In this chapter we will see how you can create a dynamic pipeline. A dynamic pipeline is a pipeline that is updated or created while data is flowing through it. We will create a partial pipeline first and add more elements while the pipeline is playing. Dynamic pipelines cause all sorts of scheduling issues and will remain a topic of research for a long time in GStreamer. We will show how to create an MPEG1 video player using dynamic pipelines. As you have seen in the pad section, we can attach a signal to an element when a pad is created. We will use this to create our MPEG1 player. We'll start with a simple main function: /* example-begin dynamic.c */ #include <string.h> #include <gst/gst.h> void eof (GstElement *src) { g_print ("have eos, quitting\n"); exit (0); } gboolean idle_func (gpointer data) { gst_bin_iterate (GST_BIN (data)); return TRUE; } void new_pad_created (GstElement *parse, GstPad *pad, GstElement *pipeline) { GstElement *decode_video = NULL; GstElement *decode_audio, *play, *color, *show; GstElement *audio_queue, *video_queue; GstElement *audio_thread, *video_thread; g_print ("***** a new pad %s was created\n", gst_pad_get_name (pad)); gst_element_set_state (GST_ELEMENT (pipeline), GST_STATE_PAUSED); /* link to audio pad */ if (strncmp (gst_pad_get_name (pad), "audio_", 6) == 0) { /* construct internal pipeline elements */ decode_audio = gst_element_factory_make ("mad", "decode_audio"); g_return_if_fail (decode_audio != NULL); play = gst_element_factory_make ("osssink", "play_audio"); g_return_if_fail (play != NULL); /* create the thread and pack stuff into it */ audio_thread = gst_thread_new ("audio_thread"); g_return_if_fail (audio_thread != NULL); /* construct queue and link everything in the main pipeline */ audio_queue = gst_element_factory_make ("queue", "audio_queue"); g_return_if_fail (audio_queue != NULL); gst_bin_add_many (GST_BIN (audio_thread), audio_queue, decode_audio, play, NULL); /* set up pad links */ gst_element_add_ghost_pad (audio_thread, gst_element_get_pad (audio_queue, "sink"), "sink"); gst_element_link (audio_queue, decode_audio); gst_element_link (decode_audio, play); gst_bin_add (GST_BIN (pipeline), audio_thread); gst_pad_link (pad, gst_element_get_pad (audio_thread, "sink")); /* set up thread state and kick things off */ g_print ("setting to READY state\n"); gst_element_set_state (GST_ELEMENT (audio_thread), GST_STATE_READY); } else if (strncmp (gst_pad_get_name (pad), "video_", 6) == 0) { /* construct internal pipeline elements */ decode_video = gst_element_factory_make ("mpeg2dec", "decode_video"); g_return_if_fail (decode_video != NULL); color = gst_element_factory_make ("colorspace", "color"); g_return_if_fail (color != NULL); show = gst_element_factory_make ("xvideosink", "show"); g_return_if_fail (show != NULL); /* construct queue and link everything in the main pipeline */ video_queue = gst_element_factory_make ("queue", "video_queue"); g_return_if_fail (video_queue != NULL); /* create the thread and pack stuff into it */ video_thread = gst_thread_new ("video_thread"); g_return_if_fail (video_thread != NULL); gst_bin_add_many (GST_BIN (video_thread), video_queue, decode_video, color, show, NULL); /* set up pad links */ gst_element_add_ghost_pad (video_thread, gst_element_get_pad (video_queue, "sink"), "sink"); gst_element_link (video_queue, decode_video); gst_element_link_many (decode_video, color, show, NULL); gst_bin_add (GST_BIN (pipeline), video_thread); gst_pad_link (pad, gst_element_get_pad (video_thread, "sink")); /* set up thread state and kick things off */ g_print ("setting to READY state\n"); gst_element_set_state (GST_ELEMENT (video_thread), GST_STATE_READY); } gst_element_set_state (GST_ELEMENT (pipeline), GST_STATE_PLAYING); } int main (int argc, char *argv[]) { GstElement *pipeline, *src, *demux; gst_init (&argc, &argv); pipeline = gst_pipeline_new ("pipeline"); g_return_val_if_fail (pipeline != NULL, -1); src = gst_element_factory_make ("filesrc", "src"); g_return_val_if_fail (src != NULL, -1); if (argc < 2) g_error ("Please specify a video file to play !"); g_object_set (G_OBJECT (src), "location", argv[1], NULL); demux = gst_element_factory_make ("mpegdemux", "demux"); g_return_val_if_fail (demux != NULL, -1); gst_bin_add_many (GST_BIN (pipeline), src, demux, NULL); g_signal_connect (G_OBJECT (demux), "new_pad", G_CALLBACK (new_pad_created), pipeline); g_signal_connect (G_OBJECT (src), "eos", G_CALLBACK (eof), NULL); gst_element_link (src, demux); gst_element_set_state (GST_ELEMENT (pipeline), GST_STATE_PLAYING); g_idle_add (idle_func, pipeline); gst_main (); return 0; } /* example-end dynamic.c */ We create two elements: a file source and an MPEG demuxer. There's nothing special about this piece of code except for the signal 'new_pad' that we linked to the mpegdemux element using: g_signal_connect (G_OBJECT (demux), "new_pad", G_CALLBACK (new_pad_created), pipeline); When an elementary stream has been detected in the system stream, mpegdemux will create a new pad that will provide the data of the elementary stream. A function 'new_pad_created' will be called when the pad is created. In the above example, we created new elements based on the name of the newly created pad. We then added them to a new thread. There are other possibilities to check the type of the pad, for example by using the MIME type and the properties of the pad.