.
afb_req_context_set: store the context data of the plugin.
afb_req_context: retrieves the context data of the plugin, if needed, creates the context and store it.
afb_req_context_clear: reset the stored data.
The plugin tictactoe use a convenient function to retrieve its context: the board. This function is board_of_req:
/*
* 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);
}
The function afb_req_context 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.
Here is the definition of the function afb_req_context
/*
* 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;
}
The second argument if the function that creates the context. For the plugin tic-tac-toe it is the function get_new_board. The function get_new_board 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.
The third argument if the function that frees the context. For the plugin tic-tac-toe it is the function release_board. The function release_board decrease the the count of use of the board given as argument. If the use count decrease to zero, the board data are freed.
The definition of the other functions for dealing with contexts are:
/*
* 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);
}
Two kinds of replies can be made: successful replies and failure replies.
Sending a reply to a request must be done at most one time.
The two functions to send a reply of kind “success” are afb_req_success and afb_req_success_f.
/*
* 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).
*/
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.
*/
void afb_req_success_f(struct afb_req req, struct json_object *obj, const char *info, ...);
The two functions to send a reply of kind “failure” are afb_req_fail and afb_req_fail_f.
/*
* 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.
*/
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.
*/
void afb_req_fail_f(struct afb_req req, const char *status, const char *info, ...);
Many verbs expect arguments. Afb-daemon let plugins retrieve their arguments by name not by position.
Arguments are given by the requests either through HTTP or through WebSockets.
For example, the verb join of the plugin tic-tac-toe expects one argument: the boardid to join. Here is an extract:
/*
* 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;
...
The function afb_req_value search in the request req for an argument whose name is given. When no argument of the given name was passed, afb_req_value returns NULL.
The search is case sensitive. So the name boardid is not the same name than BoardId. But this must not be assumed so two expected names of argument should not differ only by case.
The function afb_req_value is defined as below:
/*
* 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;
}
It is defined as a shortcut to call the function afb_req_get. That function is defined as below:
/*
* 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);
That function takes 2 parameters: the request and the name of the argument to retrieve. It returns a PLAIN structure of type struct afb_arg.
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 “” (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.
The definition of struct afb_arg is:
/*
* 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 */
};
The structure returns the data arguments that are known for the request. This data include a field named path. This path can be accessed using the function afb_req_path defined as below:
/*
* 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;
}
The path is only defined for HTTP/POST requests that send file.
As it is explained just above, clients can send files using HTTP/POST requests.
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 post/upload-image with 2 arguments named file and hidden.
<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>
In that case, the argument named file has its value and its path defined and not NULL.
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.
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.
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.
Plugins can get all the arguments as one single object. This feature is provided by the function afb_req_json that is defined as below:
/*
* 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);
It returns a json object. This object depends on how the request was made:
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”: “…” }
For WebSockets requests, the returned object is the object given by the client transparently transported.
In fact, for Websockets requests, the function afb_req_value can be seen as a shortcut to json_object_get_string(json_object_object_get(afb_req_json(req), name))
To be active, the verbs of the plugin should be declared to afb-daemon. And even more, the plugin itself must be recorded.
The mechanism for doing this is very simple: when afb-need starts, it loads the plugins that are listed in its argument or configuration.
Loading a plugin follows the following steps:
It loads the plugin using dlopen.
It searchs for the symbol named pluginAfbV1Register using dlsym. This symbol is assumed to be the exported initialisation function of the plugin.
It build an interface object for the plugin.
It calls the found function pluginAfbV1Register and pass it the pointer to its interface.
The function pluginAfbV1Register setup the plugin, initialize it.
The function pluginAfbV1Register returns the pointer to a structure that describes the plugin: its version, its name (prefix or API name), and the list of its verbs.
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.
Here is the listing of the function pluginAfbV1Register of the plugin tic-tac-toe:
/*
* 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
}
This is a very small function because the tic-tac-toe plugin doesn’t have initialisation step. It merely record the daemon’s interface and returns its descritption.
The variable afbitf 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:
/*
* the interface to afb-daemon
*/
const struct AFB_interface *afbitf;
The description of the plugin is defined as below.
/*
* 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 */
}
};
The structure plugin_description describes the plugin. It declares the type and version of the plugin, its name, a description and a list of its verbs.
The list of verbs is an array of structures describing the verbs and terminated by a marker: a verb whose name is NULL.
The description of the verbs for this version is made of 4 fields:
the name of the verbs,
the session management flags,
the implementation function to be call for the verb,
a short description.
The structure describing verbs is defined as follows:
/*
* 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 */
};
For technical reasons, the enumeration enum AFB_session_v1 is not exactly an enumeration but the wrapper of constant definitions that can be mixed using bitwise or (the C operator |).
The constants that can bit mixed are:
| Constant name | Meaning |
|---|---|
| AFB_SESSION_CREATE | Equals to AFB_SESSION_LOA_EQ_0|AFB_SESSION_RENEW |
| AFB_SESSION_CLOSE | Closes the session after the reply and set the LOA to 0 |
| AFB_SESSION_RENEW | Refreshes the token of authentification |
| AFB_SESSION_CHECK | Just requires the token authentification |
| AFB_SESSION_LOA_LE_0 | Requires the current LOA to be lesser then or equal to 0 |
| AFB_SESSION_LOA_LE_1 | Requires the current LOA to be lesser then or equal to 1 |
| AFB_SESSION_LOA_LE_2 | Requires the current LOA to be lesser then or equal to 2 |
| AFB_SESSION_LOA_LE_3 | Requires the current LOA to be lesser then or equal to 3 |
| AFB_SESSION_LOA_GE_0 | Requires the current LOA to be greater then or equal to 0 |
| AFB_SESSION_LOA_GE_1 | Requires the current LOA to be greater then or equal to 1 |
| AFB_SESSION_LOA_GE_2 | Requires the current LOA to be greater then or equal to 2 |
| AFB_SESSION_LOA_GE_3 | Requires the current LOA to be greater then or equal to 3 |
| AFB_SESSION_LOA_EQ_0 | Requires the current LOA to be equal to 0 |
| AFB_SESSION_LOA_EQ_1 | Requires the current LOA to be equal to 1 |
| AFB_SESSION_LOA_EQ_2 | Requires the current LOA to be equal to 2 |
| AFB_SESSION_LOA_EQ_3 | Requires the current LOA to be equal to 3 |
If any of this flags is set, afb-daemon requires the token authentification as if the flag AFB_SESSION_CHECK had been set.
The special value AFB_SESSION_NONE is zero and can be used to avoid any check.
Note that AFB_SESSION_CREATE and AFB_SESSION_CLOSE might be removed in later versions.
Afb-daemon provides 4 levels of verbosity and 5 verbs for logging messages.
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.
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.
The 5 logging verbs are:
| Macro | Verbosity | Meaning | syslog level |
|---|---|---|---|
| ERROR | 0 | Error conditions | 3 |
| WARNING | 1 | Warning conditions | 4 |
| NOTICE | 1 | Normal but significant condition | 5 |
| INFO | 2 | Informational | 6 |
| DEBUG | 3 | Debug-level messages | 7 |
You can note that the 2 verbs WARNING and INFO have the same level of verbosity. But they don’t have the same syslog level. It means that they are output with a different level on the logging system.
All of these verbs have the same signature:
void ERROR(const struct AFB_interface *afbitf, const char *message, ...);
The first argument afbitf is the interface to afb daemon that the plugin received at its initialisation when pluginAfbV1Register was called.
The second argument message is a formatting string compatible with printf/sprintf.
The remaining arguments are arguments of the formating message like for printf.
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.
| Level of verbosity | Outputed macro |
|---|---|
| 0 | ERROR |
| 1 | ERROR + WARNING + NOTICE |
| 2 | ERROR + WARNING + NOTICE + INFO |
| 3 | ERROR + WARNING + NOTICE + INFO + DEBUG |
The syslog level is used for forging a prefix to the message. The prefixes are:
| syslog level | prefix |
|---|---|
| 0 | <0> EMERGENCY |
| 1 | <1> ALERT |
| 2 | <2> CRITICAL |
| 3 | <3> ERROR |
| 4 | <4> WARNING |
| 5 | <5> NOTICE |
| 6 | <6> INFO |
| 7 | <7> DEBUG |
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 StandardError: It can be journal, syslog or kmsg. (See man sd-daemon).
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.
The plugin tic-tac-toe broadcasts events when the board changes. This is done in the function changed:
/*
* 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);
}
The description of the changed board is pushed via the daemon interface.
Within the plugin tic-tac-toe, the reason indicates the origin of the change. For the function afb_daemon_broadcast_event, the second parameter is the name of the broadcasted event. The third argument is the object that is transmitted with the event.
The function afb_daemon_broadcast_event is defined as below:
/*
* 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.
*/
void afb_daemon_broadcast_event(struct afb_daemon daemon, const char *name, struct json_object *object);
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 move and then the clients receive the event tictactoe/move.
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.
/ * 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);
}
Afb-daemon provides a pkg-config configuration file that can be queried by the name afb-daemon. This configuration file provides data that should be used for compiling plugins. Examples:
$ pkg-config --cflags afb-daemon
$ pkg-config --libs afb-daemon
This example is the extract for building the plugin afm-main using CMAKE.
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 ${plugin_dir})
else()
message(STATUS "Not creating the plugin for AFB-DAEMON")
endif()
Let now describe some of these lines.
pkg_check_modules(afb afb-daemon)
This first lines searches to the pkg-config configuration file for afb-daemon. Resulting data are stored in the following variables:
| Variable | Meaning |
|---|---|
| afb_FOUND | Set to 1 if afb-daemon plugin development files exist |
| afb_LIBRARIES | Only the libraries (w/o the ‘-l’) for compiling afb-daemon plugins |
| afb_LIBRARY_DIRS | The paths of the libraries (w/o the ‘-L’) for compiling afb-daemon plugins |
| afb_LDFLAGS | All required linker flags for compiling afb-daemon plugins |
| afb_INCLUDE_DIRS | The ‘-I’ preprocessor flags (w/o the ‘-I’) for compiling afb-daemon plugins |
| afb_CFLAGS | All required cflags for compiling afb-daemon plugins |
If development files are found, the plugin can be added to the set of target to build.
add_library(afm-main-plugin MODULE afm-main-plugin.c)
This line asks to create a shared library having only the source file afm-main-plugin.c (that is compiled). The default name of the created shared object is libafm-main-plugin.so.
set_target_properties(afm-main-plugin PROPERTIES
PREFIX ""
LINK_FLAGS "-Wl,--version-script=${CMAKE_CURRENT_SOURCE_DIR}/afm-main-plugin.export-map"
)
This lines are doing two things:
It renames the built library from libafm-main-plugin.so to afm-main-plugin.so by removing the implicitely added prefix lib. 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 .so but this convention is used only when afb-daemon discovers plugin from a directory hierarchy.
It applies a version script at link to only export the conventional name of the entry point: pluginAfbV1Register. See below. By default, the linker that creates the shared object exports all the public symbols (C functions that are not static).
Next line are:
target_include_directories(afm-main-plugin PRIVATE ${afb_INCLUDE_DIRS})
target_link_libraries(afm-main-plugin utils ${afb_LIBRARIES})
As you can see it uses the variables computed by pkg_check_modules(afb afb-daemon) to configure the compiler and the linker.
The function pluginAfbV1Register must be exported. This can be achieved using a version script when linking. Here is the version script that is used for tic-tac-toe (plugins/samples/export.map).
{ global: pluginAfbV1Register; local: *; };
This sample version script exports as global the symbol pluginAfbV1Register and hides any other symbols.
This version script is added to the link options using the option –version-script=export.map is given directly to the linker or using th option -Wl,–version-script=export.map when the option is given to the C compiler.
Adding a dependency to afb-daemon is enough. See below:
DEPENDS += " afb-daemon "