gstreamer/docs/manual/helloworld.xml

<chapter id="chapter-hello-world">
  <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,
    pausing and stopping the pipeline.
  </para>

  <sect1 id="section-hello-world">
    <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 to the program.
    </para>

    <programlisting>
/* example-begin helloworld.c */      
#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;mp3 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);

  /* 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 */      
    </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 three elements and one pipeline. Since all
      elements share the same base type, <ulink type="http"
      url="../../gstreamer/html/GstElement.html"><classname>GstElement</classname></ulink>,
      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 pack 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 ().
    </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 play back the audio using OSS.
    </para>
    <programlisting>
  /* and an audio sink */
  audiosink = gst_element_factory_make ("osssink", "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 link the different pads of the elements together like this:
    </para>
    <programlisting>
  /* link src to sink */
  gst_element_link_many (filesrc, decoder, audiosink, NULL);
    </programlisting>
    
    <para>
      We now have created a complete pipeline.  We can visualise the
      pipeline as follows:
    </para>
    <figure float="1" id="section-hello-img">
      <title>The "hello world" pipeline</title>
      <mediaobject>
        <imageobject>
          <imagedata fileref="images/hello-world.&image;" format="&IMAGE;" />
        </imageobject>
      </mediaobject>  

    </figure>

    <para>
     Everything is now set up to start 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 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 happened 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 id="section-hello-world-compile">
    <title>Compiling helloworld.c</title>
    <para>
      To compile the helloworld example, use: 
    </para>
    <programlisting>
       gcc -Wall `pkg-config gstreamer-&GST_MAJORMINOR; --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 id="section-hello-world-conclusion">
    <title>Conclusion</title>
    <para>
      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.
    </para>
    <para>
      It should be clear from the example that we can very easily replace the
      filesrc element with the gnomevfssrc element, giving you instant streaming
      from any gnomevfs URL.
    </para>
    <para>
      We can also choose to use another type of sink instead of the audiosink.
      We could use a filesink 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>