gstreamer/docs/pwg/building-boiler.xml

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<chapter id="cha-building-boiler" xreflabel="Constructing the Boilerplate">
  <title>Constructing the Boilerplate</title>
  <para>
    In this chapter you will learn how to construct the bare minimum code for a
    new plugin. Starting from ground zero, you will see how to get the
    &GStreamer; template source. Then you will learn how to use a few basic
    tools to copy and modify a template plugin to create a new plugin. If you
    follow the examples here, then by the end of this chapter you will have a
    functional audio filter plugin that you can compile and use in &GStreamer;
    applications.
  </para>

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  <sect1 id="sect1-boiler-source" xreflabel="Getting the Gstreamer Plugin Templates">
    <title>Getting the Gstreamer Plugin Templates</title>
    <para>
      There are currently two ways to develop a new plugin for &GStreamer;: You
      can write the entire plugin by hand, or you can copy an existing plugin
      template and write the plugin code you need. The second method is by far
      the simpler of the two, so the first method will not even be described
      here. (Errm, that is, <quote>it is left as an exercise to the
      reader.</quote>)
    </para>
    <para>
      The first step is to check out a copy of the
      <filename>gst-template</filename> CVS module to get an important tool and
      the source code template for a basic &GStreamer; plugin. To check out the
      <filename>gst-template</filename> module, make sure you are connected to
      the internet, and type the following commands at a command console:
    </para>
    <screen>
<prompt>shell $ </prompt><userinput>cd .</userinput>
<prompt>shell $ </prompt><userinput>cvs -d:pserver:anonymous@cvs.gstreamer.sourceforge.net:/cvsroot/gstreamer login</userinput>
Logging in to :pserver:anonymous@cvs.gstreamer.sourceforge.net:2401/cvsroot/gstreamer
CVS password:
<prompt>shell $ </prompt><userinput>cvs -z3 -d:pserver:anonymous@cvs.gstreamer.sourceforge.net:/cvsroot/gstreamer co gst-template</userinput>
U gst-template/README
U gst-template/gst-app/AUTHORS
U gst-template/gst-app/ChangeLog
U gst-template/gst-app/Makefile.am
U gst-template/gst-app/NEWS
U gst-template/gst-app/README
U gst-template/gst-app/autogen.sh
U gst-template/gst-app/configure.ac
U gst-template/gst-app/src/Makefile.am
...
    </screen>
    <para>
      After the first command, you will have to press <keycap>ENTER</keycap> to
      log in to the CVS server. (You might have to log in twice.) The second
      command will check out a series of files and directories into <filename
      class="directory">./gst-template</filename>. The template you will be
      using is in <filename
      class="directory">./gst-template/gst-plugin/</filename> directory. You
      should look over the files in that directory to get a general idea of the
      structure of a source tree for a plugin.
    </para>
  </sect1>

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  <sect1 id="sect1-boiler-project-stamp" xreflabel="Using the Project Stamp">
    <title>Using the Project Stamp</title>
    <para>
      The first thing to do when making a new element is to specify some basic
      details about it: what its name is, who wrote it, what version number it
      is, etc. We also need to define an object to represent the element and to
      store the data the element needs. These details are collectively known as
      the <emphasis>boilerplate</emphasis>.
    </para>
    <para>
      The standard way of defining the boilerplate is simply to write some code,
      and fill in some structures. As mentioned in the previous section, the
      easiest way to do this is to copy a template and add functionality
      according to your needs. To help you do so, there are some tools in the
      <filename class="directory">./gst-template/tools/</filename> directory.
      One tool, <filename>gst-quick-stamp</filename>, is a quick command line
      tool. The other, <filename>gst-project-stamp</filename>, is a full GNOME
      druid application that takes you through the steps of creating a new
      project (either a plugin or an application).
    </para>
    <para>
      To use <command>pluginstamp.sh</command>, first open up a terminal window.
      Change to the <filename class="directory">gst-template</filename>
      directory, and then run the <command>pluginstamp.sh</command> command. The
      arguments to the <command>pluginstamp.sh</command> are:
    </para>
    <orderedlist>
      <listitem>
        <para>the name of the plugin, and</para>
      </listitem>
      <listitem>
        <para>
          the directory that should hold a new subdirectory for the source tree
          of the plugin.
        </para>
      </listitem>
    </orderedlist>
    <para>
      Note that capitalization is important for the name of the plugin. Under
      some operating systems, capitalization is also important when specifying
      directory names. For example, the following commands create the
      ExampleFilter plugin based on the plugin template and put the output files
      in a new directory called <filename
      class="directory">~/src/examplefilter/</filename>:
    </para>
    <screen>
<prompt>shell $ </prompt><userinput>cd gst-template</userinput>
<prompt>shell $ </prompt><userinput>tools/pluginstamp.sh ExampleFilter ~/src</userinput>
    </screen>
  </sect1>

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  <sect1 name="sect1-boiler-examine" xreflabel="Examining the Basic Code">
    <title>Examining the Basic Code</title>
    <para>
      First we will examine the code you would be likely to place in a header
      file (although since the interface to the code is entirely defined by the
      pluging system, and doesn't depend on reading a header file, this is not
      crucial.)

      The code here can be found in
      <filename>examples/pwg/examplefilter/boiler/gstexamplefilter.h</filename>.
    </para>

    <example name="ex-boiler-examine-h" xreflabel="Example Plugin Header File">
      <title>Example Plugin Header File</title>
      <programlisting>
  /* Definition of structure storing data for this element. */
  typedef struct _GstExample GstExample;

  struct _GstExample {
    GstElement element;

    GstPad *sinkpad,*srcpad;

    gint8 active;
  };

  /* Standard definition defining a class for this element. */
  typedef struct _GstExampleClass GstExampleClass;
  struct _GstExampleClass {
    GstElementClass parent_class;
  };

  /* Standard macros for defining types for this element.  */
  #define GST_TYPE_EXAMPLE \
    (gst_example_get_type())
  #define GST_EXAMPLE(obj) \
    (GTK_CHECK_CAST((obj),GST_TYPE_EXAMPLE,GstExample))
  #define GST_EXAMPLE_CLASS(klass) \
    (GTK_CHECK_CLASS_CAST((klass),GST_TYPE_EXAMPLE,GstExample))
  #define GST_IS_EXAMPLE(obj) \
    (GTK_CHECK_TYPE((obj),GST_TYPE_EXAMPLE))
  #define GST_IS_EXAMPLE_CLASS(obj) \
    (GTK_CHECK_CLASS_TYPE((klass),GST_TYPE_EXAMPLE))

  /* Standard function returning type information. */
  GtkType gst_example_get_type(void);
      </programlisting>
    </example>
  </sect1>

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  <sect1 id="sect1-boiler-filterfactory" xreflabel="Creating a Filter With FilterFactory">
    <title>Creating a Filter With FilterFactory (Future)</title>
    <para>
      A plan for the future is to create a FilterFactory, to make the process of
      making a new filter a simple process of specifying a few details, and
      writing a small amount of code to perform the actual data processing.
      Ideally, a FilterFactory would perform the tasks of boilerplate creation,
      code functionality implementation, and filter registration.
    </para>
    <para>
      Unfortunately, this has not yet been implemented. Even when someone
      eventually does write a FilterFactory, this element will not be able to
      cover all the possibilities available for filter writing. Thus, some
      plugins will always need to be manually coded and registered.
    </para>
    <para>
      Here is a rough outline of what is planned: You run the FilterFactory and
      give the factory a list of appropriate function pointers and data
      structures to define a filter. With a reasonable measure of preprocessor
      magic, you just need to provide a name for the filter and definitions of
      the functions and data structures desired. Then you call a macro from
      within plugin_init() that registers the new filter. All the fluff that
      goes into the definition of a filter is thus be hidden from view.
    </para>
  </sect1>

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  <sect1 id="sect1-boiler-details">
    <title>GstElementDetails</title>
    <para>
      The GstElementDetails structure gives a heirarchical type for the element,
      a human-readable description of the element, as well as author and version
      data. The entries are:
    </para>
    <itemizedlist>
      <listitem><para>
        A long, english, name for the element.
      </para></listitem><listitem><para>
        The type of the element, as a heirarchy. The heirarchy is defined by
        specifying the top level category, followed by a "/", followed by the
        next level category, etc. The type should be defined according to the
        guidelines elsewhere in this document. (FIXME: write the guidelines, and
        give a better reference to them)
      </para></listitem><listitem><para>
        A brief description of the purpose of the element.
      </para></listitem><listitem><para>
        The version number of the element. For elements in the main GStreamer
        source code, this will often simply be VERSION, which is a macro defined
        to be the version number of the current GStreamer version. The only
        requirement, however, is that the version number should increase
        monotonically.
      </para>
      <para>
        Version numbers should be stored in major.minor.patch form: ie, 3
        (decimal) numbers, separated by periods (.).
      </para></listitem><listitem><para>
        The name of the author of the element, optionally followed by a contact
        email address in angle brackets.
      </para></listitem><listitem><para>
        The copyright details for the element.
      </para></listitem>
    </itemizedlist>
    <para>
      For example:
    </para>
    <programlisting>
  static GstElementDetails example_details = {
      "An example plugin",
      "Example/FirstExample",
      "Shows the basic structure of a plugin",
      VERSION,
      "your name &lt;your.name@your.isp&gt;",
      "(C) 2001",
  };
    </programlisting>
  </sect1>

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  <sect1 id="sect1-boiler-constructors">
    <title>Constructor Functions</title>
    <para>
      Each element has two functions which are used for construction of an
      element. These are the _class_init() function, which is used to initialise
      the class (specifying what signals and arguments the class has and setting
      up global state), and the _init() function, which is used to initialise a
      specific instance of the class.
    </para>
  </sect1>

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  <sect1 id="sect1-boiler-plugininit">
    <title>The plugin_init function</title>
    <para>
      Once we have written code defining all the parts of the plugin, we need to
      write the plugin_init() function. This is a special function, which is
      called as soon as the plugin is loaded, and must return a pointer to a newly
      allocated GstPlugin structure. This structure contains the details of all
      the facilities provided by the plugin, and is the mechanism by which the
      definitions are made available to the rest of the &GStreamer; system. Helper
      functions are provided to help fill the structure: for future compatability
      it is recommended that these functions are used, as documented below, rather
      than attempting to access the structure directly.
    </para>
    <para>
      Note that the information returned by the plugin_init() function will be
      cached in a central registry. For this reason, it is important that the same
      information is always returned by the function: for example, it must not
      make element factories available based on runtime conditions. If an element
      can only work in certain conditions (for example, if the soundcard is not
      being used by some other process) this must be reflected by the element
      being unable to enter the READY state if unavailable, rather than the plugin
      attempting to deny existence of the plugin.
    </para>
  </sect1>
</chapter>