# Your first application

This chapter describes the most rudimentary aspects of a GStreamer application, including initializing the libraries, creating elements, packing them into a pipeline and playing, pausing and stopping the pipeline.

## Hello world

We will create a simple first application, a complete MP3 player, using standard GStreamer components. The player will read from a file that is given as the first argument to the program.

    /* example-begin helloworld.c */
    #include <gst/gst.h>

    int
    main (int argc, char *argv[])
    {
      GstElement *pipeline, *filesrc, *decoder, *audiosink;

      gst_init(&argc, &argv);

      if (argc != 2) {
        g_print ("usage: %s <mp3 filename>\n", argv[0]);
        exit (-1);
      }

      /* create a new pipeline to hold the elements */
      pipeline = gst_pipeline_new ("pipeline");

      /* create a disk reader */
      filesrc = gst_element_factory_make ("filesrc", "disk_source");
      g_object_set (G_OBJECT (filesrc), "location", argv[1], NULL);

      /* now it's time to get the decoder */
      decoder = gst_element_factory_make ("mad", "decoder");

      /* and an audio sink */
      audiosink = gst_element_factory_make ("osssink", "play_audio");

      /* add objects to the main pipeline */
      gst_bin_add_many (GST_BIN (pipeline), filesrc, decoder, audiosink, NULL);

      /* link src to sink */
      gst_element_link_many (filesrc, decoder, audiosink, NULL);

      /* start playing */
      gst_element_set_state (pipeline, GST_STATE_PLAYING);

      while (gst_bin_iterate (GST_BIN (pipeline)));

      /* stop the pipeline */
      gst_element_set_state (pipeline, GST_STATE_NULL);

      /* we don't need a reference to these objects anymore */
      gst_object_unref (GST_OBJECT (pipeline));
      /* unreffing the pipeline unrefs the contained elements as well */

      exit (0);
    }
    /* example-end helloworld.c */

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);
      ...

We are going to create three elements and one pipeline. Since all elements share the same base type, `GstElement`, we can define them as:

      ...
      GstElement *pipeline, *filesrc, *decoder, *audiosink;
      ...

Next, we are going to create an empty pipeline. As you have seen in the basic introduction, this pipeline will hold and manage all the elements we are going to pack into it.

      /* create a new pipeline to hold the elements */
      pipeline = gst_pipeline_new ("pipeline");

We use the standard constructor for a pipeline: gst_pipeline_new ().

We then create a disk source element. The disk source element is able to read from a file. We use the standard GObject property mechanism to set a property of the element: the file to read from.

      /* create a disk reader */
      filesrc = gst_element_factory_make ("filesrc", "disk_source");
      g_object_set (G_OBJECT (filesrc), "location", argv[1], NULL);

You can check if the filesrc != NULL to verify the creation of the disk source element.

We now create the MP3 decoder element. This assumes that the 'mad' plugin is installed on the system where this application is executed.

      /* now it's time to get the decoder */
      decoder = gst_element_factory_make ("mad", "decoder");

gst_element_factory_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/pipeline.

Finally we create our audio sink element. This element will be able to play back the audio using OSS.

      /* and an audio sink */
      audiosink = gst_element_factory_make ("audiosink", "play_audio");

We then add the elements to the pipeline.

      /* add objects to the main pipeline */
      gst_bin_add_many (GST_BIN (pipeline), filesrc, decoder, audiosink, NULL);

We link the different pads of the elements together like this:

      /* link src to sink */
      gst_element_link_many (filesrc, decoder, audiosink, NULL);

We now have a created a complete pipeline. We can visualise the pipeline as follows:

### The Hello world pipeline

Everything is now set up to start the streaming. We use the following statements to change the state of the pipeline:

      /* start playing */
      gst_element_set_state (pipeline, GST_STATE_PLAYING);

GStreamer will take care of the READY and PAUSED state for you when going from NULL to PLAYING.

Since we do not use threads, nothing will happen yet. We have to call gst_bin_iterate() to execute one iteration of the pipeline.

      while (gst_bin_iterate (GST_BIN (pipeline)));

The gst_bin_iterate() function will return TRUE as long as something interesting happened inside the pipeline. When the end-of-file has been reached the _iterate function will return FALSE and we can end the loop.

      /* stop the pipeline */
      gst_element_set_state (pipeline, GST_STATE_NULL);

      gst_object_unref (GST_OBJECT (pipeline));

      exit (0);

Don't forget to set the state of the pipeline to NULL. This will free all of the resources held by the elements.

## Compiling helloworld.c

To compile the helloworld example, use:

           gcc -Wall `pkg-config gstreamer-0.6 --cflags --libs` helloworld.c \
                 -o helloworld

We use pkg-config to get the compiler flags needed to compile this application. Make sure to have your PKG_CONFIG_PATH environment variable set to the correct location if you are building this application against the uninstalled location.

You can run the example with (substitute helloworld.mp3 with you favorite MP3 file):

          ./helloworld helloworld.mp3

## Conclusion

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 custom MP3 element with a higher-level API.

It should be clear from the example that we can very easily replace the filesrc element with an httpsrc element, giving you instant network streaming. An element could be built 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 already existing elements.