examples/plugins/example.c

/* Gnome-Streamer
 * Copyright (C) <1999> Erik Walthinsen <omega@cse.ogi.edu>
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Library General Public
 * License as published by the Free Software Foundation; either
 * version 2 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Library General Public License for more details.
 *
 * You should have received a copy of the GNU Library General Public
 * License along with this library; if not, write to the
 * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 * Boston, MA 02111-1307, USA.
 */

/* First, include the header file for the plugin, to bring in the
 * object definition and other useful things.
 */
#include "example.h"

/* The ElementDetails structure gives a human-readable description
 * of the plugin, as well as author and version data.
 */
static GstElementDetails example_details = {
  "An example plugin",
  "Example/FirstExample",
  "Shows the basic structure of a plugin",
  VERSION,
  "your name <your.name@your.isp>",
  "(C) 2001",
};

/* These are the signals that this element can fire.  They are zero-
 * based because the numbers themselves are private to the object.
 * LAST_SIGNAL is used for initialization of the signal array.
 */
enum {
  ASDF,
  /* FILL ME */
  LAST_SIGNAL
};

/* Arguments are identified the same way, but cannot be zero, so you
 * must leave the ARG_0 entry in as a placeholder.
 */
enum {
  ARG_0,
  ARG_ACTIVE,
  /* FILL ME */
};

/* The PadFactory structures describe what pads the element has or
 * can have.  They can be quite complex, but for this example plugin
 * they are rather simple.
 */
static GstPadTemplate*
sink_factory (void)
{
  return 
    gst_padtemplate_new (
  	"sink",			/* The name of the pad */
  	GST_PAD_SINK,		/* Direction of the pad */
  	GST_PAD_ALWAYS,	/* The pad exists for every instance */
	gst_caps_new (
  	  "example_sink",				/* The name of the caps */
     	  "unknown/unknown",				/* The overall MIME/type */
	  gst_props_new (
     	    "foo",	GST_PROPS_INT (1),		/* An integer property */
     	    "bar",	GST_PROPS_BOOLEAN (TRUE),	/* A boolean */
     	    "baz",	GST_PROPS_LIST (		/* A list of values for */
			  GST_PROPS_INT (1),
			  GST_PROPS_INT (3)
			),
	    NULL)));
}

/* This factory is much simpler, and defines the source pad. */
static GstPadTemplate*
src_factory (void)
{
  return
    gst_padtemplate_new (
  	"src",
  	GST_PAD_SRC,
  	GST_PAD_ALWAYS,
	gst_caps_new (
  	  "example_src",
    	  "unknown/unknown",
	  NULL));
}


/* A number of functon prototypes are given so we can refer to them later. */
static void	gst_example_class_init	(GstExampleClass *klass);
static void	gst_example_init	(GstExample *example);

static void	gst_example_chain	(GstPad *pad, GstBuffer *buf);

static void	gst_example_set_arg	(GtkObject *object,GtkArg *arg,guint id);
static void	gst_example_get_arg	(GtkObject *object,GtkArg *arg,guint id);

/* These hold the constructed pad templates, which are created during
 * plugin load, and used during element instantiation.
 */
static GstPadTemplate *src_template, *sink_template;

/* The parent class pointer needs to be kept around for some object
 * operations.
 */
static GstElementClass *parent_class = NULL;

/* This array holds the ids of the signals registered for this object.
 * The array indexes are based on the enum up above.
 */
static guint gst_example_signals[LAST_SIGNAL] = { 0 };

/* This function is used to register and subsequently return the type
 * identifier for this object class.  On first invocation, it will
 * register the type, providing the name of the class, struct sizes,
 * and pointers to the various functions that define the class.
 */
GtkType
gst_example_get_type(void)
{
  static GtkType example_type = 0;

  if (!example_type) {
    static const GtkTypeInfo example_info = {
      "GstExample",
      sizeof(GstExample),
      sizeof(GstExampleClass),
      (GtkClassInitFunc)gst_example_class_init,
      (GtkObjectInitFunc)gst_example_init,
      (GtkArgSetFunc)NULL,	/* These last three are depracated */
      (GtkArgGetFunc)NULL,
      (GtkClassInitFunc)NULL,
    };
    example_type = gtk_type_unique(GST_TYPE_ELEMENT,&example_info);
  }
  return example_type;
}

/* In order to create an instance of an object, the class must be
 * initialized by this function.  GtkObject will take care of running
 * it, based on the pointer to the function provided above.
 */
static void
gst_example_class_init (GstExampleClass *klass)
{
  /* Class pointers are needed to supply pointers to the private
   * implementations of parent class methods.
   */
  GtkObjectClass *gtkobject_class;
  GstElementClass *gstelement_class;

  /* Since the example class contains the parent classes, you can simply
   * cast the pointer to get access to the parent classes.
   */
  gtkobject_class = (GtkObjectClass*)klass;
  gstelement_class = (GstElementClass*)klass;

  /* The parent class is needed for class method overrides. */
  parent_class = gtk_type_class(GST_TYPE_ELEMENT);

  /* Here we add an argument to the object.  This argument is an integer,
   * and can be both read and written.
   */
  gtk_object_add_arg_type("GstExample::active", GTK_TYPE_INT,
                          GTK_ARG_READWRITE, ARG_ACTIVE);

  /* Here we add a signal to the object. This is avery useless signal
   * called asdf. The signal will also pass a pointer to the listeners
   * which happens to be the example element itself */
  gst_example_signals[ASDF] =
    gtk_signal_new("asdf", GTK_RUN_LAST, gtkobject_class->type,
                   GTK_SIGNAL_OFFSET (GstExampleClass, asdf),
                   gtk_marshal_NONE__POINTER, GTK_TYPE_NONE, 1,
                   GST_TYPE_EXAMPLE);

  gtk_object_class_add_signals (gtkobject_class, gst_example_signals,
                                LAST_SIGNAL);

  /* The last thing is to provide the functions that implement get and set
   * of arguments.
   */
  gtkobject_class->set_arg = gst_example_set_arg;
  gtkobject_class->get_arg = gst_example_get_arg;
}

/* This function is responsible for initializing a specific instance of
 * the plugin.
 */
static void
gst_example_init(GstExample *example)
{
  /* First we create the sink pad, which is the input to the element.
   * We will use the sink_template constructed in the plugin_init function
   * (below) to quickly generate the pad we need.
   */
  example->sinkpad = gst_pad_new_from_template (sink_template, "sink");
  /* Setting the chain function allows us to supply the function that will
   * actually be performing the work.  Without this, the element would do
   * nothing, with undefined results (assertion failures and such).
   */
  gst_pad_set_chain_function(example->sinkpad,gst_example_chain);
  /* We then must add this pad to the element's list of pads.  The base
   * element class manages the list of pads, and provides accessors to it.
   */
  gst_element_add_pad(GST_ELEMENT(example),example->sinkpad);

  /* The src pad, the output of the element, is created and registered
   * in the same way, with the exception of the chain function.  Source
   * pads don't have chain functions, because they can't accept buffers,
   * they only produce them.
   */
  example->srcpad = gst_pad_new_from_template (src_template, "src");
  gst_element_add_pad(GST_ELEMENT(example),example->srcpad);

  /* Initialization of element's private variables. */
  example->active = FALSE;
}

/* The chain function is the heart of the element.  It's where all the
 * work is done.  It is passed a pointer to the pad in question, as well
 * as the buffer provided by the peer element.
 */
static void
gst_example_chain (GstPad *pad, GstBuffer *buf)
{
  GstExample *example;
  GstBuffer *outbuf;

  /* Some of these checks are of dubious value, since if there were not
   * already true, the chain function would never be called.
   */
  g_return_if_fail(pad != NULL);
  g_return_if_fail(GST_IS_PAD(pad));
  g_return_if_fail(buf != NULL);

  /* We need to get a pointer to the element this pad belogs to. */
  example = GST_EXAMPLE(gst_pad_get_parent (pad));

  /* A few more sanity checks to make sure that the element that owns
   * this pad is the right kind of element, in case something got confused.
   */
  g_return_if_fail(example != NULL);
  g_return_if_fail(GST_IS_EXAMPLE(example));

  /* If we are supposed to be doing something, here's where it happens. */
  if (example->active) {
    /* In this example we're going to copy the buffer to another one, 
     * so we need to allocate a new buffer first. */
    outbuf = gst_buffer_new();

    /* We need to copy the size and offset of the buffer at a minimum. */
    GST_BUFFER_SIZE (outbuf) = GST_BUFFER_SIZE (buf);
    GST_BUFFER_OFFSET (outbuf) = GST_BUFFER_OFFSET (buf);

    /* Then allocate the memory for the new buffer */
    GST_BUFFER_DATA (outbuf) = (guchar *)g_malloc (GST_BUFFER_SIZE (outbuf));

    /* Then copy the data in the incoming buffer into the new buffer. */
    memcpy (GST_BUFFER_DATA (outbuf), GST_BUFFER_DATA (buf), GST_BUFFER_SIZE (outbuf));

    /* we don't need the incomming buffer anymore so we unref it. When we are
     * the last plugin with a handle to the buffer, its memory will be freed */
    gst_buffer_unref (buf);

    /* When we're done with the buffer, we push it on to the next element
     * in the pipeline, through the element's source pad, which is stored
     * in the element's structure.
     */
    gst_pad_push(example->srcpad,outbuf);

    /* For fun we'll emit our useless signal here */
    gtk_signal_emit (GTK_OBJECT (example), gst_example_signals[ASDF],
                     example);

  /* If we're not doing something, just send the original incoming buffer. */
  } else {
    gst_pad_push(example->srcpad,buf);
  }
}

/* Arguments are part of the Gtk+ object system, and these functions
 * enable the element to respond to various arguments.
 */
static void
gst_example_set_arg (GtkObject *object,GtkArg *arg,guint id)
{
  GstExample *example;

  /* It's not null if we got it, but it might not be ours */
  g_return_if_fail(GST_IS_EXAMPLE(object));

  /* Get a pointer of the right type. */
  example = GST_EXAMPLE(object);

  /* Check the argument id to see which argument we're setting. */
  switch (id) {
    case ARG_ACTIVE:
      /* Here we simply copy the value of the argument to our private
       * storage.  More complex operations can be done, but beware that
       * they may occur at any time, possibly even while your chain function
       * is running, if you are using threads.
       */
      example->active = GTK_VALUE_INT(*arg);
      g_print("example: set active to %d\n",example->active);
      break;
    default:
      break;
  }
}

/* The set function is simply the inverse of the get fuction. */
static void
gst_example_get_arg (GtkObject *object,GtkArg *arg,guint id)
{
  GstExample *example;

  /* It's not null if we got it, but it might not be ours */
  g_return_if_fail(GST_IS_EXAMPLE(object));
  example = GST_EXAMPLE(object);

  switch (id) {
    case ARG_ACTIVE:
      GTK_VALUE_INT(*arg) = example->active;
      break;
    default:
      arg->type = GTK_TYPE_INVALID;
      break;
  }
}

/* This is the entry into the plugin itself.  When the plugin loads,
 * this function is called to register everything that the plugin provides.
 */
static gboolean
plugin_init (GModule *module, GstPlugin *plugin)
{
  GstElementFactory *factory;

  /* We need to create an ElementFactory for each element we provide.
   * This consists of the name of the element, the GtkType identifier,
   * and a pointer to the details structure at the top of the file.
   */
  factory = gst_elementfactory_new("example", GST_TYPE_EXAMPLE, &example_details);
  g_return_val_if_fail(factory != NULL, FALSE);

  /* The pad templates can be easily generated from the factories above,
   * and then added to the list of padtemplates for the elementfactory.
   * Note that the generated padtemplates are stored in static global
   * variables, for the gst_example_init function to use later on.
   */
  sink_template = sink_factory ();
  gst_elementfactory_add_padtemplate (factory, sink_template);

  src_template = src_factory ();
  gst_elementfactory_add_padtemplate (factory, src_template);

  /* The very last thing is to register the elementfactory with the plugin. */
  gst_plugin_add_factory (plugin, factory);

  /* Now we can return successfully. */
  return TRUE;

  /* At this point, the GStreamer core registers the plugin, its
   * elementfactories, padtemplates, etc., for use in you application.
   */
}

/* This structure describes the plugin to the system for dynamically loading
 * plugins, so that the version number and name can be checked in a uniform
 * way.
 *
 * The symbol pointing to this structure is the only symbol looked up when
 * loading the plugin.
 */
GstPluginDesc plugin_desc = {
  GST_VERSION_MAJOR, /* The major version of the core that this was built with */
  GST_VERSION_MINOR, /* The minor version of the core that this was built with */
  "example",         /* The name of the plugin.  This must be unique: plugins with
		      * the same name will be assumed to be identical, and only
		      * one will be loaded. */
  plugin_init        /* Pointer to the initialisation function for the plugin. */
};