elements could not be linked, which elements to link?(gstreamer c) - c

Question

I've been studying about gstreamer and I don't understand where I got it wrong but every pipeline I tried turning code would give me 'Elements could not be linked' and I'm running this code on ubuntu 20.04 which has an upstream kernel version 5.13.0-35(which I think is not a concern) with gcc 9 and here is the code:
#include <gst/gst.h>
#include <stdio.h>
/* Structure to contain all our information, so we can pass it to callbacks */
typedef struct _CustomData {
GstElement *pipeline;
GstElement *source;
GstCaps *caps;
GstElement *depay;
GstElement *parse;
GstElement *decode;
GstElement *convert;
GstElement *sink;
} CustomData;
int main (int argc, char *argv[])
{
CustomData data;
GstBus *bus;
GstMessage *msg;
GstStateChangeReturn ret;
/* Initialize GStreamer */
gst_init (&argc, &argv);
/* Create the elements */
data.source = gst_element_factory_make ("udpsrc", "source");
data.caps = gst_caps_new_simple("application/x-rtp",
"media", G_TYPE_STRING, "video",
"clock-rate", G_TYPE_INT, 90000,
"encoding-name", G_TYPE_STRING, "H264",
"payload", G_TYPE_INT, 96,
NULL);
data.depay = gst_element_factory_make ("rtph264depay", "depay");
data.parse = gst_element_factory_make ("h264parse", "parse");
data.decode = gst_element_factory_make ("decodebin", "decode");
data.convert = gst_element_factory_make ("videoconvert", "convert");
data.sink = gst_element_factory_make ("autovideosink", "sink");
/* Create the empty pipeline */
data.pipeline = gst_pipeline_new ("test-pipeline");
if (!data.pipeline || !data.source || !data.depay || !data.parse || !data.decode || !data.convert || !data.sink) {
g_printerr ("Not all elements could be created.\n");
return -1;
}
gst_bin_add_many (GST_BIN (data.pipeline), data.source, data.depay, data.parse, data.decode, data.convert, data.sink, NULL);
if (gst_element_link_many (data.source, data.depay, data.parse, data.decode, data.convert, data.sink, NULL) != TRUE) {
g_printerr ("Elements could not be linked.\n");
gst_object_unref (data.pipeline);
return -1;
}
/* Set the port and caps to play */
g_object_set (data.source, "port", 5000, NULL);
g_object_set (data.source, "caps", data.caps, NULL);
g_object_set (data.sink, "sync", FALSE, NULL);
/* Start playing */
ret = gst_element_set_state (data.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 (data.pipeline);
return -1;
}
/* Wait until error or EOS */
bus = gst_element_get_bus (data.pipeline);
msg =
gst_bus_timed_pop_filtered (bus, GST_CLOCK_TIME_NONE,
GST_MESSAGE_ERROR | GST_MESSAGE_EOS);
/* Parse message */
if (msg != NULL) {
GError *err;
gchar *debug_info;
switch (GST_MESSAGE_TYPE (msg)) {
case GST_MESSAGE_ERROR:
gst_message_parse_error (msg, &err, &debug_info);
g_printerr ("Error received from element %s: %s\n",
GST_OBJECT_NAME (msg->src), err->message);
g_printerr ("Debugging information: %s\n",
debug_info ? debug_info : "none");
g_clear_error (&err);
g_free (debug_info);
break;
case GST_MESSAGE_EOS:
g_print ("End-Of-Stream reached.\n");
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);
}
/* Free resources */
gst_object_unref (bus);
gst_element_set_state (data.pipeline, GST_STATE_NULL);
gst_object_unref (data.pipeline);
return 0;
}
the recieving pipeline is:
gst-launch-1.0 -v udpsrc port=5000 ! "application/x-rtp, media=(string)video, clock-rate=(int)90000, encoding-name=(string)H264, payload=(int)96" ! rtph264depay ! h264parse ! decodebin ! videoconvert ! autovideosink sync=false
And the sender is:
gst-launch-1.0 -v filesrc location=test.mp4 ! qtdemux ! h264parse ! avdec_h264 ! x264enc ! rtph264pay ! udpsink host=$HOST port=5000
Thanks in advance.

Answer 1

You may try using gst_parse_launch() that will negociate caps as gst-launch does:
const gchar *pipeline_str = "udpsrc port=5000 ! application/x-rtp,encoding-name=H264 ! rtph264depay ! h264parse ! decodebin ! videoconvert ! autovideosink sync=false";
GstElement *pipeline = gst_parse_launch (pipeline_str, NULL);
if (!pipeline) {
g_error ("Failed to create pipeline\n");
exit(-1);
}
...
Otherwise, you may have to add capsfilters.