Gstreamer call backs are not hitting after setting pipeline to playing state - c

Question

I am writing a media application to grab video frames from a video file. For this, I want to get the video properties before pulling the samples from the pipeline. So, I have added a callback for auto-plug signal at the decoder and trying to get the properties. These callbacks are not getting called even after I put the pipeline into playing state, but these are called if I try to pull a sample from the pipeline using gst_app_sink_pull_sample.
Am I missing anything here? My understanding is that these callbacks will get invoked when we put the pipeline into playing state.
#include <gst/gst.h>
#include <stdio.h>
static void bus_callback (GstBus *bus, GstMessage *msg, gpointer data)
{
switch (GST_MESSAGE_TYPE (msg))
{
case GST_MESSAGE_ERROR: {
GError *err;
gchar *debug;
gst_message_parse_error (msg, &err, &debug);
g_print ("Error: %s\n", err->message);
g_error_free (err);
g_free (debug);
break;
}
default:
/* Unhandled message */
break;
}
}
static void
on_pad_added (GstElement *element, GstPad *pad, gpointer data)
{
GstPad *sinkpad;
GstElement *decoder = (GstElement *) data;
/* We can now link this pad with the decoder sink pad */
sinkpad = gst_element_get_static_pad (decoder, "sink");
gst_pad_link (pad, sinkpad);
gst_object_unref (sinkpad);
}
static void
auto_plug_select (GstElement *decoder, GstPad *pad, GstCaps *caps,
GstElementFactory *factory, int *width )
{
const gchar *klass = gst_element_factory_get_klass (factory);
/* MW_customData *cdata = (MW_customData*) data;*/
GstCaps *scaps = gst_pad_query_caps (pad, NULL);
GstStructure *str = gst_caps_get_structure (scaps, 0);
const gchar *type = gst_structure_get_name (str);
printf (" Pad cap: %s\n", type);
if (g_strrstr(type,"video"))
{
gst_structure_get_int (str, "width", width);
printf(" Width: %d\n", *width);
}
}
int main (gint argc,
gchar *argv[])
{
GstElement *pipeline, *filesrc, *decoder, *fakesink;
GstBus *bus;
/* init GStreamer */
gst_init (&argc, &argv);
/* check args */
if (argc != 2) {
g_print ("Usage: %s <filename>\n", argv[0]);
return -1;
}
/* create a new pipeline to hold the elements */
pipeline = gst_pipeline_new ("pipeline");
/* Bus call back*/
bus = gst_pipeline_get_bus (GST_PIPELINE (pipeline));
gst_bus_add_watch (bus, bus_callback, NULL);
gst_object_unref (bus);
/* create file source and typefind element */
filesrc = gst_element_factory_make ("filesrc", "source");
g_object_set (G_OBJECT (filesrc), "location", argv[1], NULL);
decoder = gst_element_factory_make ("decodebin", NULL);
fakesink = gst_element_factory_make ("fakesink", "sink");
int width = 0;
/* Connect the sink pad when decoder completes the operation */
g_signal_connect (decoder, "pad-added", G_CALLBACK (on_pad_added), &width);
g_signal_connect (decoder, "autoplug-select", G_CALLBACK (auto_plug_select), fakesink);
/* setup */
gst_bin_add_many (GST_BIN (pipeline), filesrc, decoder, fakesink, NULL);
gst_element_link (filesrc, decoder);
gst_element_set_state (GST_ELEMENT (pipeline), GST_STATE_PLAYING);
printf(" Width: %d\n", width);
gst_element_set_state (GST_ELEMENT (pipeline), GST_STATE_NULL);
return 0;
}

Answer 1

You do not leave the pipeline any time for running. You probably stop it before data can trigger the decodebin's callbacks.
For being cheap try:
gst_element_set_state (GST_ELEMENT (pipeline), GST_STATE_PLAYING);
g_usleep(100000000);
printf(" Width: %d\n", width);
gst_element_set_state (GST_ELEMENT (pipeline), GST_STATE_NULL);
But more correct would be to use a real GMainLoop and act on certain event to stop the pipeline again.
EDIT: P.S. Why not GstDiscoverer? https://gstreamer.freedesktop.org/documentation/pbutils/gstdiscoverer.html?gi-language=c