Object Handles

The protocol constructs and manipulates a set of objects referenced by 32-bit numerical IDs known as handles. Only two types of objects are referenced by handle: channels and resources. A channel is a partitioning of a connection, used to scope resources. All channel manipulations are done via control messages, whereas resources are manipulated via channel messages. Before channels can be opened and resources created on them, the connection must be initialized. Connection initialization is described in section 3.1.3.

Channels and resources are created via specific creation messages (channels via control messages, and resources via channel messages), which specify the handle that has to be associated with the newly created object. The handle can later be reclaimed with a destroy message.

The value of each handle MUST be a nonzero 32-bit integer that, at creation time, is not associated with any other object in the same scope. In other words, two objects in the same scope MUST have different handles. Two objects in different scopes, however, can have the same handle value. In particular, each connection has a single scope for channel handles, and each channel has its own dedicated scope for resource handles. The server is responsible for allocating handle values when it creates new objects. Although the protocol allows for arbitrary handle values, the server SHOULD minimize the numerical values of the handles. For example, one possible implementation strategy is to keep a table of handles currently in use for each scope, and to search that table for an unused handle value, starting at 1, whenever a new creation message is to be issued.

Although channel messages are submitted in batches, the messages in a batch are processed in the order in which they are submitted, so the handle lifetime can be tracked through a single batch. For example, in the same batch, handle 52 can be released and then reassigned to a new object, as long as it happens in that order.

After a handle is destroyed, it can be reused in the same scope for a different object, of a different type, if so desired. The object that was associated with the handle, however, could not have been destroyed if its reference count did not reach zero with the handle deletion. In that case, from that point on, the object has no handle, and therefore it is no longer addressable by the server. However, it can continue to have an effect on the behavior of the client because it can be referenced by other objects.