gstreamer/docs/pwg/appendix-porting.xml

<chapter id="chapter-porting">
  <title>Porting 0.8 plug-ins to 0.9</title>
  <para>
    This section of the appendix will discuss shortly what changes to
    plugins will be needed to quickly and conveniently port most
    applications from &GStreamer;-0.8 to &GStreamer;-0.9, with references
    to the relevant sections in this Plugin Writer's Guide where needed.
    With this list, it should be possible to port most plugins to
    &GStreamer;-0.9 in less than a day. Exceptions are elements that will
    require a base class in 0.9 (sources, sinks), in which case it may take
    a lot longer, depending on the coder's skills (however, when using the
    <classname>GstBaseSink</classname> and <classname>GstBaseSrc</classname>
    base-classes, it shouldn't be all too bad), and elements requiring
    the deprecated bytestream interface, which should take 1-2 days with
    random access. The scheduling parts of muxers will also need a rewrite,
    which will take about the same amount of time.
  </para>

  <sect1 id="section-porting-objects">
    <title>List of changes</title>
    <itemizedlist>
      <listitem>
        <para>
          Most functions returning an object or an object property have
          been changed to return its own reference rather than a constant
          reference of the one owned by the object itself. The reason for
          this change is primarily threadsafety. This means effectively
          that return values of functions such as
          <function>gst_element_get_pad ()</function>,
          <function>gst_pad_get_name ()</function> and many more like these
          have to be free'ed or unreferenced after use. Check the API
          references of each function to know for sure whether return
          values should be free'ed or not.
        </para>
      </listitem>
      <listitem>
        <para>
          In 0.8, scheduling could happen in any way. Source elements could
          be <function>_get ()</function>-based or <function>_loop
          ()</function>-based, and any other element could be  <function>_chain
          ()</function>-based or <function>_loop ()</function>-based, with
          no limitations. Scheduling in 0.9 is simpler for the scheduler,
          and the element is expected to do some more work. Pads get
          assigned a scheduling mode, based on which they can either
          operate in random access-mode, in pipeline driving mode or in
          push-mode. all this is documented in detail in <xref
          linkend="chapter-scheduling"/>. As a result of this, the bytestream
          object no longer exists. Elements requiring byte-level access should
          now use random access on their sinkpads.
        </para>
      </listitem>
      <listitem>
        <para>
          Negotiation is asynchronous. This means that downstream negotiation
          is done as data comes in and upstream negotiation is done whenever
          renegotiation is required. All details are described in 
          <xref linkend="chapter-negotiation"/>.
        </para>
      </listitem>
      <listitem>
        <para>
          For as far as possible, elements should try to use existing base
          classes in 0.9. Sink and source elements, for example, could derive
          from <classname>GstBaseSrc</classname> and
          <classname>GstBaseSink</classname>. Audio sinks or sources could even
          derive from audio-specific base classes. All existing base classes
          have been discussed in <xref linkend="chapter-other-base"/> and the
          next few chapters.
        </para>
      </listitem>
      <listitem>
        <para>
          In 0.9, event handling and buffers are separated once again. This
          means that in order to receive events, one no longer has to set the
          <classname>GST_FLAG_EVENT_AWARE</classname> flag, but can simply
          set an event handling function on the element's sinkpad(s), using
           the function <function>gst_pad_set_event_function ()</function>. The
          <function>_chain ()</function>-function will only receive buffers.
        </para>
      </listitem>
      <listitem>
        <para>
          Although core will wrap most threading-related locking for you (e.g.
          it takes the stream lock before calling your data handling
          functions), you are still responsible for locking around certain
          functions, e.g. object properties. Be sure to lock properly here,
          since applications will change those properties in a different thread
          than the thread which does the actual data passing! You can use the
          <function>GST_LOCK ()</function> and <function>GST_UNLOCK
          ()</function> helpers in most cases, fortunately, which grabs the
          default property lock of the element.
        </para>
      </listitem>
      <listitem>
        <para>
          <classname>GstValueFixedList</classname> and all
          <function>*_fixed_list_* ()</function> functions were renamed to
          <classname>GstValueArray</classname> and <function>*_array_*
          ()</function>.
        </para>
      </listitem>
      <listitem>
        <para>
          The semantics of <symbol>GST_STATE_PAUSED</symbol> and 
          <symbol>GST_STATE_PLAYING</symbol> have changed for elements that
          are not sink elements. Non-sink elements need to be able to accept
          and process data already in the <symbol>GST_STATE_PAUSED</symbol> 
          state now (ie. when prerolling the pipeline). More details can be
          found in <xref linkend="chapter-statemanage-states"/>.
        </para>
      </listitem>
      <listitem>
        <para>
          If your plugin's state change function hasn't been superseded by
          virtual start() and stop() methods of one of the new base classes,
          then your plugin's state change functions may need to be changed in 
          order to safely handle concurrent access by multiple threads. Your 
          typical state change function will now first handle upwards state 
          changes, then chain up to the state change function of the parent 
          class (usually GstElementClass in these cases), and only then handle
          downwards state changes. See the vorbis decoder plugin in
          gst-plugins-base for an example.
        </para>
        <para>
          The reason for this is that in the case of downwards state changes
          you don't want to destroy allocated resources while your plugin's 
          chain function (for example) is still accessing those resources in
          another thread. Whether your chain function might be running or not
          depends on the state of your plugin's pads, and the state of those
          pads is closely linked to the state of the element. Pad states are
          handled in the GstElement class's state change function, including
          proper locking, that's why it is essential to chain up before
          destroying allocated resources.
        </para>
        <para>
          As already mentioned above, you should really rewrite your plugin
          to derive from one of the new base classes though, so you don't have
          to worry about these things, as the base class will handle it for you.
          There are no base classes for decoders and encoders yet, so the above 
          paragraphs about state changes definitively apply if your plugin is a
          decoder or an encoder.
        </para>
      </listitem>
      <listitem>
        <para>
          <function>gst_pad_set_link_function ()</function>, which used to set
          a function that would be called when a format was negotiated between
          two <classname>GstPad</classname>s, now sets a function that is
          called when two elements are linked together in an application. For
          all practical purposes, you most likely want to use the function
          <function>gst_pad_set_setcaps_function ()</function>, nowadays, which
          sets a function that is called when the format streaming over a pad
          changes (so similar to <function>_set_link_function ()</function> in
          &GStreamer;-0.8).
        </para>
        <para>
          If the element is derived from a <classname>GstBase</classname> class,
          then override the <function>set_caps ()</function>.
        </para>
      </listitem>
    </itemizedlist>
  </sect1>
</chapter>