# Initialization and General Programming Information

## Extension Header Files

The following include files are part of the Xkb standard:

- `<X11/XKBlib.h>`

  `XKBlib.h` is the main header file for Xkb; it declares constants, types, and functions.

- `<X11/extensions/XKBstr.h>`

  `XKBstr.h` declares types and constants for Xkb. It is included automatically from `<X11/XKBlib.h>`; you should never need to reference it directly in your application code.

- `<X11/extensions/XKB.h>`

  `XKB.h` defines constants for Xkb. It is included automatically from `<X11/XKBstr.h>`; you should never need to reference it directly in your application code.

- `<X11/extensions/XKBgeom.h>`

  `XKBgeom.h` declares types, symbolic constants, and functions for manipulating keyboard geometry descriptions.

## Extension Name

The name of the Xkb extension is given in `<X11/extensions/Xkb.h>`:

    #define XkbName "XKEYBOARD"

Most extensions to the X protocol are initialized by calling `XInitExtension` and passing the extension name. However, as explained in section 2.4, Xkb requires a more complex initialization sequence, and a client program should not call `XInitExtension` directly.

XkbName

## Determining Library Compatibility

If an application is dynamically linked, both the X server and the client-side X library must contain the Xkb extension in order for the client to use the Xkb extension capabilities. Therefore a dynamically linked application must check both the library and the server for compatibility before using Xkb function calls. A properly written program must check for compatibility between the version of the Xkb library that is dynamically loaded and the one used when the application was built. It must then check the server version for compatibility with the version of Xkb in the library.

If your application is statically linked, you must still check for server compatibility and may check library compatibility. (It is possible to compile against one set of header files and link against a different, incompatible, version of the library, although this should not normally occur.)

To determine the compatibility of a library at runtime, call `XkbLibraryVersion`.

Bool `XkbLibraryVersion` int *lib_major_in_out int *lib_minor_in_out

lib_major_in_out

  specifies and returns the major Xkb library version.

lib_minor_in_out

  specifies and returns the minor Xkb library version.

Pass the symbolic value XkbMajorVersion in lib_major_in_out and XkbMinorVersion in lib_minor_in_out. These arguments represent the version of the library used at compile time. The `XkbLibraryVersion` function backfills the major and minor version numbers of the library used at run time in lib_major_in_out and lib_minor_in_out. If the versions of the compile time and run time libraries are compatible, `XkbLibraryVersion` returns True, otherwise, it returns False.

In addition, in order to use the Xkb extension, you must ensure that the extension is present in the server and that the server supports the version of the extension expected by the client. Use `XkbQueryExtension` to do this, as described in the next section.

XkbLibraryVersion

## Initializing the Keyboard Extension

Call `XkbQueryExtension` to check for the presence and compatibility of the extension in the server and to initialize the extension. Because of potential version mismatches, you cannot use the generic extension mechanism functions (`XQueryExtension` and `XInitExtension`) for checking for the presence of, and initializing the Xkb extension.

You must call `XkbQueryExtension` or `XkbOpenDisplay` before using any other Xkb library interfaces, unless such usage is explicitly allowed in the interface description in this document. The exceptions are: `XkbIgnoreExtension`, `XkbLibraryVersion`, and a handful of audible-bell functions. You should not use any other Xkb functions if the extension is not present or is uninitialized. In general, calls to Xkb library functions made prior to initializing the Xkb extension cause BadAccess protocol errors.

errors BadAccess
BadAccess

`XkbQueryExtension` both determines whether a compatible Xkb extension is present in the X server and initializes the extension when it is present.

Bool `XkbQueryExtension` Display *dpy int *opcode_rtrn int *event_rtrn int *error_rtrn int *major_in_out int *minor_in_out

dpy

  connection to the X server

opcode_rtrn

  backfilled with the major extension opcode

event_rtrn

  backfilled with the extension base event code

error_rtrn

  backfilled with the extension base error code

major_in_out

  compile time lib major version in, server major version out

minor_in_out

  compile time lib min version in, server minor version out

The `XkbQueryExtension` function determines whether a compatible version of the X Keyboard Extension is present in the server. If a compatible extension is present, `XkbQueryExtension` returns True; otherwise, it returns False.

If a compatible version of Xkb is present, `XkbQueryExtension` initializes the extension. It backfills the major opcode for the keyboard extension in opcode_rtrn, the base event code in event_rtrn, the base error code in error_rtrn, and the major and minor version numbers of the extension in major_in_out and minor_in_out. The major opcode is reported in the req_major fields of some Xkb events. For a discussion of the base event code, see section 4.1.

As a convenience, you can use the function `XkbOpenDisplay` to perform these three tasks at once: open a connection to an X server, check for a compatible version of the Xkb extension in both the library and the server, and initialize the extension for use.

Display *`XkbOpenDisplay` char *display_name int *event_rtrn int *error_rtrn int *major_in_out int *minor_in_out int *reason_rtrn

display_name

  hardware display name, which determines the display and communications domain to be used

event_rtrn

  backfilled with the extension base event code

error_rtrn

  backfilled with the extension base error code

major_in_out

  compile time lib major version in, server major version out

minor_in_out

  compile time lib minor version in, server minor version out

reason_rtrn

  backfilled with a status code

`XkbOpenDisplay` is a convenience function that opens an X display connection and initializes the X keyboard extension. In all cases, upon return reason_rtrn contains a status value indicating success or the type of failure. If major_in_out and minor_in_out are not NULL, `XkbOpenDisplay` first calls `XkbLibraryVersion` to determine whether the client library is compatible, passing it the values pointed to by major_in_out and minor_in_out. If the library is incompatible, `XkbOpenDisplay` backfills major_in_out and minor_in_out with the major and minor extension versions of the library being used and returns NULL. If the library is compatible, `XkbOpenDisplay` next calls `XOpenDisplay` with the display_name. If this fails, the function returns NULL. If successful, `XkbOpenDisplay` calls `XkbQueryExtension` and backfills the major and minor Xkb server extension version numbers in major_in_out and minor_in_out. If the server extension version is not compatible with the library extension version or if the server extension is not present, `XkbOpenDisplay` closes the display and returns NULL. When successful, the function returns the display connection.

The possible values for reason_rtrn are:

- XkbOD_BadLibraryVersion indicates `XkbLibraryVersion` returned False.

- XkbOD_ConnectionRefused indicates the display could not be opened.

- XkbOD_BadServerVersion indicates the library and the server have incompatible extension versions.

- XkbOD_NonXkbServer indicates the extension is not present in the X server.

- XkbOD_Success indicates that the function succeeded.

XkbQueryExtension
XkbOpenDisplay

## Disabling the Keyboard Extension

If a server supports the Xkb extension, the X library normally implements preXkb keyboard functions using the Xkb keyboard description and state. The server Xkb keyboard state may differ from the preXkb keyboard state. This difference does not affect most clients, but there are exceptions. To allow these clients to work properly, you may instruct the extension not to use Xkb functionality.

Call `XkbIgnoreExtension` to prevent core X library keyboard functions from using the X Keyboard Extension. You must call `XkbIgnoreExtension` before you open a server connection; Xkb does not provide a way to enable or disable use of the extension once a connection is established.

Bool `XkbIgnoreExtension` Bool ignore

ignore

  True means ignore the extension

`XkbIgnoreExtension` tells the X library whether to use the X Keyboard Extension on any subsequently opened X display connections. If ignore is True, the library does not initialize the Xkb extension when it opens a new display. This forces the X server to use compatibility mode and communicate with the client using only core protocol requests and events. If ignore is False, the library treats subsequent calls to `XOpenDisplay` normally and uses Xkb extension requests, events, and state. Do not explicitly use Xkb on a connection for which it is disabled. `XkbIgnoreExtension` returns False if it was unable to apply the ignore request.

XkbIgnoreExtension

## Protocol Errors

Many of the Xkb extension library functions described in this document can cause the X server to report an error, referred to in this document as a BadXxx protocol error, where Xxx is some name. These errors are fielded in the normal manner, by the default Xlib error handler or one replacing it. Note that X protocol errors are not necessarily reported immediately because of the buffering of X protocol requests in Xlib and the server.

Table 2.1 lists the protocol errors that can be generated, and their causes.

Xkb Protocol Errors

Error | Cause
--- | ---
BadAccess | The Xkb extension has not been properly initialized
BadKeyboard | The device specified was not a valid core or input extension device
BadImplementation | Invalid reply from server
BadAlloc | Unable to allocate storage
BadMatch | A compatible version of Xkb was not available in the server or an argument has correct type and range, but is otherwise invalid
BadValue | An argument is out of range
BadAtom | A name is neither a valid Atom or None
BadDevice | Device, Feedback Class, or Feedback ID invalid

The Xkb extension adds a single protocol error, BadKeyboard, to the core protocol error set. This error code will be reported as the error_rtrn when `XkbQueryExtension` is called. When a BadKeyboard error is reported in an XErrorEvent, additional information is reported in the resourceid field. The most significant byte of the resource_id is a further refinement of the error cause, as defined in Table 2.2. The least significant byte will contain the device, class, or feedback ID as indicated in the table.

errors BadKeyboard
BadKeyboard

BadKeyboard Protocol Error resource_id Values

high-order byte | value | meaning | low-order byte
--- | --- | --- | ---
XkbErr_BadDevice | 0xff | device not found | device ID
XkbErr_BadClass | 0xfe | device found, but it is of the wrong class | class ID
XkbErr_BadId | 0xfd | device found, class ok, but device does not contain a feedback with the indicated ID | feedback ID

errors

## Display and Device Specifications in Function Calls

Where a connection to the server is passed as an argument (Display*) and an XkbDescPtr is also passed as an argument, the Display* argument must match the dpy field of the XkbDescRec pointed to by the XkbDescPtr argument, or else the dpy field of the XkbDescRec must be NULL. If they don’t match or the dpy field is not NULL, a BadMatch error is returned (either in the return value or a backfilled Status variable). Upon successful return, the dpy field of the XkbDescRec always contains the Display* value passed in.

The Xkb extension can communicate with the X input extension if it is present. Consequently, there can potentially be more than one input device connected to the server. Most Xkb library calls that require communicating with the server involve both a server connection (Display * dpy) and a device identifier (unsigned int device_spec). In some cases, the device identifier is implicit and is taken as the device_spec field of an XkbDescRec structure passed as an argument.

The device identifier can specify any X input extension device with a KeyClass component, or it can specify the constant, XkbUseCoreKbd. The use of XkbUseCoreKbd allows applications to indicate the core keyboard without having to determine its device identifier.

XkbUseCoreKbd

Where an Xkb device identifier is passed as an argument and an XkbDescPtr is also passed as an argument, if either the argument or the XkbDescRec device_spec field is XkbUseCoreKbd, and if the function returns successfully, the XkbDescPtr device_spec field will have been converted from XkbUseCoreKbd to a real Xkb device ID. If the function does not complete successfully, the device_spec field remains unchanged. Subsequently, the device id argument must match the device_spec field of the XkbDescPtr argument. If they don’t match, a BadMatch error is returned (either in the return value or a backfilled Status variable).

When the Xkb extension in the server hands an application a device identifier to use for the keyboard, that ID is the input extension identifier for the device if the server supports the X Input Extension. If the server does not support the input extension, the meaning of the identifier is undefined — the only guarantee is that when you use XkbUseCoreKbd, XkbUseCoreKbd will work and the identifier returned by the server will refer to the core keyboard device.