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  <chapter id="bbv2.overview">
    <title>Overview</title>

    <para>
      This section will provide the information necessary to create your own
      projects using Boost.Build. The information provided here is relatively
      high-level, and <xref linkend="bbv2.reference"/> as well as the on-line
      help system must be used to obtain low-level documentation (see <xref
      linkend="bbv2.reference.init.options.help"/>).
    </para>

    <para>
      Boost.Build has two parts&mdash;a build engine
      with its own interpreted language, and Boost.Build itself, implemented in
      that language. The chain of events when you type
      <command>b2</command> on the command line is as follows:
      <orderedlist>
        <listitem>
          <para>
            The Boost.Build executable tries to find Boost.Build modules and
            loads the top-level module. The exact process is described in <xref linkend=
            "bbv2.reference.init"/>
          </para>
        </listitem>
        <listitem>
          <para>
            The top-level module loads user-defined configuration files,
            <filename>user-config.jam</filename> and
            <filename>site-config.jam</filename>, which define available toolsets.
          </para>
        </listitem>
        <listitem>
          <para>
            The Jamfile in the current directory is read. That in turn might
            cause reading of further Jamfiles. As a result, a tree of projects
            is created, with targets inside projects.
          </para>
        </listitem>
        <listitem>
          <para>
            Finally, using the build request specified on the command line,
            Boost.Build decides which targets should be built and how. That
            information is passed back to Boost.Jam, which takes care of
            actually running the scheduled build action commands.
          </para>
        </listitem>
      </orderedlist>
    </para>

    <para>
      So, to be able to successfully use Boost.Build, you need to know only four
      things:
      <itemizedlist>
        <listitem>
          <para>
            <link linkend="bbv2.overview.configuration">How to configure
            Boost.Build</link>
          </para>
        </listitem>
        <listitem>
          <para>
            <link linkend="bbv2.overview.targets">How to declare targets in
              Jamfiles</link>
          </para>
        </listitem>
        <listitem>
          <para>
            <link linkend="bbv2.overview.build_process">How the build process
              works</link>
          </para>
        </listitem>
        <listitem>
          <para>
            Some Basics about the Boost.Jam language. See <xref linkend=
            "bbv2.overview.jam_language"/>.
          </para>
        </listitem>
      </itemizedlist>
    </para>

    <section id="bbv2.overview.concepts">
      <title>Concepts</title>

      <para>Boost.Build has a few unique concepts that are introduced in this section. The best
      way to explain the concepts is by comparison with more classical build tools.</para>

      <para>
        When using any flavour of make, you directly specify <firstterm>targets</firstterm>
        and commands that are used to create them from other target. The below example
        creates <filename>a.o</filename> from <filename>a.c</filename> using a hardcoded
        compiler invocation command.
<programlisting>
a.o: a.c
    g++ -o a.o -g a.c
</programlisting>
        This is a rather low-level description mechanism and it's hard to adjust commands, options,
        and sets of created targets depending on the compiler and operating system used.
      </para>

      <para>
        To improve portability, most modern build system provide a set of higher-level
        functions that can be used in build description files. Consider this example:
<programlisting>
add_program ("a", "a.c")
</programlisting>
        This is a function call that creates the targets necessary to create a executable file
        from the source file <filename>a.c</filename>. Depending on configured properties,
        different command lines may be used. However, <code>add_program</code> is higher-level,
        but rather thin level. All targets are created immediately when the build description
        is parsed, which makes it impossible to perform multi-variant builds. Often, change
        in any build property requires a complete reconfiguration of the build tree.
      </para>

      <para>
        In order to support true multivariant builds, Boost.Build introduces the concept of a
        <indexterm> <primary>metatarget</primary> <secondary>definition</secondary></indexterm>
        <indexterm> <primary>main target</primary> <see>metataget</see> </indexterm>
        <firstterm>metatarget</firstterm>&mdash;an object that is created when the build description
        is parsed and can be called later with specific build properties to generate
        actual targets.
      </para>

      <para>
        Consider an example:
<programlisting>
exe a : a.cpp ;
</programlisting>
        When this declaration is parsed, Boost.Build creates a metatarget, but does not
        yet decide what files must be created, or what commands must be used. After
        all build files are parsed, Boost.Build considers the properties requested on the
        command line. Supposed you have invoked Boost.Build with:
<screen>
b2 toolset=gcc toolset=msvc
</screen>
        In that case, the metatarget will be called twice, once with <code>toolset=gcc</code>
        and once with <code>toolset=msvc</code>. Both invocations will produce concrete
        targets, that will have different extensions and use different command lines.
      </para>

      <para>
        Another key concept is
        <indexterm><primary>property</primary><secondary>definition</secondary></indexterm>
        <firstterm>build property</firstterm>. A build property is a variable
        that affects the build process. It can be specified on the command line, and is
        passed when calling a metatarget. While all build tools have a similar mechanism,
        Boost.Build differs by requiring that all build properties are declared in advance,
        and providing a large set of properties with portable semantics.
      </para>

      <para>
        The final concept is <indexterm><primary>property</primary><secondary>propagation</secondary></indexterm>
        <firstterm>property propagation</firstterm>. Boost.Build does not require that every
        metatarget is called with the same properties. Instead, the
        "top-level" metatargets are called with the properties specified on the command line.
        Each metatarget can elect to augment or override some properties (in particular,
        using the requirements mechanism, see <xref linkend="bbv2.overview.targets.requirements"/>).
        Then, the dependency metatargets are called with the modified properties and produce
        concrete targets that are then used in the build process. Of course, dependency metatargets
        maybe in turn modify build properties and have dependencies of their own.
      </para>

      <para>For a more in-depth treatment of the requirements and concepts, you may refer
      to <ulink url="http://syrcose.ispras.ru/2009/files/04_paper.pdf">SYRCoSE 2009 Boost.Build article</ulink>.
      </para>

    </section>

    <section id="bbv2.overview.jam_language">
      <title>Boost.Jam Language</title>

      <para>
        This section will describe the basics of the Boost.Jam language&#x2014;just
        enough for writing Jamfiles. For more information, please see the
        <link linkend="bbv2.jam">Boost.Jam</link> documentation.
      </para>

      <para>
        <link linkend="bbv2.jam">Boost.Jam</link> has an interpreted, procedural
        language. On the lowest level, a <link linkend="bbv2.jam">Boost.Jam
        </link> program consists of variables and <indexterm><primary>rule
        </primary></indexterm> <firstterm>rules</firstterm> (the Jam term for
        functions). They are grouped into modules&#x2014;there is one global
        module and a number of named modules. Besides that, a <link linkend=
        "bbv2.jam">Boost.Jam</link> program contains classes and class
        instances.
      </para>

      <para>
        Syntantically, a <link linkend="bbv2.jam">Boost.Jam</link> program
        consists of two kind of elements&#x2014;keywords (which have a special
        meaning to <link linkend="bbv2.jam">Boost.Jam</link>) and literals.
        Consider this code:
<programlisting>
a = b ;
</programlisting>
        which assigns the value <literal>b</literal> to the variable <literal>a
        </literal>. Here, <literal>=</literal> and <literal>;</literal> are
        keywords, while <literal>a</literal> and <literal>b</literal> are
        literals.
        <warning>
          <para>
            All syntax elements, even keywords, must be separated by spaces. For
            example, omitting the space character before <literal>;</literal>
            will lead to a syntax error.
          </para>
        </warning>
        If you want to use a literal value that is the same as some keyword, the
        value can be quoted:
<programlisting>
a = "=" ;
</programlisting>
      </para>

      <para>
        All variables in <link linkend="bbv2.jam">Boost.Jam</link> have the same
        type&#x2014;list of strings. To define a variable one assigns a value to
        it, like in the previous example. An undefined variable is the same as a
        variable with an empty value. Variables can be accessed using the
        <code>$(<replaceable>variable</replaceable>)</code> syntax. For example:
<programlisting>
a = $(b) $(c) ;
</programlisting>
      </para>

      <para>
        Rules are defined by specifying the rule name, the parameter names, and
        the allowed value list size for each parameter.
<programlisting>
rule <replaceable>example</replaceable>
 (
     <replaceable>parameter1</replaceable> :
     <replaceable>parameter2 ?</replaceable> :
     <replaceable>parameter3 +</replaceable> :
     <replaceable>parameter4 *</replaceable>
 )
 {
    # rule body
 }
 </programlisting>
        When this rule is called, the list passed as the first argument must
        have exactly one value. The list passed as the second argument can
        either have one value of be empty. The two remaining arguments can be
        arbitrarily long, but the third argument may not be empty.
      </para>

      <para>
        The overview of <link linkend="bbv2.jam">Boost.Jam</link> language
        statements is given below:
<programlisting>
helper 1 : 2 : 3 ;
x = [ helper 1 : 2 : 3 ] ;
</programlisting>
        This code calls the named rule with the specified arguments. When the
        result of the call must be used inside some expression, you need to add
        brackets around the call, like shown on the second line.
<programlisting>
if cond { statements } [ else { statements } ]
</programlisting>
        This is a regular if-statement. The condition is composed of:
        <itemizedlist>
          <listitem>
            <para>
              Literals (true if at least one string is not empty)
            </para>
          </listitem>
          <listitem>
            <para>
              Comparisons: <code>a <replaceable>operator</replaceable> b</code>
              where <replaceable>operator</replaceable> is one of
              <code>=</code>, <code>!=</code>, <code>&lt;</code>,
              <code>&gt;</code>, <code>&lt;=</code> or <code>&gt;=</code>. The
              comparison is done pairwise between each string in the left and
              the right arguments.
            </para>
          </listitem>
          <listitem>
            <para>
              Logical operations: <code>! a</code>, <code>a &amp;&amp; b</code>,
              <code>a || b</code>
            </para>
          </listitem>
          <listitem>
            <para>
              Grouping: <code>( cond )</code>
            </para>
          </listitem>
        </itemizedlist>
<programlisting>
for var in list { statements }
</programlisting>
        Executes statements for each element in list, setting the variable
        <varname>var</varname> to the element value.
<programlisting>
while cond { statements }
</programlisting>
        Repeatedly execute statements while cond remains true upon entry.
<programlisting>
return values ;
</programlisting>
        This statement should be used only inside a rule and assigns
        <code>values</code> to the return value of the rule.
        <warning>
          <para>
            The <code>return</code> statement does not exit the rule. For
            example:
<programlisting>
rule test ( )
{
   if 1 = 1
   {
      return "reasonable" ;
   }
   return "strange" ;
}
</programlisting>
            will return <literal>strange</literal>, not
            <literal>reasonable</literal>.
          </para>
        </warning>
<programlisting>
import <replaceable>module</replaceable> ;
import <replaceable>module</replaceable> : <replaceable>rule</replaceable> ;
</programlisting>
        The first form imports the specified module. All rules from that
        module are made available using the qualified name: <code><replaceable>
        module</replaceable>.<replaceable>rule</replaceable></code>. The second
        form imports the specified rules only, and they can be called using
        unqualified names.
      </para>

      <para id="bbv2.overview.jam_language.actions">
        Sometimes, you need to specify the actual command lines to be used
        when creating targets. In the jam language, you use named actions to do
        this. For example:
<programlisting>
actions create-file-from-another
{
    create-file-from-another $(&lt;) $(&gt;)
}
</programlisting>
        This specifies a named action called <literal>
        create-file-from-another</literal>. The text inside braces is the
        command to invoke. The <literal>$(&lt;)</literal> variable will be
        expanded to a list of generated files, and the <literal>$(&gt;)
        </literal> variable will be expanded to a list of source files.
      </para>

      <para>
        To adjust the command line flexibly, you can define a rule with the same
        name as the action and taking three parameters&mdash;targets, sources and
        properties. For example:
<programlisting>
rule create-file-from-another ( targets * : sources * : properties * )
{
   if &lt;variant&gt;debug in $(properties)
   {
       OPTIONS on $(targets) = --debug ;
   }
}
actions create-file-from-another
{
    create-file-from-another $(OPTIONS) $(&lt;) $(&gt;)
}
</programlisting>
        In this example, the rule checks if a certain build property is specified.
        If so, it sets the variable <varname>OPTIONS</varname> that is then used
        inside the action. Note that the variables set "on a target" will be
        visible only inside actions building that target, not globally. Were
        they set globally, using variable named <varname>OPTIONS</varname> in
        two unrelated actions would be impossible.
      </para>

      <para>
        More details can be found in the Jam reference, <xref
        linkend="jam.language.rules"/>.
      </para>
    </section>

  <section id="bbv2.overview.configuration">
    <title>Configuration</title>

    <para>
      On startup, Boost.Build searches and reads two configuration files:
      <filename>site-config.jam</filename> and <filename>user-config.jam</filename>.
      The first one is usually installed and maintained by a system administrator, and
      the second is for the user to modify.  You can edit the one in the top-level
      directory of your Boost.Build installation or create a copy in your home
      directory and edit the copy.  The following table explains where both files
      are searched.
    </para>

    <table id="bbv2.reference.init.config">
      <title>Search paths for configuration files</title>

      <tgroup cols="3">
        <thead>

          <row>
            <entry></entry>

            <entry>site-config.jam</entry>

            <entry>user-config.jam</entry>
          </row>

        </thead>
        <tbody>

          <row>
            <entry>Linux</entry>

            <entry>
              <simpara><code>/etc</code></simpara>
              <simpara><code>$HOME</code></simpara>
              <simpara><code>$BOOST_BUILD_PATH</code></simpara>
            </entry>

            <entry>
              <simpara><code>$HOME</code></simpara>
              <simpara><code>$BOOST_BUILD_PATH</code></simpara>
            </entry>
          </row>

          <row>
            <entry>Windows</entry>

            <entry>
              <simpara><code>%SystemRoot%</code></simpara>
              <simpara><code>%HOMEDRIVE%%HOMEPATH%</code></simpara>
              <simpara><code>%HOME%</code></simpara>
              <simpara><code>%BOOST_BUILD_PATH%</code></simpara>
            </entry>

            <entry>
              <simpara><code>%HOMEDRIVE%%HOMEPATH%</code></simpara>
              <simpara><code>%HOME%</code></simpara>
              <simpara><code>%BOOST_BUILD_PATH%</code></simpara>
            </entry>
          </row>
        </tbody>
      </tgroup>
    </table>

    <tip>
      <para>
        You can use the <command>--debug-configuration</command> option to
        find which configuration files are actually loaded.
      </para>
    </tip>

    <para>
      Usually, <filename>user-config.jam</filename> just defines the available compilers
      and other tools (see <xref linkend="bbv2.recipies.site-config"/> for more advanced
      usage). A tool is configured using the following syntax:
    </para>

<programlisting>
using <replaceable>tool-name</replaceable> : ... ;
</programlisting>
<para>
      The <code language="jam">using</code> rule is given the name of tool, and
      will make that tool available to Boost.Build. For example,
<programlisting>
using gcc ;
</programlisting> will make the <ulink url="http://gcc.gnu.org">GCC</ulink> compiler available.
    </para>

    <para>
      All the supported tools are documented in <xref linkend="bbv2.reference.tools"/>,
      including the specific options they take. Some general notes that apply to most
      C++ compilers are below.
    </para>

    <para>
      For all the C++ compiler toolsets that Boost.Build supports
      out-of-the-box, the list of parameters to
      <code language="jam">using</code> is the same: <parameter
      class="function">toolset-name</parameter>, <parameter
      class="function">version</parameter>, <parameter
      class="function">invocation-command</parameter>, and <parameter
      class="function">options</parameter>.
    </para>

    <para>If you have a single compiler, and the compiler executable
      <itemizedlist>
      <listitem><para>has its &#x201C;usual name&#x201D; and is in the
      <envar>PATH</envar>, or</para></listitem>
      <listitem><para>was installed in a standard &#x201C;installation
      directory&#x201D;, or</para></listitem>
      <listitem><para>can be found using a global system like the Windows
      registry.</para></listitem>
      </itemizedlist>
    it can be configured by simply:</para>
<programlisting>
using <replaceable>tool-name</replaceable> ;
</programlisting>
    <!-- TODO: mention auto-configuration? -->

    <para>If the compiler is installed in a custom directory, you should provide the
    command that invokes the compiler, for example:</para>
<programlisting>
using gcc : : g++-3.2 ;
using msvc : : "Z:/Programs/Microsoft Visual Studio/vc98/bin/cl" ;
</programlisting>
    <para>
      Some Boost.Build toolsets will use that path to take additional actions
      required before invoking the compiler, such as calling vendor-supplied
      scripts to set up its required environment variables. When the compiler
      executables for C and C++ are different, the path to the C++ compiler
      executable must be specified. The command can
      be any command allowed by the operating system. For example:
<programlisting>
using msvc : : echo Compiling &#x26;&#x26; foo/bar/baz/cl ;
</programlisting>
      will work.
    </para>

    <para>
      To configure several versions of a toolset, simply invoke the
      <code language="jam">using</code> rule multiple times:
<programlisting>
using gcc : 3.3 ;
using gcc : 3.4 : g++-3.4 ;
using gcc : 3.2 : g++-3.2 ;
</programlisting>
      Note that in the first call to <code language="jam">using</code>, the
      compiler found in the <envar>PATH</envar> will be used, and there is no
      need to explicitly specify the command.
    </para>

<!-- TODO: This is not actually relevant for gcc now, and we need to rethink this
    <para>As shown above, both the <parameter
    class="function">version</parameter> and <parameter
    class="function">invocation-command</parameter> parameters are
    optional, but there's an important restriction: if you configure
    the same toolset more than once, you must pass the <parameter
    class="function">version</parameter>
    parameter every time. For example, the following is not allowed:
<programlisting>
using gcc ;
using gcc : 3.4 : g++-3.4 ;
</programlisting>
      because the first <functionname>using</functionname> call does
      not specify a <parameter class="function">version</parameter>.
    </para>  -->

    <para>
      Many of toolsets have an <parameter class="function">options</parameter>
      parameter to fine-tune the configuration. All of
      Boost.Build's standard compiler toolsets accept four options
      <varname>cflags</varname>, <varname>cxxflags</varname>,
      <varname>compileflags</varname> and <varname>linkflags</varname> as <parameter
      class="function">options</parameter> specifying flags that will be
      always passed to the corresponding tools. Values of the
      <varname>cflags</varname> feature are passed directly to the C
      compiler, values of the <varname>cxxflags</varname> feature are
      passed directly to the C++ compiler, and values of the
      <varname>compileflags</varname> feature are passed to both. For
      example, to configure a <command>gcc</command> toolset so that it
      always generates 64-bit code you could write:
<programlisting>
        using gcc : 3.4 : : &lt;compileflags&gt;-m64 &lt;linkflags&gt;-m64 ;
</programlisting>
    </para>

    <warning>
      <para>
        Although the syntax used to specify toolset options is very similar
        to syntax used to specify requirements in Jamfiles, the toolset options
        are not the same as features. Don't try to specify a feature value
        in toolset initialization.
      </para>
    </warning>

    </section>

    <section id="bbv2.overview.invocation">
      <title>Invocation</title>

      <para>To invoke Boost.Build, type <command>b2</command> on the command line. Three kinds
      of command-line tokens are accepted, in any order:</para>
      <variablelist>
        <varlistentry>
          <term>options</term>

          <listitem><para>Options start with either one or two dashes.  The standard options
          are listed below, and each project may add additional options</para></listitem>
        </varlistentry>

        <varlistentry>
          <term>properties</term>

          <listitem><para>Properties specify details of what you want to build (e.g. debug
          or release variant).  Syntactically, all command line tokens with an equal sign in them
          are considered to specify properties.  In the simplest form, a property looks like
          <command><replaceable>feature</replaceable>=<replaceable>value</replaceable></command>
          </para></listitem>
        </varlistentry>

        <varlistentry>
          <term>target</term>

          <listitem><para>All tokens that are neither options nor properties specify
          what targets to build.  The available targets entirely depend on the project
          you are building.</para></listitem>
        </varlistentry>
      </variablelist>

      <section id="bbv2.overview.invocation.examples">
        <title>Examples</title>

        <para>To build all targets defined in the Jamfile in the current directory with the default properties, run:
<screen>
b2
</screen>
        </para>

        <para>To build specific targets, specify them on the command line:
<screen>
b2 lib1 subproject//lib2
</screen>
        </para>

        <para>To request a certain value for some property, add <literal>
        <replaceable>property</replaceable>=<replaceable>value</replaceable></literal> to the command line:
<screen>
b2 toolset=gcc variant=debug optimization=space
</screen>
        </para>
      </section>

      <section id="bbv2.overview.invocation.options">
        <title>Options</title>

        <para>Boost.Build recognizes the following command line options.</para>

        <variablelist>

          <varlistentry id="bbv2.reference.init.options.help">
            <term><option>--help</option></term>
            <listitem>
              <para>Invokes the online help system. This prints general
              information on how to use the help system with additional
              --help* options.
              </para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term><option>--clean</option></term>
            <listitem>
              <para>Cleans all targets in the current directory and
              in any subprojects. Note that unlike the <literal>clean</literal>
              target in make, you can use <literal>--clean</literal>
              together with target names to clean specific targets.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term><option>--clean-all</option></term>
            <listitem>
              <para>Cleans all targets,
              no matter where they are defined. In particular, it will clean targets
              in parent Jamfiles, and targets defined under other project roots.
              </para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term><option>--build-dir</option></term>
            <listitem>
              <para>Changes the build directories for all project roots being built. When
              this option is specified, all Jamroot files must declare a project name.
              The build directory for the project root will be computed by concatanating
              the value of the <option>--build-dir</option> option, the project name
              specified in Jamroot, and the build dir specified in Jamroot
              (or <literal>bin</literal>, if none is specified).
              </para>

              <para>The option is primarily useful when building from read-only
              media, when you can't modify Jamroot.
              </para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term><option>--abbreviate-paths</option></term>
            <listitem>
              <para>Compresses target paths by abbreviating each component.
              This option is useful to keep paths from becoming longer than
              the filesystem supports.  See also <xref linkend="bbv2.reference.buildprocess.targetpath"/>.
              </para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term><option>--hash</option></term>
            <listitem>
              <para>Compresses target paths using an MD5 hash.  This option is
              useful to keep paths from becoming longer than the filesystem
              supports.  This option produces shorter paths than --abbreviate-paths
              does, but at the cost of making them less understandable.
              See also <xref linkend="bbv2.reference.buildprocess.targetpath"/>.
              </para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term><option>--version</option></term>
            <listitem>
              <para>Prints information on the Boost.Build and Boost.Jam
              versions.
              </para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term><option>-a</option></term>
            <listitem>
              <para>Causes all files to be rebuilt.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term><option>-n</option></term>
            <listitem>
              <para>Do not execute the commands, only print them.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term><option>-q</option></term>
            <listitem>
              <para>Stop at the first error, as opposed to continuing to build targets
              that don't depend on the failed ones.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term><option>-j <replaceable>N</replaceable></option></term>
            <listitem>
              <para>Run up to <replaceable>N</replaceable> commands in parallel.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term><option>--debug-configuration</option></term>
            <listitem>
              <para>Produces debug information about the loading of Boost.Build
              and toolset files.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term><option>--debug-building</option></term>
            <listitem>
              <para>Prints what targets are being built and with what properties.
              </para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term><option>--debug-generators</option></term>
            <listitem>
              <para>Produces debug output from the generator search process.
              Useful for debugging custom generators.
              </para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term><option>-d0</option></term>
            <listitem>
              <para>Suppress all informational messages.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term><option>-d <replaceable>N</replaceable></option></term>
            <listitem>
              <para>Enable cummulative debugging levels from 1 to n. Values are:
              <orderedlist>
                <listitem>Show the actions taken for building targets, as they are executed (the default).</listitem>
                <listitem>Show "quiet" actions and display all action text, as they are executed.</listitem>
                <listitem>Show dependency analysis, and target/source timestamps/paths.</listitem>
                <listitem>Show arguments and timming of shell invocations.</listitem>
                <listitem>Show rule invocations and variable expansions.</listitem>
                <listitem>Show directory/header file/archive scans, and attempts at binding to targets.</listitem>
                <listitem>Show variable settings.</listitem>
                <listitem>Show variable fetches, variable expansions, and evaluation of '"if"' expressions.</listitem>
                <listitem>Show variable manipulation, scanner tokens, and memory usage.</listitem>
                <listitem>Show profile information for rules, both timing and memory.</listitem>
                <listitem>Show parsing progress of Jamfiles.</listitem>
                <listitem>Show graph of target dependencies.</listitem>
                <listitem>Show change target status (fate).</listitem>
              </orderedlist>
              </para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term><option>-d +<replaceable>N</replaceable></option></term>
            <listitem>
              <para>Enable debugging level <replaceable>N</replaceable>.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term><option>-o <replaceable>file</replaceable></option></term>
            <listitem>
              <para>Write the updating actions to the specified file instead of running them.
              </para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term><option>-s <replaceable>var</replaceable>=<replaceable>value</replaceable></option></term>
            <listitem>
              <para>Set the variable <replaceable>var</replaceable> to
              <replaceable>value</replaceable> in the global scope of the jam
              language interpreter, overriding variables imported from the
              environment.
              </para>
            </listitem>
          </varlistentry>
        </variablelist>
      </section>

      <section id="bbv2.overview.invocation.properties">
        <title>Properties</title>

        <para>In the simplest case, the build is performed with a single set of properties,
        that you specify on the command line with elements in the form
        <command><replaceable>feature</replaceable>=<replaceable>value</replaceable></command>.
        The complete list of features can be found in <xref linkend="bbv2.overview.builtins.features"/>.
        The most common features are summarized below.</para>

        <table>
          <tgroup cols="3">
            <thead>

              <row>
                <entry>Feature</entry>

                <entry>Allowed values</entry>

                <entry>Notes</entry>
              </row>

            </thead>
            <tbody>

              <row>
                <entry>variant</entry>

                <entry>debug,release</entry>

                <entry></entry>
              </row>

              <row>
                <entry>link</entry>

                <entry>shared,static</entry>

                <entry>Determines if Boost.Build creates shared or static libraries</entry>
              </row>

              <row>
                <entry>threading</entry>

                <entry>single,multi</entry>

                <entry>Cause the produced binaries to be thread-safe.  This requires proper support in the source code itself.</entry>
              </row>

              <row>
                <entry>address-model</entry>

                <entry>32,64</entry>

                <entry>Explicitly request either 32-bit or 64-bit code generation. This typically
                requires that your compiler is appropriately configured. Please refer to
                <xref linkend="bbv2.reference.tools.compilers"/> and your compiler documentation
                in case of problems.</entry>
              </row>

              <row>
                <entry>toolset</entry>

                <entry>(Depends on configuration)</entry>

                <entry>The C++ compiler to use. See <xref linkend="bbv2.reference.tools.compilers"/> for a detailed list.</entry>
              </row>

              <row>
                <entry>include</entry>

                <entry>(Arbitrary string)</entry>

                <entry>Additional include paths for C and C++ compilers.</entry>
              </row>

              <row>
                <entry>define</entry>

                <entry>(Arbitrary string)</entry>

                <entry>Additional macro definitions for C and C++ compilers. The string should be either
                <code>SYMBOL</code> or <code>SYMBOL=VALUE</code></entry>
              </row>

              <row>
                <entry>cxxflags</entry>

                <entry>(Arbitrary string)</entry>

                <entry>Custom options to pass to the C++ compiler.</entry>
              </row>

              <row>
                <entry>cflags</entry>

                <entry>(Arbitrary string)</entry>

                <entry>Custom options to pass to the C compiler.</entry>
              </row>

              <row>
                <entry>linkflags</entry>

                <entry>(Arbitrary string)</entry>

                <entry>Custom options to pass to the C++ linker.</entry>
              </row>

              <row>
                <entry>runtime-link</entry>

                <entry>shared,static</entry>

                <entry>Determines if shared or static version of C and C++ runtimes should be used.</entry>
              </row>

            </tbody>
          </tgroup>
        </table>

        <para>If you have more than one version of a given C++ toolset (e.g. configured in
        <filename>user-config.jam</filename>, or autodetected, as happens with msvc), you can
        request the specific version by passing
        <code><replaceable>toolset</replaceable>-<replaceable>version</replaceable></code> as
        the value of the <code>toolset</code> feature, for example <code>toolset=msvc-8.0</code>.
        </para>


        <para>
          If a feature has a fixed set of values it can be specified more than
          once on the command line. <!-- define 'base' and link to it -->
          In which case, everything will be built several times --
          once for each specified value of a feature.  For example, if you use
        </para>
<screen>
b2 link=static link=shared threading=single threading=multi
</screen>
        <para>
          Then a total of 4 builds will be performed.  For convenience,
          instead of specifying all requested values of a feature in separate command line elements,
          you can separate the values with commas, for example:
        </para>
<screen>
b2 link=static,shared threading=single,multi
</screen>
        <para>
          The comma has this special meaning only if the feature has a fixed set of values, so
        </para>
<screen>
b2 include=static,shared
</screen>
        <para>is not treated specially.</para>

      </section>

      <section id="bbv2.overview.invocation.targets">
        <title>Targets</title>

        <para>All command line elements that are neither options nor properties are the names of the
        targets to build. See <xref linkend="bbv2.reference.ids"/>. If no target is specified,
        the project in the current directory is built.</para>
      </section>

    </section>

    <section id="bbv2.overview.targets">
      <title>Declaring Targets</title>

      <para id="bbv2.overview.targets.main">
        A <firstterm>Main target</firstterm> is a user-defined named
        entity that can be built, for example an executable file.
        Declaring a main target is usually done using one of the main
        target rules described in <xref linkend=
        "bbv2.reference.rules"/>.  The user can also declare
        custom main target rules as shown in <xref
        linkend="bbv2.extending.rules"/>.
      </para>

      <indexterm><primary>main target</primary><secondary>declaration
      syntax</secondary></indexterm>
      <para>Most main target rules in Boost.Build have the same common
      signature:</para>

      <!-- I think we maybe ought to be talking about a common
           _signature_ here, having already explained Boost.Jam function
           signatures at the beginning of this chapter.  Then we could show
           ( main-target-name : sources * : requirements * : default-build * : usage-requirements * )
           instead.  More precise.

           Also, I suggest replacing "default-build" by "default-properties" everywhere.
      -->

<indexterm><primary>common signature</primary></indexterm>
<anchor id="bbv2.main-target-rule-syntax"/>
<programlisting>
rule <replaceable>rule-name</replaceable> (
     main-target-name :
     sources + :
     requirements * :
     default-build * :
     usage-requirements * )
</programlisting>

      <itemizedlist>
        <listitem>
          <simpara>
            <parameter>main-target-name</parameter> is the name used
            to request the target on command line and to use it from
            other main targets. A main target name may contain
            alphanumeric characters, dashes
            (&#x2018;<code>-</code>&#x2019;), and underscores
            (&#x2018;<code>_</code>&#x2019;).
          </simpara>
        </listitem>

        <listitem>
          <simpara>
            <parameter>sources</parameter> is the list of source files and other main
            targets that must be combined.
          </simpara>
        </listitem>

        <listitem>
          <simpara>
            <parameter>requirements</parameter> is the list of properties that must always
            be present when this main target is built.
          </simpara>
        </listitem>

        <listitem>
          <simpara>
            <parameter>default-build</parameter> is the list of properties that will be used
            unless some other value of the same feature is already
            specified, e.g. on the command line or by propagation from a dependent target.
          </simpara>
        </listitem>

        <listitem>
          <simpara>
            <parameter>usage-requirements</parameter> is the list of properties that will be
            propagated to all main targets that use this one, i.e. to all its
            dependents.
          </simpara>
        </listitem>
      </itemizedlist>

      <para>
        Some main target rules have a different list of parameters as explicitly
        stated in their documentation.
      </para>

      <para>The actual requirements for a target are obtained by refining
      the requirements of the project where the target is declared with the
      explicitly specified requirements. The same is true for
      usage-requirements. More details can be found in
      <xref linkend="bbv2.reference.variants.proprefine"/>
      </para>

      <section>
        <title>Name</title>

        <!-- perphaps we should use 'name-target-name' to closer
        bind this description to the rule's signature. Here, and for
        other parameters.  -->
        <para>The name of main target has two purposes. First, it's used to refer to this target from
        other targets and from command line. Second, it's used to compute the names of the generated files.
        Typically, filenames are obtained from main target name by appending system-dependent suffixes and
        prefixes.
        </para>

        <para>The name of a main target can contain alphanumeric characters,
        dashes, undescores and dots. The entire
        name is significant when resolving references from other targets. For determining filenames, only the
        part before the first dot is taken. For example:</para>
<programlisting>
obj test.release : test.cpp : &lt;variant&gt;release ;
obj test.debug : test.cpp : &lt;variant&gt;debug ;
</programlisting>
        <para>will generate two files named <filename>test.obj</filename> (in two different directories), not
        two files named <filename>test.release.obj</filename> and <filename>test.debug.obj</filename>.
        </para>

      </section>

      <section>
        <title>Sources</title>

        <para>The list of sources specifies what should be processed to
        get the resulting targets. Most of the time, it's just a list of
        files. Sometimes, you'll want to automatically construct the
        list of source files rather than having to spell it out
        manually, in which case you can use the
        <link linkend="bbv2.reference.rules.glob">glob</link> rule.
        Here are two examples:</para>
<programlisting>
exe a : a.cpp ;           # a.cpp is the only source file
exe b : [ glob *.cpp ] ;  # all .cpp files in this directory are sources
</programlisting>
        <para>
        Unless you specify a file with an absolute path, the name is
        considered relative to the source directory&#x200A;&#x2014;&#x200A;which is typically
        the directory where the Jamfile is located, but can be changed as
        described in <xref linkend=
          "bbv2.overview.projects.attributes.projectrule"/>.
        </para>

        <para>
          <!-- use "project-id" here? -->
          The list of sources can also refer to other main targets. Targets in
          the same project can be referred to by name, while targets in other
          projects must be qualified with a directory or a symbolic project
          name. The directory/project name is separated from the target name by
          a double forward slash. There is no special syntax to distinguish the
          directory name from the project name&#x2014;the part before the double
          slash is first looked up as project name, and then as directory name.
          For example:
        </para>
<programlisting>
lib helper : helper.cpp ;
exe a : a.cpp helper ;
# Since all project ids start with slash, ".." is a directory name.
exe b : b.cpp ..//utils ;
exe c : c.cpp /boost/program_options//program_options ;
</programlisting>
        <para>
          The first exe uses the library defined in the same project. The second
          one uses some target (most likely a library) defined by a Jamfile one
          level higher. Finally, the third target uses a <ulink url=
          "http://boost.org">C++ Boost</ulink> library, referring to it using
          its absolute symbolic name. More information about target references
          can be found in <xref linkend="bbv2.tutorial.libs"/> and <xref
          linkend="bbv2.reference.ids"/>.
        </para>
      </section>

      <section id="bbv2.overview.targets.requirements">
        <title>Requirements</title>
        <indexterm><primary>requirements</primary></indexterm>
        <para>Requirements are the properties that should always be present when
        building a target. Typically, they are includes and defines:
<programlisting>
exe hello : hello.cpp : &lt;include&gt;/opt/boost &lt;define&gt;MY_DEBUG ;
</programlisting>
        There are a number of other features, listed in
        <xref linkend="bbv2.overview.builtins.features"/>. For example if
        a library can only be built statically, or a file can't be compiled
        with optimization due to a compiler bug, one can use
<programlisting>
lib util : util.cpp : &lt;link&gt;static ;
obj main : main.cpp : &lt;optimization&gt;off ;
</programlisting>
        </para>

        <para id="bbv2.overview.targets.requirements.conditional">
          <indexterm><primary>requirements</primary><secondary>conditional</secondary></indexterm>
          Sometimes, particular relationships need to be maintained
        among a target's build properties. This can be achieved with
        <firstterm>conditional
        requirements</firstterm>. For example, you might want to set
        specific <code>#defines</code> when a library is built as shared,
        or when a target's <code>release</code> variant is built in
        release mode.
<programlisting>
lib network : network.cpp
    : <emphasis role="bold">&lt;link&gt;shared:&lt;define&gt;NETWORK_LIB_SHARED</emphasis>
     &lt;variant&gt;release:&lt;define&gt;EXTRA_FAST
    ;
</programlisting>

        In the example above, whenever <filename>network</filename> is
        built with <code>&lt;link&gt;shared</code>,
        <code>&lt;define&gt;NETWORK_LIB_SHARED</code> will be in its
        properties, too.
        </para>

        <para>You can use several properties in the condition, for example:
<programlisting>
lib network : network.cpp
    : &lt;toolset&gt;gcc,&lt;optimization&gt;speed:&lt;define&gt;USE_INLINE_ASSEMBLER
    ;
</programlisting>
        </para>

        <para id="bbv2.overview.targets.requirements.indirect">
          <indexterm><primary>requirements</primary><secondary>indirect</secondary></indexterm>
          A more powerful variant of conditional requirements
          is <firstterm>indirect conditional requirements</firstterm>.
          You can provide a rule that will be called with the current build properties and can compute additional properties
          to be added. For example:
<programlisting>
lib network : network.cpp
    : &lt;conditional&gt;@my-rule
    ;
rule my-rule ( properties * )
{
    local result ;
    if &lt;toolset&gt;gcc &lt;optimization&gt;speed in $(properties)
    {
        result += &lt;define&gt;USE_INLINE_ASSEMBLER ;
    }
    return $(result) ;
}
</programlisting>
        This example is equivalent to the previous one, but for complex cases, indirect conditional
        requirements can be easier to write and understand.
        </para>

        <para>Requirements explicitly specified for a target are usually
        combined with the requirements specified for the containing project. You
        can cause a target to completely ignore a specific project requirement
        using the syntax by adding a minus sign before the property, for example:
<programlisting>
exe main : main.cpp : <emphasis role="bold">-&lt;define&gt;UNNECESSARY_DEFINE</emphasis> ;
</programlisting>
        This syntax is the only way to ignore free properties, such as defines,
        from a parent. It can be also useful for ordinary properties. Consider
        this example:
<programlisting>
project test : requirements &lt;threading&gt;multi ;
exe test1 : test1.cpp ;
exe test2 : test2.cpp : &lt;threading&gt;single ;
exe test3 : test3.cpp : -&lt;threading&gt;multi ;
</programlisting>
        Here, <code>test1</code> inherits the project requirements and will always
        be built in multi-threaded mode. The <code>test2</code> target
        <emphasis>overrides</emphasis> the project's requirements and will
        always be built in single-threaded mode. In contrast, the
        <code>test3</code> target <emphasis>removes</emphasis> a property
        from the project requirements and will be built either in single-threaded or
        multi-threaded mode depending on which variant is requested by the
        user.</para>

        <para>Note that the removal of requirements is completely textual:
        you need to specify exactly the same property to remove it.</para>

      </section>

      <section>
        <title>Default Build</title>

        <para>The <varname>default-build</varname> parameter
        is a set of properties to be used if the build request does
        not otherwise specify a value for features in the set. For example:
<programlisting>
exe hello : hello.cpp : : &lt;threading&gt;multi ;
</programlisting>
        would build a multi-threaded target unless the user
        explicitly requests a single-threaded version. The difference between
        the requirements and the default-build is that the requirements cannot be
        overridden in any way.
        </para>
      </section>

      <section>
        <title>Additional Information</title>

        <para>
          The ways a target is built can be so different that
          describing them using conditional requirements would be
          hard. For example, imagine that a library actually uses
          different source files depending on the toolset used to build
          it. We can express this situation using <firstterm>target
          alternatives</firstterm>:
<programlisting>
lib demangler : dummy_demangler.cpp ;                # alternative 1
lib demangler : demangler_gcc.cpp : &lt;toolset&gt;gcc ;   # alternative 2
lib demangler : demangler_msvc.cpp : &lt;toolset&gt;msvc ; # alternative 3
</programlisting>
          In the example above, when built with <literal>gcc</literal>
          or <literal>msvc</literal>, <filename>demangler</filename>
          will use a source file specific to the toolset.  Otherwise, it
          will use a generic source file,
          <filename>dummy_demangler.cpp</filename>.
        </para>

        <para>It is possible to declare a target inline, i.e. the "sources"
        parameter may include calls to other main rules. For example:</para>

<programlisting>
exe hello : hello.cpp
    [ obj helpers : helpers.cpp : &lt;optimization&gt;off ] ;</programlisting>

         <para>
           Will cause "helpers.cpp" to be always compiled without
           optimization. When referring to an inline main target, its declared
           name must be prefixed by its parent target's name and two dots. In
           the example above, to build only helpers, one should run
           <code>b2 hello..helpers</code>.
         </para>

         <para>When no target is requested on the command line, all targets in the
         current project will be built. If a target should be built only by
         explicit request, this can be expressed by the
         <link linkend="bbv2.reference.rules.explicit">explicit</link> rule:
         <programlisting>
explicit install_programs ;</programlisting>
         </para>

      </section>
    </section>

    <section id="bbv2.overview.projects">
      <title>Projects</title>

      <para>As mentioned before, targets are grouped into projects,
      and each Jamfile is a separate project. Projects are useful
      because they allow us to group related targets together, define
      properties common to all those targets, and assign a symbolic
      name to the project that can be used in referring to its
      targets.
      </para>

      <para>Projects are named using the
      <code language="jam">project</code> rule, which has the
      following syntax:
<programlisting>
project <replaceable>id</replaceable> : <replaceable>attributes</replaceable> ;
</programlisting>
        Here, <replaceable>attributes</replaceable> is a sequence of
        rule arguments, each of which begins with an attribute-name
        and is followed by any number of build properties. The list
        of attribute names along with its handling is also shown in
        the table below. For example, it is possible to write:
<programlisting>
project tennis
    : requirements &lt;threading&gt;multi
    : default-build release
    ;
</programlisting>
      </para>

      <para>The possible attributes are listed below.</para>

      <para><emphasis>Project id</emphasis> 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". <link linkend=
          "bbv2.reference.ids">Target references</link> make use of project ids to
        specify a target.</para>
        <!--
             This is actually spelled "project-id," isn't it?  You
             have to fix all of these and use a code font.  Also below
             in the table.
        -->

      <para><emphasis>Source location</emphasis> specifies the directory where sources
        for the project are located.</para>

      <para><emphasis>Project requirements</emphasis> are requirements that apply to
        all the targets in the projects as well as all subprojects.</para>

      <para><emphasis>Default build</emphasis> is the build request that should be
        used when no build request is specified explicitly.</para>
        <!--
             This contradicts your earlier description of default
             build and I believe it is incorrect.  Specifying a build
             request does not neccessarily render default build
             ineffective, because it may cover different features.
             This description is repeated too many times in the
             documentation; you almost *had* to get it wrong once.
        -->

      <para id="bbv2.overview.projects.attributes.projectrule">
        The default values for those attributes are
        given in the table below.

        <table>
          <title/>
          <tgroup cols="4">
            <thead>
              <row>
                <entry>Attribute</entry>

                <entry>Name</entry>

                <entry>Default value</entry>

                <entry>Handling by the <code language="jam">project</code>
                rule</entry>

              </row>
            </thead>

            <tbody>

              <row>
                <entry>Project id</entry>

                <entry>none</entry>

                <entry>none</entry>

                <entry>Assigned from the first parameter of the 'project' rule.
                  It is assumed to denote absolute project id.</entry>
              </row>

              <row>
                <entry>Source location</entry>

                <entry><literal>source-location</literal></entry>

                <entry>The location of jamfile for the project</entry>

                <entry>Sets to the passed value</entry>
              </row>

              <row>
                <entry>Requirements</entry>

                <entry><literal>requirements</literal></entry>

                <entry>The parent's requirements</entry>

                <entry>The parent's requirements are refined with the passed
                  requirement and the result is used as the project
                  requirements.</entry>
              </row>

              <row>
                <entry>Default build</entry>

                <entry><literal>default-build</literal></entry>

                <entry>none</entry>

                <entry>Sets to the passed value</entry>
              </row>

              <row>
                <entry>Build directory</entry>

                <entry><literal>build-dir</literal></entry>

                <entry>Empty if the parent has no build directory set.
                Otherwise, the parent's build directory with the
                relative path from parent to the current project
                appended to it.
                </entry>

                <entry>Sets to the passed value, interpreted as relative to the
                  project's location.</entry>
              </row>
            </tbody>
          </tgroup>
        </table>
        </para>

      <para>Besides defining projects and main targets, Jamfiles
      often invoke various utility rules. For the full list of rules
      that can be directly used in Jamfile see
      <xref linkend="bbv2.reference.rules"/>.
      </para>

      <para>Each subproject inherits attributes, constants and rules
      from its parent project, which is defined by the nearest
      Jamfile in an ancestor directory above
      the subproject.  The top-level project is declared in a file
      called <filename>Jamroot</filename> rather than
      <filename>Jamfile</filename>.  When loading a project,
      Boost.Build looks for either <filename>Jamroot</filename> or
      <code>Jamfile</code>.  They are handled identically, except
      that if the file is called <filename>Jamroot</filename>, the
      search for a parent project is not performed.
      </para>

      <para>Even when building in a subproject directory, parent
      project files are always loaded before those of their
      subprojects, so that every definition made in a parent project
      is always available to its children. The loading order of any
      other projects is unspecified.  Even if one project refers to
      another via the <code>use-project</code> or a target reference,
      no specific order should be assumed.
      </para>

      <note>
        <para>Giving the root project the special name
        &#x201C;<filename>Jamroot</filename>&#x201D; ensures that
        Boost.Build won't misinterpret a directory above it as the
        project root just because the directory contains a Jamfile.
        <!-- The logic of the previous reasoning didn't hang together -->
        </para>
      </note>

      <!-- All this redundancy with the tutorial is bad.  The tutorial
           should just be made into the introductory sections of this
           document, which should be called the "User Guide."  It's
           perfectly appropriate to start a user guide with that kind
           of material. -->
    </section>

  <section id="bbv2.overview.build_process">
    <title>The Build Process</title>

    <para>When you've described your targets, you want Boost.Build to run the
      right tools and create the needed targets.
      <!-- That sentence is awkward and doesn't add much. -->
      This section will describe
      two things: how you specify what to build, and how the main targets are
      actually constructed.
    </para>

    <para>The most important thing to note is that in Boost.Build, unlike
      other build tools, the targets you declare do not correspond to specific
      files. What you declare in a Jamfile is more like a “metatarget.”
      <!-- Do we need a new word?  We already have “main target.” If
           you're going to introduce “metatarget” you should at least
           tie it together with the main target concept. It's too
           strange to have been saying “main target” all along and now
           suddenly start saying “what you declare in a jamfile” -->
      Depending on the properties you specify on the command line,
      each metatarget will produce a set of real targets corresponding
      to the requested properties. It is quite possible that the same
      metatarget is built several times with different properties,
      producing different files.
    </para>
    <tip>
      <para>
        This means that for Boost.Build, you cannot directly obtain a build
        variant from a Jamfile. There could be several variants requested by the
        user, and each target can be built with different properties.
      </para>
    </tip>

    <section id="bbv2.overview.build_request">
      <title>Build Request</title>

      <para>
        The command line specifies which targets to build and with which
        properties. For example:
<programlisting>
b2 app1 lib1//lib1 toolset=gcc variant=debug optimization=full
</programlisting>
        would build two targets, "app1" and "lib1//lib1" with the specified
        properties. You can refer to any targets, using
        <link linkend="bbv2.reference.ids">target id</link> and specify arbitrary
        properties. Some of the properties are very common, and for them the name
        of the property can be omitted. For example, the above can be written as:
<programlisting>
b2 app1 lib1//lib1 gcc debug optimization=full
</programlisting>
        The complete syntax, which has some additional shortcuts, is
        described in <xref linkend="bbv2.overview.invocation"/>.
      </para>
    </section>

    <section><title>Building a main target</title>

      <para>When you request, directly or indirectly, a build of a main target
        with specific requirements, the following steps are done. Some brief
        explanation is provided, and more details are given in <xref
        linkend="bbv2.reference.buildprocess"/>.
        <orderedlist>

          <listitem><para>Applying default build. If the default-build
          property of a target specifies a value of a feature that is not
          present in the build request, that value is added.</para>
          <!--
              Added to what?  Don't say “the build request!” The
              request is what was requested; if its meaning changes
              the reader will be confused.
          -->
          </listitem>

          <listitem><para>Selecting the main target alternative to use. For
              each alternative we look how many properties are present both in
              alternative's requirements, and in build request. The
              alternative with large number of matching properties is selected.
            </para></listitem>

          <listitem><para>Determining "common" properties.
          <!-- It would be nice to have a better name for this.  But
               even more importantly, unless you say something about
               the reason for choosing whatever term you use, the
               reader is going to wonder what it means. -->
              The build request
              is <link linkend="bbv2.reference.variants.proprefine">refined</link>
              with target's requirements.
              <!-- It's good that you have the links here and below,
                   but I'm concerned that it doesn't communicate well
                   in print and there's not enough information for the
                   print reader.  Maybe we need separate XSL for PDF
                   printing that generates a readable footnote. -->
              The conditional properties in
              requirements are handled as well. Finally, default values of
              features are added.
            </para></listitem>

          <listitem><para>Building targets referred by the sources list and
              dependency properties. The list of sources and the properties
              can refer to other target using <link
              linkend="bbv2.reference.ids">target references</link>. For each
              reference, we take all <link
              linkend="bbv2.reference.features.attributes.propagated">propagated</link>
              properties, refine them by explicit properties specified in the
              target reference, and pass the resulting properties as build
              request to the other target.
            </para></listitem>

          <listitem><para>Adding the usage requirements produced when building
              dependencies to the "common" properties. When dependencies are
              built in the previous step, they return
              <!-- don't assume reader has a mental model for BB internals! -->
              both the set of created
              "real" targets, and usage requirements. The usage requirements
              are added to the common properties and the resulting property
              set will be used for building the current target.
            </para></listitem>

          <listitem><para>Building the target using generators. To convert the
              sources to the desired type, Boost.Build uses "generators" ---
              objects that correspond to tools like compilers and linkers. Each
              generator declares what type of targets it can produce and what
              type of sources it requires. Using this information, Boost.Build
              determines which generators must be run to produce a specific
              target from specific sources. When generators are run, they return
              the "real" targets.
            </para></listitem>

          <listitem><para>Computing the usage requirements to be returned. The
          conditional properties in usage requirements are expanded
          <!-- what does "expanded" mean? -->
          and the result is returned.</para></listitem>
        </orderedlist>
      </para>
    </section>

    <section><title>Building a Project</title>

      <para>Often, a user builds a complete project, not just one main
      target. In fact, invoking <command>b2</command> without
      arguments
      <!-- do you know the difference between parameters and
           arguments?  I only learned this year -->
      builds the project defined in the current
      directory.</para>

      <para>When a project is built, the build request is passed without
        modification to all main targets in that project.
        <!-- What does it mean to pass a build request to a target?
             -->
        It's is possible to
        prevent implicit building of a target in a project with the
        <code>explicit</code> rule:
<programlisting>
explicit hello_test ;
</programlisting>
        would cause the <code>hello_test</code> target to be built only if
        explicitly requested by the user or by some other target.
      </para>

      <para>The Jamfile for a project can include a number of
      <code>build-project</code> rule calls that specify additional projects to
      be built.
      </para>

    </section>

  </section>

  </chapter>

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