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<title>The Build Process</title>
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<div class="section" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="bbv2.advanced.build_process"></a>The Build Process</h2></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="build_process.html#id1704308">Build request</a></span></dt>
<dt><span class="section"><a href="build_process.html#id1704350">Building a main target</a></span></dt>
<dt><span class="section"><a href="build_process.html#id1704448">Building a project</a></span></dt>
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<p>When you've described your targets, you want Boost.Build to run the
right tools and create the needed targets.
This section will describe
two things: how you specify what to build, and how the main targets are
actually constructed.
</p>
<p>The most important thing to note is that in Boost.Build, unlike
other build tools, the targets you declare do not correspond to specific
files. What you declare in a Jamfile is more like a “metatarget.”
Depending on the properties you specify on the command line,
each metatarget will produce a set of real targets corresponding
to the requested properties. It is quite possible that the same
metatarget is built several times with different properties,
producing different files.
</p>
<div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Tip</h3>
<p>
This means that for Boost.Build, you cannot directly obtain a build
variant from a Jamfile. There could be several variants requested by the
user, and each target can be built with different properties.
</p>
</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id1704308"></a>Build request</h3></div></div></div>
<p>
The command line specifies which targets to build and with which
properties. For example:
</p>
<pre class="programlisting">
bjam app1 lib1//lib1 toolset=gcc variant=debug optimization=full
</pre>
<p>
would build two targets, "app1" and "lib1//lib1" with the specified
properties. You can refer to any targets, using
<a href="../reference/definitions.html#bbv2.reference.ids" title="Target identifiers and references">target id</a> and specify arbitrary
properties. Some of the properties are very common, and for them the name
of the property can be omitted. For example, the above can be written as:
</p>
<pre class="programlisting">
bjam app1 lib1//lib1 gcc debug optimization=full
</pre>
<p>
The complete syntax, which has some additional shortcuts, is
described in <a href="../reference.html#bbv2.reference.commandline" title="Command line">the section called “Command line”</a>.
</p>
</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id1704350"></a>Building a main target</h3></div></div></div>
<p>When you request, directly or indirectly, a build of a main target
with specific requirements, the following steps are made. Some brief
explanation is provided, and more details are given in <a href="../reference/buildprocess.html" title="Build process">the section called “Build process”</a>.
</p>
<div class="orderedlist"><ol type="1">
<li><p>Applying default build. If the default-build
property of a target specifies a value of a feature that is not
present in the build request, that value is added.</p></li>
<li><p>Selecting the main target alternative to use. For
each alternative we look how many properties are present both in
alternative's requirements, and in build request. The
alternative with large number of matching properties is selected.
</p></li>
<li><p>Determining "common" properties.
The build request
is <a href="../reference/definitions.html#bbv2.reference.variants.proprefine" title="Property refinement">refined</a>
with target's requirements.
The conditional properties in
requirements are handled as well. Finally, default values of
features are added.
</p></li>
<li><p>Building targets referred by the sources list and
dependency properties. The list of sources and the properties
can refer to other target using <a href="../reference/definitions.html#bbv2.reference.ids" title="Target identifiers and references">target references</a>. For each
reference, we take all <a href="../reference/definitions.html#bbv2.reference.features.attributes.propagated">propagated</a>
properties, refine them by explicit properties specified in the
target reference, and pass the resulting properties as build
request to the other target.
</p></li>
<li><p>Adding the usage requirements produced when building
dependencies to the "common" properties. When dependencies are
built in the previous step, they return
both the set of created
"real" targets, and usage requirements. The usage requirements
are added to the common properties and the resulting property
set will be used for building the current target.
</p></li>
<li><p>Building the target using generators. To convert the
sources to the desired type, Boost.Build uses "generators" ---
objects that correspond to tools like compilers and
linkers. Each generator declares what type of targets it
can
produce and what type of sources it requires. Using this
information, Boost.Build determines which generators must be run
to produce a specific target from specific sources. When
generators are run, they return the "real" targets.
</p></li>
<li><p>Computing the usage requirements to be returned. The
conditional properties in usage requirements are expanded
and the
result is returned.</p></li>
</ol></div>
</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id1704448"></a>Building a project</h3></div></div></div>
<p>Often, a user builds a complete project, not just one main
target. In fact, invoking <span><strong class="command">bjam</strong></span> without
arguments
builds the project defined in the current
directory.</p>
<p>When a project is built, the build request is passed without
modification to all main targets in that project.
It's is possible to
prevent implicit building of a target in a project with the
<code class="computeroutput">explicit</code> rule:
</p>
<pre class="programlisting">
explicit hello_test ;
</pre>
<p>
would cause the <code class="computeroutput">hello_test</code> target to be built only if
explicitly requested by the user or by some other target.
</p>
<p>The Jamfile for a project can include a number of
<code class="computeroutput">build-project</code> rule calls
that specify additional projects
to be built.
</p>
</div>
</div>
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