# Specifying the pads

As explained before, pads are the port through which data goes in and out of your element, and that makes them a very important item in the process of element creation. In the boilerplate code, we have seen how static pad templates take care of registering pad templates with the element class. Here, we will see how to create actual elements, use `_link ()` and `_getcaps ()` functions to let other elements know their capabilities and how to register functions to let data flow through the element.

In the element `_init ()` function, you create the pad from the pad template that has been registered with the element class in the `_base_init ()` function. After creating the pad, you have to set a `_link ()` function pointer and a `_getcaps ()` function pointer. Optionally, you can set a `_chain ()` function pointer (on sink pads in filter and sink elements) through which data will come in to the element, or (on source pads in source elements) a `_get ()` function pointer through which data will be pulled from the element. After that, you have to register the pad with the element. This happens like this:

    static GstPadLinkReturn	gst_my_filter_link	(GstPad        *pad,
    						 const GstCaps *caps);
    static GstCaps *	gst_my_filter_getcaps	(GstPad        *pad);
    static void		gst_my_filter_chain	(GstPad        *pad,
    						 GstData       *data);

    static void
    gst_my_filter_init (GstMyFilter *filter)
    {
      GstElementClass *klass = GST_ELEMENT_GET_CLASS (filter);

      /* pad through which data comes in to the element */
      filter->sinkpad = gst_pad_new_from_template (
    	gst_element_class_get_pad_template (klass, "sink"), "sink");
      gst_pad_set_link_function (filter->sinkpad, gst_my_filter_link);
      gst_pad_set_getcaps_function (filter->sinkpad, gst_my_filter_getcaps);
      gst_pad_set_chain_function (filter->sinkpad, gst_my_filter_chain);
      gst_element_add_pad (GST_ELEMENT (filter), filter->sinkpad);

      /* pad through which data goes out of the element */
      filter->srcpad = gst_pad_new_from_template (
    	gst_element_class_get_pad_template (klass, "src"), "src");
      gst_pad_set_link_function (filter->srcpad, gst_my_filter_link);
      gst_pad_set_getcaps_function (filter->srcpad, gst_my_filter_getcaps);
      gst_element_add_pad (GST_ELEMENT (filter), filter->srcpad);
    [..]
    }

## The link function

The `_link ()` is called during caps negotiation. This is the process where the linked pads decide on the streamtype that will transfer between them. A full list of type-definitions can be found in . A `_link ()` receives a pointer to a `GstCaps` struct that defines the proposed streamtype, and can respond with either yes (`GST_PAD_LINK_OK`), no (`GST_PAD_LINK_REFUSED`) or don't know yet (`GST_PAD_LINK_DELAYED`). If the element responds positively towards the streamtype, that type will be used on the pad. An example:

    static GstPadLinkReturn
    gst_my_filter_link (GstPad        *pad,
    		    const GstCaps *caps)
    {
      GstStructure *structure = gst_caps_get_structure (caps, 0);
      GstMyFilter *filter = GST_MY_FILTER (gst_pad_get_parent (pad));
      GstPad *otherpad = (pad == filter->srcpad) ? filter->sinkpad :
    					       filter->srcpad;
      GstPadLinkReturn ret;
      const gchar *mime;

      /* Since we're an audio filter, we want to handle raw audio
       * and from that audio type, we need to get the samplerate and
       * number of channels. */
      mime = gst_structure_get_name (structure);
      if (strcmp (mime, "audio/x-raw-int") != 0) {
        GST_WARNING ("Wrong mimetype %s provided, we only support %s",
    		 mime, "audio/x-raw-int");
        return GST_PAD_LINK_REFUSED;
      }

      /* we're a filter and don't touch the properties of the data.
       * That means we can set the given caps unmodified on the next
       * element, and use that negotiation return value as ours. */
      ret = gst_pad_try_set_caps (otherpad, gst_caps_copy (caps));
      if (GST_PAD_LINK_FAILED (ret))
        return ret;

      /* Capsnego succeeded, get the stream properties for internal
       * usage and return success. */
      gst_structure_get_int (structure, "rate", &filter->samplerate);
      gst_structure_get_int (structure, "channels", &filter->channels);

      g_print ("Caps negotiation succeeded with %d Hz @ %d channels\n",
    	   filter->samplerate, filter->channels);

      return ret;
    }

In here, we check the mimetype of the provided caps. Normally, you don't need to do that in your own plugin/element, because the core does that for you. We simply use it to show how to retrieve the mimetype from a provided set of caps. Types are stored in `GstStructure` internally. A `GstCaps` is nothing more than a small wrapper for 0 or more structures/types. From the structure, you can also retrieve properties, as is shown above with the function `gst_structure_get_int ()`.

If your `_link ()` function does not need to perform any specific operation (i.e. it will only forward caps), you can set it to `gst_pad_proxy_link`. This is a link forwarding function implementation provided by the core. It is useful for elements such as `identity`.

## The getcaps function

The `_getcaps ()` funtion is used to request the list of supported formats and properties from the element. In some cases, this will be equal to the formats provided by the pad template, in which case this function can be omitted. In some cases, too, it will not depend on anything inside this element, but it will rather depend on the input from another element linked to this element's sink or source pads. In that case, you can use `gst_pad_proxy_getcaps` as implementation, it provides getcaps forwarding in the core. However, in many cases, the format supported by this element cannot be defined externally, but is more specific than those provided by the pad template. In this case, you should use a `_getcaps ()` function. In the case as specified below, we assume that our filter is able to resample sound, so it would be able to provide any samplerate (indifferent from the samplerate specified on the other pad) on both pads. It explains how a `_getcaps ()` can be used to do this.

    static GstCaps *
    gst_my_filter_getcaps (GstPad *pad)
    {
      GstMyFilter *filter = GST_MY_FILTER (gst_pad_get_parent (pad));
      GstPad *otherpad = (pad == filter->srcpad) ? filter->sinkpad :
    					       filter->srcpad;
      GstCaps *othercaps = gst_pad_get_allowed_caps (otherpad), *caps;
      gint n;

      if (gst_caps_is_empty (othercaps))
        return othercaps;

      /* We support *any* samplerate, indifferent from the samplerate
       * supported by the linked elements on both sides. */
      for (i = 0; i < gst_caps_get_size (othercaps); i++) {
        GstStructure *structure = gst_caps_get_structure (othercaps, i);

        gst_structure_remove_field (structure, "rate");
      }
      caps = gst_caps_intersect (othercaps, gst_pad_get_pad_template_caps (pad));
      gst_caps_free (othercaps);

      return caps;
    }

## Explicit caps

Obviously, many elements will not need this complex mechanism, because they are much simpler than that. They only support one format, or their format is fixed but the contents of the format depend on the stream or something else. In those cases, explicit caps are an easy way of handling caps. Explicit caps are an easy way of specifying one, fixed, supported format on a pad. Pads using explicit caps do not implement their own `_getcaps ()` or `_link ()` functions. When the exact format is known, an elements uses `gst_pad_set_explicit_caps ()` to specify the exact format. This is very useful for demuxers, for example.

    static void
    gst_my_filter_init (GstMyFilter *filter)
    {
      GstElementClass *klass = GST_ELEMENT_GET_CLASS (filter);
    [..]
      filter->srcpad = gst_pad_new_from_template (
    	gst_element_class_get_pad_template (klass, "src"), "src");
      gst_pad_use_explicit_caps (filter->srcpad);
    [..]
    }

    static void
    gst_my_filter_somefunction (GstMyFilter *filter)
    {
      GstCaps *caps = ..;
    [..]
      gst_pad_set_explicit_caps (filter->srcpad, caps);
    [..]
    }