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>requires</TITLE
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>The <SPAN
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>eCos</SPAN
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> Component Writer's Guide</TH
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><H1
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NAME="REF.REQUIRES"><SPAN
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CLASS="PROPERTY"
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>requires</SPAN
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></H1
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><DIV
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CLASS="REFNAMEDIV"
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><A
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NAME="AEN4921"
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></A
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><H2
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>Name</H2
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>Property <SPAN
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CLASS="PROPERTY"
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>requires</SPAN
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>&nbsp;--&nbsp;List constraints that the configuration should satisfy if
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a given option is active and enabled..</DIV
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><DIV
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CLASS="REFSYNOPSISDIV"
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><A
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NAME="AEN4925"><H2
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>Synopsis</H2
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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="SYNOPSIS"
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>cdl_option &lt;name&gt; {
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    requires &lt;goal expression&gt;
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    &#8230;
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}</PRE
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></TD
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></TR
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="AEN4927"
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></A
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><H2
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>Description</H2
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><P
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>Configuration options are not independent. For example the C library
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can provide thread-safe implementations of certain functions, but only
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if the kernel is present, if the kernel provides multi-threading, and
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if the kernel options related to per-thread data are enabled. It is
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possible to express such constraints using <SPAN
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CLASS="PROPERTY"
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>requires</SPAN
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> properties.</P
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><P
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>The arguments to a <SPAN
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CLASS="PROPERTY"
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>requires</SPAN
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> property should constitute a goal
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expression, as described in <A
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HREF="language.values.html#LANGUAGE.LIST-EXPRESSION"
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>the Section called <I
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>List Expressions</I
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> in Chapter 3</A
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>.
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Most goal expressions are relatively simple because the constraints
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being described are simple, but complicated expressions can be used
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when necessary. The body of an option or other CDL entity can contain
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any number of <SPAN
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CLASS="PROPERTY"
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>requires</SPAN
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> constraints. If the option is active and
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enabled then all these constraints should be satisfied, and any goal
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expressions which evaluate to <TT
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CLASS="LITERAL"
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>0</TT
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> will result in
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conflicts being raised. It is possible for users to ignore such
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conflicts and attempt to build the current configuration anyway, but
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there is no guarantee that anything will work. If an option is
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inactive or disabled then its <SPAN
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CLASS="PROPERTY"
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>requires</SPAN
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> constraints will be ignored. </P
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><P
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>The configuration system contains an inference engine which can
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resolve many types of conflicts automatically. For example, if option
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<TT
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CLASS="LITERAL"
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>A</TT
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> is enabled and requires an option
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<TT
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CLASS="LITERAL"
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>B</TT
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> that is currently disabled then the inference
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engine may attempt to resolve the conflict by enabling
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<TT
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CLASS="LITERAL"
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>B</TT
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>. However this will not always be possible, for
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example there may be other constraints in the configuration which
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force <TT
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CLASS="LITERAL"
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>B</TT
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> to be disabled at present, in which case
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user intervention is required.</P
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></DIV
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><DIV
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NAME="AEN4942"
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></A
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>Example</H2
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><TR
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><PRE
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CLASS="PROGRAMLISTING"
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>cdl_component CYGPKG_IO_SERIAL_POWERPC_COGENT_SERIAL_A {
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    display       "Cogent PowerPC serial port A driver"
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    flavor        bool
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    default_value 0
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    requires      (CYGIMP_KERNEL_INTERRUPTS_CHAIN || \
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                   !CYGPKG_IO_SERIAL_POWERPC_COGENT_SERIAL_B)
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    &#8230;
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}</PRE
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></TD
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="AEN4945"
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></A
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><H2
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>See Also</H2
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><P
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>Property <A
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HREF="ref.active-if.html"
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><SPAN
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CLASS="PROPERTY"
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>active_if</SPAN
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></A
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>.</P
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