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><A
NAME="section-scheduling-randomxs"
>11.3. Providing random access</A
></H1
><P
> In the previous section, we have talked about how elements (or pads)
that are assigned to drive the pipeline using their own task, have
random access over their sinkpads. This means that all elements linked
to those pads (recursively) need to provide random access functions.
Requesting random access is done using the function
<CODE
CLASS="function"
>gst_pad_pull_range ()</CODE
>, which requests a buffer of
a specified size and offset. Source pads implementing and assigned to
do random access will have a <CODE
CLASS="function"
>_get_range ()</CODE
>-function
set using <CODE
CLASS="function"
>gst_pad_set_getrange_function ()</CODE
>, and
that function will be called when the peer pad requests some data. The
element is then responsible for seeking to the right offset and
providing the requested data. Several elements can implement random
access:
</P
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><UL
><LI
><P
> Data sources, such as a file source, that can provide data from any
offset with reasonable low latency.
</P
></LI
><LI
><P
> Filters that would like to provide a pull-based-like scheduling
mode over the whole pipeline. Note that elements assigned to do
random access-based scheduling are themselves responsible for
assigning this scheduling mode to their upstream peers! <SPAN
CLASS="application"
>GStreamer</SPAN
>
will not do that for you.
</P
></LI
><LI
><P
> Parsers who can easily provide this by skipping a small part of
their input and are thus essentially "forwarding" random access
requests literally without any own processing involved. Examples
include tag readers (e.g. ID3) or single output parsers, such as
a WAVE parser.
</P
></LI
></UL
><P
> The following example will show how a <CODE
CLASS="function"
>_get_range
()</CODE
>-function can be implemented in a source element:
</P
><PRE
CLASS="programlisting"
> #include "filter.h"
static GstFlowReturn
gst_my_filter_get_range (GstPad * pad,
guint64 offset,
guint length,
GstBuffer ** buf);
GST_BOILERPLATE (GstMyFilter, gst_my_filter, GstElement, GST_TYPE_ELEMENT);
static void
gst_my_filter_init (GstMyFilter * filter)
{
GstElementClass *klass = GST_ELEMENT_GET_CLASS (filter);
filter->srcpad = gst_pad_new_from_template (
gst_element_class_get_pad_template (klass, "src"), "src");
gst_pad_set_getrange_function (filter->srcpad,
gst_my_filter_get_range);
gst_element_add_pad (GST_ELEMENT (filter), filter->srcpad);
[..]
}
static gboolean
gst_my_filter_get_range (GstPad * pad,
guint64 offset,
guint length,
GstBuffer ** buf)
{
GstMyFilter *filter = GST_MY_FILTER (GST_OBJECT_PARENT (pad));
[.. here, you would fill *buf ..]
return GST_FLOW_OK;
}
</PRE
><P
> In practice, many elements that could theoretically do random access,
may in practice often be assigned to do push-based scheduling anyway,
since there is no downstream element able to start its own task.
Therefore, in practice, those elements should implement both a
<CODE
CLASS="function"
>_get_range ()</CODE
>-function and a <CODE
CLASS="function"
>_chain
()</CODE
>-function (for filters and parsers) or a <CODE
CLASS="function"
>_get_range
()</CODE
>-function and be prepared to start their own task by
providing <CODE
CLASS="function"
>_activate_* ()</CODE
>-functions (for
source elements), so that <SPAN
CLASS="application"
>GStreamer</SPAN
> can decide for the optimal
scheduling mode and have it just work fine in practice.
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