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<!-- Copyright (C) 2003 Red Hat, Inc. --> <!-- This material may be distributed only subject to the terms --> <!-- and conditions set forth in the Open Publication License, v1.0 --> <!-- or later (the latest version is presently available at --> <!-- http://www.opencontent.org/openpub/). --> <!-- Distribution of the work or derivative of the work in any --> <!-- standard (paper) book form is prohibited unless prior --> <!-- permission is obtained from the copyright holder. --> <HTML ><HEAD ><TITLE >calculated</TITLE ><meta name="MSSmartTagsPreventParsing" content="TRUE"> <META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+ "><LINK REL="HOME" TITLE="The eCos Component Writer's Guide" HREF="cdl-guide.html"><LINK REL="UP" TITLE="CDL Language Specification" HREF="reference.html"><LINK REL="PREVIOUS" TITLE="active_if" HREF="ref.active-if.html"><LINK REL="NEXT" TITLE="compile" HREF="ref.compile.html"></HEAD ><BODY CLASS="REFENTRY" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" >The <SPAN CLASS="APPLICATION" >eCos</SPAN > Component Writer's Guide</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="ref.active-if.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="ref.compile.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><H1 ><A NAME="REF.CALCULATED"><SPAN CLASS="PROPERTY" >calculated</SPAN ></H1 ><DIV CLASS="REFNAMEDIV" ><A NAME="AEN3722" ></A ><H2 >Name</H2 >Property <SPAN CLASS="PROPERTY" >calculated</SPAN > -- Used if the current option's value is not user-modifiable, but is calculated using a suitable CDL expression.</DIV ><DIV CLASS="REFSYNOPSISDIV" ><A NAME="AEN3726"><H2 >Synopsis</H2 ><TABLE BORDER="5" BGCOLOR="#E0E0F0" WIDTH="70%" ><TR ><TD ><PRE CLASS="SYNOPSIS" >cdl_option <name> { calculated <expression> … }</PRE ></TD ></TR ></TABLE ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN3728" ></A ><H2 >Description</H2 ><P >In some cases it is useful to have a configuration option whose value cannot be modified directly by the user. This can be achieved using a <SPAN CLASS="PROPERTY" >calculated</SPAN >, which takes a CDL expression as argument (see <A HREF="language.values.html#LANGUAGE.EXPRESSION" >the Section called <I >Ordinary Expressions</I > in Chapter 3</A > for a description of expression syntax). The configuration system evaluates the expression when the current package is loaded and whenever there is a change to any other option referenced in the expression. The result depends on the option's flavor:</P ><P ></P ><DIV CLASS="VARIABLELIST" ><DL ><DT ><TT CLASS="LITERAL" >flavor none</TT ></DT ><DD ><P >Options with this flavor have no value, so the <SPAN CLASS="PROPERTY" >calculated</SPAN > property is not applicable.</P ></DD ><DT ><TT CLASS="LITERAL" >flavor bool</TT ></DT ><DD ><P >If the expression evaluates to a non-zero result the option is enabled, otherwise it is disabled.</P ></DD ><DT ><TT CLASS="LITERAL" >flavor booldata</TT ></DT ><DD ><P >If the result of evaluating the expression is zero then the option is disabled, otherwise the option is enabled and its value is the result.</P ></DD ><DT ><TT CLASS="LITERAL" >flavor data</TT ></DT ><DD ><P >The value of the option is the result of evaluating the expression.</P ></DD ></DL ></DIV ><P >There are a number of valid uses for calculated options, and there are also many cases where some other CDL facility would be more appropriate. Valid uses of calculated options include the following:</P ><P ></P ><UL ><LI ><P >On some target hardware a particular feature may be user-configurable, while on other targets it is fixed. For example some processors can operate in either big-endian or little-endian mode, while other processors do not provide any choice. It is possible to have an option <TT CLASS="LITERAL" >CYGARC_BIGENDIAN</TT > which is calculated in some architectural HAL packages but user-modifiable in others.</P ></LI ><LI ><P >Calculated options can provide an alternative way for one package to affect the behavior of another one. Suppose a package may provide two possible implementations, a preferred one involving self-modifying code and a slower alternative. If the system involves a ROM bootstrap then the slower alternative must be used, but it would be inappropriate to modify the startup option in every HAL to impose constraints on this package. Instead it is possible to have a calculated option whose value is <TT CLASS="LITERAL" >{ CYG_HAL_STARTUP == "ROM" }</TT >, and which has appropriate consequences. Arguably this is a spurious example, and it should be a user choice whether or not to use self-modifying code with a <SPAN CLASS="PROPERTY" >default_value</SPAN > based on <TT CLASS="LITERAL" >CYG_HAL_STARTUP</TT >, but that is for the component writer to decide.</P ></LI ><LI ><P >Sometimes it should be possible to perform a particular test at compile-time, for example by using a C preprocessor <TT CLASS="LITERAL" >#if</TT > construct. However the preprocessor has only limited functionality, for example it cannot perform string comparisons. CDL expressions are more powerful.</P ></LI ><LI ><P >Occasionally a particular sub-expression may occur multiple times in a CDL script. If the sub-expression is sufficiently complex then it may be worthwhile to have a calculated option whose value is the sub-expression, and then reference that calculated option in the appropriate places. </P ></LI ></UL ><P >Alternatives to using calculated options include the following:</P ><P ></P ><UL ><LI ><P >CDL <A HREF="language.interface.html" >interfaces</A > are a form of calculated option intended as an abstraction mechanism. An interface can be used to express the concept of <SPAN CLASS="emphasis" ><I CLASS="EMPHASIS" >any scheduler</I ></SPAN >, as opposed to a specific one such as the bitmap scheduler.</P ></LI ><LI ><P >If a calculated option would serve only to add additional information to a configuration header file, it may be possible to achieve the same effect using a <A HREF="ref.define-proc.html" ><SPAN CLASS="PROPERTY" >define_proc</SPAN ></A > property or one of the other properties related to header file generation.</P ></LI ></UL ><DIV CLASS="TIP" ><BLOCKQUOTE CLASS="TIP" ><P ><B >Tip: </B >If the first entry in a <SPAN CLASS="PROPERTY" >calculated</SPAN > expression is a negative number, for example <TT CLASS="LITERAL" >calculated -1</TT > then this can be misinterpreted as an option instead of as part of the expression. Currently the <SPAN CLASS="PROPERTY" >calculated</SPAN > property does not take any options, but this may change in future. Option processing halts at the sequence <TT CLASS="LITERAL" >--</TT >, so the desired value can be expressed safely using <TT CLASS="LITERAL" >calculated -- -1</TT ></P ></BLOCKQUOTE ></DIV ><DIV CLASS="WARNING" ><P ></P ><TABLE CLASS="WARNING" BORDER="1" WIDTH="100%" ><TR ><TD ALIGN="CENTER" ><B >Warning</B ></TD ></TR ><TR ><TD ALIGN="LEFT" ><P >Some of the CDL scripts in current <SPAN CLASS="APPLICATION" >eCos</SPAN > releases make excessive use of calculated options. This is partly because the recommended alternatives were not always available at the time the scripts were written. It is also partly because there is still some missing functionality, for example <SPAN CLASS="PROPERTY" >define_proc</SPAN > properties cannot yet access the configuration data so it may be necessary to use <SPAN CLASS="PROPERTY" >calculated</SPAN > properties to access the data and perform the desired manipulation via a <SPAN CLASS="APPLICATION" >CDL</SPAN > expression. New scripts should use calculated options only in accordance with the guidelines.</P ></TD ></TR ></TABLE ></DIV ><DIV CLASS="NOTE" ><BLOCKQUOTE CLASS="NOTE" ><P ><B >Note: </B >For options with the booldata flavor the current CDL syntax does not allow the enabled flag and the value to be calculated separately. Functionality to permit this may be added in a future release.</P ></BLOCKQUOTE ></DIV ><DIV CLASS="NOTE" ><BLOCKQUOTE CLASS="NOTE" ><P ><B >Note: </B >It has been suggested that having options which are not user-modifiable is potentially confusing, and that a top-level <TT CLASS="LITERAL" >cdl_constant</TT > command should be added to the language instead of or in addition to the <SPAN CLASS="PROPERTY" >calculated</SPAN > property. Such a change is under consideration. However because the value of a calculated option can depend on other options, it is not necessarily constant.</P ></BLOCKQUOTE ></DIV ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN3799" ></A ><H2 >Example</H2 ><TABLE BORDER="5" BGCOLOR="#E0E0F0" WIDTH="70%" ><TR ><TD ><PRE CLASS="PROGRAMLISTING" ># A constant on some target hardware, perhaps user-modifiable on other # targets. cdl_option CYGNUM_HAL_RTC_PERIOD { display "Real-time clock period" flavor data calculated 12500 }</PRE ></TD ></TR ></TABLE ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN3802" ></A ><H2 >See Also</H2 ><P >Properties <A HREF="ref.default-value.html" ><SPAN CLASS="PROPERTY" >default_value</SPAN ></A >, <A HREF="ref.flavor.html" ><SPAN CLASS="PROPERTY" >flavor</SPAN ></A > and <A HREF="ref.legal-values.html" ><SPAN CLASS="PROPERTY" >legal_values</SPAN ></A >,</P ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="ref.active-if.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="cdl-guide.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="ref.compile.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><SPAN CLASS="PROPERTY" >active_if</SPAN ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="reference.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><SPAN CLASS="PROPERTY" >compile</SPAN ></TD ></TR ></TABLE ></DIV ></BODY ></HTML >
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