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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 >requires</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="parent" HREF="ref.parent.html"><LINK REL="NEXT" TITLE="script" HREF="ref.script.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.parent.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="ref.script.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><H1 ><A NAME="REF.REQUIRES"><SPAN CLASS="PROPERTY" >requires</SPAN ></H1 ><DIV CLASS="REFNAMEDIV" ><A NAME="AEN4921" ></A ><H2 >Name</H2 >Property <SPAN CLASS="PROPERTY" >requires</SPAN > -- List constraints that the configuration should satisfy if a given option is active and enabled..</DIV ><DIV CLASS="REFSYNOPSISDIV" ><A NAME="AEN4925"><H2 >Synopsis</H2 ><TABLE BORDER="5" BGCOLOR="#E0E0F0" WIDTH="70%" ><TR ><TD ><PRE CLASS="SYNOPSIS" >cdl_option <name> { requires <goal expression> … }</PRE ></TD ></TR ></TABLE ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN4927" ></A ><H2 >Description</H2 ><P >Configuration options are not independent. For example the C library can provide thread-safe implementations of certain functions, but only if the kernel is present, if the kernel provides multi-threading, and if the kernel options related to per-thread data are enabled. It is possible to express such constraints using <SPAN CLASS="PROPERTY" >requires</SPAN > properties.</P ><P >The arguments to a <SPAN CLASS="PROPERTY" >requires</SPAN > property should constitute a goal expression, as described in <A HREF="language.values.html#LANGUAGE.LIST-EXPRESSION" >the Section called <I >List Expressions</I > in Chapter 3</A >. Most goal expressions are relatively simple because the constraints being described are simple, but complicated expressions can be used when necessary. The body of an option or other CDL entity can contain any number of <SPAN CLASS="PROPERTY" >requires</SPAN > constraints. If the option is active and enabled then all these constraints should be satisfied, and any goal expressions which evaluate to <TT CLASS="LITERAL" >0</TT > will result in conflicts being raised. It is possible for users to ignore such conflicts and attempt to build the current configuration anyway, but there is no guarantee that anything will work. If an option is inactive or disabled then its <SPAN CLASS="PROPERTY" >requires</SPAN > constraints will be ignored. </P ><P >The configuration system contains an inference engine which can resolve many types of conflicts automatically. For example, if option <TT CLASS="LITERAL" >A</TT > is enabled and requires an option <TT CLASS="LITERAL" >B</TT > that is currently disabled then the inference engine may attempt to resolve the conflict by enabling <TT CLASS="LITERAL" >B</TT >. However this will not always be possible, for example there may be other constraints in the configuration which force <TT CLASS="LITERAL" >B</TT > to be disabled at present, in which case user intervention is required.</P ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN4942" ></A ><H2 >Example</H2 ><TABLE BORDER="5" BGCOLOR="#E0E0F0" WIDTH="70%" ><TR ><TD ><PRE CLASS="PROGRAMLISTING" >cdl_component CYGPKG_IO_SERIAL_POWERPC_COGENT_SERIAL_A { display "Cogent PowerPC serial port A driver" flavor bool default_value 0 requires (CYGIMP_KERNEL_INTERRUPTS_CHAIN || \ !CYGPKG_IO_SERIAL_POWERPC_COGENT_SERIAL_B) … }</PRE ></TD ></TR ></TABLE ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN4945" ></A ><H2 >See Also</H2 ><P >Property <A HREF="ref.active-if.html" ><SPAN CLASS="PROPERTY" >active_if</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.parent.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.script.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><SPAN CLASS="PROPERTY" >parent</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" >script</SPAN ></TD ></TR ></TABLE ></DIV ></BODY ></HTML >
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