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docs/manual/advanced-dataaccess.xml: Don't imply that it's okay to unconditionally change buffer data or buffer metad...

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Original commit message from CVS:
* docs/manual/advanced-dataaccess.xml:
Don't imply that it's okay to unconditionally change
buffer data or buffer metadata in a pad probe callback,
and a bunch of other comments. Fixes #430031.
This commit is contained in:
Tim-Philipp Müller 2007-10-09 10:41:41 +00:00
parent ba9fe25f2d
commit c3a2e0699f
2 changed files with 62 additions and 8 deletions
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7
ChangeLog
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@ -1,3 +1,10 @@
2007-10-09 Tim-Philipp Müller <tim at centricular dot net>
* docs/manual/advanced-dataaccess.xml:
Don't imply that it's okay to unconditionally change
buffer data or buffer metadata in a pad probe callback,
and a bunch of other comments. Fixes #430031.
2007-10-08 Tim-Philipp Müller <tim at centricular dot net>
* win32/common/gstenumtypes.c:

63
docs/manual/advanced-dataaccess.xml
View file

@ -20,21 +20,32 @@
nothing more than a signal callback that can be attached to a pad.
Those signals are by default not fired at all (since that may have a
negative impact on performance), but can be enabled by attaching a
probe using <function>gst_pad_add_data_probe ()</function> or one of
the similar functions. Those functions attach the signal handler and
probe using <function>gst_pad_add_buffer_probe ()</function>,
<function>gst_pad_add_event_probe ()</function>, or
<function>gst_pad_add_data_probe ()</function>.
Those functions attach the signal handler and
enable the actual signal emission. Similarly, one can use the
<function>gst_pad_remove_data_probe ()</function> or related functions
to remove the signal handlers again. It is also possible to only listen
to events or only to buffers (and ignore the other).
<function>gst_pad_remove_buffer_probe ()</function>,
<function>gst_pad_remove_event_probe ()</function>, or
<function>gst_pad_remove_data_probe ()</function>
to remove the signal handlers again.
</para>
<para>
Probes run in pipeline threading context, so callbacks should try to
not block and generally not do any weird stuff, since this could
have a negative impact on pipeline performance or, in case of bugs,
cause deadlocks or crashes. However, most common buffer operations
cause deadlocks or crashes. More precisely, one should usually not
call any GUI-related functions from within a probe callback, nor try
to change the state of the pipeline. An application may post custom
messages on the pipeline's bus though to communicate with the main
application thread and have it do things like stop the pipeline.
</para>
<para>
In any case, most common buffer operations
that elements can do in <function>_chain ()</function> functions, can
be done in probe callbacks as well. The example below gives a short
impression on how to use them.
impression on how to use them (even if this usage is not entirely
correct, but more on that below):
</para>
<programlisting><!-- example-begin probe.c -->
#include &lt;gst/gst.h&gt;
@ -133,6 +144,35 @@ main (gint argc,
videotestsrc ! xvimagesink</quote>, just so you know what you're
looking for.
</para>
<para>
The above example is not really correct though. Strictly speaking, a
pad probe callback is only allowed to modify the buffer content if the
buffer is writable, and it is only allowed to modify buffer metadata like
timestamps, caps, etc. if the buffer metadata is writable. Whether this
is the case or not depends a lot on the pipeline and the elements
involved. Often enough, this is the case, but sometimes it is not,
and if it is not then unexpected modification of the data or metadata
can introduce bugs that are very hard to debug and track down. You can
check if a buffer and its metadata are writable with
<function>gst_buffer_is_writable ()</function> and
<function>gst_buffer_is_metadata_writable ()</function>. Since you
can't pass back a different buffer than the one passed in, there is no
point of making a buffer writable in the callback function.
</para>
<para>
Pad probes are suited best for looking at data as it passes through
the pipeline. If you need to modify data, you should write your own
GStreamer element. Base classes like GstAudioFilter, GstVideoFilter or
GstBaseTransform make this fairly easy.
</para>
<para>
If you just want to inspect buffers as they pass through the pipeline,
you don't even need to set up pad probes. You could also just insert
an identity element into the pipeline and connect to its "handoff"
signal. The identity element also provides a few useful debugging tools
like the "dump" property or the "last-message" property (the latter is
enabled by passing the '-v' switch to gst-launch).
</para>
</sect1>
<sect1 id="section-data-spoof">
@ -146,12 +186,19 @@ main (gint argc,
there is no support for those methods.</emphasis> If it doesn't work,
you're on your own. Also, synchronization, thread-safety and other
things that you've been able to take for granted so far are no longer
guanranteed if you use any of those methods. It's always better to
guaranteed if you use any of those methods. It's always better to
simply write a plugin and have the pipeline schedule and manage it.
See the Plugin Writer's Guide for more information on this topic. Also
see the next section, which will explain how to embed plugins statically
in your application.
</para>
<note><para>
New API is being developed at the moment to make data insertion and
extraction less painful for applications. It can be found as GstAppSrc
and GstAppSink in the gst-plugins-bad module. At the time of writing
(October 2007), this API is not quite stable and ready yet, even though
it may work fine for your purposes.
</para></note>
<para>
After all those disclaimers, let's start. There's three possible
elements that you can use for the above-mentioned purposes. Those are