Commit a6cf1984 authored by Yong Bakos's avatar Yong Bakos Committed by Pekka Paalanen

util: Document wl_message

Signed-off-by: 's avatarYong Bakos <>
Reviewed-by: Peter Hutterer's avatarPeter Hutterer <>
Reviewed-by: Pekka Paalanen's avatarPekka Paalanen <>
parent ed7a6880
......@@ -68,9 +68,77 @@ extern "C" {
#define WL_PRINTF(x, y)
* Protocol message signature
* A wl_message describes the signature of an actual protocol message, such as a
* request or event, that adheres to the Wayland protocol wire format. The
* protocol implementation uses a wl_message within its demarshal machinery for
* decoding messages between a compositor and its clients. In a sense, a
* wl_message is to a protocol message like a class is to an object.
* The `name` of a wl_message is the name of the corresponding protocol message.
* The `signature` is an ordered list of symbols representing the data types
* of message arguments and, optionally, a protocol version and indicators for
* nullability. A leading integer in the `signature` indicates the _since_
* version of the protocol message. A `?` preceding a data type symbol indicates
* that the following argument type is nullable. When no arguments accompany a
* message, `signature` is an empty string.
* * `i`: int
* * `u`: uint
* * `f`: fixed
* * `s`: string
* * `o`: object
* * `n`: new_id
* * `a`: array
* * `h`: fd
* * `?`: following argument is nullable
* While demarshaling primitive arguments is straightforward, when demarshaling
* messages containing `object` or `new_id` arguments, the protocol
* implementation often must determine the type of the object. The `types` of a
* wl_message is an array of wl_interface references that correspond to `o` and
* `n` arguments in `signature`, with `NULL` placeholders for arguments with
* non-object types.
* Consider the protocol event wl_display `delete_id` that has a single `uint`
* argument. The wl_message is:
* \code
* { "delete_id", "u", [NULL] }
* \endcode
* Here, the message `name` is `"delete_id"`, the `signature` is `"u"`, and the
* argument `types` is `[NULL]`, indicating that the `uint` argument has no
* corresponding wl_interface since it is a primitive argument.
* In contrast, consider a `wl_foo` interface supporting protocol request `bar`
* that has existed since version 2, and has two arguments: a `uint` and an
* object of type `wl_baz_interface` that may be `NULL`. Such a `wl_message`
* might be:
* \code
* { "bar", "2u?o", [NULL, &wl_baz_interface] }
* \endcode
* Here, the message `name` is `"bar"`, and the `signature` is `"2u?o"`. Notice
* how the `2` indicates the protocol version, the `u` indicates the first
* argument type is `uint`, and the `?o` indicates that the second argument
* is an object that may be `NULL`. Lastly, the argument `types` array indicates
* that no wl_interface corresponds to the first argument, while the type
* `wl_baz_interface` corresponds to the second argument.
* \sa wl_argument
* \sa wl_interface
* \sa <a href="">Wire Format</a>
struct wl_message {
/** Message name */
const char *name;
/** Message signature */
const char *signature;
/** Object argument interfaces */
const struct wl_interface **types;
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