gstreamer/docs/manual/helloworld.xml

<chapter id="cha-hello">
  <title>Your first application</title>
  <para> 
    This chapter describes the most rudimentary aspects of a <application>GStreamer</application> application, 
    including initializing the libraries, creating elements, packing them into
    a pipeline and playing, pause and stop the pipeline.
  </para>

  <sect1>
    <title>Hello world</title>
    <para>
      We will create a simple first application, a complete MP3 player, using standard
      <application>GStreamer</application> components. The player will read from a file that is
      given as the first argument of the program.
    </para>

    <programlisting>
      
#include &lt;gst/gst.h&gt;

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

  gst_init(&amp;argc, &amp;argv);

  if (argc != 2) {
    g_print ("usage: %s &lt;filename&gt;\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);

  /* connect src to sink */
  gst_element_connect_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);
}

    </programlisting>

    <para>
      Let's go through this example step by step.
    </para>

    <para>
      The first thing you have to do is to include the standard <application>GStreamer</application> headers and 
      initialize the framework.
    </para>
    <programlisting>

#include &lt;gst/gst.h&gt;

  ...

int 
main (int argc, char *argv[]) 
{
  ...
  gst_init(&amp;argc, &amp;argv);
  ...

    </programlisting>

    <para>
      We are going to create 3 elements and one pipeline. Since all elements share the same base
      type, <classname>GstElement</classname>, we can define them as:
    </para>
    <programlisting>
  ...
  GstElement *pipeline, *filesrc, *decoder, *audiosink;
  ...
    </programlisting>

    <para>
      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 stuff into it.
    </para>
    <programlisting>
  /* create a new pipeline to hold the elements */
  pipeline = gst_pipeline_new ("pipeline");
    </programlisting>
    <para>
      We use the standard constructor for a pipeline: gst_pipeline_new ("name").
    </para>

    <para>
      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.
    </para>
    <programlisting>
  /* create a disk reader */
  filesrc = gst_element_factory_make ("filesrc", "disk_source");
  g_object_set (G_OBJECT (filesrc), "location", argv[1], NULL);
    </programlisting>
    <note>
      <para>
        You can check if the filesrc != NULL to verify the creation of the
	disk source element.
      </para>
    </note>

    <para>
      We now create the MP3 decoder element. This assumes that the 'mad' plugin
      is installed on the system where this application is executed.
    </para>
    <programlisting>
  /* now it's time to get the decoder */
  decoder = gst_element_factory_make ("mad", "decoder");
    </programlisting>
    <para>
      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.
    </para>

    <para>
      Finally we create our audio sink element. This element will be able
      to playback the audio using OSS.
    </para>
    <programlisting>
  /* and an audio sink */
  audiosink = gst_element_factory_make ("audiosink", "play_audio");
    </programlisting>

    <para>
      We then add the elements to the pipeline.
    </para>
    <programlisting>
  /* add objects to the main pipeline */
  gst_bin_add_many (GST_BIN (pipeline), filesrc, decoder, audiosink, NULL);
    </programlisting>

    <para>
      We connect the different pads of the elements together like this:
    </para>
    <programlisting>
  /* connect src to sink */
  gst_element_connect_many (filesrc, decoder, audiosink, NULL);
    </programlisting>
    
    <para>
      We now have a created a complete pipeline.  We can visualise the
      pipeline as follows:
    </para>
    <figure float="1" id="sec-hello-img">
      <title>The Hello world pipeline</title>
      <mediaobject>
        <imageobject>
          <imagedata fileref="images/hello-world.&magic;" format="&magic;" />
        </imageobject>
      </mediaobject>  

    </figure>

    <para>
     Everything is now set up to start the streaming. We use the following
     statements to change the state of the pipeline:
    </para>
    <programlisting>
  /* start playing */
  gst_element_set_state (pipeline, GST_STATE_PLAYING);

    </programlisting>
    <note>
      <para>
        <application>GStreamer</application> will take care of the READY and PAUSED state for 
	you when going from NULL to PLAYING.
      </para>
    </note>

    <para>
      Since we do not use threads, nothing will happen yet. We manually have to
      call gst_bin_iterate() to execute one iteration of the pipeline. 
    </para>
    <programlisting>
  while (gst_bin_iterate (GST_BIN (pipeline)));
    </programlisting>
    <para>
      The gst_bin_iterate() function will return TRUE as long as something interesting
      happended inside the pipeline. When the end-of-file has been reached the _iterate 
      function will return FALSE and we can end the loop.
    </para>
    <programlisting>
  /* stop the pipeline */
  gst_element_set_state (pipeline, GST_STATE_NULL);

  gst_object_unref (GST_OBJECT (pipeline));

  exit (0);
    </programlisting>
    <note>
      <para>
        Don't forget to set the state of the pipeline to NULL. This will free
        all of the resources held by the elements.
      </para>
    </note>
    
  </sect1>

  <sect1>
    <title>Compiling helloworld.c</title>
    <para>
      To compile the helloworld example, use: 
    </para>
    <programlisting>
       gcc -Wall `pkg-config gstreamer --cflags --libs` helloworld.c \
             -o helloworld 
    </programlisting>
    <para>
      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.
    </para>
    <para>
      You can run the example with (substitute helloworld.mp3 with you favorite MP3 file):
    </para>
    <programlisting>
      ./helloworld helloworld.mp3
    </programlisting>
  </sect1>

  <sect1>
    <title>Conclusion</title>
    <para>
      This concludes our first example. As you see, setting up a pipeline
      is very lowlevel but powerful. You will later in this manual how
      you can create a custom MP3 element with a more high level API.
    </para>
    <para>
      It should be clear from the example that we can very easily replace the 
      filesrc element with an httpsrc, giving you instant network streaming.
      An element could be build to handle icecast connections, for example.
    </para>
    <para>
      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.
    </para>
  </sect1>
</chapter>