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>19.2. Writing a source</A
></H1
><P
> In the previous part, particularly <A
HREF="section-scheduling-randomxs.html"
>Providing random access</A
>, we have learned that some types
of elements can provide random access. This applies most definitely to
source elements reading from a randomly seekable location, such as file
sources. However, other source elements may be better described as a
live source element, such as a camera source, an audio card source and
such; those are not seekable and do not provide byte-exact access. For
all such use cases, <SPAN
CLASS="application"
>GStreamer</SPAN
> provides two base classes:
<CODE
CLASS="classname"
>GstBaseSrc</CODE
> for the basic source functionality, and
<CODE
CLASS="classname"
>GstPushSrc</CODE
>, which is a non-byte exact source
base-class. The pushsource base class itself derives from basesource as
well, and thus all statements about the basesource apply to the
pushsource, too.
</P
><P
> The basesrc class does several things automatically for derived classes,
so they no longer have to worry about it:
</P
><P
></P
><UL
><LI
><P
> Fixes to <CODE
CLASS="classname"
>GstBaseSrc</CODE
> apply to all derived
classes automatically.
</P
></LI
><LI
><P
> Automatic pad activation handling, and task-wrapping in case we get
assigned to start a task ourselves.
</P
></LI
></UL
><P
> The <CODE
CLASS="classname"
>GstBaseSrc</CODE
> may not be suitable for all cases,
though; it has limitations:
</P
><P
></P
><UL
><LI
><P
> There is one and only one sourcepad. Source elements requiring
multiple sourcepads cannot use this base-class.
</P
></LI
><LI
><P
> Since the base-class owns the pad and derived classes can only
control it as far as the virtual functions allow, you are limited
to the functionality provided by the virtual functions. If you need
more, you cannot use this base-class.
</P
></LI
></UL
><P
> It is possible to use special memory, such as X server memory pointers
or <CODE
CLASS="function"
>mmap ()</CODE
>'ed memory areas, as data pointers in
buffers returned from the <CODE
CLASS="function"
>create()</CODE
> virtual function.
In almost all cases, you will want to subclass
<CODE
CLASS="classname"
>GstBuffer</CODE
> so that your own set of functions can
be called when the buffer is destroyed.
</P
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><H2
CLASS="sect2"
><A
NAME="section-base-audiosrc"
>19.2.1. Writing an audio source</A
></H2
><P
> An audio source is nothing more but a special case of a pushsource.
Audio sources would be anything that reads audio, such as a source
reading from a soundserver, a kernel interface (such as ALSA) or a
test sound / signal generator. <SPAN
CLASS="application"
>GStreamer</SPAN
> provides two base classes,
similar to the two audiosinks described in <A
HREF="chapter-other-base.html#section-base-audiosink"
>Writing an audio sink</A
>; one is ringbuffer-based, and
requires the derived class to take care of its own scheduling,
synchronization and such. The other is based on this
<CODE
CLASS="classname"
>GstBaseAudioSrc</CODE
> and is called
<CODE
CLASS="classname"
>GstAudioSrc</CODE
>, and provides a simple
<CODE
CLASS="function"
>open ()</CODE
>, <CODE
CLASS="function"
>close ()</CODE
> and
<CODE
CLASS="function"
>read ()</CODE
> interface, which is rather simple to
implement and will suffice for most soundserver sources and audio
interfaces (e.g. ALSA or OSS) out there.
</P
><P
> The <CODE
CLASS="classname"
>GstAudioSrc</CODE
> base-class has several benefits
for derived classes, on top of the benefits of the
<CODE
CLASS="classname"
>GstPushSrc</CODE
> base-class that it is based on:
</P
><P
></P
><UL
><LI
><P
> Does syncronization and provides a clock.
</P
></LI
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><P
> New features can be added to it and will apply to all derived
classes automatically.
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