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><H1
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><A
NAME="section-basics-data"
>2.3. Data, Buffers and Events</A
></H1
><P
> All streams of data in <SPAN
CLASS="application"
>GStreamer</SPAN
> are chopped up into chunks that are
passed from a source pad on one element to a sink pad on another element.
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>Data</I
></SPAN
> are structures used to hold these chunks of
data.
</P
><P
> Data contains the following important types:
<P
></P
><UL
><LI
><P
> An exact type indicating what type of data (control, content, ...)
this Data is.
</P
></LI
><LI
><P
> A reference count indicating the number of elements currently
holding a reference to the buffer. When the buffer reference count
falls to zero, the buffer will be unlinked, and its memory will be
freed in some sense (see below for more details).
</P
></LI
></UL
>
</P
><P
> There are two types of data defined: events (control) and buffers
(content).
</P
><P
> Buffers may contain any sort of data that the two linked pads
know how to handle. Normally, a buffer contains a chunk of some sort of
audio or video data that flows from one element to another.
</P
><P
> Buffers also contain metadata describing the buffer's contents. Some of
the important types of metadata are:
<P
></P
><UL
><LI
><P
> A pointer to the buffer's data.
</P
></LI
><LI
><P
> An integer indicating the size of the buffer's data.
</P
></LI
><LI
><P
> A timestamp indicating the preferred display timestamp of the
content in the buffer.
</P
></LI
></UL
>
</P
><P
> Events
contain information on the state of the stream flowing between the two
linked pads. Events will only be sent if the element explicitely supports
them, else the core will (try to) handle the events automatically. Events
are used to indicate, for example, a clock discontinuity, the end of a
media stream or that the cache should be flushed.
</P
><P
> Events may contain several of the following items:
<P
></P
><UL
><LI
><P
> A subtype indicating the type of the contained event.
</P
></LI
><LI
><P
> The other contents of the event depend on the specific event type.
</P
></LI
></UL
>
</P
><P
> Events will be discussed extensively in <A
HREF="chapter-advanced-events.html"
>Chapter 20</A
>.
Until then, the only event that will be used is the <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>EOS</I
></SPAN
>
event, which is used to indicate the end-of-stream (usually end-of-file).
</P
><P
> See the <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>GStreamer Library Reference</I
></SPAN
> for the current implementation details of a <A
HREF="../gstreamer/gstreamer-gstdata.html"
TARGET="_top"
><CODE
CLASS="classname"
>GstData</CODE
></A
>, <A
HREF="../gstreamer/gstreamer-gstbuffer.html"
TARGET="_top"
><CODE
CLASS="classname"
>GstBuffer</CODE
></A
> and <A
HREF="../gstreamer/gstreamer-gstevent.html"
TARGET="_top"
><CODE
CLASS="classname"
>GstEvent</CODE
></A
>.
</P
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="sect2-buffer-allocation"
>2.3.1. Buffer Allocation</A
></H2
><P
> Buffers are able to store chunks of memory of several different
types. The most generic type of buffer contains memory allocated
by malloc(). Such buffers, although convenient, are not always
very fast, since data often needs to be specifically copied into
the buffer.
</P
><P
> Many specialized elements create buffers that point to special
memory. For example, the filesrc element usually
maps a file into the address space of the application (using mmap()),
and creates buffers that point into that address range. These
buffers created by filesrc act exactly like generic buffers, except
that they are read-only. The buffer freeing code automatically
determines the correct method of freeing the underlying memory.
Downstream elements that recieve these kinds of buffers do not
need to do anything special to handle or unreference it.
</P
><P
> Another way an element might get specialized buffers is to
request them from a downstream peer. These are called
downstream-allocated buffers. Elements can ask a
peer connected to a source pad to create an empty buffer of
a given size. If a downstream element is able to create a
special buffer of the correct size, it will do so. Otherwise
<SPAN
CLASS="application"
>GStreamer</SPAN
> will automatically create a generic buffer instead.
The element that requested the buffer can then copy data into
the buffer, and push the buffer to the source pad it was
allocated from.
</P
><P
> Many sink elements have accelerated methods for copying data
to hardware, or have direct access to hardware. It is common
for these elements to be able to create downstream-allocated
buffers for their upstream peers. One such example is
ximagesink. It creates buffers that contain XImages. Thus,
when an upstream peer copies data into the buffer, it is copying
directly into the XImage, enabling ximagesink to draw the
image directly to the screen instead of having to copy data
into an XImage first.
</P
><P
> Filter elements often have the opportunity to either work on
a buffer in-place, or work while copying from a source buffer
to a destination buffer. It is optimal to implement both
algorithms, since the <SPAN
CLASS="application"
>GStreamer</SPAN
> framework can choose the
fastest algorithm as appropriate. Naturally, this only makes
sense for strict filters -- elements that have exactly the
same format on source and sink pads.
</P
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