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<span><b>Boost.Build Tutorial</b></span>
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<hr class="hrhead">
<p>Written by Boris Schäling.</p>
<div class="toc">
<h3>Table of Contents</h3>
<ul>
<li><a href="#introduction">1. Introduction</a></li>
<li><a href="#buildprocess">2. Build process</a></li>
<li><a href="#basic_tasks">3. Basic tasks</a></li>
<li><a href="#project_management">4. Project management</a></li>
<li><a href="#best_practices">5. Best practices</a></li>
<li><a href="#rule_reference">6. Rule reference</a></li>
<li><a href="#feature_reference">7. Feature reference</a></li>
</ul>
</div>
<hr>
<h2 id="introduction">Introduction<br><small>Compiler- and
platform-independent build system</small></h2>
<div>
<p>Boost.Build is a high-level build system which makes it as easy as
possible to manage C++ projects. The idea is to specify in
configuration files just as much as necessary to build a program. For
example it is not required to tell Boost.Build how to use a certain
compiler. Boost.Build supports many compilers out of the box and knows
how to use them. If you create a configuration file you just need to
tell Boost.Build where to find the source files, what the executable
should be called and which compiler Boost.Build should use. Boost.Build
will then try to find the compiler and automatically build the
program.</p>
<p>As Boost.Build supports many compilers configuration files never
contain any compiler-specific options. Configuration files are entirely
compiler-independent. Of course it is possible to set options like
whether code should be optimized. However these options are written in
a language only understood by Boost.Build. Once a compiler is picked to
build a program Boost.Build translates options in configuration files
to command line options expected by the selected compiler. This makes
it possible to write configuration files once and build a program on
different platforms with different compilers.</p>
<p>As nice as it sounds Boost.Build can only be used for C++ and C
projects. Boost.Build doesn't know how to use other compilers like a
Java compiler. Although Boost.Build is extensible it makes more sense
to use a different build system for programs implemented in other
programming languages.</p>
<p>Boost.Build was created to build and install the <a class="link"
href="http://www.boost.org/" target="_top">Boost C++ libraries</a>
easily with different compilers on different platforms. Although
Boost.Build is part of and shipped with the Boost C++ libraries it can
be used separately for any C++ or C project. It's even possible to
<a class="link" href="http://sourceforge.net/projects/boost/files/"
target="_top">download only Boost.Build</a> in case you don't want to
use the Boost C++ libraries.</p>
<p>This article is an introduction to help you using Boost.Build for
your own C++ or C projects. It gives you a basic understanding of how
Boost.Build works and how you start using it. After reading the article
you should not only be able to use Boost.Build for your own projects,
it will also be easier to understand the <a class="link" href=
"http://www.boost.org/doc/tools/build/doc/html/index.html" target=
"_top">Boost.Build documentation</a> as you'll know the big
picture.</p>
</div>
<hr>
<h2 id="buildprocess">Build process<br>
<small>Jamfiles and an interpreter called b2</small>
</h2>
<div>
<p>The program you use to build a project managed by Boost.Build is
called <span class="command"><strong>b2</strong></span>. If you
downloaded and built the Boost C++ libraries you have used <span class=
"command"><strong>b2</strong></span> already. <span class=
"command"><strong>b2</strong></span> looks for configuration files,
reads them and builds a project accordingly. It also accepts various
command line options which can be useful for example to show all
commands executed by <span class="command"><strong>b2</strong></span>
to build a project.</p>
<p>Projects can be large and can consist of many components whose
source code is distributed over many directories. Instead of creating
one big configuration file for the entire project components typically
get their own configuration files. This is no different with
Boost.Build: In a large project there will be many configuration files
which have to be found and interpreted by <span class=
"command"><strong>b2</strong></span>.</p>
<p>For Boost.Build every directory with a configuration file is a
project: If there is a configuration file in a directory something can
be built. Whether it's a component in a subdirectory or a software
consisting of many components doesn't make a difference for
Boost.Build.</p>
<p>When <span class="command"><strong>b2</strong></span> is started
it doesn't run a search for configuration files on the entire file
system. It searches for a configuration file in the current working
directory only. If it doesn't find a configuration file it doesn't do
anything. <span class="command"><strong>b2</strong></span> does not
search for configuration files in any other directory if there is no
configuration file in the current working directory.</p>
<p>The configuration file <span class=
"command"><strong>b2</strong></span> is looking for is called
<code class="filename">Jamfile.jam</code>. Files with the extension
<code class="filename">jam</code> are called Jamfiles. If <span class=
"command"><strong>b2</strong></span> finds a Jamfile in the current
working directory it searches for more Jamfiles in parent directories.
<span class="command"><strong>b2</strong></span> climbs up parent
directories until it finds a configuration file called <code class=
"filename">Jamroot.jam</code>. <code class=
"filename">Jamroot.jam</code> is no different from <code class=
"filename">Jamfile.jam</code>. It only indicates that <span class=
"command"><strong>b2</strong></span> doesn't need to look
further.</p>
<p>The reason why <span class="command"><strong>b2</strong></span>
looks for Jamfiles in parent directories is that it makes it possible
to group settings. If there are some components which should be built
with similar settings they can be stored in a Jamfile in a parent
directory which will be automatically used if a component in a
subdirectory is built.</p>
<p>Please note that <span class="command"><strong>b2</strong></span>
must find a file called <code class="filename">Jamroot.jam</code>. It
is an error if no <code class="filename">Jamroot.jam</code> exists. If
<code class="filename">Jamroot.jam</code> is in the current working
directory no other file <code class="filename">Jamfile.jam</code> is
required. If <code class="filename">Jamroot.jam</code> is in a parent
directory a file <code class="filename">Jamfile.jam</code> must exist
in the current working directory - otherwise <span class=
"command"><strong>b2</strong></span> doesn't do anything.</p>
<p>If you copy <span class="command"><strong>b2</strong></span> to a
directory which contains no Jamfiles and start the program you get an
error message. However <span class=
"command"><strong>b2</strong></span> doesn't complain that it can't
find a Jamfile. It complains about not finding the build system.</p>
<pre class="screen">
Unable to load Boost.Build: could not find "boost-build.jam"
---------------------------------------------------------------
Attempted search from C:\Users\Boris\Desktop up to the root
Please consult the documentation at 'http://www.boost.org'.
</pre>
<p>The first thing <span class="command"><strong>b2</strong></span>
does is not looking for a Jamfile but loading the build system. But
what exactly is the build system?</p>
<p><span class="command"><strong>b2</strong></span> is an
interpreter. It doesn't really know how to build anything. What
<span class="command"><strong>b2</strong></span> does is interpreting
Jamfiles. Boost.Build is really implemented in Jamfiles. And they
contain all the logic which makes Boost.Build such a powerful tool. As
<span class="command"><strong>b2</strong></span> only does what it
reads in Jamfiles it needs to know where to find the Jamfiles
Boost.Build is made of.</p>
<p>When <span class="command"><strong>b2</strong></span> is started
it looks for a file <code class="filename">boost-build.jam</code> in
the current working directory. If it doesn't find the file it searches
all parent directories. This file needs to contain only one line to
tell <span class="command"><strong>b2</strong></span> where to find
the build system.</p>
<pre class="programlisting">
boost-build C:/boost_1_57_0/tools/build/src ;
</pre>
<p>The path after <code class="code">boost-build</code> must refer to a
directory which contains a file called <code class=
"filename">bootstrap.jam</code>. This is the file <span class=
"command"><strong>b2</strong></span> needs to load the build system.
As the Boost C++ libraries ship Boost.Build you can refer to the
subdirectory <code class="filename">tools/build</code> of the root
directory of the Boost C++ libraries. And you can always use a slash as
a path separator - even if you are on Windows.</p>
<p>Please note that there must be a space between the path and the
semicolon at the end of the line. It is an error if the space is
missing. You'll learn more about the syntax used in Jamfiles later in
this article.</p>
<p>If <span class="command"><strong>b2</strong></span> finds
<code class="filename">boost-build.jam</code> it uses the path within
the file to load the build system. When the build system is loaded it
also prepares itself to use a certain compiler, linker and maybe other
tools required to build a project. Boost.Build refers to these programs
as a toolset. If no command line option is used to start <span class=
"command"><strong>b2</strong></span> the build system tries to find a
toolset it can use automatically. On Windows for example it searches
for Visual C++. And if it detects that Visual C++ is installed it uses
the toolset msvc.</p>
<pre class="screen">
warning: No toolsets are configured.
warning: Configuring default toolset "msvc".
warning: If the default is wrong, your build may not work correctly.
warning: Use the "toolset=xxxxx" option to override our guess.
warning: For more configuration options, please consult
warning: http://boost.org/boost-build2/doc/html/bbv2/advanced/configuration.html
</pre>
<p>If you start <span class="command"><strong>b2</strong></span>
without specifying which toolset should be used you see a warning.
<span class="command"><strong>b2</strong></span> tells you which
toolset it detected and decided to use. If you want to suppress the
warning you must specify the toolset yourself. For example you tell the
build system to use Visual C++ with <span class="command"><strong>b2
toolset=msvc</strong></span>. If you want GCC to be used you enter
<span class="command"><strong>b2 toolset=gcc</strong></span>.</p>
<p>As of today there are more than 10 toolsets supported. There is a
good chance that Boost.Build will work with the compiler you use out of
the box.</p>
<p>Once the build system has been found, loaded and knows which toolset
to use - either because you specified one or the build system detected
one automatically - <span class="command"><strong>b2</strong></span>
looks for a file <code class="filename">Jamfile.jam</code> in the
current working directory. If it doesn't find a Jamfile an error
message is printed.</p>
<pre class="screen">
error: error: no Jamfile in current directory found, and no target references specified.
</pre>
<p>If you create an empty file <code class=
"filename">Jamfile.jam</code> and start <span class=
"command"><strong>b2</strong></span> again another error message is
printed.</p>
<pre class="screen">
error: Could not find parent for project at '.'
error: Did not find Jamfile.jam or Jamroot.jam in any parent directory.
</pre>
<p><span class="command"><strong>b2</strong></span> is ultimately
looking for a Jamfile called <code class="filename">Jamroot.jam</code>.
If it doesn't exist in the current working directory <span class=
"command"><strong>b2</strong></span> expects to find it in a parent
directory.</p>
<p>If you create an empty file <code class=
"filename">Jamroot.jam</code> and start <span class=
"command"><strong>b2</strong></span> the error message is gone.
Obviously there is nothing done by Boost.Build. But now you know how
<span class="command"><strong>b2</strong></span> proceeds to build a
program and what the minimum Boost.Build configuration looks like.</p>
<p>Please note that if you work on a small project and you need only
one configuration file you can simply call it <code class=
"filename">Jamroot.jam</code>. You don't need another file called
<code class="filename">Jamfile.jam</code>.</p>
</div>
<hr>
<h2 id="basic_tasks">Basic tasks<br>
<small>Rules and features</small>
</h2>
<div class="sect1">
<p>If you look at Jamfiles the syntax might remind you of configuration
files used by other build systems. Simple Jamfiles can look like plain
old configuration files where for example values seem to be assigned to
keys. What is important to understand though is that Jamfiles are
really script files. There is a programming language used to write
Jamfiles. <span class="command"><strong>b2</strong></span> isn't the
core component of Boost.Build which knows how to build programs. The
logic of Boost.Build is in the Jamfiles which tell <span class=
"command"><strong>b2</strong></span> how to build programs.</p>
<p>Even though Boost.Build is based on a programming language you don't
need to think of programming when you create Jamfiles. The syntax of
the programming language used by Boost.Build tries to remind you more
of creating plain old configuration files. The idea is to have the best
of two worlds: A powerful and flexible programming language but a
simple syntax you might be familiar with from other build systems.</p>
<p>This article doesn't introduce you into the programming language
Boost.Build is based on. The programming language is proprietary and
not really a joy to use. It is no competitor to popular scripting
languages like Javascript or Python. The developers of Boost.Build
recognize it and work on another version of Boost.Build based on
Python. However all of this shouldn't matter to developers who plan to
manage their projects with Boost.Build. It helps to understand the
syntax of Jamfiles better once one realizes that there is a programming
language inside Boost.Build. But it's not required to learn the details
of the programming language.</p>
<p>Let's look at a simple Jamfile which can be used to build an
executable <span class="command"><strong>hello</strong></span> from a
source file <code class="filename">hello.cpp</code>.</p>
<pre class="programlisting">
exe hello : hello.cpp ;
</pre>
<p>Boost.Build provides a lot of built-in rules and <code class=
"code">exe</code> is one of them. While the documentation of
Boost.Build refers to <code class="code">exe</code> as a rule you know
already that the above Jamfile is actually built using a programming
language. As it turns out rules are simply functions. And the Jamfile
above contains a function call.</p>
<p>For the majority of tasks which are typically required to build
programs Boost.Build provides predefined rules - or functions if you
like. As with functions in other programming languages it is possible
to pass parameters. In the Jamfile above the function <code class=
"code">exe</code> is called with the two parameters hello and
hello.cpp.</p>
<p>The programming language Boost.Build is based on knows only one data
type: Everything is a list of strings. A list can be empty or contain
one or more strings. In the Jamfile above the function <code class=
"code">exe</code> is called with two parameters each one a list
containing one string.</p>
<pre class="programlisting">
exe "hello" : "hello.cpp" ;
</pre>
<p>It is possible to use quotes. It's not necessary though as after all
every item in a list has the data type string anyway. Quotes are only
used if parameters contain spaces.</p>
<p>While there is no special delimiter between a rule and the first
parameter a colon must be used to separate other parameters. It is also
required to end a line with a semicolon just as you are used to from
C++.</p>
<p>Please note that the programming language of Boost.Build requires
that there is a space around all tokens. For example there must be a
space on the left and on the right of the colon and there must be a
space on the left of the semicolon. Without spaces around tokens
<span class="command"><strong>b2</strong></span> won't be able to
parse Jamfiles correctly.</p>
<p>If <span class="command"><strong>b2</strong></span> is run in a
directory which contains the Jamfile above and a source file
<code class="filename">hello.cpp</code>, and if the msvc toolset is
used on Windows a subdirectory <code class=
"filename">bin\msvc-9.0\debug</code> is created to build an executable
<code class="filename">hello.exe</code>.</p>
<pre class="screen">
...found 9 targets...
...updating 5 targets...
common.mkdir bin
common.mkdir bin\msvc-9.0
common.mkdir bin\msvc-9.0\debug
compile-c-c++ bin\msvc-9.0\debug\hello.obj
hello.cpp
msvc.link bin\msvc-9.0\debug\hello.exe
msvc.manifest bin\msvc-9.0\debug\hello.exe
...updated 5 targets...
</pre>
<p>As you see it takes only one line in a Jamfile to build an
executable from a source file. And if the program is built on Windows
there is even the correct file extension <code class=
"filename">exe</code> appended.</p>
<p>The main advantage of Boost.Build is that you specify just as much
as necessary for a build system to know how to build a program.
Anything Boost.Build can do automatically is done automatically. You
don't need to detect the platform a program is built on to decide if a
file extension like <code class="filename">exe</code> should be
appended or not. And you don't need to specify how a compiler like
Visual C++ has actually to be invoked to compile source code.</p>
<p>Boost.Build supports a lot of toolsets out of the box. As a program
can be built using different toolsets Boost.Build uses toolset-specific
directories. This way it is possible to build a program with different
toolsets without a toolset constantly overwriting files produced by
another toolset.</p>
<p>There are not only toolset-specific directories but also
variant-specific directories. A variant is a debug or release version
of a program. For each variant another directory is used to build a
program - again for the reason not to overwrite files produced by
another variant. By default the debug variant is used. That's why the
subdirectory <code class="filename">bin\msvc-9.0\debug</code> was
created. If you want a release version to be created you can specify
the variant on the command line with <span class="command"><strong>b2
variant=release</strong></span> or, even simpler, <span class="command">
<strong>b2 release </strong></span>.</p>
<pre class="screen">
...found 9 targets...
...updating 5 targets...
common.mkdir bin
common.mkdir bin\msvc-9.0
common.mkdir bin\msvc-9.0\release
compile-c-c++ bin\msvc-9.0\release\hello.obj
hello.cpp
msvc.link bin\msvc-9.0\release\hello.exe
msvc.manifest bin\msvc-9.0\release\hello.exe
...updated 5 targets...
</pre>
<p>With the variant set to release the subdirectory <code class=
"filename">bin\msvc-9.0\release</code> is used to create the executable
<code class="filename">hello.exe</code>.</p>
<p>Choosing a variant is something which is done so often that it's
sufficient to enter <span class="command"><strong>b2
release</strong></span>. Boost.Build figures out that release is meant
to choose the variant.</p>
<p>If you don't want to specify the variant on the command line but
want to build release versions of <code class=
"filename">hello.exe</code> by default the Jamfile has to be
changed.</p>
<pre class="programlisting">
exe hello : hello.cpp : &lt;variant&gt;release ;
</pre>
<p>The <code class="code">exe</code> rule (or, if you prefer, function)
accepts a few more parameters which are optional. The third parameter
is a list of requirements. You can think of command line options which
are always set and passed to commands run to build an executable.</p>
<p>In order to force a release version to be built the variant has to
be set to release just as it was done before on the command line. The
syntax to set the variant in a Jamfile is different though.</p>
<p>Boost.Build defines features which look like XML tags. One of the
features supported by Boost.Build is <code class=
"code">&lt;variant&gt;</code>. If a feature should be set to a value it
has to be put next to it - without a space in between. Some features
are free which means they can be set to any value you want.
<code class="code">&lt;variant&gt;</code> is a non-free feature as it
can only be set to debug or release. No other value is allowed. If
another value is set <code class="code">b2</code> will report an
error.</p>
<p>If you run <code class="code">b2 variant=debug</code> and try to
build a debug version of <code class="filename">hello.exe</code> it
won't work as the Jamfile contains the requirement that <code class=
"filename">hello.exe</code> is built as a release version. If you want
to be able to overwrite the feature on the command line you have to
pass the feature as the fourth parameter instead of the third.</p>
<pre class="programlisting">
exe hello : hello.cpp : : &lt;variant&gt;release ;
</pre>
<p>The fourth parameter contains features which are used by default but
which can be overwritten.</p>
<p>If you want both a debug and a release version of <code class=
"filename">hello.exe</code> to be built by default the <code class=
"code">&lt;variant&gt;</code> feature needs to be set twice to debug
and release.</p>
<pre class="programlisting">
exe hello : hello.cpp : : &lt;variant&gt;debug &lt;variant&gt;release ;
</pre>
<p>It is important that <code class="code">&lt;variant&gt;</code> is
set twice in the fourth parameter where default values are specified.
If it was the third parameter where requirements are specified
<span class="command"><strong>b2</strong></span> would report an
error. It is possible to set a feature multiple times in the
requirements but only if values are not mutually exclusive. As a
program can't be a debug and a release version at the same time
<code class="code">&lt;variant&gt;</code> must be set in the default
values. Only then Boost.Build understands that two versions of
<code class="filename">hello.exe</code> should be built.</p>
<pre class="programlisting">
exe hello : hello.cpp : &lt;define&gt;WIN32 &lt;define&gt;_WIN32 : &lt;variant&gt;debug &lt;variant&gt;release ;
</pre>
<p>The above Jamfile is an example for setting a feature multiple times
in the requirements. The feature <code class=
"code">&lt;define&gt;</code> is used to define preprocessor directives.
It is no problem to define several preprocessor directives. Thus there
are now two versions of <code class="filename">hello.exe</code> built
both with the two directives <code class="code">WIN32</code> and
<code class="code">_WIN32</code> defined.</p>
<pre class="programlisting">
exe hello : hello.cpp : : &lt;variant&gt;debug &lt;variant&gt;release &lt;define&gt;WIN32 &lt;define&gt;_WIN32 ;
</pre>
<p>If the definitions are moved to the fourth parameter and you run
<span class="command"><strong>b2</strong></span> you get the same two
versions of <code class="filename">hello.exe</code> built with the two
directives <code class="code">WIN32</code> and <code class=
"code">_WIN32</code>. As <code class="code">&lt;define&gt;</code> does
not expect mutually exclusive values there is no other set of
executables generated. The only difference between this Jamfile and the
previous one is that directives passed in the fourth parameter are
default values which can be dropped while anything passed as a third
parameter is an immutable requirement.</p>
<p>Here is another example of a feature whose values are mutually
exclusive.</p>
<pre class="programlisting">
exe hello : hello.cpp : : &lt;variant&gt;debug &lt;variant&gt;release &lt;optimization&gt;speed &lt;optimization&gt;off ;
</pre>
<p><span class="command"><strong>b2</strong></span> creates four
versions of <code class="filename">hello.exe</code>: A debug version
optimized for speed, a debug version with no optimization, a release
version optimized for speed and a release version with no optimization.
All of these versions are built in seperate directories which are
automatically created.</p>
<p>So far the only rule used was <code class="code">exe</code>. But of
course Boost.Build provides many more built-in rules. Another important
rule is <code class="code">lib</code>. It is used to build a
library.</p>
<pre class="programlisting">
lib world : world.cpp ;
</pre>
<p>The above Jamfile builds a shared library from the source file
<code class="filename">world.cpp</code>. On Windows a file <code class=
"filename">world.dll</code> is created. The usual file extension is
again automatically appended by Boost.Build.</p>
<p>By default a shared library is built. If you want a static library
to be generated you set the <code class="code">&lt;link&gt;</code>
feature to static.</p>
<pre class="programlisting">
lib world : world.cpp : &lt;link&gt;static ;
</pre>
<p>Another useful rule is <code class="code">install</code>. After
executables and libraries have been built this rule can be used to
install them.</p>
<pre class="programlisting">
exe hello : hello.cpp ;
install "C:/Program Files/hello" : hello ;
</pre>
<p>The above Jamfile installs the executable <code class=
"filename">hello.exe</code> to the directory <code class=
"filename">C:\Program Files\hello</code>. The second parameter hello is
a reference to the target hello defined in the first line. Please note
that the path has to be put in quotes as it contains a space.</p>
<p>Here concepts known from other build systems shine through: Instead
of thinking of function calls every line defines a target. Dependencies
are created by referencing other targets. That's how Boost.Build knows
in what order it should build targets.</p>
<p>Typically the rule <code class="code">install</code> is written
differently though. Instead of passing the installation directory as
the first parameter a feature <code class=
"code">&lt;location&gt;</code> is used to set the installation
directory in the third parameter.</p>
<pre class="programlisting">
exe hello : hello.cpp ;
install install-bin : hello : &lt;location&gt;"C:/Program Files/hello" ;
</pre>
<p>The main reason why it's better to use <code class=
"code">&lt;location&gt;</code> is that the first parameter always
defines a target. Other rules might refer to a target. That's why it is
a good idea to use target names which don't have to be changed later.
Imagine a program should be installed to a different directory. It's
easier to change the installation directory if the <code class=
"code">&lt;location&gt;</code> feature has been used as no other rules
which might refer to install-bin have to be updated.</p>
<p>There is another reason why it makes sense to use a feature.
Boost.Build supports conditional properties which make it possible to
use different installation directories depending on the platform a
program is built on.</p>
<pre class="programlisting">
exe hello : hello.cpp ;
install install-bin : hello : &lt;target-os&gt;windows:&lt;location&gt;"C:/Program Files/hello" &lt;target-os&gt;linux:&lt;location&gt;/usr/local/bin ;
</pre>
<p>The feature <code class="code">&lt;target-os&gt;</code> is another
feature with mutually exclusive values. It can be set for example to
windows or linux but not to both.</p>
<p>The feature <code class="code">&lt;location&gt;</code> follows
<code class="code">&lt;target-os&gt;</code> only delimited by a colon.
Such a construct is called conditional property: Boost.Build selects
the installation directory depending on the operating system.</p>
<p>Of course conditional properties can also be used with other rules.
It is for example possible to define different preprocessor directives
depending on the variant when building a program or a library.</p>
<p>Boost.Build provides many more built-in rules. Another useful rule
is <code class="code">glob</code> which makes it possible to use
wildcards. In a big project with many source files it's then not
required to list them all one by one but refer to all of them with
<code class="code">glob</code>.</p>
<pre class="programlisting">
exe hello : [ glob *.cpp ] ;
</pre>
<p>The above Jamfile contains a nested function call: The result of the
rule <code class="code">glob</code> is passed as the second parameter
to <code class="code">exe</code>. Due to requirements of the
programming language Boost.Build is based on brackets must be used for
nested function calls.</p>
</div>
<hr>
<h2 id="project_management">Project management<br>
<small>Multiple Jamfiles</small>
</h2>
<div>
<p>In large projects with many Jamfiles it's necessary to connect
Jamfiles somehow. There is typically a <code class=
"filename">Jamroot.jam</code> file in the project's root directory and
many <code class="filename">Jamfile.jam</code> files in subdirectories.
If <span class="command"><strong>b2</strong></span> is run in the
root directory developers probably expect that the entire project
including all components in subdirectories is built. As <span class=
"command"><strong>b2</strong></span> looks for Jamfiles in parent
directories but not in subdirectories Jamfiles need to refer to
Jamfiles in subdirectories explicitly.</p>
<pre class="programlisting">
build-project hello ;
</pre>
<p>If a Jamfile looks like the sample above it refers to a Jamfile in a
subdirectory <code class="filename">hello</code>. <code class=
"code">build-project</code> is a rule which expects a path as its sole
parameter. The path is then used to lookup a Jamfile.</p>
<pre class="programlisting">
build-project hello ;
build-project world ;
</pre>
<p>If you want several projects to be built you must use <code class=
"code">build-project</code> multiple times.</p>
<p>Apart from referring to Jamfiles in subdirectories it makes also
sense to group options which should be used when building components in
a project.</p>
<pre class="programlisting">
project : default-build release ;
build-project hello ;
build-project world ;
</pre>
<p>The <code class="code">project</code> rule accepts various
parameters to set options for the Jamfile in the current working
directory and in subdirectories.</p>
<p>While other rules like <code class="code">exe</code> and
<code class="code">lib</code> expect parameters to be passed in a
certain order <code class="code">project</code> uses named arguments.
In the sample above the argument's name is default-build. That's why it
is possible to pass the value release in a very different
parameter.</p>
<pre class="programlisting">
project : : : : : : : : : default-build release ;
build-project hello ;
build-project world ;
</pre>
<p>It doesn't make sense to pass release as the tenth parameter. But it
works as <code class="code">project</code> doesn't care about the
order. As the tenth parameter is called default-build it is
accepted.</p>
<p><code class="code">project</code> supports only a few named
arguments. Another one is requirements which can be used to set options
which can't be overwritten.</p>
<pre class="programlisting">
project : requirements &lt;variant&gt;release ;
build-project hello ;
build-project world ;
</pre>
<p>The Jamfile above builds only release versions. It is not possible
to build a debug version anymore as requirements can not be
overwritten. That's the difference to the named argument called
default-build which was used in the previous sample: It can be
overwritten.</p>
<p>When <code class="code">build-project</code> is used Boost.Build
assumes that the parameter is a reference to a subdirectory. We had
seen another type of reference before.</p>
<pre class="programlisting">
exe hello : hello.cpp ;
install install-bin : hello : &lt;location&gt;"C:/Program Files/hello" ;
</pre>
<p>In the above Jamfile the <code class="code">install</code> rule
refers to the target hello defined in the first line.</p>
<p>In a large project it might be necessary to refer to targets which
are defined in Jamfiles in other directories. It is possible to
concatenate a path to a Jamfile and a target with a double slash.</p>
<pre class="programlisting">
install install-bin : subdir//hello : &lt;location&gt;"C:/Program Files/hello" ;
</pre>
<p>Now the <code class="code">install</code> rule refers to a target
hello in a Jamfile in the subdirectory <code class=
"filename">subdir</code>.</p>
<p>Let's assume that the executable <span class=
"command"><strong>hello</strong></span> depends on a library in another
directory <code class="filename">world</code>. The library is also
built with Boost.Build using the rule <code class=
"code">lib</code>.</p>
<pre class="programlisting">
lib world : world.cpp ;
</pre>
<p>In the Jamfile to build the executable a reference is required to
the Jamfile of the library. It's not necessary to refer to the target
world directly as all targets in a Jamfile are built by default.</p>
<pre class="programlisting">
exe hello : hello.cpp world : : &lt;variant&gt;debug &lt;variant&gt;release ;
</pre>
<p>The above Jamfile assumes that the library and its Jamfile are in a
subdirectory <code class="filename">world</code>.</p>
<p>When the executable is built there are two versions generated - a
debug and a release version. The Jamfile of the library however doesn't
set the <code class="code">&lt;variant&gt;</code> feature. But
Boost.Build assumes that it should build two versions of the library,
too. The feature <code class="code">&lt;variant&gt;</code> is said to
be propagated.</p>
<p>Propagating features simplify project management as you don't need
to set the same features in various Jamfiles. However it also makes it
a bit more complicated to understand how components are built as it all
depends on what features are propagated. You can assume that
Boost.Build knows what it should do. But of course it doesn't mean that
you easily understand what it does.</p>
<p>Let's look at another example using the feature <code class=
"code">&lt;define&gt;</code>.</p>
<pre class="programlisting">
exe hello : hello.cpp world : &lt;define&gt;WIN32 : &lt;variant&gt;debug &lt;variant&gt;release ;
</pre>
<p>The above Jamfile defines a preprocessor directive <code class=
"code">WIN32</code> for the program <span class=
"command"><strong>hello</strong></span>. But will <code class=
"code">WIN32</code> be defined for the library, too?</p>
<p>It won't as <code class="code">&lt;define&gt;</code> is not a
propagating feature. If you wonder how you should know: The only way to
find out which features are propagated is to lookup the
documentation.</p>
<p>If you installed the Boost C++ libraries you probably want to link
against some of them. You somehow have to add a dependency to the
respective Boost C++ library to your project's Jamfile. If you didn't
delete the directories you had unzipped the source files of the Boost
C++ libraries to you can refer to a target in a Jamfile in the root
directory.</p>
<pre class="programlisting">
exe hello : hello.cpp world C:/boost_1_39_0//filesystem/ ;
</pre>
<p>Now <span class="command"><strong>hello</strong></span> also depends
on the Boost.Filesystem library. As the target filesystem is defined in
a Jamfile in the root directory of the Boost C++ libraries the
<code class="code">exe</code> rule can refer to it. Not only will the
appropriate Boost C++ libraries be linked - an include directory is
also passed to the compiler to find the header files. If <code class=
"filename">hello.cpp</code> includes <code class=
"filename">boost/filesystem.hpp</code> the header file will be
found.</p>
<p>In the above Jamfile the path to the root directory of the Boost C++
libraries is hardcoded. Somehow <span class=
"command"><strong>b2</strong></span> needs to know where to find the
Boost C++ libraries. But it would be better if the path was hardcoded
only once in case several components in a project need to link against
some Boost C++ libraries.</p>
<pre class="programlisting">
project : requirements &lt;variant&gt;release ;
use-project /boost : C:/boost_1_39_0 ;
build-project hello ;
build-project world ;
</pre>
<p>The <code class="code">use-project</code> rule is used to define an
alias to a Jamfile in another directory. Jamfiles in subdirectories use
then the alias to refer to a Boost C++ library.</p>
<pre class="programlisting">
exe hello : hello.cpp world /boost//filesystem ;
</pre>
<p><span class="command"><strong>b2</strong></span> figures out that
<code class="filename">hello.cpp</code> is a source file, <code class=
"filename">world</code> a subdirectory and /boost//filesystem a
reference to a target filesystem in a Jamfile in <code class=
"filename">C:\boost_1_39_0</code>.</p>
<p>Please note that a reference must start with a slash if it should
refer to a project.</p>
<p>As libraries can be linked differently it is possible to set
features relevant to the linker.</p>
<pre class="programlisting">
exe hello : hello.cpp world /boost//filesystem/&lt;link&gt;static ;
</pre>
<p>By default libraries are linked dynamically. If libraries should be
linked statically the feature <code class="code">&lt;link&gt;</code>
has to be set to static.</p>
<p>Features can be appended with a slash. If more than one feature
should be set it is appended with another slash to the previous
feature.</p>
<pre class="programlisting">
exe hello : hello.cpp world /boost//filesystem/&lt;link&gt;static/&lt;threading&gt;multi ;
</pre>
<p><code class="code">&lt;threading&gt;</code> is another feature which
can be set to single or multi. If <span class=
"command"><strong>hello</strong></span> should be linked against the
thread-safe version of Boost.Filesystem the feature can be set
accordingly.</p>
<p>Linking a Boost C++ library by referencing a Jamfile might not
always work. If the Boost C++ libraries were installed differently
because they weren't built from source for example there won't be any
Jamfile to reference.</p>
<pre class="programlisting">
lib filesystem : : &lt;name&gt;libboost_filesystem &lt;search&gt;C:/libs ;
exe hello : hello.cpp world filesystem : &lt;include&gt;C:/include ;
</pre>
<p>The <code class="code">lib</code> rule can not only be used to build
a library from source. It also has to be used to refer to an existing
and pre-built library.</p>
<p>If <code class="code">lib</code> shouldn't build a library from
source the second parameter must be empty. Instead in the third
parameter the features <code class="code">&lt;name&gt;</code> and
<code class="code">&lt;search&gt;</code> are used to specify the
library's name and a location where Boost.Build will find the
library.</p>
<p>It is important to specify the library's name in a
platform-independent way. For example for the Jamfile above Boost.Build
will try to find a file <code class=
"filename">libboost_filesystem.lib</code> on Windows. The usual file
extension is again automatically appended.</p>
<p>If you want to reference a file by specifying its exact name you can
use the <code class="code">&lt;file&gt;</code> feature.</p>
<p>If a system library should be referenced for which you can expect
Boost.Build to know where to find it the feature <code class=
"code">&lt;search&gt;</code> can be dropped.</p>
<p>It is also possible to use the <code class="code">project</code>
rule to make sure all targets in a project are automatically linked
against a library.</p>
<pre class="programlisting">
lib filesystem : : &lt;name&gt;libboost_filesystem &lt;search&gt;C:/libs ;
explicit filesystem ;
project : requirements &lt;include&gt;C:/include &lt;library&gt;filesystem ;
lib world : world.cpp ;
</pre>
<p>A feature called <code class="code">&lt;library&gt;</code> must be
used to add a library dependency to a <code class="code">project</code>
rule. <code class="code">&lt;library&gt;</code> must refer to a
<code class="code">lib</code> rule which uses the already known
features <code class="code">&lt;name&gt;</code> and <code class=
"code">&lt;search&gt;</code>.</p>
<p>It is now very important to make the <code class="code">lib</code>
rule explicit. This is done by using the <code class=
"code">explicit</code> rule. It is important as by default all targets
in a Jamfile are built. As the <code class="code">project</code> rule
defines requirements for all targets in the Jamfile they are also
requirements for the <code class="code">lib</code> rule. Thus the
<code class="code">lib</code> rule refers to itself. If the
<code class="code">lib</code> rule is made explicit though it's not
built and no recursive reference occurs.</p>
<p>Please note that the order of rules in a Jamfile matters only if a
rule refers to a target: Before a target can be referenced it must have
been defined.</p>
</div>
<hr>
<h2 id="best_practices">Best practices<br>
<small>How Boost.Build is used by others</small>
</h2>
<div>
<p>As Boost.Build is a high-level build system you benefit most if you
keep Jamfiles platform- and compiler-independent. After all the idea is
to build your C++ or C projects on any platform with any compiler
without being required to modify or maintain several Jamfiles.</p>
<p>A typical problem you'll run into is that third-party libraries you
want to use will be installed in different directories. If you want to
build your project on Windows and Unix platforms paths also look very
different. Furthermore you might need to link against some system
libraries on a platform but not on another.</p>
<p>Instead of trying to put paths for various platforms in a project's
Jamfiles it is better to rely on configuration files on every system
for system-specific settings. As it turns out <span class=
"command"><strong>b2</strong></span> does indeed look for two more
configuration files when it starts.</p>
<p>The file <code class="filename">site-config.jam</code> should be
used to set options for an entire system. As it is machine-dependent
<span class="command"><strong>b2</strong></span> expects to find it
in <code class="filename">C:\Windows</code> on Windows platforms and in
<code class="filename">/etc</code> on Unix systems. As <code class=
"filename">site-config.jam</code> is machine-dependent paths to local
libraries are no problem.</p>
<p>Users might not be able to create or change <code class=
"filename">site-config.jam</code> though. They would either need to
wait for system administrators to update the file or be forced again to
add system-specific paths to their own Jamfiles. As neither is a good
solution, <span class="command"><strong>b2</strong></span> also looks
for a file <code class="filename">user-config.jam</code> in a user's
home directory. On Windows it is a subdirectory of <code class=
"filename">C:\Users</code>, on Unix a subdirecory of <code class=
"filename">/home</code>. As the file <code class=
"filename">user-config.jam</code> can be maintained by users it is
probably used more often than <code class=
"filename">site-config.jam</code>.</p>
<p>You use <code class="filename">site-config.jam</code> and
<code class="filename">user-config.jam</code> just like any other
Jamfile. As these configuration files do not belong to a project but to
a machine or a user on a machine they are allowed to contain
machine-specific options. For example they could contain a <code class=
"code">using</code> rule.</p>
<pre class="programlisting">
using msvc ;
</pre>
<p>The <code class="code">using</code> rule above tells <span class=
"command"><strong>b2</strong></span> to use the msvc toolset. If you
know that there is only Visual C++ installed on a system it makes sense
to put this line into a configuration file. Then <span class=
"command"><strong>b2</strong></span> doesn't need to guess anymore
which toolset to use and won't omit a warning.</p>
<p>If you define targets in <code class=
"filename">site-config.jam</code> or <code class=
"filename">user-config.jam</code> and want to refer to these targets in
Jamfiles the <code class="code">project</code> rule must be used to set
a name.</p>
<pre class="programlisting">
using msvc ;
project user-config ;
lib xml : : &lt;name&gt;libxml &lt;search&gt;C:/lib : : &lt;include&gt;C:/include ;
</pre>
<p>The <code class="code">lib</code> rule is used to refer to a
pre-built library whose basename is libxml and can be found in
<code class="filename">C:\lib</code>. A program which uses this XML
library probably needs to include header files from this library.
That's why in the usage requirements - this is the fifth parameter -
the feature <code class="code">&lt;include&gt;</code> is set to
<code class="filename">C:\include</code>: Whoever uses this rule will
inherit the <code class="code">&lt;include&gt;</code> feature.</p>
<p>As the <code class="code">project</code> rule has been used to set
the name user-config a Jamfile can refer to the XML library via
/user-config//xml.</p>
<pre class="programlisting">
exe xmlparser : xmlparser.cpp : &lt;library&gt;/user-config//xml ;
</pre>
<p>In order to build <span class=
"command"><strong>xmlparser</strong></span> the program must be linked
against the XML library. Even though the location of the library and
its header files might vary the Jamfile does not contain any
system-specific paths. The Jamfile expects to find the target xml in
the project user-config. If this is a configuration file it's no
problem to use system-specific paths as after all configuration files
are bound to a machine or to a user on a machine.</p>
<p>As Boost.Build has been created to build and install the Boost C++
libraries there is built-in support to use pre-built Boost C++
libraries more easily.</p>
<pre class="programlisting">
using msvc ;
project user-config ;
using boost : 1.39 : &lt;include&gt;C:/include/boost-1_39 &lt;library&gt;C:/lib ;
</pre>
<p>The <code class="code">using</code> rule must be used to refer to a
toolset called boost. This toolset is different from toolsets like msvc
which you've read about so far: It doesn't contain any programs which
will be run later. As support for pre-built Boost C++ libraries has
been implemented in a toolset though it's required to use the
<code class="code">using</code> rule.</p>
<p>Just as with other libraries the location of the Boost C++ libraries
might vary. Thus it makes sense to put the <code class=
"code">using</code> rule into one of the two configuration files.</p>
<p>It is possible to pass parameters to the <code class=
"code">using</code> rule: The first one is the version number, the
second a list of options. In the Jamfile above the Boost C++ libraries
1.39 are used which can be found in the directories passed as
options.</p>
<p>Once the boost toolset is used it is possible to use Boost C++
libraries without defining targets yourself.</p>
<pre class="programlisting">
import boost ;
boost.use-project 1.39 ;
exe hello : hello.cpp : &lt;library&gt;/boost//thread ;
</pre>
<p>If a program uses a Boost C++ library it can refer to targets in a
project called boost. In order to recognize the project boost though
the boost module must be imported and the rule <code class=
"code">boost.use-project</code> used: Importing the boost module makes
the <code class="code">boost.use-project</code> rule available. This
rule expects a version number as its sole argument. As it is possible
to use the <code class="code">using</code> rule to refer to various
versions of the Boost C++ libraries a project can specify which version
it wants to use. In the Jamfile above the program <span class=
"command"><strong>hello</strong></span> uses Boost.Thread from version
1.39.</p>
</div>
<hr>
<h2 id="rule_reference">Rule reference<br>
<small>Building blocks for Jamfiles</small>
</h2>
<div>
<p>If you manage a project with Boost.Build and create Jamfiles you use
rules all the time. Thus you should know which rules exist and how they
are used. The following table gives you an overview about the most
important rules.</p>
<p>There is a star, plus sign or question mark behind some parameters.
The star means there can be arbitrary many values, the plus sign there
must be at least one value and the question mark there must be zero or
exactly one value.</p>
<table border="0" cellspacing="0" cellpadding="0" id="id369340">
<caption>
Table&nbsp;1.&nbsp;Rules
</caption>
<tbody>
<tr>
<th class="col-md-2">Name</th>
<th>Parameters</th>
<th>Description</th>
</tr>
<tr>
<td>alias</td>
<td>name : sources * : requirements * : default-build * :
usage-requirements *</td>
<td>Refer to sources or any other targets via a new name.</td>
</tr>
<tr>
<td>build-project</td>
<td>dir</td>
<td>Refer to a Jamfile in another directory to build a
project.</td>
</tr>
<tr>
<td>conditional</td>
<td>condition + : requirements *</td>
<td>Create conditional requirements without using conditional
properties.</td>
</tr>
<tr>
<td>exe</td>
<td>name : sources * : requirements * : default-build * :
usage-requirements *</td>
<td>Build an executable.</td>
</tr>
<tr>
<td>explicit</td>
<td>target-names *</td>
<td>Make targets explicit.</td>
</tr>
<tr>
<td>glob</td>
<td>wildcards + : excludes *</td>
<td>Reference files in a directory via wildcards.</td>
</tr>
<tr>
<td>glob-tree</td>
<td>wildcards + : excludes *</td>
<td>Reference files in a directory and all subdirectories via
wildcards.</td>
</tr>
<tr>
<td>install</td>
<td>name-and-dir : sources * : requirements * : default-build
*</td>
<td>Install files to a directory.</td>
</tr>
<tr>
<td>lib</td>
<td>names + : sources * : requirements * : default-build * :
usage-requirements *</td>
<td>Build a library.</td>
</tr>
<tr>
<td>project</td>
<td>id ? : options * : *</td>
<td>Set project options.</td>
</tr>
<tr>
<td>unit-test</td>
<td>target : source : properties *</td>
<td>Build and run an executable.</td>
</tr>
<tr>
<td>use-project</td>
<td>id : where</td>
<td>Reference a Jamfile in another directory to use the project
id as a target.</td>
</tr>
<tr>
<td>using</td>
<td>toolset-module : *</td>
<td>Select a toolset.</td>
</tr>
</tbody>
</table>
<p>Your Boost.Build version might support more rules than listed above.
If you want to find out which rules are supported you should check out
the files in the subdirectory <code class="filename">build</code> of
your Boost.Build installation.</p>
</div>
<hr>
<h2 id="feature_reference">Feature reference<br>
<small>Configuration options for the build process</small>
</h2>
<div>
<p>Features allow you to specify exactly how binaries are built. As
there are many configuration options available the list of features is
pretty long. The following table introduces you to the most important
features.</p>
<table border="0" cellspacing="0" cellpadding="0" id="id369624">
<caption>
Table&nbsp;2.&nbsp;Features
</caption>
<tbody>
<tr>
<th class="col-md-2">Name</th>
<th>Values</th>
<th>Description</th>
</tr>
<tr>
<td>&lt;address-model&gt;</td>
<td>16, 32, 64, 32_64</td>
<td>Generate 16-, 32- or 64-bit code.</td>
</tr>
<tr>
<td>&lt;architecture&gt;</td>
<td>x86, ia64, sparc, power, mips1, mips2, mips3, mips4, mips32,
mips32r2, mips64, parisc, arm, combined, combined-x86-power</td>
<td>Set processor family to generate code for.</td>
</tr>
<tr>
<td>&lt;c++-template-depth&gt;</td>
<td>1, 2, 3, ...</td>
<td>Set maximum template depth.</td>
</tr>
<tr>
<td>&lt;cflags&gt;</td>
<td>...</td>
<td>Pass flags to C compiler.</td>
</tr>
<tr>
<td>&lt;cxxflags&gt;</td>
<td>...</td>
<td>Pass flags to C++ compiler</td>
</tr>
<tr>
<td>&lt;debug-symbols&gt;</td>
<td>on, off</td>
<td>Create debug symbols.</td>
</tr>
<tr>
<td>&lt;def-file&gt;</td>
<td>...</td>
<td>Set path to <code class="filename">def</code> file (specific
to Windows DLLs).</td>
</tr>
<tr>
<td>&lt;define&gt;</td>
<td>...</td>
<td>Define preprocessor directives.</td>
</tr>
<tr>
<td>&lt;embed-manifest&gt;</td>
<td>on, off</td>
<td>Embed manifest (specific to msvc toolset).</td>
</tr>
<tr>
<td>&lt;host-os&gt;</td>
<td>aix, bsd, cygwin, darwin, freebsd, hpux, iphone, linux,
netbsd, openbsd, osf, qnx, qnxnto, sgi, solaris, unix, unixware,
windows</td>
<td>Use in conditional properties if features depend on host
operating systems.</td>
</tr>
<tr>
<td>&lt;include&gt;</td>
<td>...</td>
<td>Set include directories.</td>
</tr>
<tr>
<td>&lt;inlining&gt;</td>
<td>off, on, full</td>
<td>Inline functions.</td>
</tr>
<tr>
<td>&lt;library&gt;</td>
<td>...</td>
<td>Link to a library (use in <code class="code">project</code>
rule).</td>
</tr>
<tr>
<td>&lt;link&gt;</td>
<td>shared, static</td>
<td>Link to shared or static version of a library.</td>
</tr>
<tr>
<td>&lt;linkflags&gt;</td>
<td>...</td>
<td>Pass flags to linker.</td>
</tr>
<tr>
<td>&lt;location&gt;</td>
<td>...</td>
<td>Set directory (use in <code class="code">install</code>
rule).</td>
</tr>
<tr>
<td>&lt;name&gt;</td>
<td>...</td>
<td>Set basename of a library (use in <code class=
"code">lib</code> rule).</td>
</tr>
<tr>
<td>&lt;optimization&gt;</td>
<td>off, speed, space</td>
<td>Generate optimized code.</td>
</tr>
<tr>
<td>&lt;profiling&gt;</td>
<td>off, on</td>
<td>Generate profiled code.</td>
</tr>
<tr>
<td>&lt;runtime-link&gt;</td>
<td>shared, static</td>
<td>Link to single-threaded or thread-safe runtime library.</td>
</tr>
<tr>
<td>&lt;search&gt;</td>
<td>...</td>
<td>Set directory to search for libraries (use in <code class=
"code">lib</code> rule together with <code class=
"code">&lt;name&gt;</code>).</td>
</tr>
<tr>
<td>&lt;source&gt;</td>
<td>...</td>
<td>Set source in requirements parameter of <code class=
"code">project</code> rule or in conditional properties.</td>
</tr>
<tr>
<td>&lt;target-os&gt;</td>
<td>aix, bsd, cygwin, darwin, freebsd, hpux, iphone, linux,
netbsd, openbsd, osf, qnx, qnxnto, sgi, solaris, unix, unixware,
windows</td>
<td>Use in conditional properties if features depend on target
operating systems.</td>
</tr>
<tr>
<td>&lt;threading&gt;</td>
<td>single, multi</td>
<td>Build singlethreaded or thread-safe version.</td>
</tr>
<tr>
<td>&lt;toolset&gt;</td>
<td>gcc, msvc, intel-linux, intel-win, acc, borland, como-linux,
cw, dmc, hp_cxx, sun</td>
<td>Use in conditional properties if features depend on
toolsets.</td>
</tr>
<tr>
<td>&lt;undef&gt;</td>
<td>...</td>
<td>Undefine preprocessor directives.</td>
</tr>
<tr>
<td>&lt;use&gt;</td>
<td>...</td>
<td>Take over only usage requirements of a referenced target but
don't do anything else.</td>
</tr>
<tr>
<td>&lt;variant&gt;</td>
<td>debug, release, profile</td>
<td>Build debug, release or profile version.</td>
</tr>
<tr>
<td>&lt;warnings&gt;</td>
<td>on, all, off</td>
<td>Switch off warnings.</td>
</tr>
<tr>
<td>&lt;warnings-as-errors&gt;</td>
<td>off, on</td>
<td>Treat warnings as errors.</td>
</tr>
</tbody>
</table>
<p>For a complete and up-to-date reference of Boost.Build features look
up the file <code class="filename">builtin.jam</code> in the
subdirectory <code class="filename">tools</code> of your Boost.Build
installation. Search for lines starting with <code class=
"code">feature.feature</code> - this is the internal rule used to
define features.</p>
</div>
<hr id="hrfoot">
<p>Copyright Boris Schäling 2009. Distributed under the Boost Software
License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
<a href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)
</div>
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