Huh?: Gstreamer Basic Tutorial / API (3: Dynamic pipelines)

R C cjvijf at
Thu May 16 03:11:59 UTC 2019

so while messing with it,  I forgot to change teh second part, and even 
though it seems incosistent, can someone explain why this works?  (I am 
just putting the declarations here,  rewritting to ugly got's into 
something nicr is not the problem)

in the main part the declarations I had are:

// Create the elements
data.source = gst_element_factory_make("uridecodebin", "source");
data.audioconvert = gst_element_factory_make("audioconvert", 
data.audiosink = gst_element_factory_make("autoaudiosink", "audiosink");
data.videoconvert = gst_element_factory_make("videoconvert", 
data.videosink = gst_element_factory_make("autovideosink", "videosink");

and I forgot to make the changes in the second part (pad_added_handler), 
surprised it worked, and seems counter-intuitive?

GstPad *audiosink_pad = gst_element_get_static_pad(data->audioconvert, 
GstPad *videosink_pad = gst_element_get_static_pad(data->videoconvert, 

but  ..  why??  How can it even find the right GstElement, since none of 
the are named "sink"?



      gst_element_get_static_pad ()

GstPad  <>  *
gst_element_get_static_pad (/|GstElement 
                             /|const gchar 

Retrieves a pad from /|element|/ by name. This version only retrieves 
already-existing (i.e. 'static') pads.


element a GstElement 
to find a static pad of.

name the name of the static GstPad 
to retrieve.


the requested GstPad 
if found, otherwise |NULL| 
unref after usage.

MT safe.




On 5/15/19 8:33 PM, R C wrote:
> Hello,
> I am trying to get a little more familiar with gstreamer, and thought 
> I'd do some
> of the "basic" programming tutorial examples.
> I did play with the 3rd example:
> If I copy the example, and compile it, it works, no surprise there. 
> But when I change it and do the "exercise", I get some weirdness.
> What I noticed is (well I probably missed something)...  but the 
> function call:
> |data.sink = gst_element_factory_make ("autoaudiosink", "sink"); 
> According to the docs, the 2nd parameter can be 'whatever' you want to 
> name it. Since I wanted to do the video part too, I figure I should 
> change the name to "audio-something", and changed the second function 
> call to: ||data.convert = gst_element_factory_make ("audioconvert", "convert"); 
> (original) data.sink = gst_element_factory_make ("autoaudiosink", 
> "audiosink"); (changed name)|
> of course in the even handler (pad_added_handler), the name needs to be changed in to:
> |GstPad *sink_pad = gst_element_get_static_pad (data->convert, 
> "audiosink");Browsing through the code, that should be it, BUT it seems "||sink_pad|"||||  always is NULL
>   and the pipeline elements don't/can't be linked. (see errors below)
> since this is supposed to be a simple example,  what am I missing?
> thanks,
> Ron
> errors:
> # ./basic-tutorial-3
> Pipeline state changed from NULL to READY:
> Received new pad 'src_0' from 'source':
> (basic-tutorial-3:14679): GStreamer-CRITICAL **: 20:22:54.991: gst_pad_is_linked: assertion 'GST_IS_PAD (pad)' failed
> It has type 'video/x-raw' which is not raw audio. Ignoring.
> (basic-tutorial-3:14679): GStreamer-CRITICAL **: 20:22:54.992: gst_object_unref: assertion 'object != NULL' failed
> Received new pad 'src_1' from 'source':
> (basic-tutorial-3:14679): GStreamer-CRITICAL **: 20:22:54.992: gst_pad_is_linked: assertion 'GST_IS_PAD (pad)' failed
> (basic-tutorial-3:14679): GStreamer-CRITICAL **: 20:22:54.992: gst_pad_link_full: assertion 'GST_IS_PAD (sinkpad)' failed
> Type is 'audio/x-raw' but link failed.
> (basic-tutorial-3:14679): GStreamer-CRITICAL **: 20:22:54.992: gst_object_unref: assertion 'object != NULL' failed
> Error received from element source: Internal data stream error.
> Debugging information: gstbasesrc.c(2950): gst_base_src_loop (): /GstPipeline:test-pipeline/GstURIDecodeBin:source/GstSoupHTTPSrc:source:
> streaming stopped, reason not-linked (-1)
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