aboutsummaryrefslogtreecommitdiff
path: root/libX11/specs/XKB/ch02.xml
blob: 57286a475e5c32bdac8b5c794867cdf6c9eede11 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
<chapter id='Initialization_and_General_Programming_Information'>
<title>Initialization and General Programming Information</title>
<sect1 id='Extension_Header_Files'>
<title>Extension Header Files</title>
<para>
The following include files are part of the Xkb standard:
</para>
<itemizedlist>
  <listitem>
    <para>
<emphasis>&lt;X11/XKBlib.h&gt;</emphasis>
    </para>
    <para><emphasis>XKBlib.h</emphasis>
is the main header file for Xkb; it declares constants, types, and
functions.
    </para>
  </listitem>
  <listitem>
    <para>
<emphasis>&lt;X11/extensions/XKBstr.h&gt;</emphasis>
    </para>
    <para>
<emphasis>XKBstr.h</emphasis> declares types and
constants for Xkb. It is included automatically from
<emphasis>&lt;X11/XKBlib.h&gt;</emphasis>
; you should never need to reference it directly in your application code.
    </para>
  </listitem>
  <listitem>
    <para>
<emphasis>&lt;X11/extensions/XKB.h&gt;</emphasis>
    </para>
    <para>
<emphasis>XKB.h</emphasis>
defines constants for Xkb. It is included automatically from <emphasis>
&lt;X11/XKBstr.h&gt;</emphasis>
; you should never need to reference it directly in your application code.
    </para>
  </listitem>
  <listitem>
    <para>
<emphasis>&lt;X11/extensions/XKBgeom.h&gt;</emphasis>
   </para>
   <para><emphasis>XKBgeom.h</emphasis>
declares types, symbolic constants, and functions for manipulating
keyboard geometry descriptions.
    </para>
  </listitem>
</itemizedlist>
</sect1>
<sect1 id='Extension_Name'>
<title>Extension Name</title>
<para>
The name of the Xkb extension is given in <emphasis>
&lt;X11/extensions/Xkb.h&gt;:</emphasis>
</para>

<para>
<emphasis>
#define XkbName "XKEYBOARD"</emphasis>
</para>

<para>
Most extensions to the X protocol are initialized by calling <emphasis>
XInitExtension</emphasis>
 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 <emphasis>
XInitExtension</emphasis>
 directly.
</para>
</sect1>
<sect1 id='Determining_Library_Compatibility'>
<title>Determining Library Compatibility</title>
<para>
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.
</para>
<para>
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.)
</para>
<para>
To determine the compatibility of a library at runtime, call <emphasis>
XkbLibraryVersion</emphasis>.
</para>
<informaltable frame='none'>
<tgroup cols='1'>
<colspec colsep='0'/>
<tbody>
  <row rowsep='0'>
    <entry role='functiondecl'>
Bool <emphasis>
XkbLibraryVersion</emphasis>
(<emphasis>
lib_major_in_out</emphasis>
, <emphasis>
lib_minor_in_out</emphasis>
)
    </entry>
  </row>
  <row rowsep='0'>
    <entry role='functionargdecl'>
int *      <emphasis>
lib_major_in_out;</emphasis>
                  /* specifies and returns the major Xkb library version. */
    </entry>
  </row>
  <row rowsep='0'>
    <entry role='functionargdecl'>
int *      <emphasis>
lib_minor_in_out;</emphasis>
                  /* specifies and returns the minor Xkb library version. */
    </entry>
</row>
</tbody>
</tgroup>
</informaltable>

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

<para>
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 <emphasis>
XkbQueryExtension</emphasis>
 to do this, as described in the next section.
</para>

</sect1>
<sect1 id='Initializing_the_Keyboard_Extension'>
<title>Initializing the Keyboard Extension</title>
<para>
Call <emphasis>
XkbQueryExtension</emphasis>
 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 (<emphasis>
XQueryExtension </emphasis>
and<emphasis>
 XInitExtension</emphasis>
) for checking for the presence of, and initializing the Xkb extension.
</para>

<para>
You must call <emphasis>
XkbQueryExtension</emphasis>
 or <emphasis>
XkbOpenDisplay</emphasis>
 before using any other Xkb library interfaces, unless such usage is explicitly
allowed in the interface description in this document. The exceptions are:
<emphasis>
XkbIgnoreExtension</emphasis>
, <emphasis>
XkbLibraryVersion</emphasis>
, 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 <emphasis>
BadAccess</emphasis>
 protocol errors.
</para>

<para>
<emphasis>
XkbQueryExtension</emphasis>
 both determines whether a compatible Xkb extension is present in the X server
and initializes the extension when it is present.
</para>

<informaltable frame='none'>
<tgroup cols='1'>
<colspec colsep='0'/>
<tbody>
  <row rowsep='0'>
    <entry role='functiondecl'>
Bool <emphasis>
XkbQueryExtension</emphasis>
(<emphasis>
dpy, opcode_rtrn, event_rtrn, error_rtrn, major_in_out, minor_in_out</emphasis>
)
    </entry>
  </row>
  <row rowsep='0'>
    <entry role='functionargdecl'>
Display *      <emphasis>
dpy;      </emphasis>
      /* connection to the X server */
    </entry>
  </row>
  <row rowsep='0'>
    <entry role='functionargdecl'>
int *      <emphasis>
opcode_rtrn</emphasis>
;            /* backfilled with the major extension opcode */
    </entry>
  </row>
  <row rowsep='0'>
    <entry role='functionargdecl'>
int *      <emphasis>
event_rtrn</emphasis>
;            /* backfilled with the extension base event code */
    </entry>
  </row>
  <row rowsep='0'>
    <entry role='functionargdecl'>
int *      <emphasis>
error_rtrn</emphasis>
;            /* backfilled with the extension base error code */
    </entry>
  </row>
  <row rowsep='0'>
    <entry role='functionargdecl'>
int *      <emphasis>
major_in_out</emphasis>
;            /* compile time lib major version in, server major version out */
    </entry>
  </row>
  <row rowsep='0'>
    <entry role='functionargdecl'>
int *      <emphasis>
minor_in_out;      </emphasis>
      /* compile time lib min version in, server minor version out */
    </entry>
</row>
</tbody>
</tgroup>
</informaltable>

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

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

<para>
As a convenience, you can use the function <emphasis>
XkbOpenDisplay</emphasis>
 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.
</para>

<informaltable frame='none'>
<tgroup cols='1'>
<colspec colsep='0'/>
<tbody>
  <row rowsep='0'>
    <entry role='functiondecl'>
Display *<emphasis>
XkbOpenDisplay</emphasis>
(<emphasis>
display_name, event_rtrn, error_rtrn, major_in_out, minor_in_out,
reason_rtrn)</emphasis>
    </entry>
  </row>
  <row rowsep='0'>
    <entry role='functionargdecl'>
char *      <emphasis>
display_name</emphasis>
;      /* hardware display name, which determines the display and
communications domain to be used */
    </entry>
  </row>
  <row rowsep='0'>
    <entry role='functionargdecl'>
int *      <emphasis>
event_rtrn</emphasis>
;      /* backfilled with the extension base event code */
    </entry>
  </row>
  <row rowsep='0'>
    <entry role='functionargdecl'>
int *      <emphasis>
error_rtrn</emphasis>
;      /* backfilled with the extension base error code */
    </entry>
  </row>
  <row rowsep='0'>
    <entry role='functionargdecl'>
int *      <emphasis>
major_in_out</emphasis>
;      /* compile time lib major version in, server major version out */
    </entry>
  </row>
  <row rowsep='0'>
    <entry role='functionargdecl'>
int *      <emphasis>
minor_in_out</emphasis>
;      /* compile time lib minor version in, server minor version out */
    </entry>
  </row>
  <row rowsep='0'>
    <entry role='functionargdecl'>
int *      <emphasis>
reason_rtrn</emphasis>
;      /* backfilled with a status code */
    </entry>
</row>
</tbody>
</tgroup>
</informaltable>

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

<para>
The possible values for <emphasis>
reason_rtrn</emphasis> are:
</para>

<itemizedlist>
<listitem>
  <para>
<emphasis>
XkbOD_BadLibraryVersion</emphasis>
 indicates <emphasis>
XkbLibraryVersion </emphasis>
returned <emphasis>
False</emphasis>.
  </para>
</listitem>
<listitem>
  <para>
<emphasis>
XkbOD_ConnectionRefused</emphasis>
 indicates the display could not be opened.
  </para>
</listitem>
<listitem>
  <para>
<emphasis>
XkbOD_BadServerVersion</emphasis>
 indicates the library and the server have incompatible extension versions.
  </para>
</listitem>
<listitem>
  <para>
<emphasis>
XkbOD_NonXkbServer</emphasis>
 indicates the extension is not present in the X server.
  </para>
</listitem>
<listitem>
  <para>
<emphasis>
XkbOD_Success</emphasis>
 indicates that the function succeeded.
  </para>
</listitem>
</itemizedlist>
</sect1>

<sect1 id='Disabling_the_Keyboard_Extension'>
<title>Disabling the Keyboard Extension</title>
<para>
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.
</para>

<para>
Call <emphasis>
XkbIgnoreExtension</emphasis>
 to prevent core X library keyboard functions from using the X Keyboard
Extension. You must call <emphasis>
XkbIgnoreExtension</emphasis>
 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.
</para>

<informaltable frame='none'>
<tgroup cols='1'>
<colspec colsep='0'/>
<tbody>
  <row rowsep='0'>
    <entry role='functiondecl'>
Bool<emphasis>
 XkbIgnoreExtension</emphasis>
(<emphasis>
ignore</emphasis>
)
    </entry>
  </row>
  <row rowsep='0'>
    <entry role='functionargdecl'>
Bool      <emphasis>
ignore</emphasis>
;      /* <emphasis>
True</emphasis>
 means ignore the extension */
    </entry>
</row>
</tbody>
</tgroup>
</informaltable>

<para>
<emphasis>
XkbIgnoreExtension</emphasis>
 tells the X library whether to use the X Keyboard Extension on any
subsequently opened X display connections. If ignore is <emphasis>
True</emphasis>
, 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
<emphasis>
False</emphasis>
, the library treats subsequent calls to <emphasis>
XOpenDisplay</emphasis>
 normally and uses Xkb extension requests, events, and state. Do not explicitly
use Xkb on a connection for which it is disabled.<emphasis>
 XkbIgnoreExtension</emphasis>
 returns <emphasis>
False</emphasis>
 if it was unable to apply the ignore request.
</para>

</sect1>
<sect1 id='Protocol_Errors'>
<title>Protocol Errors</title>
<para>
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
<emphasis>
BadXxx</emphasis>
 protocol error, where <emphasis>
Xxx</emphasis>
 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.
</para>

<para>
Table 2.1 lists the protocol errors that can be generated, and their causes. <!-- xref -->
</para>

<table frame='none'>
<title>Xkb Protocol Errors</title>
<tgroup cols='2'>
<colspec colsep='0'/>
<colspec colsep='0'/>
<thead>
  <row rowsep='1'>
    <entry>Error</entry>
    <entry>Cause</entry>
  </row>
</thead>
<tbody>
  <row rowsep='0'>
    <entry>BadAccess</entry>
    <entry>
<para>
The Xkb extension has not been properly initialized
</para>
    </entry>
  </row>
  <row rowsep='0'>
    <entry>BadKeyboard</entry>
    <entry>
<para>
The device specified was not a valid core or input extension device
</para>
    </entry>
  </row>
  <row rowsep='0'>
    <entry>BadImplementation</entry>
    <entry>
<para>
Invalid reply from server
</para>
    </entry>
  </row>
  <row rowsep='0'>
    <entry>BadAlloc</entry>
    <entry>
<para>
Unable to allocate storage
</para>
    </entry>
  </row>
  <row rowsep='0'>
    <entry>BadMatch</entry>
    <entry>
<para>
A compatible version of Xkb was not available in the server or an argument has
correct type and range, but is otherwise invalid
</para>
    </entry>
  </row>
  <row rowsep='0'>
    <entry>BadValue</entry>
    <entry>
<para>
An argument is out of range
</para>
    </entry>
  </row>
  <row rowsep='0'>
    <entry>BadAtom</entry>
    <entry>
<para>
A name is neither a valid Atom or <emphasis>
None</emphasis>
</para>
    </entry>
  </row>
  <row rowsep='0'>
    <entry>BadDevice</entry>
    <entry>
<para>
Device, Feedback Class, or Feedback ID invalid
</para>
    </entry>
  </row>
</tbody>
</tgroup>
</table>

<para>
The Xkb extension adds a single protocol error, <emphasis>
BadKeyboard</emphasis>
, to the core protocol error set. This error code will be reported as the
<emphasis>
error_rtrn</emphasis>
 when <emphasis>
XkbQueryExtension</emphasis>
 is called. When a <emphasis>
BadKeyboard</emphasis>
 error is reported in an <emphasis>
XErrorEvent</emphasis>
, additional information is reported in the <emphasis>
resource_id</emphasis>
 field. The most significant byte of the <emphasis>
resource_id</emphasis>
 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.
</para>

<table frame='none'>
<title>BadKeyboard Protocol Error resource_id Values</title>
<tgroup cols='4'>
<colspec colsep='0'/>
<colspec colsep='0'/>
<colspec colsep='0'/>
<colspec colsep='0'/>
<thead>
  <row rowsep='0'>
    <entry>high-order byte</entry>
    <entry>value</entry>
    <entry>meaning</entry>
    <entry>low-order byte</entry>
  </row>
</thead>
<tbody>
  <row rowsep='0'>
    <entry>XkbErr_BadDevice</entry>
    <entry>0xff</entry>
    <entry>
<para>
device not found
</para>
    </entry>
    <entry>device ID</entry>
  </row>
  <row rowsep='0'>
    <entry>XkbErr_BadClass</entry>
    <entry>0xfe</entry>
    <entry>
<para>
device found, but it is of the wrong class
</para>
    </entry>
    <entry>class ID</entry>
  </row>
  <row rowsep='0'>
    <entry>XkbErr_BadId</entry>
    <entry>0xfd</entry>
    <entry>
<para>
device found, class ok, but device does not contain a feedback with the
indicated ID
</para>
    </entry>
    <entry>feedback ID</entry>
  </row>
</tbody>
</tgroup>
</table>
</sect1>

<sect1 id='Display_and_Device_Specifications_in_Function_Calls'>
<title>Display and Device Specifications in Function Calls</title>
<para>
Where a connection to the server is passed as an argument (Display*) and an
<emphasis>
XkbDescPtr</emphasis>
 is also passed as an argument, the Display* argument must match the <emphasis>
dpy</emphasis>
 field of the <emphasis>
XkbDescRec</emphasis>
 pointed to by the <emphasis>
XkbDescPtr</emphasis>
 argument, or else the <emphasis>
dpy</emphasis>
 field of the <emphasis>
XkbDescRec</emphasis>
 must be <emphasis>
NULL</emphasis>
. If they don’t match or the <emphasis>
dpy</emphasis>
 field is not <emphasis>
NULL</emphasis>
, a <emphasis>
BadMatch</emphasis>
 error is returned (either in the return value or a backfilled <emphasis>
Status</emphasis>
 variable). Upon successful return, the <emphasis>
dpy</emphasis>
 field of the <emphasis>
XkbDescRec</emphasis>
 always contains the Display* value passed in.
</para>

<para>
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 * <emphasis>
dpy</emphasis>
) and a device identifier (unsigned int <emphasis>
device_spec</emphasis>
). In some cases, the device identifier is implicit and is taken as the
<emphasis>
device_spec</emphasis>
 field of an <emphasis>
XkbDescRec</emphasis>
 structure passed as an argument.
</para>

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

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

<para>
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 <emphasis>
XkbUseCoreKbd</emphasis>
, <emphasis>
XkbUseCoreKbd</emphasis>
 will work and the identifier returned by the server will refer to the core
keyboard device.
</para>
</sect1>
</chapter>