mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
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436 lines
16 KiB
XML
436 lines
16 KiB
XML
<!-- ############ chapter ############# -->
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<chapter id="chapter-building-boiler" xreflabel="Constructing the Boilerplate">
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<title>Constructing the Boilerplate</title>
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<para>
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In this chapter you will learn how to construct the bare minimum code for a
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new plugin. Starting from ground zero, you will see how to get the
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&GStreamer; template source. Then you will learn how to use a few basic
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tools to copy and modify a template plugin to create a new plugin. If you
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follow the examples here, then by the end of this chapter you will have a
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functional audio filter plugin that you can compile and use in &GStreamer;
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applications.
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</para>
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<!-- ############ sect1 ############# -->
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<sect1 id="section-boiler-source" xreflabel="Getting the GStreamer Plugin Templates">
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<title>Getting the GStreamer Plugin Templates</title>
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<para>
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There are currently two ways to develop a new plugin for &GStreamer;: You
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can write the entire plugin by hand, or you can copy an existing plugin
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template and write the plugin code you need. The second method is by far
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the simpler of the two, so the first method will not even be described
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here. (Errm, that is, <quote>it is left as an exercise to the
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reader.</quote>)
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</para>
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<para>
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The first step is to check out a copy of the
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<filename>gst-template</filename> git module to get an important tool and
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the source code template for a basic &GStreamer; plugin. To check out the
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<filename>gst-template</filename> module, make sure you are connected to
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the internet, and type the following commands at a command console:
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</para>
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<screen>
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<prompt>shell $ </prompt><userinput>git clone git://anongit.freedesktop.org/gstreamer/gst-template.git</userinput>
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Initialized empty Git repository in /some/path/gst-template/.git/
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remote: Counting objects: 373, done.
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remote: Compressing objects: 100% (114/114), done.
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remote: Total 373 (delta 240), reused 373 (delta 240)
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Receiving objects: 100% (373/373), 75.16 KiB | 78 KiB/s, done.
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Resolving deltas: 100% (240/240), done.
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</screen>
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<para>
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This command will check out a series of files and directories into
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<filename class="directory">gst-template</filename>. The template you
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will be using is in the
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<filename class="directory">gst-template/gst-plugin/</filename>
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directory. You should look over the files in that directory to get a
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general idea of the structure of a source tree for a plugin.
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</para>
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<para>
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If for some reason you can't access the git repository, you can also
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<ulink type="http"
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url="http://cgit.freedesktop.org/gstreamer/gst-template/commit/">
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download a snapshot of the latest revision</ulink> via the cgit web
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interface.
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</para>
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</sect1>
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<!-- ############ sect1 ############# -->
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<sect1 id="section-boiler-project-stamp" xreflabel="Using the Project Stamp">
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<title>Using the Project Stamp</title>
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<para>
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The first thing to do when making a new element is to specify some basic
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details about it: what its name is, who wrote it, what version number it
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is, etc. We also need to define an object to represent the element and to
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store the data the element needs. These details are collectively known as
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the <emphasis>boilerplate</emphasis>.
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</para>
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<para>
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The standard way of defining the boilerplate is simply to write some code,
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and fill in some structures. As mentioned in the previous section, the
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easiest way to do this is to copy a template and add functionality
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according to your needs. To help you do so, there is a tool in the
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<filename class="directory">./gst-plugins/tools/</filename> directory.
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This tool, <filename>make_element</filename>, is a command line utility
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that creates the boilerplate code for you.
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</para>
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<para>
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To use <command>make_element</command>, first open up a terminal window.
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Change to the <filename class="directory">gst-template/gst-plugin/src</filename>
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directory, and then run the <command>make_element</command> command. The
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arguments to the <command>make_element</command> are:
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</para>
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<orderedlist>
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<listitem>
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<para>the name of the plugin, and</para>
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</listitem>
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<listitem>
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<para>
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the source file that the tool will use. By default,
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<filename>gstplugin</filename> is used.
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</para>
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</listitem>
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</orderedlist>
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<para>
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Note that capitalization is important for the name of the plugin. Under
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some operating systems, capitalization is also important when specifying
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directory names. For example, the following commands create the
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ExampleFilter plugin based on the plugin template and put the output files
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in the <filename class="directory">gst-template/gst-plugin/src</filename>
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directory:
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</para>
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<screen>
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<prompt>shell $ </prompt><userinput>cd gst-template/gst-plugin/src</userinput>
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<prompt>shell $ </prompt><userinput>../tools/make_element ExampleFilter</userinput>
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</screen>
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<para>
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The last command creates two files:
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<filename>gstexamplefilter.c</filename> and
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<filename>gstexamplefilter.h</filename>.
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</para>
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<note>
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<para>
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It is recommended that you create a copy of the <filename
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class="directory">gst-plugin</filename>
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directory before continuing.
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</para>
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</note>
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</sect1>
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<!-- ############ sect1 ############# -->
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<sect1 id="section-boiler-examine">
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<title>Examining the Basic Code</title>
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<para>
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First we will examine the code you would be likely to place in a header
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file (although since the interface to the code is entirely defined by the
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plugin system, and doesn't depend on reading a header file, this is not
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crucial.)
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The code here can be found in
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<filename>examples/pwg/examplefilter/boiler/gstexamplefilter.h</filename>.
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</para>
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<example id="ex-boiler-examine-h">
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<title>Example Plugin Header File</title>
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<programlisting><!-- example-begin filter.h a -->
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#include <gst/gst.h>
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/* Definition of structure storing data for this element. */
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typedef struct _GstMyFilter {
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GstElement element;
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GstPad *sinkpad, *srcpad;
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gboolean silent;
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<!-- example-end filter.h a -->
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<!-- example-begin filter.h b --><!--
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gint samplerate, channels;
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gint from_samplerate, to_samplerate;
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gboolean passthrough;
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guint64 offset;
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--><!-- example-end filter.h b -->
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<!-- example-begin filter.h c -->
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} GstMyFilter;
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/* Standard definition defining a class for this element. */
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typedef struct _GstMyFilterClass {
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GstElementClass parent_class;
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} GstMyFilterClass;
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/* Standard macros for defining types for this element. */
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#define GST_TYPE_MY_FILTER (gst_my_filter_get_type())
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#define GST_MY_FILTER(obj) \
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(G_TYPE_CHECK_INSTANCE_CAST((obj),GST_TYPE_MY_FILTER,GstMyFilter))
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#define GST_MY_FILTER_CLASS(klass) \
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(G_TYPE_CHECK_CLASS_CAST((klass),GST_TYPE_MY_FILTER,GstMyFilterClass))
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#define GST_IS_MY_FILTER(obj) \
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(G_TYPE_CHECK_INSTANCE_TYPE((obj),GST_TYPE_MY_FILTER))
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#define GST_IS_MY_FILTER_CLASS(klass) \
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(G_TYPE_CHECK_CLASS_TYPE((klass),GST_TYPE_MY_FILTER))
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/* Standard function returning type information. */
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GType gst_my_filter_get_type (void);
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<!-- example-end filter.h c --></programlisting>
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</example>
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<para>
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Using this header file, you can use the following macro to setup
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the <classname>GObject</classname> basics in your source file so
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that all functions will be called appropriately:
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</para>
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<programlisting><!-- example-begin boilerplate.c a -->
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#include "filter.h"
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GST_BOILERPLATE (GstMyFilter, gst_my_filter, GstElement, GST_TYPE_ELEMENT);
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<!-- example-end boilerplate.c a --></programlisting>
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</sect1>
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<!-- ############ sect1 ############# -->
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<sect1 id="section-boiler-details">
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<title>GstElementDetails</title>
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<para>
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The GstElementDetails structure gives a hierarchical type for the element,
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a human-readable description of the element, as well as author and version
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data. The entries are:
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</para>
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<itemizedlist>
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<listitem><para>
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A long, English, name for the element.
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</para></listitem><listitem><para>
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The type of the element, see the docs/design/draft-klass.txt document
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in the GStreamer core source tree for details and examples.
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</para></listitem><listitem><para>
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A brief description of the purpose of the element.
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</para></listitem><listitem><para>
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The name of the author of the element, optionally followed by a contact
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email address in angle brackets.
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</para></listitem>
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</itemizedlist>
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<para>
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For example:
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</para>
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<programlisting><!-- example-begin boilerplate.c b -->
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static const GstElementDetails my_filter_details = {
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"An example plugin",
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"Example/FirstExample",
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"Shows the basic structure of a plugin",
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"your name <your.name@your.isp>"
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};
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<!-- example-end boilerplate.c b --></programlisting>
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<para>
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The element details are registered with the plugin during
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the <function>_base_init ()</function> function, which is part of
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the GObject system. The <function>_base_init ()</function> function
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should be set for this GObject in the function where you register
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the type with GLib.
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</para>
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<programlisting><!-- example-begin boilerplate.c c -->
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static void
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gst_my_filter_base_init (gpointer klass)
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{
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GstElementClass *element_class = GST_ELEMENT_CLASS (klass);
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<!-- example-end boilerplate.c c -->
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static const GstElementDetails my_filter_details = {
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[..]
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};
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[..]<!-- example-begin boilerplate.c d -->
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gst_element_class_set_details (element_class, &my_filter_details);
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<!-- example-end boilerplate.c d -->
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}
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</programlisting>
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</sect1>
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<!-- ############ sect1 ############# -->
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<sect1 id="section-boiler-padtemplates">
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<title>GstStaticPadTemplate</title>
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<para>
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A GstStaticPadTemplate is a description of a pad that the element will
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(or might) create and use. It contains:
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</para>
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<itemizedlist>
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<listitem>
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<para>A short name for the pad.</para>
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</listitem>
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<listitem>
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<para>Pad direction.</para>
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</listitem>
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<listitem>
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<para>
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Existence property. This indicates whether the pad exists always (an
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<quote>always</quote> pad), only in some cases (a
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<quote>sometimes</quote> pad) or only if the application requested
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such a pad (a <quote>request</quote> pad).
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</para>
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</listitem>
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<listitem>
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<para>Supported types by this element (capabilities).</para>
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</listitem>
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</itemizedlist>
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<para>
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For example:
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</para>
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<programlisting><!-- example-begin boilerplate.c e -->
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static GstStaticPadTemplate sink_factory =
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GST_STATIC_PAD_TEMPLATE (
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"sink",
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GST_PAD_SINK,
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GST_PAD_ALWAYS,
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GST_STATIC_CAPS ("ANY")
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);
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<!-- example-end boilerplate.c e -->
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<!-- example-begin boilerplate.c f --><!--
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static GstStaticPadTemplate src_factory =
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GST_STATIC_PAD_TEMPLATE (
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"src",
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GST_PAD_SRC,
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GST_PAD_ALWAYS,
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GST_STATIC_CAPS ("ANY")
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);
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--><!-- example-end boilerplate.c f -->
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</programlisting>
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<para>
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Those pad templates are registered during the
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<function>_base_init ()</function> function. Pads are created from these
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templates in the element's <function>_init ()</function> function using
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<function>gst_pad_new_from_template ()</function>. The template can be
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retrieved from the element class using
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<function>gst_element_class_get_pad_template ()</function>. See below
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for more details on this. In order to create a new pad from this
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template using <function>gst_pad_new_from_template ()</function>, you
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will need to declare the pad template as a global variable. More on
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this subject in <xref linkend="chapter-building-pads"/>.
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</para>
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<programlisting>
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static GstStaticPadTemplate sink_factory = [..],
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src_factory = [..];
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static void
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gst_my_filter_base_init (gpointer klass)
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{
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GstElementClass *element_class = GST_ELEMENT_CLASS (klass);
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[..]
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<!-- example-begin boilerplate.c g -->
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gst_element_class_add_pad_template (element_class,
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gst_static_pad_template_get (&src_factory));
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gst_element_class_add_pad_template (element_class,
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gst_static_pad_template_get (&sink_factory));
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}
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<!-- example-end boilerplate.c g -->
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<!-- example-begin boilerplate.c h --><!--
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static void
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gst_my_filter_class_init (GstMyFilterClass * klass)
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{
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}
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static void
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gst_my_filter_init (GstMyFilter * filter)
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{
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}
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#include "register.func"
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--><!-- example-end boilerplate.c h --></programlisting>
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<para>
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The last argument in a template is its type
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or list of supported types. In this example, we use 'ANY', which means
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that this element will accept all input. In real-life situations, you
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would set a mimetype and optionally a set of properties to make sure
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that only supported input will come in. This representation should be
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a string that starts with a mimetype, then a set of comma-separates
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properties with their supported values. In case of an audio filter that
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supports raw integer 16-bit audio, mono or stereo at any samplerate, the
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correct template would look like this:
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</para>
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<programlisting>
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static GstStaticPadTemplate sink_factory =
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GST_STATIC_PAD_TEMPLATE (
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"sink",
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GST_PAD_SINK,
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GST_PAD_ALWAYS,
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GST_STATIC_CAPS (
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"audio/x-raw-int, "
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"width = (int) 16, "
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"depth = (int) 16, "
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"endianness = (int) BYTE_ORDER, "
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"channels = (int) { 1, 2 }, "
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"rate = (int) [ 8000, 96000 ]"
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)
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);
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</programlisting>
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<para>
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Values surrounded by curly brackets (<quote>{</quote> and
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<quote>}</quote>) are lists, values surrounded by square brackets
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(<quote>[</quote> and <quote>]</quote>) are ranges.
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Multiple sets of types are supported too, and should be separated by
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a semicolon (<quote>;</quote>). Later, in the chapter on pads, we will
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see how to use types to know the exact format of a stream:
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<xref linkend="chapter-building-pads"/>.
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</para>
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</sect1>
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<!-- ############ sect1 ############# -->
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<sect1 id="section-boiler-constructors">
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<title>Constructor Functions</title>
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<para>
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Each element has three functions which are used for construction of an
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element. These are the <function>_base_init()</function> function which
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is meant to initialize class and child class properties during each new
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child class creation; the <function>_class_init()</function> function,
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which is used to initialise the class only once (specifying what signals,
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arguments and virtual functions the class has and setting up global
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state); and the <function>_init()</function> function, which is used to
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initialise a specific instance of this type.
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</para>
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</sect1>
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<!-- ############ sect1 ############# -->
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<sect1 id="section-boiler-plugininit">
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<title>The plugin_init function</title>
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<para>
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Once we have written code defining all the parts of the plugin, we need to
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write the plugin_init() function. This is a special function, which is
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called as soon as the plugin is loaded, and should return TRUE or FALSE
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depending on whether it loaded initialized any dependencies correctly.
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Also, in this function, any supported element type in the plugin should
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be registered.
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</para>
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<programlisting><!-- example-begin register.func -->
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static gboolean
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plugin_init (GstPlugin *plugin)
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{
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return gst_element_register (plugin, "my_filter",
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GST_RANK_NONE,
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GST_TYPE_MY_FILTER);
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}
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GST_PLUGIN_DEFINE (
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GST_VERSION_MAJOR,
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GST_VERSION_MINOR,
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"my_filter",
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"My filter plugin",
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plugin_init,
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VERSION,
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"LGPL",
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"GStreamer",
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"http://gstreamer.net/"
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)
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<!-- example-end register.func --></programlisting>
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<para>
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Note that the information returned by the plugin_init() function will be
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cached in a central registry. For this reason, it is important that the
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same information is always returned by the function: for example, it
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must not make element factories available based on runtime conditions.
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If an element can only work in certain conditions (for example, if the
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soundcard is not being used by some other process) this must be reflected
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by the element being unable to enter the READY state if unavailable,
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rather than the plugin attempting to deny existence of the plugin.
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</para>
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</sect1>
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</chapter>
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