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<!-- Copyright (C) 2003 Red Hat, Inc.                                -->
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<HTML
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><HEAD
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><TITLE
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>An Introduction to Tcl</TITLE
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><meta name="MSSmartTagsPreventParsing" content="TRUE">
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<META
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NAME="GENERATOR"
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"><LINK
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REL="HOME"
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TITLE="The eCos Component Writer's Guide"
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HREF="cdl-guide.html"><LINK
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TITLE="The CDL Language"
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HREF="language.html"><LINK
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TITLE="Option Naming Convention"
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HREF="language.naming.html"><LINK
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TITLE="Values and Expressions"
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HREF="language.values.html"></HEAD
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><BODY
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CLASS="SECT1"
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><TABLE
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SUMMARY="Header navigation table"
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>The <SPAN
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CLASS="APPLICATION"
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>eCos</SPAN
52 
> Component Writer's Guide</TH
53 
></TR
54 
><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="language.naming.html"
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ACCESSKEY="P"
62 
>Prev</A
63 
></TD
64 
><TD
65 
WIDTH="80%"
66 
ALIGN="center"
67 
VALIGN="bottom"
68 
>Chapter 3. The CDL Language</TD
69 
><TD
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WIDTH="10%"
71 
ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="language.values.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="SECT1"
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><H1
86 
CLASS="SECT1"
87 
><A
88 
NAME="LANGUAGE.TCL">An Introduction to Tcl</H1
89 
><P
90 
>All <SPAN
91 
CLASS="APPLICATION"
92 
>CDL</SPAN
93 
> scripts are implemented as <SPAN
94 
CLASS="APPLICATION"
95 
>Tcl</SPAN
96 
> scripts, and are read in by
97 
running the data through a standard <SPAN
98 
CLASS="APPLICATION"
99 
>Tcl</SPAN
100 
> interpreter, extended with a
101 
small number of additional commands such as
102 
<TT
103 
CLASS="LITERAL"
104 
>cdl_option</TT
105 
> and <TT
106 
CLASS="LITERAL"
107 
>cdl_component</TT
108 
>.
109 
Often it is not necessary to know the full details of <SPAN
110 
CLASS="APPLICATION"
111 
>Tcl</SPAN
112 
> syntax.
113 
Instead it is possible to copy an existing script, perform some copy
114 
and paste operations, and make appropriate changes to names and to
115 
various properties. However there are also cases where an
116 
understanding of <SPAN
117 
CLASS="APPLICATION"
118 
>Tcl</SPAN
119 
> syntax is very desirable, for example:</P
120 
><TABLE
121 
BORDER="5"
122 
BGCOLOR="#E0E0F0"
123 
WIDTH="70%"
124 
><TR
125 
><TD
126 
><PRE
127 
CLASS="PROGRAMLISTING"
128 
>cdl_option CYGDAT_UITRON_MEMPOOLFIXED_EXTERNS {
129 
    display       "Externs for initialization"
130 
    flavor        data
131 
    default_value {"static char fpool1[ 2000 ], \\\n\
132 
                                fpool2[ 2000 ], \\\n\
133 
                                fpool3[ 2000 ];"}
134 
    &#8230;
135 
}</PRE
136 
></TD
137 
></TR
138 
></TABLE
139 
><P
140 
>This causes the <TT
141 
CLASS="LITERAL"
142 
>cdl_option</TT
143 
> command to be executed, which in turn
144 
evaluates its body in a recursive invocation of the <SPAN
145 
CLASS="APPLICATION"
146 
>Tcl</SPAN
147 
> interpreter.
148 
When the <SPAN
149 
CLASS="PROPERTY"
150 
>default_value</SPAN
151 
> property is encountered the braces around the
152 
value part are processed by the interpreter, stopping it from doing
153 
further processing of the braced contents (except for backslash
154 
processing at the end of a line, that is special). In particular it
155 
prevents command substitution for
156 
<TT
157 
CLASS="LITERAL"
158 
>[&nbsp;2000&nbsp;]</TT
159 
>. A single argument will be
160 
passed to the <SPAN
161 
CLASS="PROPERTY"
162 
>default_value</SPAN
163 
> command which expects a <SPAN
164 
CLASS="APPLICATION"
165 
>CDL</SPAN
166 
>
167 
expression, so the expression parsing code is passed the following:</P
168 
><TABLE
169 
BORDER="5"
170 
BGCOLOR="#E0E0F0"
171 
WIDTH="70%"
172 
><TR
173 
><TD
174 
><PRE
175 
CLASS="SCREEN"
176 
>"static char fpool1[ 2000 ], \\\n fpool2[ 2000 ], \\\n fpool3[ 2000 ];"</PRE
177 
></TD
178 
></TR
179 
></TABLE
180 
><P
181 
>The <SPAN
182 
CLASS="APPLICATION"
183 
>CDL</SPAN
184 
> expression parsing code will treat this as a simple string
185 
constant, as opposed to a more complicated expression involving other
186 
options and various operators. The string parsing code will perform
187 
the usual backslash substitutions so the actual default value will be:</P
188 
><TABLE
189 
BORDER="5"
190 
BGCOLOR="#E0E0F0"
191 
WIDTH="70%"
192 
><TR
193 
><TD
194 
><PRE
195 
CLASS="SCREEN"
196 
>static char fpool1[ 2000 ], \
197 
 fpool2[ 2000 ], \
198 
 fpool3[ 2000 ];</PRE
199 
></TD
200 
></TR
201 
></TABLE
202 
><P
203 
>If the user does not modify the option's value then the following
204 
will be generated in the appropriate configuration header file:</P
205 
><TABLE
206 
BORDER="5"
207 
BGCOLOR="#E0E0F0"
208 
WIDTH="70%"
209 
><TR
210 
><TD
211 
><PRE
212 
CLASS="PROGRAMLISTING"
213 
>#define CYGDAT_UITRON_MEMPOOLFIXED_EXTERNS static char fpool1[ 2000 ], \
214 
 fpool2[ 2000 ], \
215 
 fpool3[ 2000 ];</PRE
216 
></TD
217 
></TR
218 
></TABLE
219 
><P
220 
>Getting this desired result usually requires an understanding of both
221 
<SPAN
222 
CLASS="APPLICATION"
223 
>Tcl</SPAN
224 
> syntax and <SPAN
225 
CLASS="APPLICATION"
226 
>CDL</SPAN
227 
> expression syntax. Sometimes it is possible to
228 
substitute a certain amount of trial and error instead, but this may
229 
prove frustrating. It is also worth pointing out that many <SPAN
230 
CLASS="APPLICATION"
231 
>CDL</SPAN
232 
>
233 
scripts do not involve this level of complexity. On the other hand,
234 
some of the more advanced features of the <SPAN
235 
CLASS="APPLICATION"
236 
>CDL</SPAN
237 
> language involve
238 
fragments of <SPAN
239 
CLASS="APPLICATION"
240 
>Tcl</SPAN
241 
> code, for example the <SPAN
242 
CLASS="PROPERTY"
243 
>define_proc</SPAN
244 
> property. To
245 
use these component writers will need to know about the full <SPAN
246 
CLASS="APPLICATION"
247 
>Tcl</SPAN
248 
>
249 
language as well as the syntax.</P
250 
><P
251 
>Although the current example may seem to suggest that <SPAN
252 
CLASS="APPLICATION"
253 
>Tcl</SPAN
254 
> is rather
255 
complicated, it is actually a very simple yet powerful scripting
256 
language: the syntax is defined by just eleven rules. On occasion this
257 
simplicity means that Tcl's behavior is subtly different from other
258 
languages, which can confuse newcomers.</P
259 
><P
260 
>When the Tcl interpreter is passed some data such as
261 
<TT
262 
CLASS="LITERAL"
263 
>puts&nbsp;Hello</TT
264 
>, it splits this data into a command
265 
and its arguments. The command will be terminated by a newline or by a
266 
semicolon, unless one of the quoting mechanisms is used. The command
267 
and each of its arguments are separated by white space. So in the
268 
following example:</P
269 
><TABLE
270 
BORDER="5"
271 
BGCOLOR="#E0E0F0"
272 
WIDTH="70%"
273 
><TR
274 
><TD
275 
><PRE
276 
CLASS="SCREEN"
277 
>puts Hello
278 
set x 42</PRE
279 
></TD
280 
></TR
281 
></TABLE
282 
><P
283 
>This will result in two separate commands being executed. The first
284 
command is <TT
285 
CLASS="LITERAL"
286 
>puts</TT
287 
> and is passed a single argument,
288 
<TT
289 
CLASS="LITERAL"
290 
>Hello</TT
291 
>. The second command is <TT
292 
CLASS="LITERAL"
293 
>set</TT
294 
>
295 
and is passed two arguments, <TT
296 
CLASS="LITERAL"
297 
>x</TT
298 
> and
299 
<TT
300 
CLASS="LITERAL"
301 
>42</TT
302 
>. The intervening newline character serves to
303 
terminate the first command, and a semi-colon separator could be used
304 
instead: </P
305 
><TABLE
306 
BORDER="5"
307 
BGCOLOR="#E0E0F0"
308 
WIDTH="70%"
309 
><TR
310 
><TD
311 
><PRE
312 
CLASS="SCREEN"
313 
>puts Hello;set x 42</PRE
314 
></TD
315 
></TR
316 
></TABLE
317 
><P
318 
>Any white space surrounding the semicolon is just ignored because it
319 
does not serve to separate arguments.</P
320 
><P
321 
>Now consider the following:</P
322 
><TABLE
323 
BORDER="5"
324 
BGCOLOR="#E0E0F0"
325 
WIDTH="70%"
326 
><TR
327 
><TD
328 
><PRE
329 
CLASS="SCREEN"
330 
>set x Hello world</PRE
331 
></TD
332 
></TR
333 
></TABLE
334 
><P
335 
>This is not valid <SPAN
336 
CLASS="APPLICATION"
337 
>Tcl</SPAN
338 
>. It is an attempt to invoke the
339 
<TT
340 
CLASS="LITERAL"
341 
>set</TT
342 
> command with three arguments:
343 
<TT
344 
CLASS="LITERAL"
345 
>x</TT
346 
>, <TT
347 
CLASS="LITERAL"
348 
>Hello</TT
349 
>, and
350 
<TT
351 
CLASS="LITERAL"
352 
>world</TT
353 
>. The <TT
354 
CLASS="LITERAL"
355 
>set</TT
356 
> only takes two
357 
arguments, a variable name and a value, so it is necessary to combine
358 
the data into a single argument by quoting:</P
359 
><TABLE
360 
BORDER="5"
361 
BGCOLOR="#E0E0F0"
362 
WIDTH="70%"
363 
><TR
364 
><TD
365 
><PRE
366 
CLASS="SCREEN"
367 
>set x "Hello world"</PRE
368 
></TD
369 
></TR
370 
></TABLE
371 
><P
372 
>When the <SPAN
373 
CLASS="APPLICATION"
374 
>Tcl</SPAN
375 
> interpreter encounters the first quote character it
376 
treats all subsequent data up to but not including the closing quote
377 
as part of the current argument. The quote marks are removed by the
378 
interpreter, so the second argument passed to the
379 
<TT
380 
CLASS="LITERAL"
381 
>set</TT
382 
> command is just <TT
383 
CLASS="LITERAL"
384 
>Hello world</TT
385 
>
386 
without the quote characters. This can be significant in the context
387 
of <SPAN
388 
CLASS="APPLICATION"
389 
>CDL</SPAN
390 
> scripts. For example:</P
391 
><TABLE
392 
BORDER="5"
393 
BGCOLOR="#E0E0F0"
394 
WIDTH="70%"
395 
><TR
396 
><TD
397 
><PRE
398 
CLASS="PROGRAMLISTING"
399 
>cdl_option CYG_HAL_STARTUP {
400 
    &#8230;
401 
    default_value "RAM"
402 
}</PRE
403 
></TD
404 
></TR
405 
></TABLE
406 
><P
407 
>The <SPAN
408 
CLASS="APPLICATION"
409 
>Tcl</SPAN
410 
> interpreter strips off the quote marks so the <SPAN
411 
CLASS="APPLICATION"
412 
>CDL</SPAN
413 
>
414 
expression parsing code sees <TT
415 
CLASS="LITERAL"
416 
>RAM</TT
417 
> instead of
418 
<TT
419 
CLASS="LITERAL"
420 
>"RAM"</TT
421 
>. It will treat this as a reference to
422 
some unknown option <TT
423 
CLASS="VARNAME"
424 
>RAM</TT
425 
> rather than as a string
426 
constant, and the expression evaluation code will use a value of
427 
<TT
428 
CLASS="LITERAL"
429 
>0</TT
430 
> when it encounters an option that is not
431 
currently loaded. Therefore the option
432 
<TT
433 
CLASS="VARNAME"
434 
>CYG_HAL_STARTUP</TT
435 
> ends up with a default value of
436 
<TT
437 
CLASS="LITERAL"
438 
>0</TT
439 
>. Either braces or backslashes should be used to
440 
avoid this, for example
441 
<TT
442 
CLASS="LITERAL"
443 
>default_value&nbsp;{&nbsp;"RAM"&nbsp;}</TT
444 
>. </P
445 
><DIV
446 
CLASS="NOTE"
447 
><BLOCKQUOTE
448 
CLASS="NOTE"
449 
><P
450 
><B
451 
>Note: </B
452 
>There are long-term plans to implement some sort of <SPAN
453 
CLASS="APPLICATION"
454 
>CDL</SPAN
455 
> validation
456 
utility <SPAN
457 
CLASS="APPLICATION"
458 
>cdllint</SPAN
459 
> which
460 
could catch common errors like this one.</P
461 
></BLOCKQUOTE
462 
></DIV
463 
><P
464 
>A quoted argument continues until the closing quote character is
465 
encountered, which means that it can span multiple lines. Newline or
466 
semicolon characters do not terminate the current command in such
467 
cases. <SPAN
468 
CLASS="PROPERTY"
469 
>description</SPAN
470 
> properties usually make use of this:</P
471 
><TABLE
472 
BORDER="5"
473 
BGCOLOR="#E0E0F0"
474 
WIDTH="70%"
475 
><TR
476 
><TD
477 
><PRE
478 
CLASS="PROGRAMLISTING"
479 
>cdl_package CYGPKG_ERROR {
480 
    description   "
481 
        This package contains the common list of error and
482 
        status codes. It is held centrally to allow
483 
        packages to interchange error codes and status
484 
        codes in a common way, rather than each package
485 
        having its own conventions for error/status
486 
        reporting. The error codes are modelled on the
487 
        POSIX style naming e.g. EINVAL etc. This package
488 
        also provides the standard strerror() function to
489 
        convert error codes to textual representation."
490 
    &#8230;
491 
}</PRE
492 
></TD
493 
></TR
494 
></TABLE
495 
><P
496 
>The <SPAN
497 
CLASS="APPLICATION"
498 
>Tcl</SPAN
499 
> interpreter supports much the same forms of backslash
500 
substitution as other common programming languages. Some backslash
501 
sequences such as <TT
502 
CLASS="LITERAL"
503 
>\n</TT
504 
> will be replaced by the
505 
appropriate character. The sequence <TT
506 
CLASS="LITERAL"
507 
>\\</TT
508 
> will be
509 
replaced by a single backslash. A backslash at the very end of a line
510 
will cause that backslash, the newline character, and any white space
511 
at the start of the next line to be replaced by a single space. Hence
512 
the following two Tcl commands are equivalent:</P
513 
><TABLE
514 
BORDER="5"
515 
BGCOLOR="#E0E0F0"
516 
WIDTH="70%"
517 
><TR
518 
><TD
519 
><PRE
520 
CLASS="PROGRAMLISTING"
521 
>puts  "Hello\nworld\n"
522 
puts \
523 
"Hello
524 
world
525 
"</PRE
526 
></TD
527 
></TR
528 
></TABLE
529 
><P
530 
>If a <SPAN
531 
CLASS="PROPERTY"
532 
>description</SPAN
533 
> string needs to contain quote marks or other
534 
special characters then backslash escapes can be used. In addition to
535 
quote and backslash characters, the Tcl interpreter treats square
536 
brackets, the <TT
537 
CLASS="LITERAL"
538 
>$</TT
539 
> character, and braces specially.
540 
Square brackets are used for command substitution, for example:</P
541 
><TABLE
542 
BORDER="5"
543 
BGCOLOR="#E0E0F0"
544 
WIDTH="70%"
545 
><TR
546 
><TD
547 
><PRE
548 
CLASS="SCREEN"
549 
>puts "The answer is [expr 6 * 9]"</PRE
550 
></TD
551 
></TR
552 
></TABLE
553 
><P
554 
>When the Tcl interpreter encounters the square brackets it will treat
555 
the contents as another command that should be executed first, and the
556 
result of executing that is used when continuing to process the
557 
script. In this case the Tcl interpreter will execute the command
558 
<TT
559 
CLASS="LITERAL"
560 
>expr 6 * 9</TT
561 
>, yielding a result of 42
562 
<A
563 
NAME="AEN1270"
564 
HREF="#FTN.AEN1270"
565 
>[1]</A
566 
>
567 
and then the
568 
Tcl interpreter will execute <TT
569 
CLASS="LITERAL"
570 
>puts "The answer is 42"</TT
571 
>.
572 
It should be noted that the interpreter performs only one level
573 
of substitution: if the result of performing command substitution
574 
performs further special characters such as square brackets then these
575 
will not be treated specially.</P
576 
><P
577 
>Command substitution will not prove useful for many <SPAN
578 
CLASS="APPLICATION"
579 
>CDL</SPAN
580 
> scripts,
581 
except for e.g. a <SPAN
582 
CLASS="PROPERTY"
583 
>define_proc</SPAN
584 
> property which involves a fragment of
585 
<SPAN
586 
CLASS="APPLICATION"
587 
>Tcl</SPAN
588 
> code. Potentially there are some interesting uses, for example
589 
to internationalize <SPAN
590 
CLASS="PROPERTY"
591 
>display</SPAN
592 
> strings. However care does have to be
593 
taken to avoid unexpected command substitution, for example if an
594 
option description involves square brackets then typically these would
595 
require backslash-escapes.</P
596 
><P
597 
>The <TT
598 
CLASS="LITERAL"
599 
>$</TT
600 
> character is used in Tcl scripts to perform
601 
variable substitution:</P
602 
><TABLE
603 
BORDER="5"
604 
BGCOLOR="#E0E0F0"
605 
WIDTH="70%"
606 
><TR
607 
><TD
608 
><PRE
609 
CLASS="PROGRAMLISTING"
610 
>set x [expr 6 * 9]
611 
puts "The answer is $x"</PRE
612 
></TD
613 
></TR
614 
></TABLE
615 
><P
616 
>Variable substitution, like command substitution, is unlikely to
617 
prove useful for many <SPAN
618 
CLASS="APPLICATION"
619 
>CDL</SPAN
620 
> scripts except in the context of
621 
<SPAN
622 
CLASS="APPLICATION"
623 
>Tcl</SPAN
624 
> fragments. If it is necessary to have a <TT
625 
CLASS="LITERAL"
626 
>$</TT
627 
>
628 
character then a backslash escape may have to be used.</P
629 
><P
630 
>Braces are used to collect a sequence of characters into a single
631 
argument, just like quotes. The difference is that variable, command
632 
and backslash substitution do not occur inside braces (with the
633 
sole exception of backslash substitution at the end of a line).
634 
Therefore given a line in a <SPAN
635 
CLASS="APPLICATION"
636 
>CDL</SPAN
637 
> script such as:</P
638 
><TABLE
639 
BORDER="5"
640 
BGCOLOR="#E0E0F0"
641 
WIDTH="70%"
642 
><TR
643 
><TD
644 
><PRE
645 
CLASS="PROGRAMLISTING"
646 
>default_value {"RAM"}</PRE
647 
></TD
648 
></TR
649 
></TABLE
650 
><P
651 
>The braces are stripped off by the <SPAN
652 
CLASS="APPLICATION"
653 
>Tcl</SPAN
654 
> interpreter, leaving
655 
<TT
656 
CLASS="LITERAL"
657 
>"RAM"</TT
658 
> which will be handled as a string constant by
659 
the expression parsing code. The same effect could be achieved using
660 
one of the following:</P
661 
><TABLE
662 
BORDER="5"
663 
BGCOLOR="#E0E0F0"
664 
WIDTH="70%"
665 
><TR
666 
><TD
667 
><PRE
668 
CLASS="PROGRAMLISTING"
669 
>default_value \"RAM\"
670 
default_value "\"RAM\""</PRE
671 
></TD
672 
></TR
673 
></TABLE
674 
><P
675 
>Generally the use of braces is less confusing. At this stage it is
676 
worth noting that the basic format of <SPAN
677 
CLASS="APPLICATION"
678 
>CDL</SPAN
679 
> data makes use of
680 
braces:</P
681 
><TABLE
682 
BORDER="5"
683 
BGCOLOR="#E0E0F0"
684 
WIDTH="70%"
685 
><TR
686 
><TD
687 
><PRE
688 
CLASS="PROGRAMLISTING"
689 
>cdl_option &lt;name&gt; {
690 
     &#8230;
691 
};</PRE
692 
></TD
693 
></TR
694 
></TABLE
695 
><P
696 
>The <TT
697 
CLASS="LITERAL"
698 
>cdl_option</TT
699 
> command is passed two arguments, a name and a body,
700 
where the body consists of everything inside the braces but not the
701 
braces themselves. This body can then be executed in a recursive
702 
invocation of the <SPAN
703 
CLASS="APPLICATION"
704 
>Tcl</SPAN
705 
> interpreter. If a <SPAN
706 
CLASS="APPLICATION"
707 
>CDL</SPAN
708 
> script contains
709 
mismatched braces then the interpreter is likely to get rather
710 
confused and the resulting diagnostics may be difficult to understand. </P
711 
><P
712 
>Comments in Tcl scripts are introduced by a hash character
713 
<TT
714 
CLASS="LITERAL"
715 
>#</TT
716 
>. However, a hash character only introduces a
717 
comment if it occurs where a command is expected. Consider the
718 
following:</P
719 
><TABLE
720 
BORDER="5"
721 
BGCOLOR="#E0E0F0"
722 
WIDTH="70%"
723 
><TR
724 
><TD
725 
><PRE
726 
CLASS="PROGRAMLISTING"
727 
># This is a comment
728 
puts "Hello" # world</PRE
729 
></TD
730 
></TR
731 
></TABLE
732 
><P
733 
>The first line is a valid comment, since the hash character occurs
734 
right at the start where a command name is expected. The second line
735 
does not contain a comment. Instead it is an attempt to invoke the
736 
<TT
737 
CLASS="LITERAL"
738 
>puts</TT
739 
> command with three arguments:
740 
<TT
741 
CLASS="LITERAL"
742 
>Hello</TT
743 
>, <TT
744 
CLASS="LITERAL"
745 
>#</TT
746 
> and
747 
<TT
748 
CLASS="LITERAL"
749 
>world</TT
750 
>. These are not valid arguments for the
751 
<TT
752 
CLASS="LITERAL"
753 
>puts</TT
754 
> command so an error will be raised.
755 
If the second line was rewritten as:</P
756 
><TABLE
757 
BORDER="5"
758 
BGCOLOR="#E0E0F0"
759 
WIDTH="70%"
760 
><TR
761 
><TD
762 
><PRE
763 
CLASS="PROGRAMLISTING"
764 
>puts "Hello"; # world</PRE
765 
></TD
766 
></TR
767 
></TABLE
768 
><P
769 
>then this is a valid Tcl script. The semicolon identifies the end of
770 
the current command, so the hash character occurs at a point where the
771 
next command would start and hence it is interpreted as the start of a
772 
comment.</P
773 
><P
774 
>This handling of comments can lead to subtle behavior. Consider the
775 
following:</P
776 
><TABLE
777 
BORDER="5"
778 
BGCOLOR="#E0E0F0"
779 
WIDTH="70%"
780 
><TR
781 
><TD
782 
><PRE
783 
CLASS="PROGRAMLISTING"
784 
>cdl_option WHATEVER {
785 
# This is a comment }
786 
    default_value 0
787 
    &#8230;
788 
}</PRE
789 
></TD
790 
></TR
791 
></TABLE
792 
><P
793 
>Consider the way the Tcl interpreter processes this. The command name
794 
and the first argument do not pose any special difficulties. The
795 
opening brace is interpreted as the start of the next argument, which
796 
continues until a closing brace is encountered. In this case the
797 
closing brace occurs on the second line, so the second argument passed
798 
to <TT
799 
CLASS="LITERAL"
800 
>cdl_option</TT
801 
> is
802 
<TT
803 
CLASS="LITERAL"
804 
>\n&nbsp;&nbsp;&nbsp;&nbsp;#&nbsp;This&nbsp;is&nbsp;a&nbsp;comment</TT
805 
>. This second argument is processed in a recursive
806 
invocation of the Tcl interpreter and does not contain any commands,
807 
just a comment. Top-level script processing then resumes, and the next
808 
command that is encountered is <TT
809 
CLASS="LITERAL"
810 
>default_value</TT
811 
>. Since
812 
the parser is not currently processing a configuration option this is
813 
an error. Later on the Tcl interpreter would encounter a closing brace
814 
by itself, which is also an error.</P
815 
><P
816 
>For component writers who need more information about <SPAN
817 
CLASS="APPLICATION"
818 
>Tcl</SPAN
819 
>,
820 
especially about the language rather than the syntax, various
821 
resources are available. A reasonable starting point is the
822 
<A
823 
HREF="http://www.tcl.tk/scripting/"
824 
TARGET="_top"
825 
>Scriptics developer
826 
web site</A
827 
>.</P
828 
></DIV
829 
><H3
830 
CLASS="FOOTNOTES"
831 
>Notes</H3
832 
><TABLE
833 
BORDER="0"
834 
CLASS="FOOTNOTES"
835 
WIDTH="100%"
836 
><TR
837 
><TD
838 
ALIGN="LEFT"
839 
VALIGN="TOP"
840 
WIDTH="5%"
841 
><A
842 
NAME="FTN.AEN1270"
843 
HREF="language.tcl.html#AEN1270"
844 
>[1]</A
845 
></TD
846 
><TD
847 
ALIGN="LEFT"
848 
VALIGN="TOP"
849 
WIDTH="95%"
850 
><P
851 
>It is possible that some versions of the Tcl interpreter will instead
852 
produce a result of 54 when asked to multiply six by nine. Appropriate
853 
<A
854 
HREF="http://www.douglasadams.com/creations/hhgg.html"
855 
TARGET="_top"
856 
>reference
857 
documentation</A
858 
> should be consulted for more information on why
859 
42 is in fact the correct answer.</P
860 
></TD
861 
></TR
862 
></TABLE
863 
><DIV
864 
CLASS="NAVFOOTER"
865 
><HR
866 
ALIGN="LEFT"
867 
WIDTH="100%"><TABLE
868 
SUMMARY="Footer navigation table"
869 
WIDTH="100%"
870 
BORDER="0"
871 
CELLPADDING="0"
872 
CELLSPACING="0"
873 
><TR
874 
><TD
875 
WIDTH="33%"
876 
ALIGN="left"
877 
VALIGN="top"
878 
><A
879 
HREF="language.naming.html"
880 
ACCESSKEY="P"
881 
>Prev</A
882 
></TD
883 
><TD
884 
WIDTH="34%"
885 
ALIGN="center"
886 
VALIGN="top"
887 
><A
888 
HREF="cdl-guide.html"
889 
ACCESSKEY="H"
890 
>Home</A
891 
></TD
892 
><TD
893 
WIDTH="33%"
894 
ALIGN="right"
895 
VALIGN="top"
896 
><A
897 
HREF="language.values.html"
898 
ACCESSKEY="N"
899 
>Next</A
900 
></TD
901 
></TR
902 
><TR
903 
><TD
904 
WIDTH="33%"
905 
ALIGN="left"
906 
VALIGN="top"
907 
>Option Naming Convention</TD
908 
><TD
909 
WIDTH="34%"
910 
ALIGN="center"
911 
VALIGN="top"
912 
><A
913 
HREF="language.html"
914 
ACCESSKEY="U"
915 
>Up</A
916 
></TD
917 
><TD
918 
WIDTH="33%"
919 
ALIGN="right"
920 
VALIGN="top"
921 
>Values and Expressions</TD
922 
></TR
923 
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924 
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925 
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926 
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927 
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