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 a _setcaps ()-functions to configure for a particular format 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 _setcaps () function pointer and optionally a _getcaps () function pointer. Also, you have to set a _chain () function pointer. Alternatively, pads can also operate in looping mode, which means that they can pull data themselves. More on this topic later. After that, you have to register the pad with the element. This happens like this: static gboolean gst_my_filter_setcaps (GstPad *pad, GstCaps *caps); static GstFlowReturn gst_my_filter_chain (GstPad *pad, GstBuffer *buf); static void gst_my_filter_init (GstMyFilter *filter, GstMyFilterClass *filter_klass) { GstElementClass *klass = GST_ELEMENT_CLASS (filter_klass); /* 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_setcaps_function (filter->sinkpad, gst_my_filter_setcaps); 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_element_add_pad (GST_ELEMENT (filter), filter->srcpad); /* properties initial value */ filter->silent = FALSE; } The _setcaps ()-function is called during caps negotiation, which is discussed in great detail in . 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 (TRUE) or no (FALSE). If the element responds positively towards the streamtype, that type will be used on the pad. An example: static gboolean gst_my_filter_setcaps (GstPad *pad, GstCaps *caps) { GstStructure *structure = gst_caps_get_structure (caps, 0); GstMyFilter *filter = GST_MY_FILTER (GST_OBJECT_PARENT (pad)); 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") != 0) { GST_WARNING ("Wrong mimetype %s provided, we only support %s", mime, "audio/x-raw"); return FALSE; } /* 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. */ if (!gst_pad_set_caps (filter->srcpad, caps)) return FALSE; /* 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 TRUE; } 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.