source: downloads/boost_1_34_1/tools/build/v2/doc/src/architecture.xml @ 29

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<!DOCTYPE appendix PUBLIC "-//Boost//DTD BoostBook XML V1.0//EN"
  "http://www.boost.org/tools/boostbook/dtd/boostbook.dtd">

  <appendix id="bbv2.arch">
    <title>Boost.Build v2 architecture</title>

  <sidebar>
    <para>This document is work-in progress. Don't expect much from it
      yet.</para>
  </sidebar>
  
  <section id="bbv2.arch.overview">
    <title>Overview</title>

    <para>The Boost.Build code is structured in four different components:
    "kernel", "util", "build" and "tools". The first two are relatively
    uninteresting, so we'll focus on the remaining pair. The "build" component
    provides classes necessary to declare targets, determine which properties
    should be used for their building, and for creating the dependency
    graph. The "tools" component provides user-visible functionality. It
    mostly allows to declare specific kind of main targets, and declare
    avaiable tools, which are then used when creating the dependency graph.
    </para>
    
  </section>

  <section id="bbv2.arch.build">
    <title>The build layer</title>

      <para>The build layer has just four main parts -- metatargets (abstract targets),
        virtual targets, generators and properties. 
        <itemizedlist>
          <listitem><para>Metatargets (see the "targets.jam" module) represent
              all the user-defined entities which can be built. The "meta" prefix
              signify that they don't really corrspond to files -- depending of
              build request, they can produce different set of
              files. Metatargets are created when Jamfiles are loaded. Each
              metagarget has a <code>generate</code> method which is given a
              property set and produces virtual targets for the passed properties.
            </para></listitem>
          <listitem><para>Virtual targets (see the "virtual-targets.jam"
              module) correspond to the atomic things which can be updated --
              most typically files. 
            </para></listitem>
          <listitem><para>Properties are just (name, value) pairs, specified
              by the user and describing how the targets should be
              built. Properties are stored using the <code>property-set</code> class.
              </para></listitem>
          <listitem><para>Generators are the objects which encapsulate tools
              -- they can take a list of source virtual targets and produce new
              virtual targets from them.
            </para></listitem>
        </itemizedlist>
      </para>

      <para>The build process includes those steps:
        <orderedlist>
          <listitem><para>Top-level code calls the <code>generate</code>
              method of a metatarget with some properties.  </para></listitem>


          <listitem><para>The metatarget combines the requested properties
              with requirements and passes the result, together with the list
              of sources, to the <code>generators.construct</code>
              function</para></listitem> 

          
          <listitem><para>A generator appropriate for the build properties is
              selected and its <code>run</code> method is
              called. The method returns a list of virtual targets
            </para></listitem>

          <listitem><para>The targets are returned to the top level code. They
              are converted into bjam targets (via
              <code>virtual-target.actualize</code>) and passed to bjam for building.
            </para></listitem>
        </orderedlist>
      </para>

      <section id="bbv2.arch.metatargets">
        <title>Metatargets</title>

        <para>There are several classes derived from "abstract-target". The
          "main-target" class represents top-level main target, the "project-target"
          acts like container for all main targets, and "basic-target" class is a
          base class for all further target types.
        </para>
        
        <para>Since each main target can have several alternatives, all top-level
          target objects are just containers, referring to "real" main target
          classes. The type is that container is "main-target". For example, given:
<programlisting>
alias a ;
lib a : a.cpp : &lt;toolset&gt;gcc ;
</programlisting>
          we would have one-top level instance of "main-target-class", which will
          contain one instance of "alias-target-class" and one instance of
          "lib-target-class". The "generate" method of "main-target" decides
          which of the alternative should be used, and call "generate" on the
          corresponding instance.
        </para>

        <para>Each alternative is a instance of a class derived from
          "basic-target". The "basic-target.generate" does several things that are
          always should be done:
          <itemizedlist>
            <listitem>
              <para>Determines what properties should be used for building the
                target. This includes looking at requested properties, requirements,
                and usage requirements of all sources.</para>
            </listitem>
            <listitem>
              <para>Builds all sources</para>
            </listitem>
            <listitem>
              <para>Computes the usage requirements which should be passes back.</para>
            </listitem>
          </itemizedlist>
          For the real work of constructing virtual target, a new method
          "construct" is called.
        </para>

        <para>The "construct" method can be implemented in any way by classes
          derived from "basic-target", but one specific derived class plays the
          central role -- "typed-target". That class holds the desired type of file
          to be produces, and calls the generators modules to do the job.
        </para>

        <para>This means that a specific metatarget subclass may avoid using
          generators at all. However, this is deprecated and we're trying to
          eliminate all such subsclasses at the moment.
        </para>

        <para>Note that the <filename>build/targets.jam</filename> file contains
            an UML diagram which might help.</para>

      </section>

      <section id="bbv2.arch.virtual">
        <title>Virtual targets</title>

        <para>Virtual targets correspond to the atomic things which can be
          updated. Each virtual target can be assigned an updating action --
          instance of the <code>action</code> class. The action class, in
          turn, contains a list of source targets, properties, and a name of
          bjam action block which should be executed.
        </para>

        <para>We try hard to never create equal instances of the
          <code>virtual-target</code> class. Each code which creates virtual
          targets passes them though the <code>virtual-target.register</code>
          function, which detects if a target with the same name, sources, and
          properties was created. In that case, existing target is returned.
        </para>
        
        <para>When all virtual targets are produced, they are
          "actualized". This means that the real file names are computed, and
          the commands that should be run are generated. This is done by the
          <code>virtual-target.actualize</code> method and the
          <code>action.actualize</code> methods. The first is conceptually
          simple, while the second need additional explanation. The commands
          in bjam are generated in two-stage process. First, a rule with the
          appropriate name (for example
          "gcc.compile") is called and is given the names of targets. The rule
          sets some variables, like "OPTIONS". After that, the command string
          is taken, and variable are substitutes, so use of OPTIONS inside the
          command string become the real compile options.
        </para>

        <para>Boost.Build added a third stage to simplify things. It's now
          possible to automatically convert properties to appropriate assignments to
          variables. For example, &lt;debug-symbols&gt;on would add "-g" to the
          OPTIONS variable, without requiring to manually add this logic to
          gcc.compile. This functionality is part of the "toolset" module.
        </para>

        <para>Note that the <filename>build/virtual-targets.jam</filename> file
            contains an UML diagram which might help.</para>
      </section>

      <section id="bbv2.arch.properties">
        <para>Above, we noted that metatargets are built with a set of
          properties. That set is represented with the
          <code>property-set</code> class. An important point is that handling
          of property sets can get very expensive. For that reason, we make
          sure that for each set of (name, value) pairs only one
          <code>property-set</code> instance is created. The
          <code>property-set</code> uses extensive caching for all operation,
          so most work is avoided. The <code>property-set.create</code> is the 
          factory function which should be used to create instances of the
          <code>property-set</code> class.
        </para>
      </section>
      
        
  </section>

  <section id="bbv2.arch.tools">
    <title>The tools layer</title>

    <para>Write me!</para>

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

  <para>NOTE: THIS SECTION IS NOT EXPECTED TO BE READ!
        There are two user-visible kinds of targets in Boost.Build.
  First are "abstract" &#x2014; they correspond to things declared
  by user, for example, projects and executable files. The primary
  thing about abstract target is that it's possible to request them
  to be build with a particular values of some properties. Each
  combination of properties may possible yield different set of
  real file, so abstract target do not have a direct correspondence
  with files.</para>

  <para>File targets, on the contary, are associated with concrete
  files. Dependency graphs for abstract targets with specific
  properties are constructed from file targets. User has no was to
  create file targets, however it can specify rules that detect
  file type for sources, and also rules for transforming between
  file targets of different types. That information is used in
  constructing dependency graph, as desribed in the "next section".
  [ link? ] <emphasis role="bold">Note:</emphasis>File targets are not
  the same as targets in Jam sense; the latter are created from
  file targets at the latest possible moment. <emphasis role="bold">Note:</emphasis>"File
  target" is a proposed name for what we call virtual targets. It
  it more understandable by users, but has one problem: virtual
  targets can potentially be "phony", and not correspond to any
  file.</para>

    <section id="bbv2.arch.depends">
      <title>Dependency scanning</title>

  <para>Dependency scanning is the process of finding implicit
  dependencies, like "#include" statements in C++. The requirements
  for right dependency scanning mechanism are:</para>

  <itemizedlist>
    <listitem>
      <simpara>
        Support for different scanning algorithms. C++ and XML have
    quite different syntax for includes and rules for looking up
    included files.
      </simpara>
    </listitem>

    <listitem>
      <simpara>
        Ability to scan the same file several times. For example,
    single C++ file can be compiled with different include
    paths.
      </simpara>
    </listitem>

    <listitem>
      <simpara>
        Proper detection of dependencies on generated files.
      </simpara>
    </listitem>

    <listitem>
      <simpara>
        Proper detection of dependencies from generated file.
      </simpara>
    </listitem>
  </itemizedlist>

      <section>
        <title>Support for different scanning algorithms</title>

  <para>Different scanning algorithm are encapsulated by objects
  called "scanners". Please see the documentation for "scanner"
  module for more details.</para>

      </section>

      <section>
        <title>Ability to scan the same file several times</title>

  <para>As said above, it's possible to compile a C++ file twice, with
  different include paths. Therefore, include dependencies for
  those compilations can be different. The problem is that bjam
  does not allow several scans of the same target.</para>

  <para>The solution in Boost.Build is straigtforward. When a virtual
  target is converted to bjam target (via
  <literal>virtual-target.actualize</literal> method), we specify the scanner
  object to be used. The actualize method will create different
  bjam targets for different scanners.</para>

  <para>All targets with specific scanner are made dependent on target
  without scanner, which target is always created. This is done in
  case the target is updated. The updating action will be
  associated with target without scanner, but if sources for that
  action are touched, all targets &#x2014; with scanner and without
  should be considered outdated.</para>

  <para>For example, assume that "a.cpp" is compiled by two compilers
  with different include path. It's also copied into some install
  location. In turn, it's produced from "a.verbatim". The
  dependency graph will look like:</para>

<programlisting>
a.o (&lt;toolset&gt;gcc)  &lt;--(compile)-- a.cpp (scanner1) ----+
a.o (&lt;toolset&gt;msvc) &lt;--(compile)-- a.cpp (scanner2) ----|
a.cpp (installed copy)    &lt;--(copy) ----------------------- a.cpp (no scanner)
                                                                 ^
                                                                 |
                       a.verbose --------------------------------+
</programlisting>

      </section>
      <section>
        <title>Proper detection of dependencies on generated files.</title>

  <para>This requirement breaks down to the following ones.</para>

  <orderedlist>
    <listitem>
      <simpara>
        If when compiling "a.cpp" there's include of "a.h", the
    "dir" directory is in include path, and a target called "a.h"
    will be generated to "dir", then bjam should discover the
    include, and create "a.h" before compiling "a.cpp".
      </simpara>
    </listitem>

    <listitem>
      <simpara>
      Since almost always Boost.Build generates targets to a
    "bin" directory, it should be supported as well. I.e. in the
    scanario above, Jamfile in "dir" might create a main target,
    which generates "a.h". The file will be generated to "dir/bin"
    directory, but we still have to recornize the dependency.
      </simpara>
    </listitem>
  </orderedlist>

  <para>The first requirement means that when determining what "a.h"
  means, when found in "a.cpp", we have to iterate over all
  directories in include paths, checking for each one:</para>

  <orderedlist>
    <listitem>
      <simpara>
        If there's file "a.h" in that directory, or
      </simpara>
    </listitem>

    <listitem>
      <simpara>
        If there's a target called "a.h", which will be generated
    to that directory.
      </simpara>
    </listitem>
  </orderedlist>

  <para>Classic Jam has built-in facilities for point (1) above, but
  that's not enough. It's hard to implement the right semantic
  without builtin support. For example, we could try to check if
  there's targer called "a.h" somewhere in dependency graph, and
  add a dependency to it. The problem is that without search in
  include path, the semantic may be incorrect. For example, one can
  have an action which generated some "dummy" header, for system
  which don't have the native one. Naturally, we don't want to
  depend on that generated header on platforms where native one is
  included.</para>

  <para>There are two design choices for builtin support. Suppose we
  have files a.cpp and b.cpp, and each one includes header.h,
  generated by some action. Dependency graph created by classic jam
  would look like:</para>

<programlisting>
a.cpp -----&gt; &lt;scanner1&gt;header.h  [search path: d1, d2, d3]


                  &lt;d2&gt;header.h  --------&gt; header.y
                  [generated in d2]
           
b.cpp -----&gt; &lt;scanner2&gt;header.h [ search path: d1, d2, d4]
</programlisting>

    <para>
In this case, Jam thinks all header.h target are not
realated. The right dependency graph might be:

<programlisting>
a.cpp ---- 
          \
           \     
            &gt;----&gt;  &lt;d2&gt;header.h  --------&gt; header.y
           /       [generated in d2]
          / 
b.cpp ----
</programlisting>

or

<programlisting>
a.cpp -----&gt; &lt;scanner1&gt;header.h  [search path: d1, d2, d3]
                          |
                       (includes)
                          V
                  &lt;d2&gt;header.h  --------&gt; header.y
                  [generated in d2]
                          ^
                      (includes)  
                          |
b.cpp -----&gt; &lt;scanner2&gt;header.h [ search path: d1, d2, d4]
</programlisting>
        </para>

        <para>
The first alternative was used for some time. The problem
however is: what include paths should be used when scanning
header.h? The second alternative was suggested by Matt Armstrong.
It has similiar effect: add targets which depend on
&lt;scanner1&gt;header.h will also depend on &lt;d2&gt;header.h.
But now we have two different target with two different scanners,
and those targets can be scanned independently. The problem of
first alternative is avoided, so the second alternative is
implemented now.
        </para>

  <para>The second sub-requirements is that targets generated to "bin"
  directory are handled as well. Boost.Build implements
  semi-automatic approach. When compiling C++ files the process
  is:</para>

  <orderedlist>
    <listitem>
      <simpara>
        The main target to which compiled file belongs is found.
      </simpara>
    </listitem>

    <listitem>
      <simpara>
        All other main targets that the found one depends on are
    found. Those include main target which are used as sources, or
    present as values of "dependency" features.
      </simpara>
    </listitem>

    <listitem>
      <simpara>
        All directories where files belonging to those main target
    will be generated are added to the include path.
      </simpara>
    </listitem>
  </orderedlist>

  <para>After this is done, dependencies are found by the approach
  explained previously.</para>

  <para>Note that if a target uses generated headers from other main
  target, that main target should be explicitly specified as
  dependency property. It would be better to lift this requirement,
  but it seems not very problematic in practice.</para>

  <para>For target types other than C++, adding of include paths must
  be implemented anew.</para>

      </section>
      <section>
        <title>Proper detection of dependencies from generated files</title>

  <para>Suppose file "a.cpp" includes "a.h" and both are generated by
  some action. Note that classic jam has two stages. In first stage
  dependency graph graph is build and actions which should be run
  are determined. In second stage the actions are executed.
  Initially, neither file exists, so the include is not found. As
  the result, jam might attempt to compile a.cpp before creating
  a.h, and compilation will fail.</para>

  <para>The solution in Boost.Jam is to perform additional dependency
  scans after targets are updated. This break separation between
  build stages in jam &#x2014; which some people consider a good
  thing &#x2014; but I'm not aware of any better solution.</para>

  <para>In order to understand the rest of this section, you better
  read some details about jam dependency scanning, available
  <ulink url=
  "http://public.perforce.com:8080/@md=d&amp;cd=//public/jam/src/&amp;ra=s&amp;c=kVu@//2614?ac=10">
  at this link</ulink>.</para>

  <para>Whenever a target is updated, Boost.Jam rescans it for
  includes. Consider this graph, created before any actions are
  run.</para>

<programlisting>
A -------&gt; C ----&gt; C.pro
     /
B --/         C-includes   ---&gt; D
</programlisting>

        <para>
Both A and B have dependency on C and C-includes (the latter
dependency is not shown). Say during building we've tried to create
A, then tried to create C and successfully created C.
        </para>

  <para>In that case, the set of includes in C might well have
  changed. We do not bother to detect precisely which includes were
  added or removed. Instead we create another internal node
  C-includes-2. Then we determine what actions should be run to
  update the target. In fact this mean that we perform logic of
  first stage while already executing stage.</para>

  <para>After actions for C-includes-2 are determined, we add
  C-includes-2 to the list of A's dependents, and stage 2 proceeds
  as usual. Unfortunately, we can't do the same with target B,
  since when it's not visited, C target does not know B depends on
  it. So, we add a flag to C which tells and it was rescanned. When
  visiting B target, the flag is notices and C-includes-2 will be
  added to the list of B's dependencies.</para>

  <para>Note also that internal nodes are sometimes updated too.
  Consider this dependency graph:</para>

<programlisting>
a.o ---&gt; a.cpp
            a.cpp-includes --&gt;  a.h (scanned)
                                   a.h-includes ------&gt; a.h (generated)
                                                                 |
                                                                 |
            a.pro &lt;-------------------------------------------+
</programlisting>

  <para>Here, out handling of generated headers come into play. Say
  that a.h exists but is out of date with respect to "a.pro", then
  "a.h (generated)" and "a.h-includes" will be marking for
  updating, but "a.h (scanned)" won't be marked. We have to rescan
  "a.h" file after it's created, but since "a.h (generated)" has no
  scanner associated with it, it's only possible to rescan "a.h"
  after "a.h-includes" target was updated.</para>

  <para>Tbe above consideration lead to decision that we'll rescan a
  target whenever it's updated, no matter if this target is
  internal or not.</para>

  <warning>
    <para>
    The remainder of this document is not indended to be read at
    all. This will be rearranged in future.
    </para>
  </warning>

        <section>
          <title>File targets</title>
  
          <para>
  As described above, file targets corresponds
  to files that Boost.Build manages. User's may be concerned about
  file targets in three ways: when declaring file target types,
  when declaring transformations between types, and when
  determining where file target will be placed. File targets can
  also be connected with actions, that determine how the target is
  created. Both file targets and actions are implemented in the
  <literal>virtual-target</literal> module.
          </para>

            <section> 
              <title>Types</title>
              
              <para>A file target can be given a file, which determines
  what transformations can be applied to the file. The
  <literal>type.register</literal> rule declares new types. File type can
  also be assigned a scanner, which is used to find implicit
  dependencies. See "dependency scanning" [ link? ] below.</para>
            </section>
          </section>

          <section>
            <title>Target paths</title>

  <para>To distinguish targets build with different properties, they
  are put in different directories. Rules for determining target
  paths are given below:</para>

  <orderedlist>
    <listitem>
      <simpara>
        All targets are placed under directory corresponding to the
    project where they are defined.
      </simpara>
        </listitem>

    <listitem>
      <simpara>
        Each non free, non incidental property cause an additional
    element to be added to the target path. That element has the
    form <literal>&lt;feature-name&gt;-&lt;feature-value&gt;</literal> for
    ordinary features and <literal>&lt;feature-value&gt;</literal> for
    implicit ones. [Note about composite features].
      </simpara>
        </listitem>

    <listitem>
      <simpara>
        If the set of free, non incidental properties is different
    from the set of free, non incidental properties for the project
    in which the main target that uses the target is defined, a
    part of the form <literal>main_target-&lt;name&gt;</literal> is added to
    the target path. <emphasis role="bold">Note:</emphasis>It would be nice to completely
    track free features also, but this appears to be complex and
    not extremely needed.
      </simpara>
        </listitem>
  </orderedlist>

  <para>For example, we might have these paths:</para>

<programlisting>
debug/optimization-off
debug/main-target-a
</programlisting>

          </section>
        </section>
      </section>
    </section>
  </appendix>

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