OpenCores
URL https://opencores.org/ocsvn/openrisc/openrisc/trunk

Subversion Repositories openrisc

[/] [openrisc/] [trunk/] [rtos/] [ecos-2.0/] [doc/] [html/] [cdl-guide/] [ref.default-value.html] - Blame information for rev 588

Go to most recent revision | Details | Compare with Previous | View Log

Line No. Rev Author Line
1 28 unneback
<!-- Copyright (C) 2003 Red Hat, Inc.                                -->
2
<!-- This material may be distributed only subject to the terms      -->
3
<!-- and conditions set forth in the Open Publication License, v1.0  -->
4
<!-- or later (the latest version is presently available at          -->
5
<!-- http://www.opencontent.org/openpub/).                           -->
6
<!-- Distribution of the work or derivative of the work in any       -->
7
<!-- standard (paper) book form is prohibited unless prior           -->
8
<!-- permission is obtained from the copyright holder.               -->
9
<HTML
10
><HEAD
11
><TITLE
12
>default_value</TITLE
13
><meta name="MSSmartTagsPreventParsing" content="TRUE">
14
<META
15
NAME="GENERATOR"
16
CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+
17
"><LINK
18
REL="HOME"
19
TITLE="The eCos Component Writer's Guide"
20
HREF="cdl-guide.html"><LINK
21
REL="UP"
22
TITLE="CDL Language Specification"
23
HREF="reference.html"><LINK
24
REL="PREVIOUS"
25
TITLE="compile"
26
HREF="ref.compile.html"><LINK
27
REL="NEXT"
28
TITLE="define"
29
HREF="ref.define.html"></HEAD
30
><BODY
31
CLASS="REFENTRY"
32
BGCOLOR="#FFFFFF"
33
TEXT="#000000"
34
LINK="#0000FF"
35
VLINK="#840084"
36
ALINK="#0000FF"
37
><DIV
38
CLASS="NAVHEADER"
39
><TABLE
40
SUMMARY="Header navigation table"
41
WIDTH="100%"
42
BORDER="0"
43
CELLPADDING="0"
44
CELLSPACING="0"
45
><TR
46
><TH
47
COLSPAN="3"
48
ALIGN="center"
49
>The <SPAN
50
CLASS="APPLICATION"
51
>eCos</SPAN
52
> Component Writer's Guide</TH
53
></TR
54
><TR
55
><TD
56
WIDTH="10%"
57
ALIGN="left"
58
VALIGN="bottom"
59
><A
60
HREF="ref.compile.html"
61
ACCESSKEY="P"
62
>Prev</A
63
></TD
64
><TD
65
WIDTH="80%"
66
ALIGN="center"
67
VALIGN="bottom"
68
></TD
69
><TD
70
WIDTH="10%"
71
ALIGN="right"
72
VALIGN="bottom"
73
><A
74
HREF="ref.define.html"
75
ACCESSKEY="N"
76
>Next</A
77
></TD
78
></TR
79
></TABLE
80
><HR
81
ALIGN="LEFT"
82
WIDTH="100%"></DIV
83
><H1
84
><A
85
NAME="REF.DEFAULT-VALUE"><SPAN
86
CLASS="PROPERTY"
87
>default_value</SPAN
88
></H1
89
><DIV
90
CLASS="REFNAMEDIV"
91
><A
92
NAME="AEN3887"
93
></A
94
><H2
95
>Name</H2
96
>Property <SPAN
97
CLASS="PROPERTY"
98
>default_value</SPAN
99
>&nbsp;--&nbsp;Provide a default value for this option using a CDL expression.</DIV
100
><DIV
101
CLASS="REFSYNOPSISDIV"
102
><A
103
NAME="AEN3891"><H2
104
>Synopsis</H2
105
><TABLE
106
BORDER="5"
107
BGCOLOR="#E0E0F0"
108
WIDTH="70%"
109
><TR
110
><TD
111
><PRE
112
CLASS="SYNOPSIS"
113
>cdl_option &lt;name&gt; {
114
    default_value &lt;expression&gt;
115
    &#8230;
116
}</PRE
117
></TD
118
></TR
119
></TABLE
120
></DIV
121
><DIV
122
CLASS="REFSECT1"
123
><A
124
NAME="AEN3893"
125
></A
126
><H2
127
>Description</H2
128
><P
129
>The <SPAN
130
CLASS="PROPERTY"
131
>default_value</SPAN
132
> property allows the initial value of a
133
configuration option to depend on other configuration options. The
134
arguments to the property should be a <SPAN
135
CLASS="APPLICATION"
136
>CDL</SPAN
137
> expression, see
138
<A
139
HREF="language.values.html#LANGUAGE.EXPRESSION"
140
>the Section called <I
141
>Ordinary Expressions</I
142
> in Chapter 3</A
143
> for the syntactic details.
144
In many cases a simple constant value will suffice, for example:</P
145
><TABLE
146
BORDER="5"
147
BGCOLOR="#E0E0F0"
148
WIDTH="70%"
149
><TR
150
><TD
151
><PRE
152
CLASS="PROGRAMLISTING"
153
>cdl_component CYGPKG_KERNEL_EXCEPTIONS {
154
    &#8230;
155
    default_value 1
156
 
157
    cdl_option CYGSEM_KERNEL_EXCEPTIONS_DECODE {
158
        &#8230;
159
        default_value 0
160
    }
161
}</PRE
162
></TD
163
></TR
164
></TABLE
165
><P
166
>However it is also possible for an option's default value to depend on
167
other options. For example the common HAL package provides some
168
support functions that are needed by the <SPAN
169
CLASS="APPLICATION"
170
>eCos</SPAN
171
> kernel, but are
172
unlikely to be useful if the kernel is not being used. This
173
relationship can be expressed using:</P
174
><TABLE
175
BORDER="5"
176
BGCOLOR="#E0E0F0"
177
WIDTH="70%"
178
><TR
179
><TD
180
><PRE
181
CLASS="PROGRAMLISTING"
182
>cdl_option CYGFUN_HAL_COMMON_KERNEL_SUPPORT {
183
    ...
184
    default_value CYGPKG_KERNEL
185
}</PRE
186
></TD
187
></TR
188
></TABLE
189
><P
190
>If the kernel is loaded then this HAL option is automatically enabled,
191
although the user can still disable it explicitly should this prove
192
necessary. If the kernel is not loaded then the option is disabled,
193
although it can still be enabled by the user if desired.
194
<SPAN
195
CLASS="PROPERTY"
196
>default_value</SPAN
197
> expressions can be more complicated than this if
198
appropriate, and provide a very powerful facility for component
199
writers who want their code to &#8220;just do the right thing&#8221;
200
in a wide variety of configurations.</P
201
><P
202
>The <SPAN
203
CLASS="APPLICATION"
204
>CDL</SPAN
205
> configuration system evaluates the <SPAN
206
CLASS="PROPERTY"
207
>default_value</SPAN
208
>
209
expression when the current package is loaded and whenever there is a
210
change to any other option referenced in the expression. The result
211
depends on the option's flavor:</P
212
><P
213
></P
214
><DIV
215
CLASS="VARIABLELIST"
216
><DL
217
><DT
218
><TT
219
CLASS="LITERAL"
220
>flavor none</TT
221
></DT
222
><DD
223
><P
224
>Options with this flavor have no value, so the <SPAN
225
CLASS="PROPERTY"
226
>default_value</SPAN
227
>
228
property is not applicable.</P
229
></DD
230
><DT
231
><TT
232
CLASS="LITERAL"
233
>flavor bool</TT
234
></DT
235
><DD
236
><P
237
>If the expression evaluates to a non-zero result the option is
238
enabled by default, otherwise it is disabled.</P
239
></DD
240
><DT
241
><TT
242
CLASS="LITERAL"
243
>flavor booldata</TT
244
></DT
245
><DD
246
><P
247
>If the result of evaluating the expression is zero then the option is
248
disabled, otherwise the option is enabled and its value is the result.</P
249
></DD
250
><DT
251
><TT
252
CLASS="LITERAL"
253
>flavor data</TT
254
></DT
255
><DD
256
><P
257
>The default value of the option is the result of evaluating the
258
expression.</P
259
></DD
260
></DL
261
></DIV
262
><P
263
>A <TT
264
CLASS="LITERAL"
265
>cdl_option</TT
266
> or other entity can have at most one <SPAN
267
CLASS="PROPERTY"
268
>default_value</SPAN
269
>
270
property, and it is illegal to have both a <SPAN
271
CLASS="PROPERTY"
272
>calculated</SPAN
273
> and a
274
<SPAN
275
CLASS="PROPERTY"
276
>default_value</SPAN
277
> property in one body. If an option does not have
278
either a <SPAN
279
CLASS="PROPERTY"
280
>default_value</SPAN
281
> or a <SPAN
282
CLASS="PROPERTY"
283
>calculated</SPAN
284
> property and it does not
285
have the flavor <TT
286
CLASS="LITERAL"
287
>none</TT
288
> then the configuration tools
289
will assume a default value expression of <TT
290
CLASS="LITERAL"
291
>0</TT
292
>.</P
293
><P
294
>On occasion it is useful to have a configuration option
295
<TT
296
CLASS="LITERAL"
297
>A</TT
298
> which has both a <SPAN
299
CLASS="PROPERTY"
300
>requires</SPAN
301
> constraint on some
302
other option <TT
303
CLASS="LITERAL"
304
>B</TT
305
> and a <SPAN
306
CLASS="PROPERTY"
307
>default_value</SPAN
308
> expression of
309
<TT
310
CLASS="LITERAL"
311
>B</TT
312
>. If option <TT
313
CLASS="LITERAL"
314
>B</TT
315
> is not enabled
316
then <TT
317
CLASS="LITERAL"
318
>A</TT
319
> will also be disabled by default and no
320
conflict arises. If <TT
321
CLASS="LITERAL"
322
>B</TT
323
> is enabled then
324
<TT
325
CLASS="LITERAL"
326
>A</TT
327
> also becomes enabled and again no conflict
328
arises. If a user attempts to enable <TT
329
CLASS="LITERAL"
330
>B</TT
331
> but not
332
<TT
333
CLASS="LITERAL"
334
>A</TT
335
> then there will be a conflict. Users should be
336
able to deduce that the two options are closely interlinked and should
337
not be manipulated independently except in very unusual circumstances.</P
338
><DIV
339
CLASS="TIP"
340
><BLOCKQUOTE
341
CLASS="TIP"
342
><P
343
><B
344
>Tip: </B
345
>If the first entry in a <SPAN
346
CLASS="PROPERTY"
347
>default_value</SPAN
348
> expression is a negative
349
number, for example <TT
350
CLASS="LITERAL"
351
>default_value&nbsp;-1</TT
352
> then this
353
can be misinterpreted as an option instead of as part of the
354
expression. Currently the <SPAN
355
CLASS="PROPERTY"
356
>default_value</SPAN
357
> property does not take any
358
options, but this may change in future. Option processing halts at the
359
sequence <TT
360
CLASS="LITERAL"
361
>--</TT
362
>, so the desired value can be expressed
363
safely using
364
<TT
365
CLASS="LITERAL"
366
>default_value&nbsp;--&nbsp;-1</TT
367
></P
368
></BLOCKQUOTE
369
></DIV
370
><DIV
371
CLASS="NOTE"
372
><BLOCKQUOTE
373
CLASS="NOTE"
374
><P
375
><B
376
>Note: </B
377
>In many cases it would be useful to calculate default values using
378
some global preferences, for example:</P
379
><TABLE
380
BORDER="5"
381
BGCOLOR="#E0E0F0"
382
WIDTH="70%"
383
><TR
384
><TD
385
><PRE
386
CLASS="PROGRAMLISTING"
387
>cdl_option CYGIMP_LIBC_STRING_PREFER_SMALL_TO_FAST {
388
    &#8230;
389
    default_value CYGGLO_CODESIZE &#62; CYGGLO_SPEED
390
}</PRE
391
></TD
392
></TR
393
></TABLE
394
><P
395
>Such global preference options do not yet exist, but are likely to be
396
added in a future version.</P
397
></BLOCKQUOTE
398
></DIV
399
><DIV
400
CLASS="NOTE"
401
><BLOCKQUOTE
402
CLASS="NOTE"
403
><P
404
><B
405
>Note: </B
406
>For options with the booldata flavor the current syntax does not allow
407
the default values of the enabled flag and the value to be controlled
408
separately. Functionality to permit this may be added in a future
409
release.</P
410
></BLOCKQUOTE
411
></DIV
412
></DIV
413
><DIV
414
CLASS="REFSECT1"
415
><A
416
NAME="AEN3964"
417
></A
418
><H2
419
>Example</H2
420
><TABLE
421
BORDER="5"
422
BGCOLOR="#E0E0F0"
423
WIDTH="70%"
424
><TR
425
><TD
426
><PRE
427
CLASS="PROGRAMLISTING"
428
>cdl_option CYGDBG_HAL_DEBUG_GDB_THREAD_SUPPORT {
429
    display       "Include GDB multi-threading debug support"
430
    requires      CYGDBG_KERNEL_DEBUG_GDB_THREAD_SUPPORT
431
    default_value CYGDBG_KERNEL_DEBUG_GDB_THREAD_SUPPORT
432
    description   "
433
        This option enables some extra HAL code which is needed
434
        to support multi-threaded source level debugging."
435
}</PRE
436
></TD
437
></TR
438
></TABLE
439
></DIV
440
><DIV
441
CLASS="REFSECT1"
442
><A
443
NAME="AEN3967"
444
></A
445
><H2
446
>See Also</H2
447
><P
448
>Properties <A
449
HREF="ref.calculated.html"
450
><SPAN
451
CLASS="PROPERTY"
452
>calculated</SPAN
453
></A
454
>,
455
<A
456
HREF="ref.flavor.html"
457
><SPAN
458
CLASS="PROPERTY"
459
>flavor</SPAN
460
></A
461
> and
462
<A
463
HREF="ref.legal-values.html"
464
><SPAN
465
CLASS="PROPERTY"
466
>legal_values</SPAN
467
></A
468
>.</P
469
></DIV
470
><DIV
471
CLASS="NAVFOOTER"
472
><HR
473
ALIGN="LEFT"
474
WIDTH="100%"><TABLE
475
SUMMARY="Footer navigation table"
476
WIDTH="100%"
477
BORDER="0"
478
CELLPADDING="0"
479
CELLSPACING="0"
480
><TR
481
><TD
482
WIDTH="33%"
483
ALIGN="left"
484
VALIGN="top"
485
><A
486
HREF="ref.compile.html"
487
ACCESSKEY="P"
488
>Prev</A
489
></TD
490
><TD
491
WIDTH="34%"
492
ALIGN="center"
493
VALIGN="top"
494
><A
495
HREF="cdl-guide.html"
496
ACCESSKEY="H"
497
>Home</A
498
></TD
499
><TD
500
WIDTH="33%"
501
ALIGN="right"
502
VALIGN="top"
503
><A
504
HREF="ref.define.html"
505
ACCESSKEY="N"
506
>Next</A
507
></TD
508
></TR
509
><TR
510
><TD
511
WIDTH="33%"
512
ALIGN="left"
513
VALIGN="top"
514
><SPAN
515
CLASS="PROPERTY"
516
>compile</SPAN
517
></TD
518
><TD
519
WIDTH="34%"
520
ALIGN="center"
521
VALIGN="top"
522
><A
523
HREF="reference.html"
524
ACCESSKEY="U"
525
>Up</A
526
></TD
527
><TD
528
WIDTH="33%"
529
ALIGN="right"
530
VALIGN="top"
531
><SPAN
532
CLASS="PROPERTY"
533
>define</SPAN
534
></TD
535
></TR
536
></TABLE
537
></DIV
538
></BODY
539
></HTML
540
>

powered by: WebSVN 2.1.0

© copyright 1999-2024 OpenCores.org, equivalent to Oliscience, all rights reserved. OpenCores®, registered trademark.