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<p><a href="../../index.htm"><img class="banner" height="86" width="277"
alt="C++ Boost" src="../../c++boost.gif"></a></p>
<h1>Boost.Build v2 user manual<br class="clear">
</h1>
<hr>
<div class="alert">
This is a preliminary version intended to document everything
implemeneted but not yet ready for any practical use.
</div>
<br>
<br>
<hr>
<dl class="page-index">
<dt><a href="#sec-reference">Reference documentation</a></dt>
<dd>
<dl class="page-index">
<dt><a href="#features_properties">Features and properties</a></dt>
<dt><a href="#initialization">Initialization</a></dt>
<dt><a href="#command_line">Command line</a></dt>
<dt><a href="#projects">Projects</a></dt>
<dt><a href="#targets">Targets</a></dt>
<dt><a href="#build_process">Build process</a></dt>
</dl>
</dd>
</dl>
<hr>
<h2><a name="sec-reference">Reference</a></h2>
<h3><a name="features_properties">Features and properties</a></h3>
<h4>Definition</h4>
<p>A <em>feature</em> is a normalized (toolset-independent) aspect of a
build configuration, such as whether inlining is enabled.</p>
<p>A <em>feature value</em> is a specific available setting for a
feature.</p>
<p>A <em>property</em> is a (feature,value) pair, expressed as
&lt;feature&gt;value.</p>
<p>A <em>subfeature</em> is a feature which only exists in the presence
of its parent, and whose identity can be derived (in the context of its
parent) from the name of its value.</p>
<p>A <em>value-string</em> is a string of the form
<tt>value-subvalue1-subvalue2...-subvalueN</tt>, where <tt>value</tt> is
a feature value and <tt>subvalue1...subvalueN</tt> are values of related
subfeatures. For example, by using value-string properties
&lt;toolset&gt;gcc &lt;toolset-version&gt;3.0.1, can be expressed more
conscicely, as &lt;toolset&gt;gcc-3.0.1.</p>
<p>A set of properties is called, naturally, <em>property set</em>. For
example, <tt>&lt;toolset&gt;gcc &lt;runtime-link&gt;static</tt>.
Sometimes it's better to represent a property set without spaces. In that
case <em>property path</em> is used, which consists of all properties,
joined with slashes. To continue with example, property path
representation would be
<tt>&lt;toolset&gt;gcc/&lt;runtime-link&gt;static</tt> .</p>
<p>Each feature can have any of the following attributes:</p>
<ul>
<li>
<em>incidental</em>
<p>Incidental features are assumed not affect build products at all.
Warning level is one example. As a consequence, the build system
doesn't try to avoid confusing targets with different values of
incidental properties.</p>
<p>Features which are not incidental are assumed to affect build
products, therefore they are put in different directories as
described in <a href="#target_paths">target paths</a> below.</p>
</li>
<li>
<em>propagated</em>
<p>Features of this kind are propagated to dependencies. That is, if
a <a href="#main_target">main target</a> is build with a particular
value of a propagated feature, and depends on some other main
targets, the build systems attempts to use subvariants of those main
targets with the same value of the feature. For instance, when an
optimized exectuable is requested, one usually wants it to be linked
with optimized libraries. Thus, the &lt;optimization&gt; feature is
propagated.</p>
</li>
<li>
<em>free</em>
<p>Usually, each feature takes a single value from a fixed set. In
particular, this means that only one value can be used when building
a single target. When a feature is free, it can have several values
at a time and each value can be an arbitrary string. For example, it
is possible to have several preprocessor symbols defined
simultaneously:</p>
<pre>
&lt;define&gt;NDEBUG=1 &lt;define&gt;HAS_CONFIG_H=1
</pre>
<br>
<br>
</li>
<li>
<em>optional</em>
<p>An optional feature is not required to have a value during build.
When a value of non-optional non-free feature is not specified, it is
always given the default value &mdash; which is the first value
listed in the feature's declaration.</p>
</li>
<li>
<em>symmetric</em>
<p>A symmetric feature's default value is not automatically included
in <a href="#variants">build variants</a>. Normally a feature only
generates a subvariant directory when its value differs from the
value specified by the build variant, leading to an assymmetric
subvariant directory structure for certain values of the feature. A
symmetric feature, when relevant to the toolset, always generates a
corresponding subvariant directory.</p>
</li>
<li>
<em>path</em>
<p>The value of a path feature specifies a path. The path is treated
as relative to the directory of Jamfile where path feature is used
and is translated appropriately by the build system when the build is
invoked from a different directory</p>
</li>
<li>
<em>implicit</em>
<p>Values of implicit features alone identify the feature. For
example, a user is not required to write "&lt;toolset&gt;", but can
simply write "gcc". Implicit feature names also don't appear in
variant paths, although the values do. Thus: bin/gcc/... as opposed
to bin/toolset-gcc/.... There should typically be only a few such
features, to avoid possible name clashes.</p>
</li>
<li>
<em>composite</em>
<p>Composite features actually correspond to groups of properties.
For example, a build variant is a composite feature. When generating
targets from a set of build properties, composite features are
recursively expanded and <em>added</em> to the build property set, so
rules can find them if neccessary. Non-composite non-free features
override components of composite features in a build property
set.</p>
</li>
<li>
<em>link-incompatible</em>
<p>See <a href="#link_compatibility">below</a>.</p>
</li>
<li>
<em>dependency</em>
<div class="alert">
Do we need it?
</div>
</li>
</ul>
<p>TODO: document active features..</p>
<h4 id="link_compatibility">Link compatible and incompatible
properties</h4>
<p>When the build system tries to generate a target (such as library
dependency) matching a given build request, it may find that an exact
match isn't possible &mdash; for example, the target may impose additonal
build requirements. We need to determine whether a buildable version of
that target can actually be used.</p>
<p>The build request can originate in many ways: It may come directly
from the user's command-line, from a dependency of a main target upon a
library, or from a dependency of a target upon an executable used to
build that target, for example. For each way, there are different rules
whether we can use a given subvariant or not. However we currently only
assume linking and therefore use a simple approach described in the
following paragraph.</p>
<p>In general, there are many possible situations: a libary which is
dependency of a main target and should be linked into it, target which is
directly requested on the command line, or build executable which is used
in the build process itself. At this moment we use a simple approach.</p>
<p>Two property sets are called <em>link-compatible</em> when targets
with those property sets can be used interchangably. In turn, two
property sets are link compatible when there's no link-incompatible
feature which has different values in those property sets. Whenever
requested and actual properties are link-compatible, it's OK. Otherwise,
it's an error.</p>
<h4>Definition of property refinement</h4>
<p>When a target with certain properties is requested, and that target
requires some set of properties, it is needed to find the set of
properties to use for building. This process is called <em>property
refinement</em> and is performed by these rules</p>
<ol>
<li>If original properties and required properties are not
link-compatible, refinement fails.</li>
<li>Each property in the required set is added to the original property
set</li>
<li>If the original property set includes property with a different
value of non free feature, that property is removed.</li>
</ol>
<h3><a name="initialization">Initialization</a></h3>
<p>bjam's first job upon startup is to load the Jam code which implements
the build system. To do this, it searches for a file called
"boost-build.jam", first in the invocation directory, then in its parent
and so forth up to the filesystem root, and finally in the directories
specified by the environment variable BOOST_BUILD_PATH. When found, the
file is interpreted, and should specify the build system location by
calling the boost-build rule:</p>
<pre>
rule boost-build ( location ? )
</pre>
If location is a relative path, it is treated as relative to the
directory of boost-build.jam. The directory specified by location and
directories in BOOST_BUILD_PATH are then searched for a file called
bootstrap.jam which is interpreted and is expected to bootstrap the build
system. This arrangement allows the build system to work without any
command-line or environment variable settings. For example, if the build
system files were located in a directory "build-system/" at your project
root, you might place a boost-build.jam at the project root containing:
<pre>
boost-build build-system ;
</pre>
In this case, running bjam anywhere in the project tree will
automatically find the build system.
<h3><a name="command_line">Command line</a></h3>
<p>The comamnd line may contain:</p>
<ul>
<li>Jam options,</li>
<li>Boost.Build <a href="#command_line_options">options</a>,</li>
<li>Command line arguments</li>
</ul>
<h4 id="command_line_arguments">Command line arguments</h4>
Command line arguments specify targets and build request using the
following rules.
<ul>
<li>An argument which does not contain slashes or the "=" symbol is
either a value of an implicit feature, or target to be built. It is
taken to be value of a feature if appropriate feature exists.
Otherwise, it is considered a <a href="#target_id">target id</a>.</li>
<li>
An argument with either slashes or the "=" symbol specifies a number
of <a href="#build_request">build request</a> elements. properties.
In the simplest form, it's just a set of properties, separated by
slashes, which become a single build request element, for example:
<pre>
borland/&lt;runtime-link&gt;static
</pre>
More complex form is used to save typing. For example, instead of
<pre>
borland/runtime-link=static borland/runtime-link=dynamic
</pre>
one can use
<pre>
borland/runtime-link=static,dynamic
</pre>
Exactly, the convertion from argument to build request elements is
performed by (1) splitting the argument at each slash, (2) converting
each split part into a set of properties and (3) taking all possible
combination of the property sets. Each split part should have the
either the form
<pre>
<em>feature-name</em>=<em>feature-value1</em>[","<em>feature-valueN</em>]*
</pre>
or, in case of implict feature
<pre>
<em>feature-value1</em>[","<em>feature-valueN</em>;]*
</pre>
and will be converted into property set
<pre>
&lt;feature-name&gt;feature-value1 .... &lt;feature-name&gt;feature-valueN
</pre>
</li>
</ul>
For example, the command line
<pre>
target1 debug gcc/runtime-link=dynamic,static
</pre>
would cause target called <tt>target1</tt> to be rebuild in debug mode,
except that for gcc, both dynamically and statically linked binaries
would be created.
<h4 id="command_line_options">Command line options</h4>
<p>All of the Boost.Build options start with the "--" prefix. They are
described in the following table.</p>
<table align="center">
<caption>
Command line options
</caption>
<thead>
<tr>
<th>Option</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td><tt>--debug</tt></td>
<td>Enables internal checks.</td>
</tr>
<tr>
<td><tt>--dump-projects</tt></td>
<td>Cause the project structure to be output.</td>
</tr>
<tr>
<td><tt>--help</tt></td>
<td>Access to the online help system. This prints general
information on how to use the help system with additional --help*
options.</td>
</tr>
</tbody>
</table>
<h3><a name="projects">Projects</a></h3>
<p>Boost.Build considers every software it build as organized into
projects, each of which corresponds to a single Jamfile. Projects are
organized in a hierarchical structure, so each project may have a single
parent project and a number of subprojects. (TODO: project root).</p>
<h4>Project attributes</h4>
<p>For each project, there are several attributes.</p>
<p><em>Project id</em> is a short way to denote a project, as opposed to
the Jamfile's pathname. It is a hierarchical path, unrelated to
filesystem, such as "boost/thread". There are two ways to refer to a
project using project-id:</p>
<ul>
<li>Use absolute proejct-id, which starts with "/", for example
<tt>/boost/thread</tt>.</li>
<li>Use relative project-id, which is appended to the project-id of the
project where it is used.</li>
</ul>
<p><em>Source location</em> specifies the directory where sources for the
project are located.</p>
<p><em>Project requirements</em> are requirements that apply to all the
targets in the projects as well as all subprojects.</p>
<p><em>Default build</em> is the build request that should be used when
no build request is specified explicitly.</p>
<p>The default values for those attributes are given in the table below.
In order to affect them, Jamfile may call the <tt>project</tt> rule. The
rule has this syntax:</p>
<pre>
project id : &lt;attributes&gt; ;
</pre>
Here, attributes is a sequence of (attribute-name, attribute-value)
pairs. The list of attribute names along with its handling is shown in
the table below. For example, it it possible to write:
<pre>
project tennis
: requirements &lt;threading&gt;multi
: default-build release
;
</pre>
<table>
<tr>
<th>Attribute</th>
<th>Name for the 'project' rule</th>
<th>Default value</th>
<th>Handling by the 'project' rule</th>
</tr>
<tr>
<td>Project id</td>
<td>none</td>
<td>none</td>
<td>Assigned from the first parameter of the 'project' rule. It is
assumed to denote absolute project id.</td>
</tr>
<tr>
<td>Source location</td>
<td><tt>source-location</tt></td>
<td>The location of jamfile for the project</td>
<td>Sets to the passed value</td>
</tr>
<tr>
<td>Requirements</td>
<td><tt>requirements</tt></td>
<td>The parent's requirements</td>
<td>The parent's requirements are refined with the passed requirement
and the result is used as the project requirements.</td>
</tr>
<tr>
<td>Default build</td>
<td><tt>default-build</tt></td>
<td>TODO</td>
<td>Sets to the passed value</td>
</tr>
</table>
<h4>Project relationship</h4>
<p>There are three kinds of project relationships.</p>
<p>First is parent-child. This relationship is established implicitly:
parent directories of a project are searched, and the first found Jamfile
is assumed to define the parent project. The parent-child relationship
affects only attribute values for the child project.</p>
<p>Second is build relationship. Some project may request to recursively
build other projects. Those project need not be child projects. The
<tt>build-project</tt> rule is used for that:</p>
<pre>
build-project src ;
</pre>
<p>The third kind is the 'use' relationship. In means that one project
uses targets from another. It is possible to just refer to target in
other projects using target id. However, if target id uses project id, it
is required that the project id is known. The <tt>use-project</tt> rule
is employed to guarantee that.</p>
<pre>
use-project ( id : location )
</pre>
It loads the project at the specified location, which makes its project
id available in the project which invokes the rule. It is required that
the <tt>id</tt> parameter passed to the <tt>use-project</tt> rule be
equal to the id that the loaded project declared. At this moment, the
<tt>id</tt> paremeter should be absolute project id.
<h3><a name="targets">Targets</a></h3>
<h4>Main targets and main target alternatives</h4>
<p id="main_target"><em>Main target</em> is a named entity which can be
build, for example a named executable file. To declare a main target,
user invokes some of the <a href="#main_target_rules">main target
rules</a>, passing it things like list of source and requirement.</p>
<p>It is possible to have different list of sources for different
toolsets, therefore it is possible to invoke main target rules several
times for a single main target. For example:</p>
<pre>
exe a : a_gcc.cpp : &lt;toolset&gt; ;
exe a : a.cpp ;
</pre>
Each call to the 'exe' rule defines a new <em>main target
alternative</em> for the main target <tt>a.exe</tt>. In this case, the
first alternative will be used for the <tt>gcc</tt> toolset, while the
second alternative will be used in other cases. TODO: document the exact
selection method under "Build process" below.
<h4 id="target_id">Target identifiers and references</h4>
<p><em>Target identifier</em> is used to denote a target. It is described
by the following grammar:</p>
<pre>
target-id -&gt; project-reference local-target-name
project-reference -&gt; [jamfile-location] [ "@" [project-id] ]
jamfile-location -&gt; pathname
project-id -&gt; pathname
local-target-name -&gt; identifier
</pre>
For example, valid target ids might be:
<pre>
a
lib/b
@/boost/thread
/home/ghost/build/lr_library@parser/lalr1
</pre>
To map the target id into target, the project where that target is
contained is first found:
<ol>
<li>If <tt>project-reference</tt> is empty, then the current project is
used &mdash; i.e. the project there the target id occurs.</li>
<li>If the project id is absolute, the project with that id is
used.</li>
<li>If the project id is relative, it is treated relatively to
project-id of the project at <tt>jamfile-location</tt>. If that project
does not declare project id, it is an error.</li>
</ol>
After that, the target given by <tt>local-target-name</tt> in the found
project is used.
<p><em>Target reference</em> is used to specify a source target, and may
additionally specify desired properties for that target. It has this
syntax:</p>
<pre>
target-reference -&gt; target-id [ "/" requested-properties ]
requested-properties -&gt; property-path
</pre>
For example,
<pre>
exe compiler : compiler.cpp libs/cmdline/&lt;optimization&gt;space ;
</pre>
would cause the version of <tt>cmdline</tt> library, optimized for space,
to be linked in even if the <tt>compile</tt> executable is build with
optimization for speed.
<h5>Ambiguity resolution</h5>
<p>Target reference may have the same form as a pathname, for example
<tt>lib/a</tt>. In order to determine if this is target reference or
pathname, it is checked if there's a jamfile in the specified path. If
there is one, it is loaded and if the specified target is declared by
that project it is used. Otherwise, we just treat the target reference as
a file name.</p>
<h4>Target paths</h4>
<p>To distinguish targets build with different properties, they are put
in different directories. Rules for determining target paths are given
below:</p>
<ol>
<li>All targets are placed under directory corresponding to the project
where they are defined.</li>
<li>Each non free, non incidental property cause an additional element
to be added to the target path. That element has the form
<tt>&lt;feature-name&gt;-&lt;feature-value&gt;</tt> for ordinary
features and <tt>&lt;feature-value&gt;</tt> for implicit ones. [Note
about composite features].</li>
<li>If the set of free, non incidental properties is different from the
set of free, non incidental properties for the project in which the
main target that uses the target is defined, a part of the form
<tt>main_target-&lt;name&gt;</tt> is added to the target path.
<b>Note:</b>It would be nice to completely track free features also,
but this appears to be complex and not extremely needed.</li>
</ol>
<p>For example, we might have these paths:</p>
<pre>
debug/optimization-off
debug/main-target-a
</pre>
<h3><a name="build_process">Build process</a></h3>
<p>The build works in this way. On startup, the project in the current
directory is read. In turn, it may request building of other projects,
which will be loaded recursively. Parent projects are also loaded to
inherit some of their properties. As the result, a tree of projects is
constructed. After that, the build request is constructed from the
command line. Then, the steps are:</p>
<ol>
<li>All of the projects to be build are passed the build request. If
the build request does not satisfy the project's requirements, a
warning is issued and the build of the project is skipped. Projects
mentioned in <tt>build-project</tt> rule will still be build.</li>
<li>
An attempts to make all the main targets in the project is performed.
For each main target:
<ol>
<li>All main target alternatives which requirements are satisfied
by the build request are enumerated.</li>
<li>If there are several such alternatives, the one which longer
requirements list is selected.</li>
</ol>
</li>
<li>
For each selected alternative
<ol>
<li>Each target reference in the source list are recursively
constructed.</li>
<li>The dependency graph for the target is constructed by running
so called matching process, using generated source targets and
ordinary sources.</li>
</ol>
</li>
</ol>
<p>The dependency graph constructed for each target is build of so called
"virtual targets", which do not yet correspond to jam's targets. It is
therefore converted to jam's dependency graph which is then build.</p>
<hr>
<p class="revision">Last modified: Aug 15, 2002</p>
<p>&copy; Copyright Vladimir Prus 2002. Permission to copy, use, modify,
sell and distribute this document is granted provided this copyright
notice appears in all copies. This document is provided ``as is'' without
express or implied warranty, and with no claim as to its suitability for
any purpose.</p>
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