Table of Contents
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-0.10, 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-0.10 in less than a day. Exceptions are elements that will
require a base class in 0.10 (sources, sinks), in which case it may take
a lot longer, depending on the coder's skills (however, when using the
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.
Discont events have been replaced by newsegment events. In 0.10, 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 0.10 this is no longer the case).
In 0.10, buffers have caps attached to them. Elements should allocate
new buffers with
gst_pad_alloc_buffer (). See
Caps negotiation for more details.
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 thread-safety. This means effectively
that return values of functions such as
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.
In 0.8, scheduling could happen in any way. Source elements could
_get ()-based or
()-based, and any other element could be
_loop ()-based, with
no limitations. Scheduling in 0.10 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 Different scheduling modes. As a result of this, the bytestream
object no longer exists. Elements requiring byte-level access should
now use random access on their sinkpads.
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 Caps negotiation.
For as far as possible, elements should try to use existing base
classes in 0.10. Sink and source elements, for example, could derive
GstBaseSink. Audio sinks or sources could even
derive from audio-specific base classes. All existing base classes
have been discussed in Pre-made base classes and the
next few chapters.
In 0.10, event handling and buffers are separated once again. This
means that in order to receive events, one no longer has to set the
GST_FLAG_EVENT_AWARE flag, but can simply
set an event handling function on the element's sinkpad(s), using
gst_pad_set_event_function (). The
_chain ()-function will only receive buffers.
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
GST_OBJECT_LOCK () and
() helpers in most cases, fortunately, which grabs the
default property lock of the element.
GstValueFixedList and all
*_fixed_list_* () functions were renamed to
The semantics of GST_STATE_PAUSED and GST_STATE_PLAYING have changed for elements that are not sink elements. Non-sink elements need to be able to accept and process data already in the GST_STATE_PAUSED state now (i.e. when prerolling the pipeline). More details can be found in Chapter 8, What are states?.
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.
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.
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.
gst_pad_set_link_function (), which used to set
a function that would be called when a format was negotiated between
GstPads, 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
gst_pad_set_setcaps_function (), nowadays, which
sets a function that is called when the format streaming over a pad
changes (so similar to
_set_link_function () in
If the element is derived from a
then override the
gst_pad_use_explicit_caps () has been replaced by
gst_pad_use_fixed_caps (). You can then set the
fixed caps to use on a pad with