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Implemented plugin interface
2002-05-09 16:26:56 +02:00
<html>
<head>
<title>The VDR Plugin System</title>
</head>
<body bgcolor="white">
<h1>The VDR Plugin System</h1>
VDR provides an easy to use plugin interface that allows additional functionality
to be added to the program by implementing a dynamically loadable library file.
This interface allows programmers to develop additional functionality for VDR completely
separate from the core VDR source, without the need of patching the original
VDR code (and all the problems of correlating various patches).
<p>
This document describes the "outside" interface of the plugin system.
It handles everything necessary for a plugin to get hooked into the core
VDR program and present itself to the user.
<!--<p>TODO: Link to the document about VDR base classes to use when implementing actual functionality (yet to be written).-->
<hr><h2>Quick start</h2>
<center><i><b>Can't wait, can't wait!</b></i></center><p>
Actually you should read this entire document before starting to work with VDR plugins,
but you probably want to see something happening right away <tt>;-)</tt>
<p>
So, for a quick demonstration of the plugin system, there is a demo plugin called
"hello" that comes with the VDR source. To test drive this one, do the following:
<ul>
<li>change into the VDR source directory
<li><b><tt>make</tt></b> the VDR program with your usual <tt>REMOTE=...</tt> (and maybe other) options
<li>do <b><tt>make plugins</tt></b> to build the demo plugin
<li>run VDR with <b><tt>vdr -V</tt></b> to see the version information
<li>run VDR with <b><tt>vdr -h</tt></b> to see the command line options
<li>run VDR with <b><tt>vdr -Phello</tt></b>
<li>open VDR's main menu and select the <i>Hello</i> item
<li>open the <i>Setup</i> menu from VDR's main menu and select <i>Plugins</i>
</ul>
If you enjoyed this brief glimpse into VDR plugin handling, read through the rest of
this document and eventually write your own VDR plugin.
<hr><h2>The name of the plugin</h2>
<center><i><b>Give me some I.D.!</b></i></center><p>
One of the first things to consider when writing a VDR plugin is giving the thing
a proper name. This name will be used in the VDR command line in order to load
the plugin, and will also be the name of the plugin's source directory, as well
as part of the final library name.
<p>
The plugin's name should typically be as short as possible. Three letter
abbreviations like <b><tt>dvd</tt></b> (for a DVD player) or <b><tt>mp3</tt></b>
(for an MP3 player) would be good choices. It is also recommended that the name
consists of only lowercase letters and digits.
No other characters should be used here.
<p>
A plugin can access its name through the (non virtual) member function
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
const char *Name(void);
</pre></td></tr></table><p>
The actual name is derived from the plugin's library file name, as defined in the
next chapter.
<a name="The plugin directory structure"><hr><h2>The plugin directory structure</h2>
<center><i><b>Where is everybody?</b></i></center><p>
By default plugins are located in a directory named <tt>PLUGINS</tt> below the
VDR source directory. Inside this directory the following subdirectory structure
is used:
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
VDR/PLUGINS/src
VDR/PLUGINS/src/demo
VDR/PLUGINS/src/hello
VDR/PLUGINS/lib
VDR/PLUGINS/lib/libvdr-demo.so.1.1.0
VDR/PLUGINS/lib/libvdr-hello.so.1.1.0
</pre></td></tr></table><p>
The <tt>src</tt> directory contains one subdirectory for each plugin, which carries
the name of that plugin (in the above example that would be <tt>demo</tt> and
<tt>hello</tt>, respectively). What's inside the individual source directory of a
plugin is entirely up to the author of that plugin. The only prerequisites are
that there is a <tt>Makefile</tt> that provides the targets <tt>all</tt> and
<tt>clean</tt>, and that a call to <tt>make all</tt> actually produces a dynamically
loadable library file for that plugin (we'll get to the details later).
<p>
The <tt>lib</tt> directory contains the dynamically loadable libraries of all
available plugins. Note that the names of these files are created by concatenating
<p>
<table border=2>
<tr><td align=center><b><tt>libvdr-</tt></b></td><td align=center><b><tt>demo</tt></b></td><td align=center><b><tt>.so.</tt></b></td><td align=center><b><tt>1.1.0</tt></b></td></tr>
<tr><td align=center><font size=-1>VDR plugin<br>library prefix</font></td><td align=center><font size=-1>name of<br>the plugin</font></td><td align=center><font size=-1>shared object<br>indicator</font></td><td align=center><font size=-1>VDR version number<br>this plugin was<br>compiled for</font></td></tr>
</table>
<p>
The plugin library files can be stored in any directory. If the default organization
is not used, the path to the plugin directory has be be given to VDR through the
<b><tt>-L</tt></b> option.
<p>
The VDR <tt>Makefile</tt> contains the target <tt>plugins</tt>, which calls
<tt>make all</tt> in every directory found under <tt>VDR/PLUGINS/src</tt>,
plus the target <tt>plugins-clean</tt>, which calls <tt>make clean</tt> in
each of these directories.
<p>
If you download a plugin <a href="#Building the distribution package">package</a>
from the web, it will typically have a name like
<p>
<tt>vdr-demo-0.0.1.tgz</tt>
<p>
and will unpack into a directory named
<p>
<tt>vdr-demo-0.0.1</tt>
<p>
To use the <tt>plugins</tt> and <tt>plugins-clean</tt> targets from the VDR <tt>Makefile</tt>
you need to unpack such an archive into the <tt>VDR/PLUGINS/src</tt> directory and
create a symbolic link with the basic plugin name, as in
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
ln -s vdr-demo-0.0.1 demo
</pre></td></tr></table><p>
Since the VDR <tt>Makefile</tt> only searches for directories with names consisting
of only lowercase characters and digits, it will only follow the symbolic links, which
should lead to the current version of the plugin you want to use. This way you can
have several different versions of a plugin source (like <tt>vdr-demo-0.0.1</tt> and
<tt>vdr-demo-0.0.2</tt>) and define which one to actually use through the symbolic link.
<a name="Initializing a new plugin directory"><hr><h2>Initializing a new plugin directory</h2>
<center><i><b>A room with a view</b></i></center><p>
Call the Perl script <tt>newplugin</tt> from the VDR source directory to create
a new plugin directory with a <tt>Makefile</tt> and a main source file implementing
the basic derived plugin class.
You will also find a <tt>README</tt> file there with some inital text, where you
should fill in actual information about your project.
A <tt>HISTORY</tt> file is set up with an "Initial revision" entry. As your project
evolves, you should add the changes here with date and version number.
<p>
<tt>newplugin</tt> also creates a copy of the GPL license file <tt>COPYING</tt>,
assuming that you will release your work under that license. Change this if you
have other plans.
<p>
Add further files and maybe subdirectories to your plugin source directory as
necessary. Don't forget to adapt the <tt>Makefile</tt> appropriately.
<hr><h2>The actual implementation</h2>
<center><i><b>Use the source, Luke!</b></i></center><p>
A newly initialized plugin doesn't really do very much yet.
If you <a href="#Loading plugins into VDR">load it into VDR</a> you will find a new
entry in the main menu, with the same name as your plugin (where the first character
has been converted to uppercase). There will also be a new entry named "Plugins" in
the "Setup" menu, which will bring up a list of all loaded plugins, through which you
can access each plugin's own setup parameters (if it provides any).
<p>
To implement actual functionality into your plugin you need to edit the source file
that was generated as <tt>PLUGINS/src/name.c</tt>. Read the comments in that file
to see where you can bring in your own code. The following sections of this document
will walk you through the individual member functions of the plugin class.
<p>
Depending on what your plugin shall do, you may or may not need all of the given
member functions. Except for the <tt>MainMenuEntry()</tt> function they all by default
return values that will result in no actual functionality. You can either completely
delete unused functions from your source file, or just leave them as they are.
If your plugin shall not be accessible through VDR's main menu, simply remove
(or comment out) the line implementing the <tt>MainMenuEntry()</tt> function.
<p>
At the end of the plugin's source file you will find a line that looks like this:
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
VDRPLUGINCREATOR(cPluginDemo);
</pre></td></tr></table><p>
This is the "magic" hook that allows VDR to actually load the plugin into
its memory. You don't need to worry about the details behind all this.
<p>
If your plugin requires additional source files, simply add them to your plugin's
source directory and adjust the <tt>Makefile</tt> accordingly.
<hr><h2>Construction and Destruction</h2>
<center><i><b>What goes up, must come down...</b></i></center><p>
The constructor and destructor of a plugin are defined as
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
cPlugin(void);
virtual ~cPlugin();
</pre></td></tr></table><p>
The <b>constructor</b> shall initialize any member variables the plugin defines, but
<b>must not access any global structures of VDR</b>.
It also must not create any threads or other large data structures. These things
are done in the <a href="#Getting started"><tt>Start()</tt></a> function later.
Constructing a plugin object shall not have any side effects or produce any output,
since VDR, for instance, has to create the plugin objects in order to get their
command line help - and after that immediately destroys them again.
<p>
The <b>destructor</b> has to clean up any data created by the plugin, and has to
take care that any threads the plugin may have created will be stopped.
<p>
Of course, if your plugin doesn't define any member variables that need to be
initialized (and deleted), you don't need to implement either of these functions.
<hr><h2>Version number</h2>
<center><i><b>Which incarnation is this?</b></i></center><p>
Every plugin must have a version number of its own, which does not necessarily
have to be in any way related to the VDR version number.
VDR requests a plugin's version number through a call to the function
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
virtual const char *Version(void) = 0;
</pre></td></tr></table><p>
Since this is a "pure" virtual function, any derived plugin class <b>must</b>
implement it. The returned string should identify this version of the plugin.
Typically this would be something like "0.0.1", but it may also contain other
information, like for instance "0.0.1pre2" or the like. The string should only
be as long as really necessary, and shall not contain the plugin's name itself.
Here's an example:
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
static const char *VERSION = "0.0.1";
...
const char *cPluginDemo::Version(void)
{
return VERSION;
}
</pre></td></tr></table><p>
Note that the definition of the version number is expected to be located in the
main source file, and must be written as
<pre>
static const char *VERSION = ...
</pre>
just like shown in the above example. This is a convention that allows the <tt>Makefile</tt>
to extract the version number when generating the file name for the distribution archive.
<p>
A new plugin project should start with version number <tt>0.0.1</tt> and should reach
version <tt>1.0.0</tt> once it is completely operative and well tested. Following the
Linux kernel version numbering scheme, versions with <i>even</i> release numbers
(like <tt>1.0.x</tt>, <tt>1.2.x</tt>, <tt>1.4.x</tt>...) should be stable releases,
while those with <i>odd</i> release numbers (like <tt>1.1.x</tt>, <tt>1.3.x</tt>,
<tt>1.5.x</tt>...) are usually considered "under development". The three parts of
a version number are not limited to single digits, so a version number of <tt>1.2.15</tt>
would be acceptable.
<hr><h2>Description</h2>
<center><i><b>What is it that you do?</b></i></center><p>
In order to tell the user what exactly a plugin does, it must implement the function
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
virtual const char *Description(void) = 0;
</pre></td></tr></table><p>
which returns a short, one line description of the plugin's purpose.
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
virtual const char *Description(void)
{
return "A simple demo plugin";
}
</pre></td></tr></table><p>
<hr><h2>Command line arguments</h2>
<center><i><b>Taking orders</b></i></center><p>
A VDR plugin can have command line arguments just like any normal program.
If a plugin wants to react on command line arguments, it needs to implement
the function
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
virtual bool ProcessArgs(int argc, char *argv[]);
</pre></td></tr></table><p>
The parameters <tt>argc</tt> and <tt>argv</tt> have exactly the same meaning
as in a normal C program's <tt>main()</tt> function.
<tt>argv[0]</tt> contains the name of the plugin (as given in the <b><tt>-P</tt></b>
option of the <tt>vdr</tt> call).
<p>
Each plugin has its own set of command line options, which are totally independent
from those of any other plugin or VDR itself.
<p>
You can use the <tt>getopt()</tt> or <tt>getopt_long()</tt> function to process
these arguments. As with any normal C program, the strings pointed to by <tt>argv</tt>
will survive the entire lifetime of the plugin, so it is safe to store pointers to
these values inside the plugin. Here's an example:
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
bool cPluginDemo::ProcessArgs(int argc, char *argv[])
{
static struct option long_options[] = {
{ "aaa", required_argument, NULL, 'a' },
{ "bbb", no_argument, NULL, 'b' },
{ NULL }
};
int c;
while ((c = getopt_long(argc, argv, "a:b", long_options, NULL)) != -1) {
switch (c) {
case 'a': fprintf(stderr, "option -a = %s\n", optarg);
break;
case 'b': fprintf(stderr, "option -b\n");
break;
default: return false;
}
}
return true;
}
</pre></td></tr></table><p>
The return value must be <i>true</i> if all options have been processed
correctly, or <i>false</i> in case of an error. The first plugin that returns
<i>false</i> from a call to its <tt>ProcessArgs()</tt> function will cause VDR
to exit.
<hr><h2>Command line help</h2>
<center><i><b>Tell me about it...</b></i></center><p>
If a plugin accepts command line options, it should implement the function
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
virtual const char *CommandLineHelp(void);
</pre></td></tr></table><p>
which will be called if the user enters the <b><tt>-h</tt></b> option when starting VDR.
The returned string should contain the command line help for this plugin, formatted
in the same way as done by VDR itself:
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
const char *cPluginDemo::CommandLineHelp(void)
{
return " -a ABC, --aaa=ABC do something nice with ABC\n"
" -b, --bbb activate 'plan B'\n";
}
</pre></td></tr></table><p>
This command line help will be printed directly below VDR's help texts (separated
by a line indicating the plugin's name, version and description), so if you use the
same formatting as shown here it will line up nicely.
Note that all lines should be terminated with a newline character, and should
be shorter than 80 characters.
<a name="Getting started"><hr><h2>Getting started</h2>
<center><i><b>Let's get ready to rumble!</b></i></center><p>
If a plugin implements a function that runs in the background (presumably in a
thread of its own), or wants to make use of <a href="#Internationalization">internationalization</a>,
it needs to implement the function
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
virtual void Start(void);
</pre></td></tr></table><p>
which is called once for each plugin at program startup.
Inside this function the plugin must set up everything necessary to perform
its task. This may, for instance, be a thread that collects data from the DVB
stream, which is later presented to the user via a function that is available
from the main menu.
<p>
If the plugin doesn't implement any background functionality or internationalized
texts, it doesn't need to implement this function.
<hr><h2>Main menu entry</h2>
<center><i><b>Today's special is...</b></i></center><p>
If the plugin implements a feature that the user shall be able to access
from VDR's main menu, it needs to implement the function
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
virtual const char *MainMenuEntry(void);
</pre></td></tr></table><p>
The default implementation returns a <tt>NULL</tt> pointer, which means that
this plugin will not have an item in the main menu. Here's an example of a
plugin that will have a main menu item:
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
const char *cPluginDemo::MainMenuEntry(void)
{
return "Demo";
}
</pre></td></tr></table><p>
The menu entries of all plugins will be inserted into VDR's main menu right
after the <i>Recordings</i> item, in the same sequence as they were given
in the call to VDR.
<hr><h2>User interaction</h2>
<center><i><b>It's showtime!</b></i></center><p>
If the user selects the main menu entry of a plugin, VDR calls the function
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
virtual cOsdMenu *MainMenuAction(void);
</pre></td></tr></table><p>
which can do one of two things:
<ul>
<li>Return a pointer to a <tt>cOsdMenu</tt> object which will be displayed
as a submenu of the main menu (just like the <i>Recordings</i> menu, for instance).
That menu can then implement further functionality and, for instance, could
eventually start a custom player to replay a file other than a VDR recording.
<li>Perform a specific action and return <tt>NULL</tt>. In that case the main menu
will be closed after calling <tt>MainMenuAction()</tt>.
</ul>
<b>
It is very important that a call to <tt>MainMenuAction()</tt> returns as soon
as possible! As long as the program stays inside this function, no other user
interaction is possible. If a specific action takes longer than a few seconds,
the plugin should launch a separate thread to do this.
</b>
<hr><h2>Setup parameters</h2>
<center><i><b>Remember me...</b></i></center><p>
If a plugin requires its own setup parameters, it needs to implement the following
functions to handle these parameters:
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
virtual cMenuSetupPage *SetupMenu(void);
virtual bool SetupParse(const char *Name, const char *Value);
</pre></td></tr></table><p>
The <tt>SetupMenu()</tt> function shall return the plugin's "Setup" menu
page, where the user can adjust all the parameters known to this plugin.
<p>
<tt>SetupParse()</tt> will be called for each parameter the plugin has
previously stored in the global setup data (see below). It shall return
<i>true</i> if the parameter was parsed correctly, <i>false</i> in case of
an error. If <i>false</i> is returned, an error message will be written to
the log file (and program execution will continue).
<p>
The plugin's setup parameters are stored in the same file as VDR's parameters.
In order to allow each plugin (and VDR itself) to have its own set of parameters,
the <tt>Name</tt> of each parameter will be preceeded with the plugin's
name, as in
<p>
<tt>demo.SomeParameter = 123</tt>
<p>
The prefix will be handled by the core VDR setup code, so the individual
plugins need not worry about this.
<p>
To store its values in the global setup, a plugin has to call the function
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
void SetupStore(const char *Name, <i>type</i> Value);
</pre></td></tr></table><p>
where <tt>Name</tt> is the name of the parameter (<tt>"SomeParameter"</tt> in the above
example, without the prefix <tt>"demo."</tt>) and <tt>Value</tt> is a simple data type (like
<tt>char&nbsp;*</tt>, <tt>int</tt> etc).
Note that this is not a function that the individual plugin class needs to implement!
<tt>SetupStore()</tt> is a non-virtual member function of the <tt>cPlugin</tt> class.
<p>
To remove a parameter from the setup data, call <tt>SetupStore()</tt> with the appropriate
name and without any value, as in
<p>
<tt>SetupStore("SomeParameter");</tt>
<p>
The VDR menu "Setup/Plugins" will list all loaded plugins with their name,
version number and description. Selecting an item in this list will bring up
the plugin's "Setup" menu if that plugin has implemented the <tt>SetupMenu()</tt>
function.
<p>
Finally, a plugin doesn't have to implement the <tt>SetupMenu()</tt> if it only
needs setup parameters that are not directly user adjustable. It can use
<tt>SetupStore()</tt> and <tt>SetupParse()</tt> without presenting these
parameters to the user.
<a name="Internationalization"><hr><h2>Internationalization</h2>
<center><i><b>Welcome to Babylon!</b></i></center><p>
If a plugin displays texts to the user, it should implement internationalized
versions of these texts and call the function
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
void RegisterI18n(const tI18nPhrase * const Phrases);
</pre></td></tr></table><p>
to register them with VDR's internationalization mechanism.
<p>
The call to this function must be done in the <a href="#Getting started"><tt>Start()</tt></a> function of the plugin:
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
const tI18nPhrase Phrases[] = {
{ "Hello world!",
"Hallo Welt!",
"",// TODO
"",// TODO
"",// TODO
"",// TODO
"",// TODO
"",// TODO
"",// TODO
"",// TODO
"",// TODO
"",// TODO
},
{ NULL }
};
void cPluginDemo::Start(void)
{
RegisterI18n(Phrases);
}
</pre></td></tr></table><p>
Each entry of type <tt>tI18nPhrase</tt> must have exactly as many members as defined
by the constant <tt>I18nNumLanguages</tt> in the file <tt>VDR/i18n.h</tt>, and the
sequence of the various languages must be the same as defined in <tt>VDR/i18n.c</tt>.<br>
<b>It is very important that the array is terminated with a <tt>{&nbsp;NULL&nbsp;}</tt>
entry!</b>.
<p>
Usually you won't be able to fill in all the different translations by yourself, so
you may want to contact the maintainers of these languages (listed in the file
<tt>VDR/i18n.c</tt>) and ask them to provide the additional translations.
<p>
The actual runtime selection of the texts corresponding to the selected language
is done by wrapping each internationalized text with the <tt>tr()</tt> macro:
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
const char *s = tr("Hello world!");
</pre></td></tr></table><p>
The text given here must be the first one defined in the related <i>Phrases</i>
entry (which is the English version), and the returned pointer is either a translated
version (if available) or the original string. In the latter case a message will be
written to the log file, indicating that a translation is missing.
Texts are first searched for in the <i>Phrases</i> registered for this plugin (if any)
and then in the global VDR texts. So a plugin can make use of texts defined by the
core VDR code.
<a name="Loading plugins into VDR"><hr><h2>Loading plugins into VDR</h2>
<center><i><b>Saddling up!</b></i></center><p>
Plugins are loaded into VDR using the command line option <b><tt>-P</tt></b>, as in
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
vdr -Pdemo
</pre></td></tr></table><p>
If the plugin accepts command line options, they are given as part of the argument
to the <b><tt>-P</tt></b> option, which then has to be enclosed in quotes:
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
vdr -P"demo -a abc -b"
</pre></td></tr></table><p>
Any number of plugins can be loaded this way, each with its own <b><tt>-P</tt></b> option:
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
vdr -P"demo -a abc -b" -Pdvd -Pmp3
</pre></td></tr></table><p>
If you are not starting VDR from the VDR source directory (and thus your plugins
cannot be found at their default location) you need to tell VDR the location of
the plugins through the <b><tt>-L</tt></b> option:
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
vdr -L/usr/lib/vdr -Pdemo
</pre></td></tr></table><p>
There can be any number of <b><tt>-L</tt></b> options, and each of them will apply to the
<b><tt>-P</tt></b> options following it.
<p>
When started with the <b><tt>-h</tt></b> or <b><tt>-V</tt></b> option (for <i>help</i>
or <i>version</i> information, respectively), VDR will automatically load all plugins
in the default or given directory that match the VDR plugin
<a href="#The plugin directory structure">naming convention</a>,
and display their help and/or version information in addition to its own output.
<a name="Building the distribution package"><hr><h2>Building the distribution package</h2>
<center><i><b>Let's get this show on the road!</b></i></center><p>
If you want to make your plugin available to other VDR users, you'll need to
make a package that can be easily distributed.
The <tt>Makefile</tt> that has been created by the call to
<a href="#Initializing a new plugin directory"><tt>newplugin</tt></a>
provides the target <tt>package</tt>, which does this for you.
<p>
Simply change into your source directory and execute <tt>make package</tt>:
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
cd VDR/PLUGINS/src/demo
make package
</pre></td></tr></table><p>
After this you should find a file named like
<p><table><tr><td bgcolor=#F0F0F0><pre><br>
vdr-demo-0.0.1.tgz
</pre></td></tr></table><p>
in your source directory, where <tt>demo</tt> will be replaced with your actual
plugin's name, and <tt>0.0.1</tt> will be your plugin's current version number.
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