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* boost_build_v2.html: Document new option.

* new/generators.jam (find-viable-generators): Revert part of Dave's
   commit, essentially disabling finding base type generators.
   This part breaks a test, and need to be thinked about.

* new/errors.jam: Handle "--no-error-backtrace" option.

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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 preliminary reference is intended to document everything currently
implemeneted, but is not yet ready for any practical use.
</div>
<br>
<br>
<hr>
<dl class="page-index">
<dt><a href="#sec-tutorial">Tutorial</a></dt>
<dd>
<dl class="page-index">
<dt><a href="#hello">Hello, world</a></dt>
<dt><a href="#properties">Properties</a></dt>
<dt><a href="#using_libraries">Using libraries</a></dt>
<dt><a href="#prebuilt_targets">Prebuilt targets</a></dt>
</dl>
</dd>
<dt><a href="#sec-reference">Reference documentation</a></dt>
<dd>
<dl class="page-index">
<dt><a href="#overview">Overview</a></dt>
<dt><a href="#features_properties">Features and properties</a></dt>
<dd>
<dl class="page-index">
<dt><a href="#features_defined">Defintions</a></dt>
<dt><a href="#feature_attributes">Feature Attributes</a></dt>
<dt><a href="#feature_declaration">Feature Declaration</a></dt>
</dl>
</dd>
<dt><a href="#variants">Build Variants</a></dt>
<dt><a href="#subfeatures">Subfeatures</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 id="installation">Installation</h2>
When using package management or installers, Boost.Build is ready to work
instantly. In other case, two steps are required:
<ol>
<li>Place the Boost.Jam binary, called "bjam", somewhere in your
<tt>PATH</tt>.</li>
<li>
Specify the location of Boost.Build files. You can either set
environmental variable <tt>BOOST_BUILD_PATH</tt>, or create, at the
top of your project, a file called <tt>boost-build.jam</tt>, with a
single line:
<pre>
boost-build /path/to/boost.build ;
</pre>
The directory "examples" in the distribution already has this file,
so you can build projects there and add new without doing anything.
</li>
</ol>
<p>To verify your installation, you can use <tt>bjam --version</tt> at
the command line.</p>
<h2><a name="sec-tutorial">Tutorial</a></h2>
<h3 id="hello">Hello, world</h3>
The simplest project that Boost.Build can construct is stored in
examples-v2/hello directory. Let's examine Jamfile:
<pre>
exe hello : hello.cpp ;
</pre>
With this simple setup, you can do some interesting things. First of all,
running "bjam" would build binary "hello" from hello.cpp, in debug
version. After that, you can run
<pre>
bjam release
</pre>
which would create release version of the 'hello' binary. Note that debug
and release version would be created in different directories, so they
both can be used. Let's extend the example by adding another line to
Jamfile:
<pre>
exe hello2 : hello.cpp ;
</pre>
You can now rebuild both debug and release versions:
<pre>
bjam debug release
</pre>
You'll see that two versions of "hello2" binary are linked. Of course,
hello.cpp won't be recompiled. Now you decide to remove any build
products. You do that with the following command
<pre>
bjam --clean debug release
</pre>
It should be noted that it's possible to create or clean only specific
targets. Both following commands are legal and create or clean only files
that belonging the the named binary:
<pre>
bjam hello2
bjam --clean hello2.
</pre>
<h3 id="properties">Properties</h3>
<p>Boost.Build attempts to allow building different variants of projects,
e.g. for debugging and release, or in single and multithread mode. In
order to stay portable, it uses the concept of <em>features</em>, which
is abstract aspect of build configuration. <em>Property</em> is just a
(feature,value) pair. For example, there's a feature "debug-symbols",
which can have a value of "on" or "off". When users asks to build project
is a particual value, Boost.Build will pass appropriate flags to the
compiler.</p>
<p>The "release" and "debug" in bjam invocation that we've seen are just
are short form of specifying values of feature "variant". There is a lot
of builtin features, and it's possible to write something like:</p>
<pre>
bjam release inlining=off debug-symbols=on
</pre>
The first command line element specified the value of feature "variant".
The feature is very common and is therefore special &mdash; it's possible
to specify only value. Another feature, "inlining" is not special, and
you should use
<pre>
feature-name=feature-value
</pre>
syntax for it. Complete description of features can be found <a href=
"features_properties">here</a>. While use may specify arbitrary
properties, it's not always possible to use exactly this set for building
the projects. For example, to compile a C++ file you need some include
paths. It's not reasonable to ask the user to specify this include path
with each bjam invocation. For another example, a certain application can
only be linked in multithreaded mode. For that reason, every target is
allowed to specify <em>requirements</em> -- properties that are required
to its building. Consider this example:
<pre>
exe hello
: hello.cpp
: &lt;include&gt;/home/ghost/Work/boost &lt;threading&gt;multi
</pre>
In this case, when hello is build, the two specified properties will be
present. This leads to a question: what if user explictly requested
single-threading. The answer is that requirement can affect build
properties only to a certain degree: the requested and actual properties
must be link-compatible. See <a href="link_compatibility">link
compatibility</a> below. If they are not link compatible, the bulding of
the target is skipped. Previously, we've added "hello2" target. Seems
like we have to specify the same requirements for it, which results in
duplication. But there's a better way. Each project (i.e. each Jamfile),
can specify a set of attributes, including requirements:
<pre>
project
: requirements &lt;include&gt;/home/ghost/Work/boost &gt;threading&gt;multi
;
exe hello : hello.cpp ;
exe hello2 : hello.cpp ;
</pre>
The effect would be as if we specified this requirement for both "hello"
and "hello2", but there's no duplication. QWERTY: Need to describe
project hierarchy here.
<h2><a name="sec-reference">Reference</a></h2>
This section will document mostly high-level view of Boost.Build,
mentioning appropriate modules and rules. The on-line help system must be
used to obtain low-level documentation (see the <a href=
"#help_option">help option</a>).
<h3 id="overview">Overview</h3>
<p>The most fundemental entity in Boost.Build is <em>main target</em>.
This is object that user want to construct from sources and keep up to
date with regard to those sources. Typical examples of main targets are
executable files and libraries.</p>
<p>Main targets are grouped in <em>projects</em>. They main purpose is
organization: related targets can be places in one project, then can be
build together, or share some definitions.</p>
<p>Main targets and project are described mostly in declarative fashion.
To make some use of them, user issues <a href="#build_request">build
request</a>, which specifies what targets user wants to build, and what
properties are desirable. Build request is not necessary explicit.
Invoking the build system without parameters will build the project in
current directory with defauly properties.</p>
<p>The <em>properties</em> describe various aspects of constructed
objects. For portability, they are specified in a normalized form, for
example</p>
<pre>
&lt;optimization&gt;full &lt;inlining&gt;off
</pre>
Depending on the compiler used, this will be translated into appropriate
flags.
<p>Construction of each main target begins with finding properties for
<em>this</em> main target. They are found by processing both build
request, and <em>target requirements</em>, which give properties needed
for the target to build. For example, a given main target might require
certian defines, or will not work unless compiled in multithreaded mode.
The process of finding properties for main target is described in <a
href="#property_refinement">property refinement</a>.</p>
<p>After that, dependencies (i.e. other main targets) are build
recursively. Build request for dependencies is not always equal to those
of dependent &mdash; certain properties are dropped and user can
explicitly specify desired properties for dependencies. See <a href=
"#propagated_features">propagated features</a> and <a href=
"#target_reference">target reference</a> for details.</p>
<p>When dependencies are constructed, the dependency graph for this main
target and for this property set is created, which describes which files
need to be created, on which other files they depend and what actions are
needed to construct those files. There's more that one method, and user
can define new ones, but usually, this involves <em>generators</em> and
<em>target type</em>.</p>
<p>Target type is just a way to classify targets. For example, there are
builtin types <tt>EXE</tt>, <tt>OBJ</tt> and <tt>CPP</tt>. <a href=
"#generators">Generators</a> are objects that know how to convert between
different target type. When a main target of a given type must be
created, all generators for that type, which can handle needed
properties, are found. Each is passed the list of sources, and either
fails, or returns a dependency graph. If a generator cannot produce
desired type from given sources, it may try to recursively construct
types that it can handle from type is was passed. This allows to try all
possible transformations. When all generators are tried, a dependency
graph is selected.</p>
<p>Finally, the dependency graph is passed to underlying Boost.Jam
program, which runs all actions needed to bring all main targets up-to
date. At this step, implicit dependencies are also scanned and accounted
for, as described <a href="#dependency_scanning">here</a>.</p>
<h3><a name="features_properties">Features and properties</a></h3>
<h4><a name="features_defined">Definitions</a></h4>
<p>A <em>feature</em> is a normalized (toolset-independent) aspect of a
build configuration, such as whether inlining is enabled. Feature names
may not contain the '<tt>&gt;</tt>' character.</p>
<p>Each feature in a build configuration has one or more associated
<em>value</em>s. Feature values may not contain the '<tt>&lt;</tt>',
'<tt>:</tt>', or '<tt>=</tt>' characters.</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 feature, and whose identity can be derived (in the context
of its parent) from its value.</p>
<p>A <em>value-string</em> is a string of the form
<tt>value-subvalue1-subvalue2</tt>...<tt>-subvalueN</tt>, where
<tt>value</tt> is a feature value and
<tt>subvalue1</tt>...<tt>subvalueN</tt> are values of related
subfeatures. For example, the properties <tt>&lt;toolset&gt;gcc
&lt;toolset-version&gt;3.0.1</tt> can be expressed more conscisely using
a value-string, as <tt>&lt;toolset&gt;gcc-3.0.1</tt>.</p>
<p>A <em>property set</em> is a set of properties where no property
appears twice, for instance: <tt>&lt;toolset&gt;gcc
&lt;runtime-link&gt;static</tt>.</p>
<p>A <em>property path</em> is a property set whose elements have been
joined into a single string separated by slashes. A property path
representation of the previous example would be
<tt>&lt;toolset&gt;gcc/&lt;runtime-link&gt;static</tt>.</p>
<p>A <em>build specification</em> is a property set which fully describes
the set of features used to build a target.</p>
<h4><a name="feature_attributes">Feature Attributes</a></h4>
<p>Each feature has a collection of zero or more of the following
attributes. Feature attributes are low-level descriptions of how the
build system should interpret a feature's values when they appear in a
build request. We also refer to the attributes of properties, so that a
<i>incidental</i> property, for example, is one whose feature is has the
<i>incidental</i> attribute.</p>
<ul>
<li>
<em>incidental</em>
<p>Incidental features are assumed not to affect build products at
all. As a consequence, the build system may use the same file for
targets whose build specification differs only in incidental
features. A feature which controls a compiler's warning level is one
example of a likely incidental feature.</p>
<p>Non-incidental features are assumed to affect build products, so
the files for targets whose build specification differs in
non-incidental features are placed in different directories as
described in <a href="#target_paths">target paths</a> below.</p>
</li>
<li>
<em>propagated</em>
<p id="propagated_features">Features of this kind are propagated to
dependencies. That is, if a <a href="#main_target">main target</a> is
built using a propagated property, the build systems attempts to use
the same property when building any of its dependencies as part of
that main target. For instance, when an optimized exectuable is
requested, one usually wants it to be linked with optimized
libraries. Thus, the <tt>&lt;optimization&gt;</tt> feature is
propagated.</p>
</li>
<li>
<em>free</em>
<p>Most features have a finite set of allowed values, and can only
take on a single value from that set in a given build specification.
Free features, on the other hand, 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>
</li>
<li>
<em>optional</em>
<p>An optional feature is a feature which is not required to appear
in a build specification. Every non-optional non-free feature has a
default value which is used when a value for the feature is not
otherwise specified, either in a target's requirements or in the
user's build request. [A feature's default value is given by the
first value listed in the feature's declaration. -- move this
elsewhere - dwa]</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><a name="#feature_declaration">Feature Declaration</a></h4>
The low-level feature declaration interface is the <tt>feature</tt> rule
from the <tt>feature</tt> module:
<pre>
rule feature ( name : allowed-values * : attributes * )
</pre>
A feature's allowed-values may be extended wit The build system will
provide high-level rules which define features in terms of valid and
useful combinations of attributes.
<h3><a name="#variants">Build Variants</a></h3>
A build variant, or (simply variant) is a special kind of composite
feature which automatically incorporates the default values of features
that . Typically you'll want at least two separate variants: one for
debugging, and one for your release code. [ Volodya says: "Yea, we'd need
to mention that it's a composite feature and describe how they are
declared, in pacticular that default values of non-optional features are
incorporated into build variant automagically. Also, do we wan't some
variant inheritance/extension/templates. I don't remember how it works in
V1, so can't document this for V2.". Will clean up soon -DWA ]
<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 id="property_refinement">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>
<h4 id="conditional_properties">Conditional properties</h4>
<p>Sometime it's desirable to apply certain requirements only for
specific combination of other properties. For example, one of compilers
that you use issues a poinless warning that you want to suppress by
passing a command line option to it. You would not want to pass that
option to other compilers. Condititional properties allow to do that.
Their systax is:</p>
<pre>
( property ( "," property ) ":" property
</pre>
For example, the problem above would be solved by:
<pre>
exe hello : hello.cpp : &lt;toolset&gt;yfc:&lt;cxxflags&gt;-disable-pointless-warning ;
</pre>
<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>.
Special target name "clean" has the same effect as "--clean"
option.</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>--version</tt></td>
<td>Prints information on Boost.Build and Boost.Jam versions.</td>
</tr>
<tr id="help_option">
<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>
<tr>
<td><tt>--clean</tt></td>
<td>Removes everything instead of building. Unlike <tt>clean</tt>
target in make, it is possible to clean one some targets.</td>
</tr>
<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>--no-error-backtrace</tt></td>
<td>Don't print backtrace on errors. Primary usefull for
testing.</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>
<p>There are two user-visible kinds of targets in Boost.Build. First are
"abstract" &mdash; they correspond to things declared by user, for
example, projects and executable files. The primary thing about abstract
target is that it's possible to request them to be build with a
particular values of some properties. Each combination of properties may
possible yield different set of real file, so abstract target do not have
a direct correspondence with files.</p>
<p>File targets, on the contary, are associated with concrete files.
Dependency graphs for abstract targets with specific properties are
constructed from file targets. User has no was to create file targets,
however it can specify rules that detect file type for sources, and also
rules for transforming between file targets of different types. That
information is used in constructing dependency graph, as desribed in the
<a href="#generators">next section</a>. <b>Note:</b>File targets are not
the same as targets in Jam sense; the latter are created from file
targets at the latest possible moment. <b>Note:</b>"File target" is a
proposed name for what we call virtual targets. It it more understandable
by users, but has one problem: virtual targets can potentially be
"phony", and not correspond to any file.</p>
<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 sources and requirements.</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 id="target_reference"><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>File targets</h4>
As described above, file targets corresponds to files that Boost.Build
manages. User's may be concerned about file targets in three ways: when
declaring file target types, when declaring transformations between
types, and when determining where file target will be placed. File
targets can also be connected with actions, that determine how the target
is created. Both file targets and actions are implemented in the
<tt>virtual-target</tt> module.
<h5>Types</h5>
A file target can be given a file, which determines what transformations
can be applied to the file. The <tt>type.register</tt> rule declares new
types. File type can also be assigned a scanner, which is used to find
implicit dependencies. See <a href="#dependency_scanning">dependency
scanning</a> below.
<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 id="build_request">Build request</h3>
<h3><a name="build_process">Build process</a></h3>
<p>Given a list of targets ids and a build request, building goes this
way. First, for each id we obtain the abstract targets corresponding to
it. This also loads all necessary projects. If no target id is given,
project in the current directory is used. Build request is expanded, and
for each resulting property set, the <tt>generate</tt> method of all
targets is called, which yields a list of virtual targets. After that all
virtual targets are actualized, and target "all" is set to depend on all
created actual targets. Lastly, depending on whether <tt>--clean</tt>
option was given, either target "all" or target "clean" is updated.
Generation of virtual target from abstract one is performed as
follows:</p>
<ul>
<li>For project targets, all of main targets are generated with the
same properties. Then all projects referred via "build-project" are
generated as well. If it's not possible to refine requested properties
with project requirements, the project is skipped.</li>
<li>
For main target, steps are:
<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>Properties are refined with alternative's requirements, and
active features in the resulting set are executed.</li>
<li>Conditional properties are evaluated.</li>
<li>The dependency graph for the target is constructed in a way
which depends on the kind of main target, typically using
generators.</li>
</ol>
</li>
</ul>
<h3 id="generators">Generators</h3>
<p>To construct a main target with given properties from sources, it is
required to create a dependency graph for that main target, which will
also include actions to be run. The algorithm for creating the dependency
graph is described here.</p>
<p>The fundamental concept is <em>generator</em>. If encapsulates the
notion of build tool and is capable to converting a set of input targets
into a set of output targets, with some properties. Generator matches a
build tool as closely as possible: it works only when the tool can work
with requested properties (for example, msvc compiler can't work when
requested toolset is gcc), and should produce exactly the same targets as
the tool (for example, if Borland's linker produces additional files with
debug information, generator should also).</p>
<p>Given a set of generators, the fundamental operation is to construct a
target of a given type, with given properties, from a set of targets.
That operation is performed by rule <tt>generators.construct</tt> and the
used algorithm is described below.</p>
<h4>Selecting and ranking viable generators</h4>
<p>Each generator, in addition to target types that it can produce, have
attribute that affects its applicability in particular sitiation. Those
attributes are:</p>
<ol>
<li>Required properties, which are properties absolutely necessary for
the generator to work. For example, generator encapsulating the gcc
compiler would have &lt;toolset&gt;gcc as required property.</li>
<li>Optional properties, which increase the generators suitability for
a particual build.</li>
</ol>
Generator's required and optional properties may not include either free
or incidental properties. (Allowing this would greatly complicate caching
targets).
<p>When trying to construct a target, the first step is to select all
possible generators for the requested target type, which required
properties are a subset of requested properties. Generators which were
already selected up the call stack are excluded. In addition, if any
composing generators were selected up the call stack, all other composing
generators are ignored (TODO: define composing generators). The found
generators assigned a rank, which is the number of optional properties
present in requested properties. Finally, generators with highest rank
are selected for futher processing.</p>
<h4>Running generators</h4>
<p>When generators are selected, each is run to produce a list of created
targets. This list might include targets which are not of requested
types, because generators create the same targets as some tool, and
tool's behaviour is fixed. (Note: should specify that in some cases we
actually want extra targets). If generator fails, it returns an empty
list. Generator is free to call 'construct' again, to convert sources to
the types it can handle. It also can pass modified properties to
'constuct'. However, a generator is not allowed to modify any propagated
properties, otherwise when actually consuming properties we might
discover that the set of propagated properties is different from what was
used for building sources.</p>
<p>For all targets which are not of requested types, we try to convert
them to requested type, using a second call to <tt>construct</tt>. This
is done in order to support transformation sequences where single source
file expands to several later. See <a href=
"http://groups.yahoo.com/group/jamboost/message/1667">this message</a>
for details.</p>
<h4>Selecting dependency graph</h4>
After all generators are run, it is necessary to decide which of
successfull invocation will be taken as final result. At the moment, this
is not done. Instead, it is checked whether all successfull generator
invocation returned the same target list. Error is issued otherwise.
<h4>Property adjustment</h4>
<p>Because target location is determined by the build system, it is
sometimes necessary to adjust properties, in order to not break actions.
For example, if there's an action which generates a header, say
"a_parser.h", and a source file "a.cpp" which includes that file, we must
make everything work as if a_parser.h is generated in the same directory
where it would be generated without any subvariants.</p>
<p>Correct property adjustment can be done only after all targets are
created, so the approach taken is:</p>
<ol>
<li>When dependency graph is constructed, each action can be assigned a
rule for property adjustment.</li>
<li>When virtual target is actualized, that rule is run and return the
final set of properties. At this stage it can use information of all
created virtual targets.</li>
</ol>
<p>In case of quoted includes, no adjustment can give 100% correct
results. If target dirs are not changed by build system, quoted includes
are searched in "." and then in include path, while angle includes are
searched only in include path. When target dirs are changed, we'd want to
make quoted includes to be search in "." then in additional dirs and then
in the include path and make angle includes be searched in include path,
probably with additional paths added at some position. Unless, include
path already has "." as the first element, this is not possible. So,
either generated headers should not be included with quotes, or first
element of include path should be ".", which essentially erases the
difference between quoted and angle includes. <b>Note:</b> there only way
to get "." as include path into compiler command line is via verbatim
compiler option. In all other case, Boost.Build will convert "." into
directory where it occurs.</p>
<h4>Transformations cache</h4>
Under certain conditions, an attempt is made to cache results of
transformation search. First, the sources are replaced with targets with
special name and the found target list is stored. Later, when properties,
requested type, and source type are the same, the store target list is
retrieved and cloned, with appropriate change in names.
<h4 id="dependency_scanning">Dependency scanning</h4>
<p>Dependency scanning is the process of finding implicit dependencies
due to "include" statements and similar things. It has to take into
account two things:</p>
<ul>
<li>Whether includes in a particular file need to be taken into account
depends on actions that use that file. For example, if the action is
"copy file", then includes should be ignored. Another example is when a
file is compiled with two different include paths on different
toolsets.</li>
<li>It is possible to include generated header. In which case, it may
not yet exist at the time when we scan dependencies.</li>
</ul>
<p>Dependency scanning is implemented by objects called scanners. See
documentation for the "scanner" module to detail.</p>
<p>Regarding the first problem, we really have no choice. We can't treat
the same actual target differently depending on from where it is used.
Therefore, when handling of includes differers depending on actions, we
have to duplicate targets and assign different properties to it.</p>
<p>For the reason, when actualizing a virtual target we optionally pass
the needed scanner to the "virtual-target.actualize" method. When no
scanner is passed, a new actual target is created, with it's dependencies
and updating actions set accordingly. When a particular scanner is
specified, a new actual target is created. That target will depend on
target created without scanner. In effect, this will allow to use
different scanners for the same file.</p>
<h5>Generated headers</h5>
Let me explain what I find the right semantic, first without any
subvariants. We have a target "a.cpp" which includes "a_parser.h", we
have to search through all include directories, checking:
<ol>
<li>If there's such file there, or</li>
<li>If there's a target of the same name, bound to that dir via
LOCATE_TARGET.</li>
</ol>
Jam allows to do 1 via SEARCH variable, but that's not enough. Why can't
we do simpler: first check if there's target of the same name? I.e.
including of "a_parser.h" will already pick generated "a_parser.h",
regardless of search paths? Hmm... just because there's no reason to
assume that. For example, one can have an action which generated some
"dummy" header, for system which don't have the native one. Naturally, we
don't want to depend on that generated headers. To implement proposed
semantic we'd need a new builtin. We can do this in Jam code, but really,
this belongs to core. Using GLOB and jam code would just duplicate
existing binding functionality and be inefficient. New builtin will
accept a name of new target and a list of directories. It will perform
the search as explained above and return either the name of exising
target that it found, or create a new target with that name that it was
passed. So, we'd write something like
<blockquote>
<pre>
INCLUDES $(&lt;) : [ SEARCH_FOR_TARGET $(&gt;) : $(SEARCH_PATH) ] ;
</pre>
</blockquote>
What shall we do when using subvariants. For user, subvariants must be
more or less transparent. If without subvariant a header was generated to
a certain directory, everything must work. Suppose that file a.cpp
belongs to a dependency graph of main target a. Include paths are
<blockquote>
<pre>
"/usr/include" "/home/t" "."
</pre>
</blockquote>
We start by finding all places where headers that are part of a's
dependency graph are generated. We insert those places to the include
paths, immediately after ".". For example, we might end with:
<blockquote>
<pre>
"/usr/include" "/home/t" "." "build"
</pre>
</blockquote>
As a result:
<ol>
<li>File "a.cpp" will be correctly compiled. Note that it's already
necessary to adjust paths to ensure this. We'll have to add target
paths for all generated headers, because determining the exact set of
additional include path for each source -- i.e the set of headers that
it uses --- will be hard.</li>
<li>With the proposed SEARCH_FOR_TARGET rule, dependency on generated
header will work magically --- it would find the "a_parser.h" target
bound via LOCATE_TARGET to "build" and we'll call INCLUDE on that found
target, instread of creating a completely unrelated one.</li>
</ol>
<hr>
<p class="revision">Last modified: Oct 10, 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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