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><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
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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
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><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
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></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
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