Chapter 4. Wayland Protocol and Model of Operation

The Wayland protocol is an asynchronous object oriented protocol. All requests are method invocations on some object. The requests include an object ID that uniquely identifies an object on the server. Each object implements an interface and the requests include an opcode that identifies which method in the interface to invoke.

The protocol is message-based. A message sent by a client to the server is called request. A message from the server to a client is called event. A message has a number of arguments, each of which has a certain type (see the section called “Wire Format” for a list of argument types).

Additionally, the protocol can specify enums which associate names to specific numeric enumeration values. These are primarily just descriptive in nature: at the wire format level enums are just integers. But they also serve a secondary purpose to enhance type safety or otherwise add context for use in language bindings or other such code. This latter usage is only supported so long as code written before these attributes were introduced still works after; in other words, adding an enum should not break API, otherwise it puts backwards compatibility at risk.

enums can be defined as just a set of integers, or as bitfields. This is specified via the bitfield boolean attribute in the enum definition. If this attribute is true, the enum is intended to be accessed primarily using bitwise operations, for example when arbitrarily many choices of the enum can be ORed together; if it is false, or the attribute is omitted, then the enum arguments are a just a sequence of numerical values.

The enum attribute can be used on either uint or int arguments, however if the enum is defined as a bitfield, it can only be used on uint args.

The server sends back events to the client, each event is emitted from an object. Events can be error conditions. The event includes the object ID and the event opcode, from which the client can determine the type of event. Events are generated both in response to requests (in which case the request and the event constitutes a round trip) or spontaneously when the server state changes.