doc/html/cdl-guide/ref.requires.html

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  83 ><H1
  84 ><A
  85 NAME="REF.REQUIRES"><SPAN
  86 CLASS="PROPERTY"
  87 >requires</SPAN
  88 ></H1
  89 ><DIV
  90 CLASS="REFNAMEDIV"
  91 ><A
  92 NAME="AEN4921"
  93 ></A
  94 ><H2
  95 >Name</H2
  96 >Property <SPAN
  97 CLASS="PROPERTY"
  98 >requires</SPAN
  99 >&nbsp;--&nbsp;List constraints that the configuration should satisfy if
 100 a given option is active and enabled..</DIV
 101 ><DIV
 102 CLASS="REFSYNOPSISDIV"
 103 ><A
 104 NAME="AEN4925"><H2
 105 >Synopsis</H2
 106 ><TABLE
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 111 ><TD
 112 ><PRE
 113 CLASS="SYNOPSIS"
 114 >cdl_option &lt;name&gt; {
 115     requires &lt;goal expression&gt;
 116     &#8230;
 117 }</PRE
 118 ></TD
 119 ></TR
 120 ></TABLE
 121 ></DIV
 122 ><DIV
 123 CLASS="REFSECT1"
 124 ><A
 125 NAME="AEN4927"
 126 ></A
 127 ><H2
 128 >Description</H2
 129 ><P
 130 >Configuration options are not independent. For example the C library
 131 can provide thread-safe implementations of certain functions, but only
 132 if the kernel is present, if the kernel provides multi-threading, and
 133 if the kernel options related to per-thread data are enabled. It is
 134 possible to express such constraints using <SPAN
 135 CLASS="PROPERTY"
 136 >requires</SPAN
 137 > properties.</P
 138 ><P
 139 >The arguments to a <SPAN
 140 CLASS="PROPERTY"
 141 >requires</SPAN
 142 > property should constitute a goal
 143 expression, as described in <A
 144 HREF="language.values.html#LANGUAGE.LIST-EXPRESSION"
 145 >the Section called <I
 146 >List Expressions</I
 147 > in Chapter 3</A
 148 >.
 149 Most goal expressions are relatively simple because the constraints
 150 being described are simple, but complicated expressions can be used
 151 when necessary. The body of an option or other CDL entity can contain
 152 any number of <SPAN
 153 CLASS="PROPERTY"
 154 >requires</SPAN
 155 > constraints. If the option is active and
 156 enabled then all these constraints should be satisfied, and any goal
 157 expressions which evaluate to <TT
 158 CLASS="LITERAL"
 159 >0</TT
 160 > will result in
 161 conflicts being raised. It is possible for users to ignore such
 162 conflicts and attempt to build the current configuration anyway, but
 163 there is no guarantee that anything will work. If an option is
 164 inactive or disabled then its <SPAN
 165 CLASS="PROPERTY"
 166 >requires</SPAN
 167 > constraints will be ignored. </P
 168 ><P
 169 >The configuration system contains an inference engine which can
 170 resolve many types of conflicts automatically. For example, if option
 171 <TT
 172 CLASS="LITERAL"
 173 >A</TT
 174 > is enabled and requires an option
 175 <TT
 176 CLASS="LITERAL"
 177 >B</TT
 178 > that is currently disabled then the inference
 179 engine may attempt to resolve the conflict by enabling
 180 <TT
 181 CLASS="LITERAL"
 182 >B</TT
 183 >. However this will not always be possible, for
 184 example there may be other constraints in the configuration which
 185 force <TT
 186 CLASS="LITERAL"
 187 >B</TT
 188 > to be disabled at present, in which case
 189 user intervention is required.</P
 190 ></DIV
 191 ><DIV
 192 CLASS="REFSECT1"
 193 ><A
 194 NAME="AEN4942"
 195 ></A
 196 ><H2
 197 >Example</H2
 198 ><TABLE
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 202 ><TR
 203 ><TD
 204 ><PRE
 205 CLASS="PROGRAMLISTING"
 206 >cdl_component CYGPKG_IO_SERIAL_POWERPC_COGENT_SERIAL_A {
 207     display       "Cogent PowerPC serial port A driver"
 208     flavor        bool
 209     default_value 0
 210     requires      (CYGIMP_KERNEL_INTERRUPTS_CHAIN || \
 211                    !CYGPKG_IO_SERIAL_POWERPC_COGENT_SERIAL_B)
 212     &#8230;
 213 }</PRE
 214 ></TD
 215 ></TR
 216 ></TABLE
 217 ></DIV
 218 ><DIV
 219 CLASS="REFSECT1"
 220 ><A
 221 NAME="AEN4945"
 222 ></A
 223 ><H2
 224 >See Also</H2
 225 ><P
 226 >Property <A
 227 HREF="ref.active-if.html"
 228 ><SPAN
 229 CLASS="PROPERTY"
 230 >active_if</SPAN
 231 ></A
 232 >.</P
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