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<!DOCTYPE chapter PUBLIC "-//Boost//DTD BoostBook XML V1.0//EN"
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  <chapter id="bbv2.advanced">
    <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 actually consists of two parts - Boost.Jam, a
  build engine with its own interpreted language, and Boost.Build itself,
  implemented in Boost.Jam's language. The chain of events when
  you type <command>bjam</command> on the command line is:
      <orderedlist>
        <listitem>
          <para>Boost.Jam tries to find Boost.Build 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 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.advanced.configuration">
              How to configure Boost.Build</link></para>
        </listitem>
        <listitem>
          <para><link linkend="bbv2.advanced.targets">
              How to write declares targets in Jamfiles</link></para>
        </listitem>
        <listitem>
          <para><link linkend="bbv2.advanced.build_process">
              How the build process works</link></para>
        </listitem>
        <listitem>
          <para>Some Basics about the Boost.Jam language. See 
          <xref linkend="bbv2.advanced.jam_language"/>.
          </para>
        </listitem>
      </itemizedlist>
    </para>
    
    <section id="bbv2.advanced.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>Boost.Jam has an interpreted, procedural language. 
      On the lowest level, a Boost.Jam program consists of variables and 
      <indexterm><primary>rule</primary></indexterm>
      <firstterm>rules</firstterm> (the Jam term for function). They are grouped
      in modules&#x2014;there's one global module and a number of named
      modules. Besides that, a Boost.Jam program contains classes and class
      instances.      
      </para>

      <para>Syntantically, a Boost.Jam program consists of two kind of
      elements&#x2014;keywords (which have a special meaning to Boost.Jam) 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 Boost.Jam 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 with 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 size of the list value for each parameter. 
        <programlisting>
rule <replaceable>example</replaceable> 
     (
         <replaceable>parameter1</replaceable> : 
         <replaceable>parameter2 ?</replaceable> : 
         <replaceable>parameter3 +</replaceable> :
         <replaceable>parameter4 *</replaceable> 
     )
     {
        // 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 arbitrary long, but the third argument may not be empty.
      </para>

      <para>The overview of Boost.Jam 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>, <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 bjam 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.advanced.jam_language.actions">
        Sometimes, you'd need to specify the actual command lines to be used
        when creating targets. In 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 list of
        generated files, and the <literal>$(&gt;)</literal> variable will be expanded
        to the list of source files.
      </para>

      <para>To flexibly adjust command line, you can define a rule with the
      same name as the action, and taking three parameters -- 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 certain build property is specified.
      If so, it sets variable <varname>OPIONS</varname> that's used inside
      action. Note that the variable is set "on targets" -- the value will
      be only visible inside action, not globally. Were it set globally,
      using variable named <varname>OPTIONS</varname> in two unrelated
      actions would be impossible.
      </para>

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

    </section>

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

    <para>The Boost.Build configuration is specified in the file
    <filename>user-config.jam</filename>. You can edit the one in the top-level
    directory of Boost.Build installation or create a copy in your home directory
    and edit that. (See <xref linkend="bbv2.reference.init.config"/>  for the
    exact search paths.) The primary function of that file is to declare which
    compilers and other tools are available. The simplest syntax to configure
    a tool is:

<programlisting>
using <replaceable>tool-name</replaceable> ;        
</programlisting>
      The <functionname>using</functionname> rule is given a name of tool, and will make that tool
      available to Boost.Build. For example, <code>using gcc ;</code> will make the gcc compiler
      available.      
    </para>

    <para>
      Since nothing but a tool name is specified, Boost.Build will
      pick some default settings.  For example, it will use the
      <command>gcc</command> executable found in the
      <envar>PATH</envar>, or look in some known installation
      locations. In most cases, this strategy works automatically. In
      case you have several versions of a compiler, it's installed in
      some unusual location, or you need to tweak its configuration,
      you'll need to pass additional parameters to the
      <functionname>using</functionname> rule.  The parameters to
      <functionname>using</functionname> can be different for each
      tool.  You can obtain specific documentation for any tool's
      configuration parameters by invoking
<programlisting>
bjam --help <replaceable>tool-name</replaceable>.init         
</programlisting>
    </para>
    <para>
      That said, for all the compiler toolsets Boost.Build supports
      out-of-the-box, the list of parameters to
      <functionname>using</functionname> 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>.
      <!-- the previous text here was really confusing -->
    </para>

    <para>The <parameter class="function">version</parameter>
    parameter identifies the toolset version, in case you have
    several installed. It can have any form you like, but
    it's recommended that you use a numeric identifier like
    <literal>7.1</literal>.
    </para>

    <para>
      The <parameter class="function">invocation-command</parameter>
      parameter is the command that must be executed to run the
      compiler. This parameter can usually be omitted if 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 through a global mechanism like the
      Windows registry.</para></listitem>
      </itemizedlist>

      For example:
<programlisting>
using msvc : 7.1 ;
using gcc ;
</programlisting>
If the compiler can be found in the <envar>PATH</envar> but only by a
nonstandard name, you can just supply that name:
<programlisting>
using gcc : : g++-3.2 ;
</programlisting>
Otherwise, it might be necessary to supply the complete path to the
compiler executable:
<programlisting>
using msvc : : "Z:/Programs/Microsoft Visual Studio/vc98/bin/cl" ;
</programlisting>
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 compiler executables for C and C++ are different, path to the C++
compiler executable must be specified. The &#x201C;invocation command&#x201D;
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 <functionname>using</functionname> 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
        <functionname>using</functionname>, the compiler found in the
        <envar>PATH</envar> will be used, and there's no need to
        explicitly specify the command.
    </para>

    <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>The <parameter class="function">options</parameter>
    parameter is used to fine-tune the configuration. All of
    Boost.Build's standard compiler toolsets accept properties of the
    four builtin features <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>
                
    </section>

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

      <para>This section describes how invoke Boost.Build from the command line</para>

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

      <para>To build specific targets, specify them on the command line:
<screen>
bjam 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>
bjam toolset=gcc variant=debug optimization=space
</screen>
       For often used features, like <literal>toolset</literal> and <literal>variant</literal> you can
       omit the feature name, so the above can be written as:
<screen>
bjam optimization=space
</screen>
      </para>


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

      <variablelist>
        
        <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 build directories for all project roots being built. When
            this option is specified, all Jamroot files should declare 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>--version</option></term>
          <listitem>
            <para>Prints information on Boost.Build and Boost.Jam
                  versions.
            </para>
          </listitem>
        </varlistentry>

        <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>--debug-configuration</option></term>
          <listitem>
            <para>Produces debug information about 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 generator search process. 
            Useful for debugging custom generators.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>--ignore-config</option></term>
          <listitem>
            <para>Do not load <literal>site-config.jam</literal> and
                  <literal>user-config.jam</literal> configuration files.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>--debug</option></term>
          <listitem>
            <para>Enables internal checks.
            </para>
          </listitem>
        </varlistentry>
    
      </variablelist>


      <para>For complete specification of command line syntax, see 
      <xref linkend="bbv2.reference.init.args"/>
      </para>


    </section>

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

      <para id="bbv2.advanced.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
      requirements of the project where a 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
        <functionname>glob</functionname> 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.advanced.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's 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 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 library)
          defined by Jamfile one level higher. Finally, the third target
          uses some <ulink url="http://boost.org">C++ Boost</ulink>
          library, referring to it by 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.advanced.targets.requirements">
        <title>Requirements</title>
        <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 is a number of other features, listed in
        <xref linkend="bbv2.advanced.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.advanced.targets.requirements.conditional">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;NEWORK_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;NEWORK_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.advanced.targets.requirements.indirect">
          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 specific project's requirement
        using the syntax by adding a minus sign before a 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 from a parent,
        such as defines. 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 project requirements and will always
        be built in multi-threaded mode. The <code>test2</code> target
        <emphasis>overrides</emphasis> 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 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
        requirements and default-build is that 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>bjam 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
         <functionname>explicit</functionname> rule:
         <programlisting>
explicit install_programs ;</programlisting>
         </para>

      </section>
    </section>

    <section id="bbv2.advanced.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
      <functionname>project</functionname> 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.advanced.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 <functionname>project</functionname>
                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.advanced.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.advanced.build_request">
      <title>Build Request</title>

      <para>
        The command line specifies which targets to build and with which
        properties. For example:
<programlisting>
bjam 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>
bjam app1 lib1//lib1 gcc debug optimization=full
</programlisting>
        The complete syntax, which has some additional shortcuts, is
        described in <xref linkend="bbv2.reference.commandline"/>.
      </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
              <!-- Was "in." Why are these short and unmistakable
                   words so commonly misspelled? -->
              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>bjam</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
      <!-- A comma would only be correct here in German -->
      that specify additional projects
      to be built.
      </para>

    </section>

  </section>
    
  </chapter>

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