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34<div class="section" lang="en">
35<div class="titlepage"><div><div><h2 class="title" style="clear: both">
36<a name="bbv2.reference.buildprocess"></a>Build process</h2></div></div></div>
37<div class="toc"><dl>
38<dt><span class="section"><a href="buildprocess.html#bbv2.reference.buildprocess.alternatives">Alternative selection</a></span></dt>
39<dt><span class="section"><a href="buildprocess.html#bbv2.reference.buildprocess.common">Determining common properties</a></span></dt>
40</dl></div>
41<p>The general overview of the build process was given in the
42      <a href="../advanced/build_process.html" title="The Build Process">user documentation</a>.
43      This section provides additional details, and some specific rules.
44    </p>
45<p>To recap, building a target with specific properties includes the
46      following steps:
47      </p>
48<div class="orderedlist"><ol type="1">
49<li><p>applying default build,</p></li>
50<li><p>selecting the main target alternative to use,
51          </p></li>
52<li><p>determining "common" properties</p></li>
53<li><p>building targets referred by the sources list and
54            dependency properties</p></li>
55<li><p>adding the usage requirements produces when building
56            dependencies to the "common" properties</p></li>
57<li><p>building the target using generators</p></li>
58<li><p>computing the usage requirements to be returned</p></li>
59</ol></div>
60<div class="section" lang="en">
61<div class="titlepage"><div><div><h3 class="title">
62<a name="bbv2.reference.buildprocess.alternatives"></a>Alternative selection</h3></div></div></div>
63<p>When there are several alternatives, one of them must be
64        selected. The process is as follows:</p>
65<div class="orderedlist"><ol type="1">
66<li>
67            For each alternative <span class="emphasis"><em>condition</em></span> is defined
68            as the set of base properies in requirements. [Note: it might be
69            better to specify the condition explicitly, as in
70            conditional requirements].
71          </li>
72<li>
73            An alternative is viable only if all properties in condition
74            are present in build request.
75          </li>
76<li>
77            If there's one viable alternative, it's choosen. Otherwise,
78            an attempt is made to find one best alternative. An alternative
79            a is better than another alternative b, iff set of properties
80            in b's condition is strict subset of the set of properities of
81            'a's condition. If there's one viable alternative, which is
82            better than all other, it's selected. Otherwise, an error is
83            reported.
84          </li>
85</ol></div>
86</div>
87<div class="section" lang="en">
88<div class="titlepage"><div><div><h3 class="title">
89<a name="bbv2.reference.buildprocess.common"></a>Determining common properties</h3></div></div></div>
90<p>The "common" properties is a somewhat artificial term. Those are
91        the intermediate property set from which both the build request for
92        dependencies and properties for building the target are derived.
93      </p>
94<p>Since default build and alternatives are already handled, we have
95        only two inputs: build requests and requirements. Here are the rules
96        about common properties.
97      </p>
98<div class="orderedlist"><ol type="1">
99<li><p>Non-free feature can have only one
100            value</p></li>
101<li><p>A non-conditional property in requirement in always
102            present in common properties.</p></li>
103<li><p>A property in build request is present in
104            common properties, unless (2) tells otherwise.</p></li>
105<li><p>If either build request, or requirements (non-conditional
106            or conditional) include an expandable property (either composite,
107            or property with specified subfeature value), the behaviour is
108            equivalent to explicitly adding all expanded properties to build
109            request or requirements.</p></li>
110<li><p>If requirements include a conditional property, and
111            condiiton of this property is true in context of common
112            properties, then the conditional property should be in common
113            properties as well.</p></li>
114<li><p>If no value for a feature is given by other rules
115            here, it has default value in common properties.</p></li>
116</ol></div>
117<p>Those rules are declarative, they don't specify how to compute the
118        common properties. However, they provide enough information for the
119        user. The important point is the handling of conditional
120        requirements. The condition can be satisfied either by property in
121        build request, by non-conditional requirements, or even by another
122        conditional property. For example, the following example works as
123        expected:
124</p>
125<pre class="programlisting">
126exe a : a.cpp
127      : &lt;toolset&gt;gcc:&lt;variant&gt;release
128        &lt;variant&gt;release:&lt;define&gt;FOO ;
129</pre>
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