Why the gstreamer pipeline won't play if I added a udpsink plugin to it - gstreamer

Question

I tried to send an RTP stream with gstreamer, but I found that the pipeline won't play at all. When I simplified my code I found that if the udpsink plugin was added in the pipeline, the pipeline is blocked, and the status is always READY.
My code:
#include <gst/gst.h>
int main(int argc, char *argv[]) {
GstElement *pipeline, *source, *sink, *udp, *convert;
GstBus *bus;
GstMessage *msg;
GstStateChangeReturn ret;
gboolean terminate = FALSE;
/* Initialize GStreamer */
gst_init (&argc, &argv);
/* Create the elements */
source = gst_element_factory_make ("videotestsrc", "source");
convert = gst_element_factory_make ("videoconvert", "convert");
sink = gst_element_factory_make ("autovideosink", "sink");
udp = gst_element_factory_make ("udpsink", "udp");
/* Create the empty pipeline */
pipeline = gst_pipeline_new ("test-pipeline");
/* Build the pipeline */
gst_bin_add_many (GST_BIN (pipeline), source, sink, convert, /*udp,*/ NULL);
gst_element_link_many (source, convert, sink, NULL);
/* Modify the source's properties */
g_object_set (source, "pattern", 0, NULL);
/* Start playing */
ret = gst_element_set_state (pipeline, GST_STATE_PLAYING);
if (ret == GST_STATE_CHANGE_FAILURE) {
g_printerr ("Unable to set the pipeline to the playing state.\n");
gst_object_unref (pipeline);
return -1;
}
bus = gst_element_get_bus (pipeline);
do {
msg = gst_bus_timed_pop_filtered (bus, GST_CLOCK_TIME_NONE, GST_MESSAGE_ERROR | GST_MESSAGE_STATE_CHANGED | GST_MESSAGE_EOS);
/* Parse message */
if (msg != NULL) {
GError *err;
gchar *debug_info;
switch (GST_MESSAGE_TYPE (msg)) {
case GST_MESSAGE_ERROR:
case GST_MESSAGE_EOS:
terminate = TRUE;
break;
case GST_MESSAGE_STATE_CHANGED:
if (GST_MESSAGE_SRC (msg) == GST_OBJECT (pipeline)) {
GstState old_state, new_state, pending_state;
gst_message_parse_state_changed (msg, &old_state, &new_state, &pending_state);
g_print ("Pipeline state changed from %s to %s\n", gst_element_state_get_name (old_state), gst_element_state_get_name (new_state));
}
break;
default:
/* We should not reach here because we only asked for ERRORs and EOS */
g_printerr ("Unexpected message received.\n");
break;
}
gst_message_unref (msg);
}
} while (!terminate);
/* Free resources */
// ...
}
As you can see, the pipeline works fine if the udpsink is not added. This also happens in command line:
gst-launch-1.0 -v udpsink videotestsrc ! videoconvert ! autovideosink
The command above will popup a window and the video stops at the first frame.
I don't know what's wrong with my code, can anyone give me help, thanks!

Answer 1

A pipeline ends with a sink element, e.g. (autovideosink) you cannot add anything after that. That is why autovideosink does not have src pad and you can not link it to udpsink. You have to have a second thread linked to udpsink. To create threads you can use queue and tee elements.
You can find more here (GStreamer Basic tutorial 7: Multithreading and Pad Availability)