GstElement

The GstElement is the most important object in the entire GStreamer system, as it defines the structure of the pipeline. Elements include: sources, filters, sinks, and containers (Bins). They may be an intrinsic part of the core GStreamer library, or may be loaded from a plugin. In some cases they’re even fabricated from completely different systems (see the LADSPA plugin). Elements are generally created from a GstElementFactory, which will be covered in another chapter, but for the intrinsic types they can be created with specific functions.

Elements contain GstPads (also covered in another chapter), which are subsequently used to connect the Elements together to form a pipeline capable of passing and processing data. They have a parent, which must be another Element. This allows deeply nested pipelines, and the possibility of "black-box" meta-elements.

Name

All elements are named, and while their names should ideally be unique in any given pipeline, they do not have to be. The only guaranteed unique name for an element is its complete path in the object hierarchy. In other words, an element’s name is unique inside its parent (This follows from GstObject’s name explanation). This uniqueness is guaranteed through all functions where either the parentage or name of an element is changed.

Pads

GstPads are the property of a given GstElement. They provide the connection capability allowing arbitrary structure in the graph. Every GstElement (with the exception of sources and sinks) have at least 2 GstPads. These pads are stored in a single GList within the Element. Several counters are kept in order to allow quicker determination of the type and properties of a given Element.

Pads may be added to an element with _add_pad(). Retrieval is done via _get_static_pad(), which operates on the name of the Pad (the unique key). This means that all Pads owned by a given Element must have unique names. A pointer to the GList of pads may be obtained with _iterate_pads().

gst_element_add_pad(element,pads): Sets the element as the parent of the pad, then adds the pad to the element’s list of pads, keeping the counts of total, src, and sink pads up to date. Emits the new_pad signal with the pad as argument. Fails if either the element or pad are NULL or not what they claim to be. Should fail if the pad already has a parent. Should fail if the pad is already owned by the element. Should fail if there’s already a pad by that name in the list of pads.

pad = gst_element_get_pad(element, "padname"): Searches through the list of pads

Ghost Pads

More info in ghostpad.

State

An element has a state. More info in state.