This chapter describes the most rudimentary aspects of a GStreamer application, including initializing the libraries, creating elements, packing them into a pipeline and playing, pause and stop the pipeline. We will create a simple first application. In fact it will be a complete MP3 player, using standard GStreamer components. The player will read from a file that is given as the first argument of the program. #include <gst/gst.h> gboolean playing; /* eos will be called when the src element has an end of stream */ void eos(GstSrc *src) { g_print("have eos, quitting\n"); playing = FALSE; } int main(int argc,char *argv[]) { GstElement *bin, *disksrc, *parse, *decoder, *audiosink; if (argc != 2) { g_print("usage: %s <filename>n", argv[0]); exit(-1); } gst_init(&argc,&argv); gst_plugin_load_all(); g_print("\n"); /* create a new bin to hold the elements */ bin = gst_bin_new("bin"); /* create a disk reader */ disksrc = gst_elementfactory_make("disksrc", "disk_source"); gtk_object_set(GTK_OBJECT(disksrc),"location", argv[1],NULL); gtk_signal_connect(GTK_OBJECT(disksrc),"eos", GTK_SIGNAL_FUNC(eos),NULL); /* now it's time to get the parser */ parse = gst_elementfactory_make("mp3parse","parse"); decoder = gst_elementfactory_make("mpg123","decoder"); /* and an audio sink */ audiosink = gst_elementfactory_make("audiosink", "play_audio"); /* add objects to the main pipeline */ gst_bin_add(GST_BIN(bin), disksrc); gst_bin_add(GST_BIN(bin), parse); gst_bin_add(GST_BIN(bin), decoder); gst_bin_add(GST_BIN(bin), audiosink); /* connect src to sink */ gst_pad_connect(gst_element_get_pad(disksrc,"src"), gst_element_get_pad(parse,"sink")); gst_pad_connect(gst_element_get_pad(parse,"src"), gst_element_get_pad(decoder,"sink")); gst_pad_connect(gst_element_get_pad(decoder,"src"), gst_element_get_pad(audiosink,"sink")); /* find out how to handle this bin */ gst_bin_create_plan(GST_BIN(bin)); /* make it ready */ gst_element_set_state(bin, GST_STATE_READY); /* start playing */ gst_element_set_state(bin, GST_STATE_PLAYING); playing = TRUE; while (playing) { gst_bin_iterate(GST_BIN(bin)); } /* stop the bin */ gst_element_set_state(bin, GST_STATE_NULL); gst_object_destroy(GST_OBJECT(audiosink)); gst_object_destroy(GST_OBJECT(decoder)); gst_object_destroy(GST_OBJECT(disksrc)); gst_object_destroy(GST_OBJECT(bin)); exit(0); } Let's go through this example step by step. The first thing you have to do is to include the standard GStreamer headers and initialize the framework. #include <gst/gst.h> ... int main(int argc,char *argv[]) { ... gst_init(&argc,&argv); ... For simplicity, we are going to load all known plugins. This has the effect that all the codecs known to GStreamer are registered to the system. ... gst_plugin_load_all(); ... We are going to create 4 elements and one bin. Since all objects are in fact elements, we can define them as: ... GstElement *bin, *disksrc, *parse, *decoder, *audiosink; ... Next, we are going to create an empty bin. As you have seen in the basic introduction, this bin will hold and manage all the elements we are going to stuff into it. /* create a new bin to hold the elements */ bin = gst_bin_new("bin"); We then create a disk source element. The disk source element is able to read from a file. We use the standard GTK+ argument mechanism to set a property of the element: the file to read from. /* create a disk reader */ disksrc = gst_elementfactory_make("disksrc", "disk_source"); gtk_object_set(GTK_OBJECT(disksrc),"location", argv[1],NULL); gtk_signal_connect(GTK_OBJECT(disksrc),"eos", GTK_SIGNAL_FUNC(eos),NULL); We also connected the eos signal to our function. When the disk source has reached an end-of-stream, this function will be called. We will use it to set a gboolean value to FALSE; You can check if the disksrc != NULL to verify the creation of the disk source element. gboolean playing; ... /* eos will be called when the src element has an end of stream */ void eos(GstSrc *src) { g_print("have eos, quitting\n"); playing = FALSE; } We now create the MP3 decoder element. GStreamer requires you to put a parser in front of the decoder. This parser will cut the raw data from the disk source into MP3 frames suitable for the decoder. In the advanced concepts chapter we will see how this can be avoided. /* now it's time to get the parser */ parse = gst_elementfactory_make("mp3parse","parse"); decoder = gst_elementfactory_make("mpg123","decoder"); gst_elementfactory_make() takes two arguments: a string that will identify the element you need and a second argument: how you want to name the element. The name of the element is something you can choose yourself and might be used to retrieve the element from a bin. Finally we create our audio sink element. This element will be able to playback the audio using OSS. /* and an audio sink */ audiosink = gst_elementfactory_make("audiosink", "play_audio"); We then add the elements to the bin. /* add objects to the main pipeline */ gst_bin_add(GST_BIN(bin), disksrc); gst_bin_add(GST_BIN(bin), parse); gst_bin_add(GST_BIN(bin), decoder); gst_bin_add(GST_BIN(bin), audiosink); We connect the different pads of the elements together like this: /* connect src to sink */ gst_pad_connect(gst_element_get_pad(disksrc,"src"), gst_element_get_pad(parse,"sink")); gst_pad_connect(gst_element_get_pad(parse,"src"), gst_element_get_pad(decoder,"sink")); gst_pad_connect(gst_element_get_pad(decoder,"src"), gst_element_get_pad(audiosink,"sink")); We now have a created a complete pipeline. We can visualise the pipeline as follows:

Before we can run it, we have to instruct the bin to analyse its contents and come up with a plan to handle the media streams. /* find out how to handle this bin */ gst_bin_create_plan(GST_BIN(bin)); Everything is now set up to start the streaming. We use the following statements to change the state of the bin: /* make it ready */ gst_element_set_state(bin, GST_STATE_READY); /* start playing */ gst_element_set_state(bin, GST_STATE_PLAYING); playing = TRUE; Before you set the bin to the PLAYING state, you must go through the READY state. The READY state will, in this example, open the file, open the audio device and initialise the MP3 decoder. Since we do not use threads, nothing will happen yet. We manually have to call gst_bin_iterate() to execute one iteration of the bin. while (playing) { gst_bin_iterate(GST_BIN(bin)); } Remember that the variable playing will become false if the disk source has reached an end-of-file. When that happens, the follwing code takes care of the cleanup: /* stop the bin */ gst_element_set_state(bin, GST_STATE_NULL); gst_object_destroy(GST_OBJECT(audiosink)); gst_object_destroy(GST_OBJECT(decoder)); gst_object_destroy(GST_OBJECT(disksrc)); gst_object_destroy(GST_OBJECT(bin)); exit(0); don't forget to set the state of the bin to NULL. This will free all of the resources held by the elements. To compile the helloworld example, use: gcc -Wall `gstreamer-config --cflags` `gtk-config --cflags` helloworld.c \ -o helloworld `gstreamer-config --libs` `gtk-config --libs` This uses the program gstreamer-config, which comes with GStreamer. This program "knows" what compiler switches are needed to compile programs that use GStreamer. gstreamer-config --cflags will output a list of include directories for the compiler to look in, and gstreamer-config --libs will output the list of libraries for the compiler to link with and the directories to find them in. You can run the example with (substitute helloworld.mp3 with you favorite MP3 file): ./helloworld helloworld.mp3 This concludes our first example. As you see, setting up a pipeline is very lowlevel but powerfull. You will later in this manual how you can create a custom MP3 element with a more high level API. It should be clear from the example that we can vary easily replace the disksrc element with a httpsrc, giving you instant network streaming. An element could be build to handle icecast connections, for example. We can also choose to use another type of sink instead of the audiosink. We could use a disksink to write the raw samples to a file, for example. It should also be clear that inserting filters, like a stereo effect, into the pipeline is not that hard to do. The most important thing is that you can reuse allready existing codecs.