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4.3. Wire Format

The protocol is sent over a UNIX domain stream socket, where the endpoint usually is named wayland-0 (although it can be changed via WAYLAND_DISPLAY in the environment). 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. Every message is structured as 32-bit words, values are represented in the host's byte-order.
The message header has 2 words in it:
  • The first word is the sender's object ID (32-bit).
  • The second has 2 parts of 16-bit. The upper 16-bits are the message size in bytes, starting at the header (i.e. it has a minimum value of 8).The lower is the request/event opcode.
The payload describes the request/event arguments. Every argument is always aligned to 32-bits. Where padding is required, the value of padding bytes is undefined. There is no prefix that describes the type, but it is inferred implicitly from the xml specification.
The representation of argument types are as follows:
int, uint
The value is the 32-bit value of the signed/unsigned int.
fixed
Signed 24.8 decimal numbers. It is a signed decimal type which offers a sign bit, 23 bits of integer precision and 8 bits of decimal precision. This is exposed as an opaque struct with conversion helpers to and from double and int on the C API side.
string
Starts with an unsigned 32-bit length, followed by the string contents, including terminating null byte, then padding to a 32-bit boundary.
object
32-bit object ID.
new_id
The 32-bit object ID. On requests, the client decides the ID. The only events with new_id are advertisements of globals, and the server will use IDs below 0x10000.
array
Starts with 32-bit array size in bytes, followed by the array contents verbatim, and finally padding to a 32-bit boundary.
fd
The file descriptor is not stored in the message buffer, but in the ancillary data of the UNIX domain socket message (msg_control).