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>The <SPAN
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> Component Writer's Guide</TH
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><H1
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><A
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NAME="REF.ACTIVE-IF"><SPAN
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CLASS="PROPERTY"
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>active_if</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="AEN3662"
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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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>active_if</SPAN
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>&nbsp;--&nbsp;Allow additional control over the active state of an
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option or other CDL entity.</DIV
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><DIV
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CLASS="REFSYNOPSISDIV"
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><A
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NAME="AEN3666"><H2
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>Synopsis</H2
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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="SYNOPSIS"
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>cdl_option &lt;name&gt; {
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    active_if &lt;condition&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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></TABLE
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="AEN3668"
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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 or other entities may be either active or
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inactive. Typically this is controlled by the option's location in the
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overall hierarchy. Consider the option
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<TT
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CLASS="LITERAL"
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>CYGDBG_INFRA_DEBUG_PRECONDITIONS</TT
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>, which exists
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below the component <TT
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CLASS="LITERAL"
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>CYGDBG_USE_ASSERT</TT
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>. If the whole
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component is disabled then the options it contains are inactive: there
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is no point in enabling preconditions unless there is generic
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assertion support; any <SPAN
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CLASS="PROPERTY"
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>requires</SPAN
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> constraints associated with
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preconditions are irrelevant; any <SPAN
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CLASS="PROPERTY"
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>compile</SPAN
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> property or other
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build-related property is ignored.</P
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><P
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>In some cases the hierarchy does not provide sufficient control over
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whether or not a particular option should be active. For example, the
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math library could have support for floating point exceptions which
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is only worthwhile if the hardware implements appropriate
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functionality, as specified by the architectural HAL. The relevant
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math library configuration options should remain below the
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<TT
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CLASS="LITERAL"
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>CYGPKG_LIBM</TT
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> package in the overall hierarchy, but
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should be inactive unless there is appropriate hardware support. In
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cases like this an <SPAN
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CLASS="PROPERTY"
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>active_if</SPAN
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> property is appropriate.</P
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><P
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>Another common use of <SPAN
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CLASS="PROPERTY"
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>active_if</SPAN
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> properties is to avoid excessive
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nesting in the configuration hierarchy. If some option B is only
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relevant if option A is enabled, it is possible to turn A into a
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component that contains B. However adding another level to the
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hierarchy for a component which will contain just one entry may be
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considered excessive. In such cases it is possible for B to have an
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<SPAN
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CLASS="PROPERTY"
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>active_if</SPAN
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> dependency on A.</P
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><P
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><SPAN
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CLASS="PROPERTY"
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>active_if</SPAN
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> takes a goal expression as argument. For details of goal
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expression syntax see <A
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HREF="language.values.html#LANGUAGE.GOAL-EXPRESSION"
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>the Section called <I
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>Goal Expressions</I
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> in Chapter 3</A
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>. In
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most cases the goal expression will be very simple, often involving
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just one other option, but more complicated expressions can be used
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when appropriate. It is also possible to have multiple <SPAN
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CLASS="PROPERTY"
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>active_if</SPAN
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>
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conditions in a single option, in which case all of the conditions
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have to be satisfied if the option is to be active.</P
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><P
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>The <SPAN
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CLASS="PROPERTY"
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>active_if</SPAN
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> and <SPAN
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CLASS="PROPERTY"
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>requires</SPAN
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> properties have certain similarities,
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but they serve a different purpose. Suppose there are two options A
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and B, and option B relies on functionality provided by A. This could
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be expressed as either <TT
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CLASS="LITERAL"
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>active_if&nbsp;A</TT
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> or as
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<TT
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CLASS="LITERAL"
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>requires&nbsp;A</TT
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>. The points to note are:</P
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><P
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></P
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><UL
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><LI
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><P
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>If <TT
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CLASS="LITERAL"
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>active_if&nbsp;A</TT
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> is used and A is disabled or
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inactive, then graphical tools will generally prevent any attempt at
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modifying B. For example the text for B could be grayed out, and the
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associated checkbutton (if B is a boolean option) would be disabled.
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If the user needs the functionality provided by option B then it is
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necessary to go to option A first and manipulate it appropriately.</P
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></LI
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><LI
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><P
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>If <TT
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CLASS="LITERAL"
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>requires&nbsp;A</TT
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> is used and A is disabled or
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inactive, graphical tools will still allow B to be manipulated and
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enabled. This would result in a new conflict which may get resolved
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automatically or which may need user intervention.</P
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></LI
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><LI
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><P
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>If there are hardware dependencies then an <SPAN
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CLASS="PROPERTY"
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>active_if</SPAN
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> condition is
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usually the preferred approach. There is no point in allowing the user
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to manipulate a configuration option if the corresponding
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functionality cannot possibly work on the currently-selected hardware.
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Much the same argument applies to coarse-grained dependencies, for
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example if an option depends on the presence of a TCP/IP stack then an
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<TT
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CLASS="LITERAL"
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>active_if&nbsp;CYGPKG_NET</TT
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> condition is appropriate:
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it may be possible to satisfy the condition, but it requires the
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fairly drastic step of loading another package; further more, if the
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user wanted a TCP/IP stack in the configuration then it would probably
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have been loaded already. </P
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></LI
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><LI
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><P
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>If option B exists to provide additional debugging information about
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the functionality provided by A then again an <SPAN
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CLASS="PROPERTY"
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>active_if</SPAN
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> constraint
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is appropriate. There is no point in letting users enable extra
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debugging facilities for a feature that is not actually present.</P
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></LI
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><LI
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><P
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>The configuration system's inference engine will cope equally well
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with <SPAN
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CLASS="PROPERTY"
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>active_if</SPAN
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> and <SPAN
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CLASS="PROPERTY"
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>requires</SPAN
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> properties. Suppose there is a
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conflict because some third option depends on B. If B is
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<TT
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CLASS="LITERAL"
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>active_if&nbsp;A</TT
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> then the inference engine will
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attempt to make A active and enabled, and then to enable B if
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necessary. If B <TT
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CLASS="LITERAL"
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>requires&nbsp;A</TT
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> then the inference
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engine will attempt to enable B and resolve the resulting conflict by
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causing A to be both active and enabled. Although the inference occurs
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in a different order, in most cases the effect will be the same.</P
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></LI
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></UL
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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><H2
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>Example</H2
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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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># Do not provide extra semaphore debugging if there are no semaphores
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cdl_option CYGDBG_KERNEL_INSTRUMENT_BINSEM {
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    active_if CYGPKG_KERNEL_SYNCH
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    &#8230;
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}
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# Avoid another level in the configuration hierarchy
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cdl_option CYGSEM_KERNEL_SYNCH_MUTEX_PRIORITY_INHERITANCE_SIMPLE_RELAY {
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    active_if CYGSEM_KERNEL_SYNCH_MUTEX_PRIORITY_INHERITANCE_SIMPLE
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    &#8230;
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}
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# Functionality that is only relevant if another package is loaded
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cdl_option CYGSEM_START_UITRON_COMPATIBILITY {
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    active_if CYGPKG_UITRON
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    &#8230;
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}
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# Check that the hardware or HAL provide the appropriate functionality
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cdl_option CYGDBG_HAL_DEBUG_GDB_BREAK_SUPPORT {
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    active_if CYGINT_HAL_DEBUG_GDB_STUBS_BREAK
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    &#8230;
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}</PRE
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></TD
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></TR
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><A
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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.requires.html"
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><SPAN
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CLASS="PROPERTY"
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>requires</SPAN
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></A
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>.</P
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