version: 1
Date: 30 mai 2016
Author: José Bollo
Afb-daemon binders serve files through HTTP protocol and offers to developers the capability to expose application API methods through HTTP or WebSocket protocol.
Binder plugins are used to add API to afb-daemon. This part describes how to write a plugin for afb-daemon.
Excepting this summary, this document target developers.
Before moving further through an example, here after a short overview of binder plugins fundamentals.
A plugin is an independent piece of software. A plugin is self contain and exposes application logic as sharable library. A plugin is intended to be dynamically loaded by afb-daemon to expose application API.
Technically, a binder plugin does not reference and is not linked with any afb-daemon library.
Application binder supports two kinds of plugins: application plugins and service plugins. Technically both class of plugin are equivalent are use the same coding convention. Only sharing mode and security context diverge.
Application-plugins implements the glue in between application’s UI and services. Every AGL application has a corresponding binder that typically activates one or many plugins to interface the application logic with lower platform services. When an application is started by the AGL application framework, a dedicate binder is started that loads/activates application plugin(s). API expose by application-plugin are executed within corresponding application security context.
Application plugins generally handle a unique context for a unique client. As the application framework start a dedicated instance of afb_daemon for each AGL application, if a given plugin is used within multiple application each of those application get a new and private instance of eventually “shared” plugin.
Service-plugins enable API activation within corresponding service security context and not within calling application context. Service-plugins are intended to run as a unique instance. Service-plugins can be shared in between multiple clients.
Service-plugins can either be stateless or manage client context. When managing context each client get a private context.
Sharing may either be global to the platform (ie: GPS service) or dedicated to a given user (ie: user preferences)
Application and service plugins are loaded and activated each time a new afb-daemon is started.
At launch time, every loaded plugin initialise itself. If a single plugin initialisation fail corresponding instance of afb-daemon self aborts.
Conversely, when a plugin initialisation succeeds, it should register its unique name as well as the list of verbs attached to the methods it exposes.
When initialised, on request from application clients to the right API/verb, plugin methods are activated by the afb-daemon attached to the application or service.
At exit time, no special action is enforced by afb-daemon. When a specific actions is required at afb-daemon stop, developers should use ‘atexit/on_exit’ during plugin initialisation sequence to register a custom exit function.
Afb-daemon’s plugin register two classes of objects: names and functions.
Plugins declare categories of names: - A unique plugin name to access all API expose by this plugin, - One name for each methods/verbs provided by this plugin.
Plugins declare two categories of functions: - function use for the initialisation - functions implementing exposed API methods
Afb-daemon parses URI requests to extract the API(plugin name) and the VERB(method to activate). As an example, URI foo/bar translates to plugin named foo and method named bar. To serve such a request, afb-daemon looks for an active plugin named foo and then within this plugin for a method named bar. When find afb-daemon calls corresponding method with attached parameter if any.
Afb-daemon ignores letter case when parsing URI. Thus TicTacToe/Board and tictactoe/board are equivalent.
The name of a given plugin is also known as the name of the API prefix that defines the plugin.
The name of a plugin SHOULD be unique within a given afb-daemon instance.
For example, when a client of afb-daemon calls a URI named foo/bar. Afb-daemon extracts the prefix foo and the suffix bar. foo must match a plugin name and bar a VERB attached to some method.
Each plugin exposes a set of methods that can be called by the clients of a given afb-daemon.
VERB’s name attached to a given plugin (API) MUST be unique within a plugin.
Plugins static declaration link VERBS to corresponding methods. When clients emit requests on a given API/VERB corresponding method is called by afb-daemon.
Plugin’s initialisation function serves several purposes.
It allows afb-daemon to control plugin version depending on initialisation function name. As today, the only supported initialisation function is pluginAfbV1Register. This identifies version “one” of plugins.
It allows plugins to initialise itself.
It enables names declarations: descriptions, requirements and implementations of exposed API/VERB.
When an API/VERB is called, afb-daemon constructs a request object. Then it passes this request object to the implementation function corresponding to requested method, this within attached API plugin.
An implementation function receives a request object that is used to: get arguments of the request, send answer, store session data.
A plugin MUST set an answer to every received requests.
Nevertheless it is not mandatory to set the answer before returning from API/VERB implementing function. This behaviour is important for asynchronous actions.
API/VERB implementation that set an answer before returning are called synchronous implementations. Those that do not systematically set an answer before returning are called asynchronous implementations.
Asynchronous implementations typically launch asynchronous actions. They record some context at request time and provide answer to the request only at completion of asynchronous actions.
This part explains how to write an afb-plugin. For the sake of being practical it uses many examples based on tic-tac-toe. 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
Print flags use for compilation:
$ 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
Afb-daemon automatically includes dependency to json-c. This is activated through Requires keyword in pkg-config. While almost every plugin replies on json-c this is not a must have dependency.
Internally, afb-daemon relies on libsystemd for its event loop, as well as for its binding to D-Bus. Plugins developers are encouraged to leverage libsystemd when possible. Nevertheless there is no hard dependency to libsystemd if ever you rather not use it, feel free to do so.
Afb-daemon plugin are fully self contain. They do not enforce dependency on any libraries from the application framework. Afb-daemon dependencies requirer to run AGL plugins are given at runtime through pointers leveraging read-only memory feature.
Plugin tictactoe has following includes:
#define _GNU_SOURCE
#include <stdio.h>
#include <string.h>
#include <json-c/json.h>
#include <afb/afb-plugin.h>
Header afb/afb-plugin.h is the only hard dependency, it includes all features that a plugin MUST HAVE. Outside of includes used to support application logic, common external headers used within plugins are:
The tictactoe plugin does not leverage systemd features, also only json.h is used on top of mandatory afb/afb-plugin.h.
When including afb/afb-plugin.h, the macro _GNU_SOURCE MUST be defined.
Designers of plugins should define a unique name for every API plugin as well as for methods VERBs. They should also define names for request arguments passed as name/value pair in URI.
While forging names, designers should respect few rules to ensure that created names are valid and easy to use across platforms.
All names and strings are UTF-8 encoded.
Plugin API name 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 makes no distinction between lower case and upper case when searching for API/VERB.
The names of methods VERBs are totally free and not checked.
However, the validity rules for method’s VERB name are the same as for Plugin API name except that the dot(.) character is forbidden.
Afb-daemon makes no case distinction when searching for an API by name.
Argument’s name are not restricted and can be everything you wish.
Warning arguments search is case sensitive and “index” and “Index” are not two different arguments.
The key names of javascript object can be almost anything using the arrayed notation:
object[key] = value
Nevertheless this is not the case with javascript dot notation:
object.key = value
Using the dot notation, the key must be a valid javascript identifier and dash(-) as well as few other reserved characters cannot be used.
For this reason, we advise developper to chose name compatible with both javascript and HTML notation.
It is a good practice, even for arguments not to rely on case sensitivity. This may reduce headache strength at debug time, especially with interpreted language like javascript that may not warn you that a variable was not defined.
The method 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 example shows many aspects of a synchronous method implementation. Let summarise 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: first one itf, points to functions used to handle internal request. Second one closure point onto function closure.
The structure must never be used directly. Instead developer should use the intended functions provided by afb-daemon as described here after.
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 twice req object request.
The first time, to retrieve the board attached to the session of the request.
The second time, to send the reply: an object that describes the current board.
When tic-tac-toe plugin receives a request, it musts get the board describing the game associated to the session.
For a plugin, having data associated to a session is common. This data is called “plugin context” for the session. Within tic-tac-toe plugin the context is the board.
Requests afb_req offer four functions for storing and retrieving session associated context.
These functions are:
afb_req_context_get: retrieves context data stored for current plugin.
afb_req_context_set: store context data of current plugin.
afb_req_context: if exist retrieves context data of current plugin. if context does not yet exist, creates a new context and store it.
afb_req_context_clear: reset the stored context 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 ensures an existing context for the session of the request. Its two last arguments are functions to allocate and free context. Note function type casts to avoid compilation warnings.
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 plugin tic-tac-toe (function get_new_board). The function get_new_board creates a new board and set usage its count to 1. The boards are checking usage count to free resources when not used.
The third argument is a function that frees context resources. For plugin tic-tac-toe (function release_board). The function release_board decrease usage count of the board passed in argument. When usage count falls to zero, data board are freed.
Definition of other functions dealing with contexts:
/*
* 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: successful or failure.
Sending a reply to a request MUST be done once and only once.
It exists two functions for “success” replies: 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).
*
* For conveniency, the function calls 'json_object_put' for 'obj'.
* Thus, in the case where 'obj' should remain available after
* the function returns, the function 'json_object_get' shall be used.
*/
void afb_req_success(struct afb_req req, struct json_object *obj, const char *info);
/*
* Same as 'afb_req_success' but the 'info' is a formatting
* string followed by arguments.
*
* For conveniency, the function calls 'json_object_put' for 'obj'.
* Thus, in the case where 'obj' should remain available after
* the function returns, the function 'json_object_get' shall be used.
*/
void afb_req_success_f(struct afb_req req, struct json_object *obj, const char *info, ...);
It exists two functions for “failure” replies: 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
* informational 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 recommended to NEVER use "success" for status.
*
* For conveniency, the function calls 'json_object_put' for 'obj'.
* Thus, in the case where 'obj' should remain available after
* the function returns, the function 'json_object_get' shall be used.
*/
void afb_req_fail(struct afb_req req, const char *status, const char *info);
/*
* Same as 'afb_req_fail' but the 'info' is a formatting
* string followed by arguments.
*
* For conveniency, the function calls 'json_object_put' for 'obj'.
* Thus, in the case where 'obj' should remain available after
* the function returns, the function 'json_object_get' shall be used.
*/
void afb_req_fail_f(struct afb_req req, const char *status, const char *info, ...);
For conveniency, these functions automatically call json_object_put to release obj. Because obj usage count is null after being passed to a reply function, it SHOULD not be used anymore. If exceptionally obj needs to remain usable after reply function then using json_object_get on obj to increase usage count and cancels the effect the json_object_put is possible.
Many methods expect arguments. Afb-daemon’s plugins retrieve arguments by name and not by position.
Arguments are passed by requests through either HTTP or WebSockets.
For example, the method join of 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 searches in the request req for argument name passed in the second argument. When argument name is not passed, afb_req_value returns NULL.
The search is case sensitive and boardid is not equivalent to BoardId. Nevertheless having argument names that only differ by name case is not a good idea.
The function afb_req_value is defined here after:
/*
* 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 here after:
/*
* 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 here after:
/*
* 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 above, clients can send files using HTTP/POST requests.
Received files are attached to “file” argument name. 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>
Argument named file should have both its value and path defined.
The value is the name of the file as it was set by the HTTP client. Generally it is the filename on client side.
The path is the effective path of saved file on the temporary local storage area of the application. This is a randomly generated and unique filename. It is not linked with the original filename as used on client side.
After success the plugin can use the uploaded file directly from local storage path with no restriction: read, write, remove, copy, rename… Nevertheless when request reply is set and query terminated, the uploaded temporary file at path is destroyed.
Plugins may also request every arguments of a given call as one single object. This feature is provided by the function afb_req_json defined here after:
/*
* 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 built:
For HTTP requests, this json object uses key names mapped on argument name. Values are either string for common arguments or object ie: { “file”: “…”, “path”: “…” }
For WebSockets requests, returned directly the object as provided by the client.
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, plugin’s methods should be declared to afb-daemon. Furthermore, the plugin itself must be recorded.
The registration mechanism is very basic: when afb-need starts, it loads all plugins listed in: command line or configuration file.
Loading a plugin follows the following steps:
Afb-daemon loads the plugin with dlopen.
Afb-daemon searches for a symbol named pluginAfbV1Register using dlsym. This symbol is assumed to be the exported initialisation function of the plugin.
Afb-daemon builds an interface object for the plugin.
Afb-daemon calls the found function pluginAfbV1Register with interface pointer as parameter.
Function pluginAfbV1Register setups the plugin and initialises it.
Function pluginAfbV1Register returns the pointer to a structure describing the plugin: version, name (prefix or API name), and list of methods.
Afb-daemon checks that the returned version and name can be managed. If so, plugin and its methods are register to become usable as soon as afb-daemon initialisation is finished.
Here after the code used for pluginAfbV1Register from 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
}
It is a very minimal initialisation function because tic-tac-toe plugin doesn’t have any application related initialisation step. It merely record daemon’s interface and returns its description.
The variable afbitf is a plugin global variable. It keeps the interface to afb-daemon that should be 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 here after.
/*
* array of the methods exported to afb-daemon
*/
static const struct AFB_method_desc_v1 plugin_methods[] = {
/* 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 */
.methods = plugin_methods /* the array describing the methods of the API */
}
};
The structure plugin_description describes the plugin. It declares the type and version of the plugin, its name, a short description and its methods list.
The list of methods is an array of structures describing the methods and terminated by a NULL marker.
In version one of afb-damon plugin, a method description contains 4 fields:
the name of the method,
the session management flags,
the implementation function to be call for the method,
a short description.
The structure describing methods is defined as follows:
/*
* Description of one method of the API provided by the plugin
* This enumeration is valid for plugins of type 1
*/
struct AFB_method_desc_v1
{
const char *name; /* name of the method */
enum AFB_session_v1 session; /* authorisation and session requirements of the method */
void (*callback)(struct afb_req req); /* callback function implementing the method */
const char *info; /* textual description of the method */
};
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 flag is set, afb-daemon requires an authentication token as if AFB_SESSION_CHECK flag was also set.
The special value AFB_SESSION_NONE is zero and can be used to bypass token 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 methods 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 methods 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 methods 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 methods 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 methods 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 initialisation time when pluginAfbV1Register is called.
The second argument message is a formatting string compatible with printf/sprintf.
The remaining arguments are arguments of the formating message like with 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 pushed to standard error. The final destination of the message depends on how systemd service was configured through its 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. As today only unattended even are supported. Targeted events are expected for next coming version.
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 plugin tic-tac-toe, reason indicates the origin of the change. In function afb_daemon_broadcast_event the second parameter is the name of broadcasted event. The third argument is the object that is transmitted with the event.
Function afb_daemon_broadcast_event is defined here after:
/*
* Broadcasts widely the event of 'name' with the data 'object'.
* 'object' can be NULL.
* 'daemon' MUST be the daemon given in interface when activating the plugin.
*
* For conveniency, the function calls 'json_object_put' for 'object'.
* Thus, in the case where 'object' should remain available after
* the function returns, the function 'json_object_get' shall be used.
*/
void afb_daemon_broadcast_event(struct afb_daemon daemon, const char *name, struct json_object *object);
Be aware, as with reply functions object is automatically released using json_object_put when using this function. Call json_object_get before calling afb_daemon_broadcast_event to keep object available after function returns.
Event name received by listeners is prefixed with plugin name. So when a change occurs after a move, the reason is move and every clients receive an event tictactoe/move.
Note that nothing is said about 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.
The tic-tac-toe example allows two clients or more to share the same board. This is implemented by the method join that illustrated partly how to retrieve arguments.
When two or more clients are sharing a same board, one of them can wait until the state of the board changes, but this could also be implemented using events because an even is generated each time the board changes.
In this case, the reply to the wait is sent only when the board changes. See the diagram below:
CLIENT A CLIENT B TIC-TAC-TOE
| | |
+--------------|----------------->| wait . . . . . . . .
| | | .
: : : .
: : : .
| | | .
| +----------------->| move . . . .
| | | V .
| |<-----------------+ success of move .
| | | .
|<-------------|------------------+ success of wait <
Here, this is an invocation of the plugin by an other client that unblock the suspended wait call. Nevertheless in most case this should be a timer, a hardware event, a sync with a concurrent process or thread, …
Common case of an asynchronous implementation.
Here is the listing of the function wait:
static void wait(struct afb_req req)
{
struct board *board;
struct waiter *waiter;
/* retrieves the context for the session */
board = board_of_req(req);
INFO(afbitf, "method 'wait' called for boardid %d", board->id);
/* creates the waiter and enqueues it */
waiter = calloc(1, sizeof *waiter);
waiter->req = req;
waiter->next = board->waiters;
afb_req_addref(req);
board->waiters = waiter;
}
After retrieving the board, the function adds a new waiter to waiters list and returns without setting a reply.
Before returning, it increases req request’s reference count using afb_req_addref function.
When a method returns without setting a reply, it MUST increment request’s reference count using afb_req_addref. If unpredictable behaviour may pop up.
Later, when a board changes, it calls tic-tac-toe changed function with reason of change in parameter.
Here is the full listing of the function changed:
/*
* 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);
}
The list of waiters is walked and a reply is sent to each waiter. After sending the reply, the reference count of the request is decremented using afb_req_unref to allow resources to be freed.
The reference count MUST be decremented using afb_req_unref to free resources and avoid memory leaks. This usage count decrement should happen AFTER setting reply or bad things may happen.
Afb-daemon provides a pkg-config configuration file that can be queried by providing afb-daemon in command line arguments. This configuration file provides data that should be used for plugins compilation. 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 ${plugin_dir})
else()
message(STATUS "Not creating the plugin for AFB-DAEMON")
endif()
Let now describe some of these lines.
pkg_check_modules(afb afb-daemon)
This first lines searches to the pkg-config configuration file for afb-daemon. Resulting data are stored in the following variables:
| Variable | Meaning |
|---|---|
| afb_FOUND | Set to 1 if afb-daemon plugin development files exist |
| afb_LIBRARIES | Only the libraries (w/o the ‘-l’) for compiling afb-daemon plugins |
| afb_LIBRARY_DIRS | The paths of the libraries (w/o the ‘-L’) for compiling afb-daemon plugins |
| afb_LDFLAGS | All required linker flags for compiling afb-daemon plugins |
| afb_INCLUDE_DIRS | The ‘-I’ preprocessor flags (w/o the ‘-I’) for compiling afb-daemon plugins |
| afb_CFLAGS | All required cflags for compiling afb-daemon plugins |
If development files are found, the plugin can be added to the set of target to build.
add_library(afm-main-plugin MODULE afm-main-plugin.c)
This line asks to create a shared library having a single source file named afm-main-plugin.c to be compiled. The default name of the created shared object is libafm-main-plugin.so.
set_target_properties(afm-main-plugin PROPERTIES
PREFIX ""
LINK_FLAGS "-Wl,--version-script=${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 implicitly added prefix lib. This step is not mandatory because afb-daemon doesn’t check names of files at load time. The only filename convention used by afb-daemon relates to .so termination. *.so pattern is used when afb-daemon automatically discovers plugin from a directory hierarchy.
It applies a version script at link time to only export the reserved name pluginAfbV1Register for registration entry point. By default, when building a shared library linker 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 at link time. Here after is a version script 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 the 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 "