This chapter will summarize everything you've learned in the previous chapters. It describes all aspects of a simple &GStreamer; application, including initializing libraries, creating elements, packing elements together in a pipeline and playing this pipeline. By doing all this, you will be able to build a simple Ogg/Vorbis audio player. We're going to create a simple first application, a simple Ogg/Vorbis command-line audio player. For this, we will use only standard &GStreamer; components. The player will read a file specified on the command-line. Let's get started! We've learned, in , that the first thing to do in your application is to initialize &GStreamer; by calling gst_init (). Also, make sure that the application includes gst/gst.h so all function names and objects are properly defined. Use #include <gst/gst.h> to do that. Next, you'll want to create the different elements using gst_element_factory_make (). For an Ogg/Vorbis audio player, we'll need a source element that reads files from a disk. &GStreamer; includes this element under the name filesrc. Next, we'll need something to parse the file and decode it into raw audio. &GStreamer; has two elements for this: the first parses Ogg streams into elementary streams (video, audio) and is called oggdemux. The second is a Vorbis audio decoder, it's conveniently called vorbisdec. Since oggdemux creates dynamic pads for each elementary stream, you'll need to set a pad-added event handler on the oggdemux element, like you've learned in , to link the Ogg demuxer and the Vorbis decoder elements together. At last, we'll also need an audio output element, we will use autoaudiosink, which automatically detects your audio device. The last thing left to do is to add all elements into a container element, a GstPipeline, and iterate this pipeline until we've played the whole song. We've previously learned how to add elements to a container bin in , and we've learned about element states in . We will also attach a message handler to the pipeline bus so we can retrieve errors and detect the end-of-stream. Let's now add all the code together to get our very first audio player: #include <gst/gst.h> #include <glib.h> static gboolean bus_call (GstBus *bus, GstMessage *msg, gpointer data) { GMainLoop *loop = (GMainLoop *) data; switch (GST_MESSAGE_TYPE (msg)) { case GST_MESSAGE_EOS: g_print ("End of stream\n"); g_main_loop_quit (loop); break; case GST_MESSAGE_ERROR: { gchar *debug; GError *error; gst_message_parse_error (msg, &error, &debug); g_free (debug); g_printerr ("Error: %s\n", error->message); g_error_free (error); g_main_loop_quit (loop); break; } default: break; } return TRUE; } 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 vorbis-decoder sink pad */ g_print ("Dynamic pad created, linking demuxer/decoder\n"); sinkpad = gst_element_get_static_pad (decoder, "sink"); gst_pad_link (pad, sinkpad); gst_object_unref (sinkpad); } int main (int argc, char *argv[]) { GMainLoop *loop; GstElement *pipeline, *source, *demuxer, *decoder, *conv, *sink; GstBus *bus; /* Initialisation */ gst_init (&argc, &argv); loop = g_main_loop_new (NULL, FALSE); /* Check input arguments */ if (argc != 2) { g_printerr ("Usage: %s <Ogg/Vorbis filename>\n", argv[0]); return -1; } /* Create gstreamer elements */ pipeline = gst_pipeline_new ("audio-player"); source = gst_element_factory_make ("filesrc", "file-source"); demuxer = gst_element_factory_make ("oggdemux", "ogg-demuxer"); decoder = gst_element_factory_make ("vorbisdec", "vorbis-decoder"); conv = gst_element_factory_make ("audioconvert", "converter"); sink = gst_element_factory_make ("autoaudiosink", "audio-output"); if (!pipeline || !source || !demuxer || !decoder || !conv || !sink) { g_printerr ("One element could not be created. Exiting.\n"); return -1; } /* Set up the pipeline */ /* we set the input filename to the source element */ g_object_set (G_OBJECT (source), "location", argv[1], NULL); /* we add a message handler */ bus = gst_pipeline_get_bus (GST_PIPELINE (pipeline)); gst_bus_add_watch (bus, bus_call, loop); gst_object_unref (bus); /* we add all elements into the pipeline */ /* file-source | ogg-demuxer | vorbis-decoder | converter | alsa-output */ gst_bin_add_many (GST_BIN (pipeline), source, demuxer, decoder, conv, sink, NULL); /* we link the elements together */ /* file-source -> ogg-demuxer ~> vorbis-decoder -> converter -> alsa-output */ gst_element_link (source, demuxer); gst_element_link_many (decoder, conv, sink, NULL); g_signal_connect (demuxer, "pad-added", G_CALLBACK (on_pad_added), decoder); /* note that the demuxer will be linked to the decoder dynamically. The reason is that Ogg may contain various streams (for example audio and video). The source pad(s) will be created at run time, by the demuxer when it detects the amount and nature of streams. Therefore we connect a callback function which will be executed when the "pad-added" is emitted.*/ /* Set the pipeline to "playing" state*/ g_print ("Now playing: %s\n", argv[1]); gst_element_set_state (pipeline, GST_STATE_PLAYING); /* Iterate */ g_print ("Running...\n"); g_main_loop_run (loop); /* Out of the main loop, clean up nicely */ g_print ("Returned, stopping playback\n"); gst_element_set_state (pipeline, GST_STATE_NULL); g_print ("Deleting pipeline\n"); gst_object_unref (GST_OBJECT (pipeline)); return 0; } We now have created a complete pipeline. We can visualise the pipeline as follows:

To compile the helloworld example, use: gcc -Wall $(pkg-config --cflags --libs gstreamer-&GST_MAJORMINOR;) helloworld.c -o helloworld. &GStreamer; makes use of pkg-config to get compiler and linker flags needed to compile this application. If you're running a non-standard installation, make sure the PKG_CONFIG_PATH environment variable is set to the correct location ($libdir/pkgconfig). application against the uninstalled location. You can run this example application with ./helloworld file.ogg. Substitute file.ogg with your favourite Ogg/Vorbis file. This concludes our first example. As you see, setting up a pipeline is very low-level but powerful. You will see later in this manual how you can create a more powerful media player with even less effort using higher-level interfaces. We will discuss all that in . We will first, however, go more in-depth into more advanced &GStreamer; internals. It should be clear from the example that we can very easily replace the filesrc element with some other element that reads data from a network, or some other data source element that is better integrated with your desktop environment. Also, you can use other decoders and parsers/demuxers to support other media types. You can use another audio sink if you're not running Linux, but Mac OS X, Windows or FreeBSD, or you can instead use a filesink to write audio files to disk instead of playing them back. By using an audio card source, you can even do audio capture instead of playback. All this shows the reusability of &GStreamer; elements, which is its greatest advantage.