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<!-- Copyright (C) 2003 Red Hat, Inc. -->
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<!-- and conditions set forth in the Open Publication License, v1.0 -->
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<!-- standard (paper) book form is prohibited unless prior -->
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<!-- permission is obtained from the copyright holder. -->
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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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CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+
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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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REL="UP"
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TITLE="The CDL Language"
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HREF="language.html"><LINK
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REL="PREVIOUS"
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TITLE="Option Naming Convention"
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HREF="language.naming.html"><LINK
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REL="NEXT"
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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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><DIV
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CLASS="NAVHEADER"
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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
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> Component Writer's Guide</TH
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></TR
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><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"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Chapter 3. The CDL Language</TD
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><TD
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WIDTH="10%"
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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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WIDTH="100%"></DIV
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><DIV
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CLASS="SECT1"
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><H1
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CLASS="SECT1"
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><A
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NAME="LANGUAGE.TCL">An Introduction to Tcl</H1
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><P
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>All <SPAN
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CLASS="APPLICATION"
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>CDL</SPAN
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> scripts are implemented as <SPAN
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CLASS="APPLICATION"
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>Tcl</SPAN
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> scripts, and are read in by
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running the data through a standard <SPAN
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CLASS="APPLICATION"
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>Tcl</SPAN
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> interpreter, extended with a
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small number of additional commands such as
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<TT
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CLASS="LITERAL"
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>cdl_option</TT
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> and <TT
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CLASS="LITERAL"
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>cdl_component</TT
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>.
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Often it is not necessary to know the full details of <SPAN
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CLASS="APPLICATION"
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>Tcl</SPAN
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> syntax.
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Instead it is possible to copy an existing script, perform some copy
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and paste operations, and make appropriate changes to names and to
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various properties. However there are also cases where an
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understanding of <SPAN
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CLASS="APPLICATION"
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>Tcl</SPAN
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> syntax is very desirable, for example:</P
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><TABLE
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BORDER="5"
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BGCOLOR="#E0E0F0"
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WIDTH="70%"
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>cdl_option CYGDAT_UITRON_MEMPOOLFIXED_EXTERNS {
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display "Externs for initialization"
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flavor data
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default_value {"static char fpool1[ 2000 ], \\\n\
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fpool2[ 2000 ], \\\n\
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fpool3[ 2000 ];"}
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…
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}</PRE
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></TD
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></TR
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></TABLE
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><P
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>This causes the <TT
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CLASS="LITERAL"
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>cdl_option</TT
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> command to be executed, which in turn
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evaluates its body in a recursive invocation of the <SPAN
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CLASS="APPLICATION"
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>Tcl</SPAN
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> interpreter.
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When the <SPAN
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CLASS="PROPERTY"
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>default_value</SPAN
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> property is encountered the braces around the
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value part are processed by the interpreter, stopping it from doing
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further processing of the braced contents (except for backslash
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processing at the end of a line, that is special). In particular it
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prevents command substitution for
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<TT
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CLASS="LITERAL"
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>[ 2000 ]</TT
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>. A single argument will be
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passed to the <SPAN
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CLASS="PROPERTY"
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>default_value</SPAN
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> command which expects a <SPAN
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CLASS="APPLICATION"
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>CDL</SPAN
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>
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expression, so the expression parsing code is passed the following:</P
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><TABLE
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BORDER="5"
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BGCOLOR="#E0E0F0"
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WIDTH="70%"
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><TR
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><TD
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><PRE
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CLASS="SCREEN"
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>"static char fpool1[ 2000 ], \\\n fpool2[ 2000 ], \\\n fpool3[ 2000 ];"</PRE
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></TD
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></TR
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></TABLE
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><P
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>The <SPAN
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CLASS="APPLICATION"
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>CDL</SPAN
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> expression parsing code will treat this as a simple string
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constant, as opposed to a more complicated expression involving other
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options and various operators. The string parsing code will perform
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the usual backslash substitutions so the actual default value will be:</P
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><TABLE
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BORDER="5"
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BGCOLOR="#E0E0F0"
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WIDTH="70%"
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><TR
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><TD
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><PRE
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CLASS="SCREEN"
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>static char fpool1[ 2000 ], \
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fpool2[ 2000 ], \
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fpool3[ 2000 ];</PRE
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></TD
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></TR
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></TABLE
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><P
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>If the user does not modify the option's value then the following
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will be generated in the appropriate configuration header file:</P
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><TABLE
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BORDER="5"
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BGCOLOR="#E0E0F0"
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WIDTH="70%"
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>#define CYGDAT_UITRON_MEMPOOLFIXED_EXTERNS static char fpool1[ 2000 ], \
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fpool2[ 2000 ], \
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fpool3[ 2000 ];</PRE
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></TD
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></TR
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></TABLE
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><P
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>Getting this desired result usually requires an understanding of both
|
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<SPAN
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CLASS="APPLICATION"
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>Tcl</SPAN
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> syntax and <SPAN
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CLASS="APPLICATION"
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>CDL</SPAN
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> expression syntax. Sometimes it is possible to
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substitute a certain amount of trial and error instead, but this may
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prove frustrating. It is also worth pointing out that many <SPAN
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CLASS="APPLICATION"
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>CDL</SPAN
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>
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scripts do not involve this level of complexity. On the other hand,
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some of the more advanced features of the <SPAN
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CLASS="APPLICATION"
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>CDL</SPAN
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> language involve
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fragments of <SPAN
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CLASS="APPLICATION"
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>Tcl</SPAN
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> code, for example the <SPAN
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CLASS="PROPERTY"
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>define_proc</SPAN
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> property. To
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use these component writers will need to know about the full <SPAN
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CLASS="APPLICATION"
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>Tcl</SPAN
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>
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language as well as the syntax.</P
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><P
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>Although the current example may seem to suggest that <SPAN
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CLASS="APPLICATION"
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>Tcl</SPAN
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> is rather
|
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complicated, it is actually a very simple yet powerful scripting
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language: the syntax is defined by just eleven rules. On occasion this
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simplicity means that Tcl's behavior is subtly different from other
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languages, which can confuse newcomers.</P
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><P
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>When the Tcl interpreter is passed some data such as
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<TT
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CLASS="LITERAL"
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>puts Hello</TT
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>, it splits this data into a command
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and its arguments. The command will be terminated by a newline or by a
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semicolon, unless one of the quoting mechanisms is used. The command
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and each of its arguments are separated by white space. So in the
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following example:</P
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><TABLE
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BORDER="5"
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BGCOLOR="#E0E0F0"
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WIDTH="70%"
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><TR
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><TD
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><PRE
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CLASS="SCREEN"
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>puts Hello
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set x 42</PRE
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></TD
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></TR
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></TABLE
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><P
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>This will result in two separate commands being executed. The first
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command is <TT
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CLASS="LITERAL"
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>puts</TT
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> and is passed a single argument,
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<TT
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CLASS="LITERAL"
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>Hello</TT
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>. The second command is <TT
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CLASS="LITERAL"
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>set</TT
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>
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and is passed two arguments, <TT
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CLASS="LITERAL"
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>x</TT
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> and
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<TT
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CLASS="LITERAL"
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>42</TT
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>. The intervening newline character serves to
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terminate the first command, and a semi-colon separator could be used
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instead: </P
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><TABLE
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BORDER="5"
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BGCOLOR="#E0E0F0"
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WIDTH="70%"
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><TR
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><TD
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><PRE
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CLASS="SCREEN"
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>puts Hello;set x 42</PRE
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></TD
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></TR
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></TABLE
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><P
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>Any white space surrounding the semicolon is just ignored because it
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does not serve to separate arguments.</P
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><P
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>Now consider the following:</P
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><TABLE
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BORDER="5"
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BGCOLOR="#E0E0F0"
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WIDTH="70%"
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><TR
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><TD
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><PRE
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CLASS="SCREEN"
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>set x Hello world</PRE
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></TD
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></TR
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></TABLE
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><P
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>This is not valid <SPAN
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CLASS="APPLICATION"
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>Tcl</SPAN
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|
>. It is an attempt to invoke the
|
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<TT
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CLASS="LITERAL"
|
| 341 |
|
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>set</TT
|
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|
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> command with three arguments:
|
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|
<TT
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|
CLASS="LITERAL"
|
| 345 |
|
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>x</TT
|
| 346 |
|
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>, <TT
|
| 347 |
|
|
CLASS="LITERAL"
|
| 348 |
|
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>Hello</TT
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| 349 |
|
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>, and
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| 350 |
|
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<TT
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| 351 |
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CLASS="LITERAL"
|
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|
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>world</TT
|
| 353 |
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|
>. 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 |
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BORDER="5"
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| 361 |
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BGCOLOR="#E0E0F0"
|
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WIDTH="70%"
|
| 363 |
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><TR
|
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><TD
|
| 365 |
|
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><PRE
|
| 366 |
|
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CLASS="SCREEN"
|
| 367 |
|
|
>set x "Hello world"</PRE
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></TD
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| 369 |
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></TR
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></TABLE
|
| 371 |
|
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><P
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| 372 |
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>When the <SPAN
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|
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 |
|
|
…
|
| 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 { "RAM" }</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 |
|
|
…
|
| 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 <name> {
|
| 690 |
|
|
…
|
| 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 |
|
|
…
|
| 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 # This is a 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 |
|
|
></TABLE
|
| 924 |
|
|
></DIV
|
| 925 |
|
|
></BODY
|
| 926 |
|
|
></HTML
|
| 927 |
|
|
>
|
