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<!--


=head2 The Build command

By default, Cons does not change its working directory to the directory
containing a subsidiary F<Conscript> file it is including.  This behavior
can be enabled for a build by specifying, in the top-level F<Construct>
file:

  Conscript_chdir 1;

When enabled, Cons will change to the subsidiary F<Conscript> file's
containing directory while reading in that file, and then change back
to the top-level directory once the file has been processed.

It is expected that this behavior will become the default in some future
version of Cons.  To prepare for this transition, builds that expect
Cons to remain at the top of the build while it reads in a subsidiary
F<Conscript> file should explicitly disable this feature as follows:

  Conscript_chdir 0;

=head2 Relative, top-relative, and absolute file names

(There is another file prefix, ``!'', that is interpreted specially by
Cons.  See discussion of the C<Link> command, below, for details.)


=head2 Using modules in build scripts

You may pull modules into each F<Conscript> file using the normal Perl
C<use> or C<require> statements:

  use English;
  require My::Module;

Each C<use> or C<require> only affects the one F<Conscript> file in which
it appears.  To use a module in multiple F<Conscript> files, you must
put a C<use> or C<require> statement in each one that needs the module.


=head2 Scope of variables

The top-level F<Construct> file and all F<Conscript> files begin life in
a common, separate Perl package.  B<Cons> controls the symbol table for
the package so that, the symbol table for each script is empty, except
for the F<Construct> file, which gets some of the command line arguments.
All of the variables that are set or used, therefore, are set by the
script itself, not by some external script.

Variables can be explicitly B<imported> by a script from its parent
script. To import a variable, it must have been B<exported> by the parent
and initialized (otherwise an error will occur).


=head2 The Export command

The C<Export> command is used as in the following example:

  $env = new cons();
  $INCLUDE = "#export/include";
  $LIB = "#export/lib";
  Export qw( env INCLUDE LIB );
  Build qw( util/Conscript );

The values of the simple variables mentioned in the C<Export> list will be
squirreled away by any subsequent C<Build> commands. The C<Export> command
will only export Perl B<scalar> variables, that is, variables whose name
begins with C<$>. Other variables, objects, etc. can be exported by
reference, but all scripts will refer to the same object, and this object
should be considered to be read-only by the subsidiary scripts and by the
original exporting script. It's acceptable, however, to assign a new value
to the exported scalar variable, that won't change the underlying variable
referenced. This sequence, for example, is OK:

  $env = new cons();
  Export qw( env INCLUDE LIB );
  Build qw( util/Conscript );
  $env = new cons(CFLAGS => '-O');
  Build qw( other/Conscript );

It doesn't matter whether the variable is set before or after the C<Export>
command. The important thing is the value of the variable at the time the
C<Build> command is executed. This is what gets squirreled away. Any
subsequent C<Export> commands, by the way, invalidate the first: you must
mention all the variables you wish to export on each C<Export> command.


=head2 The Import command

Variables exported by the C<Export> command can be imported into subsidiary
scripts by the C<Import> command. The subsidiary script always imports
variables directly from the superior script. Consider this example:

  Import qw( env INCLUDE );

This is only legal if the parent script exported both C<$env> and
C<$INCLUDE>. It also must have given each of these variables values. It is
OK for the subsidiary script to only import a subset of the exported
variables (in this example, C<$LIB>, which was exported by the previous
example, is not imported).

All the imported variables are automatically re-exported, so the sequence:

  Import qw ( env INCLUDE );
  Build qw ( beneath-me/Conscript );

will supply both C<$env> and C<$INCLUDE> to the subsidiary file. If only
C<$env> is to be exported, then the following will suffice:

  Import qw ( env INCLUDE );
  Export qw ( env );
  Build qw ( beneath-me/Conscript );

Needless to say, the variables may be modified locally before invoking
C<Build> on the subsidiary script.

=head2 Build script evaluation order

The only constraint on the ordering of build scripts is that superior
scripts are evaluated before their inferior scripts. The top-level
F<Construct> file, for instance, is evaluated first, followed by any
inferior scripts. This is all you really need to know about the evaluation
order, since order is generally irrelevant. Consider the following C<Build>
command:

  Build qw(
	drivers/display/Conscript
	drivers/mouse/Conscript
	parser/Conscript
	utilities/Conscript
  );

We've chosen to put the script names in alphabetical order, simply because
that's the most convenient for maintenance purposes. Changing the order will
make no difference to the build.

-->

  <para>

  The source code for large software projects
  rarely stays in a single directory,
  but is nearly always divided into a
  hierarchy of directories.
  Organizing a large software build using &SCons;
  involves creating a hierarchy of build scripts
  using the &SConscript; function.

  </para>

  <section>
  <title>&SConscript; Files</title>

    <para>

    As we've already seen,
    the build script at the top of the tree is called &SConstruct;.
    The top-level &SConstruct; file can
    use the &SConscript; function to
    include other subsidiary scripts in the build.
    These subsidiary scripts can, in turn,
    use the &SConscript; function
    to include still other scripts in the build.
    By convention, these subsidiary scripts are usually
    named &SConscript;.
    For example, a top-level &SConstruct; file might
    arrange for four subsidiary scripts to be included
    in the build as follows:

    </para>

    <programlisting>
      SConscript(['drivers/display/SConscript',
                  'drivers/mouse/SConscript',
                  'parser/SConscript',
                  'utilities/SConscript'])
    </programlisting>

    <para>

    In this case, the &SConstruct; file
    lists all of the &SConscript; files in the build explicitly.
    (Note, however, that not every directory in the tree
    necessarily has an &SConscript; file.)
    Alternatively, the <literal>drivers</literal>
    subdirectory might contain an intermediate
    &SConscript; file,
    in which case the &SConscript; call in
    the top-level &SConstruct; file
    would look like:

    </para>

    <programlisting>
      SConscript(['drivers/SConscript',
                  'parser/SConscript',
                  'utilities/SConscript'])
    </programlisting>

    <para>

    And the subsidiary &SConscript; file in the
    <literal>drivers</literal> subdirectory
    would look like:

    </para>

    <programlisting>
      SConscript(['display/SConscript',
                  'mouse/SConscript'])
    </programlisting>

    <para>

    Whether you list all of the &SConscript; files in the
    top-level &SConstruct; file,
    or place a subsidiary &SConscript; file in
    intervening directories,
    or use some mix of the two schemes,
    is up to you and the needs of your software.

    </para>

  </section>

  <section>
  <title>Path Names Are Relative to the &SConscript; Directory</title>

    <para>

    Subsidiary &SConscript; files make it easy to create a build
    hierarchy because all of the file and directory names
    in a subsidiary &SConscript; files are interpreted
    relative to the directory in which the &SConscript; file lives.
    Typically, this allows the &SConscript; file containing the
    instructions to build a target file
    to live in the same directory as the source files
    from which the target will be built,
    making it easy to update how the software is built
    whenever files are added or deleted
    (or other changes are made).

    </para>

    <para>

    For example, suppose we want to build two programs
    &prog1; and &prog2; in two separate directories
    with the same names as the programs.
    One typical way to do this would be
    with a top-level &SConstruct; file like this:

    </para>

    <programlisting>
      SConscript(['prog1/SConscript',
                  'prog2/SConscript'])
    </programlisting>

    <para>

    And subsidiary &SConscript; files that look like this:

    </para>

    
    <programlisting>
      env = Environment()
      env.Program('prog1', ['main.c', 'foo1.c', 'foo2.c'])
      </programlisting>

    <para>

    And this:

    </para>

    
    <programlisting>
      env = Environment()
      env.Program('prog2', ['main.c', 'bar1.c', 'bar2.c'])
      </programlisting>

    <para>

    Then, when we run &SCons; in the top-level directory,
    our build looks like:

    </para>

    <screen>
       % <userinput>scons -Q</userinput>
       cc -o prog1/foo1.o -c prog1/foo1.c
       cc -o prog1/foo2.o -c prog1/foo2.c
       cc -o prog1/main.o -c prog1/main.c
       cc -o prog1/prog1 prog1/main.o prog1/foo1.o prog1/foo2.o
       cc -o prog2/bar1.o -c prog2/bar1.c
       cc -o prog2/bar2.o -c prog2/bar2.c
       cc -o prog2/main.o -c prog2/main.c
       cc -o prog2/prog2 prog2/main.o prog2/bar1.o prog2/bar2.o
    </screen>

    <para>

    Notice the following:

    First, you can have files with the same names
    in multiple directories, like main.c in the above example.

    Second, unlike standard recursive use of &Make;,
    &SCons; stays in the top-level directory
    (where the &SConstruct; file lives)
    and issues commands that use the path names
    from the top-level directory to the
    target and source files within the hierarchy.

    </para>

  </section>

  <section>
  <title>Top-Level Path Names in Subsidiary &SConscript; Files</title>

    <para>

    If you need to use a file from another directory,
    it's sometimes more convenient to specify
    the path to a file in another directory
    from the top-level &SConstruct; directory,
    even when you're using that file in
    a subsidiary &SConscript; file in a subdirectory.
    You can tell &SCons; to interpret a path name
    as relative to the top-level &SConstruct; directory,
    not the local directory of the &SConscript; file,
    by appending a &hash; (hash mark)
    to the beginning of the path name:

    </para>

    <programlisting>
       env = Environment()
       env.Program('prog', ['main.c', '#lib/foo1.c', 'foo2.c'])
    </programlisting>

    <para>

    In this example,
    the <literal>lib</literal> directory is
    directly underneath the top-level &SConstruct; directory.
    If the above &SConscript; file is in a subdirectory
    named <literal>src/prog</literal>,
    the output would look like:

    </para>

    <screen>
       % <userinput>scons -Q</userinput>
       cc -o lib/foo1.o -c lib/foo1.c
       cc -o src/prog/foo2.o -c src/prog/foo2.c
       cc -o src/prog/main.o -c src/prog/main.c
       cc -o src/prog/prog src/prog/main.o lib/foo1.o src/prog/foo2.o
    </screen>

    <para>

    (Notice that the <literal>lib/foo1.o</literal> object file
    is built in the same directory as its source file.
    See <xref linkend="chap-separate"></xref>, below,
    for information about
    how to build the object file in a different subdirectory.)

    </para>

  </section>

  <section>
  <title>Absolute Path Names</title>

    <para>

    Of course, you can always specify
    an absolute path name for a file--for example:

    </para>

    <programlisting>
       env = Environment()
       env.Program('prog', ['main.c', '/usr/joe/lib/foo1.c', 'foo2.c'])
    </programlisting>

    <para>

    Which, when executed, would yield:

    </para>

    <screen>
       % <userinput>scons -Q</userinput>
       cc -o src/prog/foo2.o -c src/prog/foo2.c
       cc -o src/prog/main.o -c src/prog/main.c
       cc -o /usr/joe/lib/foo1.o -c /usr/joe/lib/foo1.c
       cc -o src/prog/prog src/prog/main.o /usr/joe/lib/foo1.o src/prog/foo2.o
    </screen>

    <para>

    (As was the case with top-relative path names,
    notice that the <literal>/usr/joe/lib/foo1.o</literal> object file
    is built in the same directory as its source file.
    See <xref linkend="chap-separate"></xref>, below,
    for information about
    how to build the object file in a different subdirectory.)

    </para>

  </section>

  <section id="sect-sharing-environments">
  <title>Sharing Environments (and Other Variables) Between &SConscript; Files</title>

    <para>

    In the previous example,
    each of the subsidiary &SConscript; files
    created its own construction environment
    by calling &Environment; separately.
    This obviously works fine,
    but if each program must be built
    with the same construction variables,
    it's cumbersome and error-prone to initialize
    separate construction environments
    in the same way over and over in each subsidiary
    &SConscript; file.

    </para>

    <para>

    &SCons; supports the ability to <emphasis>export</emphasis> variables
    from a parent &SConscript; file
    to its subsidiary &SConscript; files,
    which allows you to share common initialized
    values throughout your build hierarchy.

    </para>

    <section>
    <title>Exporting Variables</title>

      <para>

      There are two ways to export a variable,
      such as a construction environment,
      from an &SConscript; file,
      so that it may be used by other &SConscript; files.
      First, you can call the &Export;
      function with a list of variables,
      or a string of white-space separated variable names.
      Each call to &Export; adds one
      or more variables to a global list
      of variables that are available for import
      by other &SConscript; files.

      </para>

      <programlisting>
        env = Environment()
        Export('env')
      </programlisting>

      <para>

      You may export more than one variable name at a time:

      </para>

      <programlisting>
        env = Environment()
        debug = ARGUMENTS['debug']
        Export('env', 'debug')
      </programlisting>

      <para>

      Because white space is not legal in Python variable names,
      the &Export; function will even automatically split
      a string into separate names for you:

      </para>

      <programlisting>
        Export('env debug')
      </programlisting>

      <para>

      Second, you can specify a list of
      variables to export as a second argument
      to the &SConscript; function call:

      </para>

      <programlisting>
        SConscript('src/SConscript', 'env')
      </programlisting>

      <para>

      Or as the &exports; keyword argument:

      </para>

      <programlisting>
        SConscript('src/SConscript', exports='env')
      </programlisting>

      <para>

      These calls export the specified variables
      to only the listed &SConscript; files.
      You may, however, specify more than one
      &SConscript; file in a list:

      </para>

      <programlisting>
        SConscript(['src1/SConscript',
                    'src2/SConscript'], exports='env')
      </programlisting>

      <para>

      This is functionally equivalent to
      calling the &SConscript; function
      multiple times with the same &exports; argument,
      one per &SConscript; file.

      </para>

    </section>

    <section>
    <title>Importing Variables</title>

      <para>

      Once a variable has been exported from a calling
      &SConscript; file,
      it may be used in other &SConscript; files
      by calling the &Import; function:

      </para>

      <programlisting>
        Import('env')
        env.Program('prog', ['prog.c'])
      </programlisting>

      <para>

      The &Import; call makes the <literal>env</literal> construction
      environment available to the &SConscript; file,
      after which the variable can be used to build
      programs, libraries, etc.

      </para>

      <para>

      Like the &Export; function,
      the &Import; function can be used
      with multiple variable names:

      </para>

      <programlisting>
        Import('env', 'debug')
        env = env.Clone(DEBUG = debug)
        env.Program('prog', ['prog.c'])
      </programlisting>

      <para>

      And the &Import; function will similarly
      split a string along white-space
      into separate variable names:

      </para>

      <programlisting>
        Import('env debug')
        env = env.Clone(DEBUG = debug)
        env.Program('prog', ['prog.c'])
      </programlisting>

      <para>

      Lastly, as a special case,
      you may import all of the variables that
      have been exported by supplying an asterisk
      to the &Import; function:

      </para>

      <programlisting>
        Import('*')
        env = env.Clone(DEBUG = debug)
        env.Program('prog', ['prog.c'])
      </programlisting>

      <para>

      If you're dealing with a lot of &SConscript; files,
      this can be a lot simpler than keeping
      arbitrary lists of imported variables in each file.

      </para>

    </section>

    <section>
    <title>Returning Values From an &SConscript; File</title>

      <para>

      Sometimes, you would like to be able to
      use information from a subsidiary
      &SConscript; file in some way.
      For example,
      suppose that you want to create one
      library from source files
      scattered throughout a number
      of subsidiary &SConscript; files.
      You can do this by using the &Return;
      function to return values
      from the subsidiary &SConscript; files
      to the calling file.

      </para>

      <para>

      If, for example, we have two subdirectories
      &foo; and &bar;
      that should each contribute a source
      file to a Library,
      what we'd like to be able to do is
      collect the object files
      from the subsidiary &SConscript; calls
      like this:

      </para>

      <programlisting>
          env = Environment()
          Export('env')
          objs = []
          for subdir in ['foo', 'bar']:
              o = SConscript('%s/SConscript' % subdir)
              objs.append(o)
          env.Library('prog', objs)
      </programlisting>

      <para>

      We can do this by using the &Return;
      function in the
      <literal>foo/SConscript</literal> file like this:

      </para>

      
      <programlisting>
          Import('env')
          obj = env.Object('foo.c')
          Return('obj')
        </programlisting>

      <para>

      (The corresponding
      <literal>bar/SConscript</literal>
      file should be pretty obvious.)
      Then when we run &SCons;,
      the object files from the subsidiary subdirectories
      are all correctly archived in the desired library:

      </para>

      <screen>
        % <userinput>scons -Q</userinput>
        cc -o bar/bar.o -c bar/bar.c
        cc -o foo/foo.o -c foo/foo.c
        ar rc libprog.a foo/foo.o bar/bar.o
        ranlib libprog.a
      </screen>

      <!--
      XXX Return(stop=False)
      -->

    </section>

  </section>

  <!--

  <section>
  <title>Executing From a Subdirectory:  the -D, -u and -U Options</title>

    <para>

    XXX -D, -u and -U

    </para>

  </section>

  -->
