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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="bbv2.reference.buildprocess"></a>Build process</h2></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="buildprocess.html#bbv2.reference.buildprocess.alternatives">Alternative selection</a></span></dt>
<dt><span class="section"><a href="buildprocess.html#bbv2.reference.buildprocess.common">Determining common properties</a></span></dt>
</dl></div>
<p>The general overview of the build process was given in the
<a href="../advanced/build_process.html" title="The Build Process">user documentation</a>.
This section provides additional details, and some specific rules.
</p>
<p>To recap, building a target with specific properties includes the
following steps:
</p>
<div class="orderedlist"><ol type="1">
<li><p>applying default build,</p></li>
<li><p>selecting the main target alternative to use,
</p></li>
<li><p>determining "common" properties,</p></li>
<li><p>building targets referred by the sources list and
dependency properties,</p></li>
<li><p>adding the usage requirements produces when building
dependencies to the "common" properties,</p></li>
<li><p>building the target using generators,</p></li>
<li><p>computing the usage requirements to be returned.</p></li>
</ol></div>
<p>
</p>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="bbv2.reference.buildprocess.alternatives"></a>Alternative selection</h3></div></div></div>
<p>When there are several alternatives, one of them must be
selected. The process is as follows:</p>
<div class="orderedlist"><ol type="1">
<li>
For each alternative <span class="emphasis"><em>condition</em></span> is defined
as the set of base properies in requirements. [Note: it might be
better to specify the condition explicitly, as in
conditional requirements].
</li>
<li>
An alternative is viable only if all properties in condition
are present in build request.
</li>
<li>
If there's one viable alternative, it's choosen. Otherwise,
an attempt is made to find one best alternative. An alternative
a is better than another alternative b, iff the set of properties
in b's condition is a strict subset of the set of properities of
'a's condition. If there's one viable alternative, which is
better than all others, it's selected. Otherwise, an error is
reported.
</li>
</ol></div>
</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="bbv2.reference.buildprocess.common"></a>Determining common properties</h3></div></div></div>
<p>The "common" properties is a somewhat artificial term. Those are
the intermediate property set from which both the build request for
dependencies and properties for building the target are derived.
</p>
<p>Since default build and alternatives are already handled, we have
only two inputs: build requests and requirements. Here are the rules
about common properties.
</p>
<div class="orderedlist"><ol type="1">
<li><p>Non-free feature can have only one
value</p></li>
<li><p>A non-conditional property in requirement in always
present in common properties.</p></li>
<li><p>A property in build request is present in
common properties, unless (2) tells otherwise.</p></li>
<li><p>If either build request, or requirements (non-conditional
or conditional) include an expandable property (either composite,
or property with specified subfeature value), the behaviour is
equivalent to explicitly adding all expanded properties to build
request or requirements.</p></li>
<li><p>If requirements include a conditional property, and
condiiton of this property is true in context of common
properties, then the conditional property should be in common
properties as well.</p></li>
<li><p>If no value for a feature is given by other rules
here, it has default value in common properties.</p></li>
</ol></div>
<p>Those rules are declarative, they don't specify how to compute the
common properties. However, they provide enough information for the
user. The important point is the handling of conditional
requirements. The condition can be satisfied either by property in
build request, by non-conditional requirements, or even by another
conditional property. For example, the following example works as
expected:
</p>
<pre class="programlisting">
exe a : a.cpp
: <toolset>gcc:<variant>release
<variant>release:<define>FOO ;
</pre>
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