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><H1
><A
NAME="REF.DEFAULT-VALUE"><SPAN
CLASS="PROPERTY"
>default_value</SPAN
></H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN3887"
></A
><H2
>Name</H2
>Property <SPAN
CLASS="PROPERTY"
>default_value</SPAN
> -- Provide a default value for this option using a CDL expression.</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN3891"><H2
>Synopsis</H2
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="SYNOPSIS"
>cdl_option <name> {
default_value <expression>
…
}</PRE
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN3893"
></A
><H2
>Description</H2
><P
>The <SPAN
CLASS="PROPERTY"
>default_value</SPAN
> property allows the initial value of a
configuration option to depend on other configuration options. The
arguments to the property should be a <SPAN
CLASS="APPLICATION"
>CDL</SPAN
> expression, see
<A
HREF="language.values.html#LANGUAGE.EXPRESSION"
>the Section called <I
>Ordinary Expressions</I
> in Chapter 3</A
> for the syntactic details.
In many cases a simple constant value will suffice, for example:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>cdl_component CYGPKG_KERNEL_EXCEPTIONS {
…
default_value 1
cdl_option CYGSEM_KERNEL_EXCEPTIONS_DECODE {
…
default_value 0
}
}</PRE
></TD
></TR
></TABLE
><P
>However it is also possible for an option's default value to depend on
other options. For example the common HAL package provides some
support functions that are needed by the <SPAN
CLASS="APPLICATION"
>eCos</SPAN
> kernel, but are
unlikely to be useful if the kernel is not being used. This
relationship can be expressed using:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>cdl_option CYGFUN_HAL_COMMON_KERNEL_SUPPORT {
...
default_value CYGPKG_KERNEL
}</PRE
></TD
></TR
></TABLE
><P
>If the kernel is loaded then this HAL option is automatically enabled,
although the user can still disable it explicitly should this prove
necessary. If the kernel is not loaded then the option is disabled,
although it can still be enabled by the user if desired.
<SPAN
CLASS="PROPERTY"
>default_value</SPAN
> expressions can be more complicated than this if
appropriate, and provide a very powerful facility for component
writers who want their code to “just do the right thing”
in a wide variety of configurations.</P
><P
>The <SPAN
CLASS="APPLICATION"
>CDL</SPAN
> configuration system evaluates the <SPAN
CLASS="PROPERTY"
>default_value</SPAN
>
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"
>default_value</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 by default, 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 default value of the option is the result of evaluating the
expression.</P
></DD
></DL
></DIV
><P
>A <TT
CLASS="LITERAL"
>cdl_option</TT
> or other entity can have at most one <SPAN
CLASS="PROPERTY"
>default_value</SPAN
>
property, and it is illegal to have both a <SPAN
CLASS="PROPERTY"
>calculated</SPAN
> and a
<SPAN
CLASS="PROPERTY"
>default_value</SPAN
> property in one body. If an option does not have
either a <SPAN
CLASS="PROPERTY"
>default_value</SPAN
> or a <SPAN
CLASS="PROPERTY"
>calculated</SPAN
> property and it does not
have the flavor <TT
CLASS="LITERAL"
>none</TT
> then the configuration tools
will assume a default value expression of <TT
CLASS="LITERAL"
>0</TT
>.</P
><P
>On occasion it is useful to have a configuration option
<TT
CLASS="LITERAL"
>A</TT
> which has both a <SPAN
CLASS="PROPERTY"
>requires</SPAN
> constraint on some
other option <TT
CLASS="LITERAL"
>B</TT
> and a <SPAN
CLASS="PROPERTY"
>default_value</SPAN
> expression of
<TT
CLASS="LITERAL"
>B</TT
>. If option <TT
CLASS="LITERAL"
>B</TT
> is not enabled
then <TT
CLASS="LITERAL"
>A</TT
> will also be disabled by default and no
conflict arises. If <TT
CLASS="LITERAL"
>B</TT
> is enabled then
<TT
CLASS="LITERAL"
>A</TT
> also becomes enabled and again no conflict
arises. If a user attempts to enable <TT
CLASS="LITERAL"
>B</TT
> but not
<TT
CLASS="LITERAL"
>A</TT
> then there will be a conflict. Users should be
able to deduce that the two options are closely interlinked and should
not be manipulated independently except in very unusual circumstances.</P
><DIV
CLASS="TIP"
><BLOCKQUOTE
CLASS="TIP"
><P
><B
>Tip: </B
>If the first entry in a <SPAN
CLASS="PROPERTY"
>default_value</SPAN
> expression is a negative
number, for example <TT
CLASS="LITERAL"
>default_value -1</TT
> then this
can be misinterpreted as an option instead of as part of the
expression. Currently the <SPAN
CLASS="PROPERTY"
>default_value</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"
>default_value -- -1</TT
></P
></BLOCKQUOTE
></DIV
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>In many cases it would be useful to calculate default values using
some global preferences, for example:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>cdl_option CYGIMP_LIBC_STRING_PREFER_SMALL_TO_FAST {
…
default_value CYGGLO_CODESIZE > CYGGLO_SPEED
}</PRE
></TD
></TR
></TABLE
><P
>Such global preference options do not yet exist, but are likely to be
added in a future version.</P
></BLOCKQUOTE
></DIV
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>For options with the booldata flavor the current syntax does not allow
the default values of the enabled flag and the value to be controlled
separately. Functionality to permit this may be added in a future
release.</P
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN3964"
></A
><H2
>Example</H2
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>cdl_option CYGDBG_HAL_DEBUG_GDB_THREAD_SUPPORT {
display "Include GDB multi-threading debug support"
requires CYGDBG_KERNEL_DEBUG_GDB_THREAD_SUPPORT
default_value CYGDBG_KERNEL_DEBUG_GDB_THREAD_SUPPORT
description "
This option enables some extra HAL code which is needed
to support multi-threaded source level debugging."
}</PRE
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN3967"
></A
><H2
>See Also</H2
><P
>Properties <A
HREF="ref.calculated.html"
><SPAN
CLASS="PROPERTY"
>calculated</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
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