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NAME

       wl_message

SYNOPSIS

       #include <wayland-util.h>

   Data Fields
       const char * name
       const char * signature
       const struct wl_interface ** types

Detailed Description

       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. While
       it is a protocol violation to send messages with non-nullable arguments set to NULL, event
       handlers in clients might still get called with non-nullable object arguments set to NULL.
       This can happen when the client destroyed the object being used as argument on its side
       and an event referencing that object was sent before the server knew about its
       destruction. As this race cannot be prevented, clients should - as a general rule -
       program their event handlers such that they can handle object arguments declared non-
       nullable being NULL gracefully.

       When no arguments accompany a message, signature is an empty string.

       Symbols:

       · 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:

       { "delete_id", "u", [NULL] }

       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:

       { "bar", "2u?o", [NULL, &wl_baz_interface] }

       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.

       See also:
           wl_argument

           wl_interface

           Wire Format

Field Documentation

   const char* wl_message::name
       Message name

   const char* wl_message::signature
       Message signature

   const struct wl_interface** wl_message::types
       Object argument interfaces

Author

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