GStreamer pipeline hangs on gst_element_get_state - c++

Question

I have following very basic code using GStreamer library (GStreamer v1.8.1 on Xubuntu 16.04 if it important)
#include <gst/gst.h>
int main(int argc, char *argv[])
{
gst_init(&argc, &argv);
const gchar* pd =
"filesrc location=some.mp4 ! qtdemux name=d "
"d.video_0 ! fakesink "
"d.audio_0 ! fakesink ";
GError* error = nullptr;
GstElement *pipeline = gst_parse_launch(pd, &error);
GstState state; GstState pending;
switch(gst_element_set_state(pipeline, GST_STATE_PAUSED)) {
case GST_STATE_CHANGE_FAILURE:
case GST_STATE_CHANGE_NO_PREROLL:
return -1;
case GST_STATE_CHANGE_ASYNC: {
gst_element_get_state(pipeline, &state, &pending, GST_CLOCK_TIME_NONE);
}
case GST_STATE_CHANGE_SUCCESS:
break;
}
GMainLoop* loop = g_main_loop_new(nullptr, false);
g_main_loop_run(loop);
gst_object_unref(pipeline);
return 0;
}
The problem is when I try run this code it hangs on
gst_element_get_state(pipeline, &state, &pending, GST_CLOCK_TIME_NONE);
The question is - why it hangs? Especially if take into account, if I remove d.audio_0 ! fakesink from pipeline description it doesn't hang.

Answer 1

It is good practice to always add queues (or a multiqueue) after elements that produces multiple output branches in the pipeline e.g. demuxers.
The reason is that sinks will block waiting for other sinks to receive the first buffer (preroll). With a single thread, as your code, it will block the only thread available to push data into the sinks. A single thread is going from the demuxers to both sinks, once 1 blocks the there is no way for data to arrive on the second sink.
Using queues will spawn new threads and each sink will have a dedicated one.

Answer 2

That's quite an old thread but it probably hangs because you have an infinite timeout (GST_CLOCK_TIME_NONE).