mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-11-14 13:21:28 +00:00
44c548b205
Original commit message from CVS: 2005-11-21 Andy Wingo <wingo@pobox.com> * *.h: * *.c: Ran scripts/update-macros. Oh yes. * gst/gstobject.h (GST_OBJECT_GET_LOCK, GST_OBJECT_LOCK) (GST_OBJECT_TRYLOCK, GST_OBJECT_UNLOCK): Renamed from GST_GET_LOCK, etc. * scripts/update-macros: New script. Run it on your files to change GST_LOCK to GST_OBJECT_LOCK, and the same for UNLOCK as well.
188 lines
9.3 KiB
XML
188 lines
9.3 KiB
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>
|
|
Discont events have been replaced by newsegment events. In 0.9, it is
|
|
essential that you send a newsegment event downstream before you send
|
|
your first buffer (in 0.8 the scheduler would invent discont events if
|
|
you forgot them, in 0.9 this is no longer the case).
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
In 0.9, buffers have caps attached to them. Elements should allocate
|
|
new buffers with <function>gst_pad_alloc_buffer ()</function>. See
|
|
<xref linkend="chapter-negotiation"/> for more details.
|
|
</para>
|
|
</listitem>
|
|
<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>,
|
|
<function>gst_pad_get_parent ()</function>,
|
|
<function>gst_object_get_parent ()</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_OBJECT_LOCK ()</function> and <function>GST_OBJECT_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>
|
|
<listitem>
|
|
<para>
|
|
<function>gst_pad_use_explicit_caps ()</function> has been replaced by
|
|
<function>gst_pad_use_fixed_caps ()</function>. You can then set the
|
|
fixed caps to use on a pad with <function>gst_pad_set_caps ()</function>.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</sect1>
|
|
</chapter>
|