<?xml version='1.0'?>
<!DOCTYPE sconsdoc [
    <!ENTITY % scons SYSTEM "../scons.mod">
    %scons;
    
    <!ENTITY % builders-mod SYSTEM "../generated/builders.mod">
    %builders-mod;
    <!ENTITY % functions-mod SYSTEM "../generated/functions.mod">
    %functions-mod;
    <!ENTITY % tools-mod SYSTEM "../generated/tools.mod">
    %tools-mod;
    <!ENTITY % variables-mod SYSTEM "../generated/variables.mod">
    %variables-mod;

]>

<chapter id="chap-builders-writing"
         xmlns="http://www.scons.org/dbxsd/v1.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://www.scons.org/dbxsd/v1.0 http://www.scons.org/dbxsd/v1.0/scons.xsd">
<title>Writing Your Own Builders</title>

<!--

  __COPYRIGHT__

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  permit persons to whom the Software is furnished to do so, subject to
  the following conditions:

  The above copyright notice and this permission notice shall be included
  in all copies or substantial portions of the Software.

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  KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
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  LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
  OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
  WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

-->

<!--

=head2 Adding new methods

For slightly more demanding changes, you may wish to add new methods to the
C<cons> package. Here's an example of a very simple extension,
C<InstallScript>, which installs a tcl script in a requested location, but
edits the script first to reflect a platform-dependent path that needs to be
installed in the script:

  # cons::InstallScript - Create a platform dependent version of a shell
  # script by replacing string ``#!your-path-here'' with platform specific
  # path $BIN_DIR.

  sub cons::InstallScript {
	my ($env, $dst, $src) = @_;
	Command $env $dst, $src, qq(
		sed s+your-path-here+$BIN_DIR+ %< > %>
		chmod oug+x %>
	);
  }

Notice that this method is defined directly in the C<cons> package (by
prefixing the name with C<cons::>). A change made in this manner will be
globally visible to all environments, and could be called as in the
following example:

  InstallScript $env "$BIN/foo", "foo.tcl";

For a small improvement in generality, the C<BINDIR> variable could be
passed in as an argument or taken from the construction environment-,-as
C<%BINDIR>.


=head2 Overriding methods

Instead of adding the method to the C<cons> name space, you could define a
new package which inherits existing methods from the C<cons> package and
overrides or adds others. This can be done using Perl's inheritance
mechanisms.

The following example defines a new package C<cons::switch> which
overrides the standard C<Library> method. The overridden method builds
linked library modules, rather than library archives. A new
constructor is provided. Environments created with this constructor
will have the new library method; others won't.

  package cons::switch;
  BEGIN {@ISA = 'cons'}

  sub new {
	shift;
	bless new cons(@_);
  }

  sub Library {
	my($env) = shift;
	my($lib) = shift;
	my(@objs) = Objects $env @_;
	Command $env $lib, @objs, q(
		%LD -r %LDFLAGS %< -o %>
	);
  }

This functionality could be invoked as in the following example:

  $env = new cons::switch(@overrides);
  ...
  Library $env 'lib.o', 'foo.c', 'bar.c';

-->

  <para>

  Although &SCons; provides many useful methods
  for building common software products
  (programs, libraries, documents, etc.),
  you frequently want to be
  able to build some other type of file
  not supported directly by &SCons;.
  Fortunately, &SCons; makes it very easy
  to define your own &Builder; objects
  for any custom file types you want to build.
  (In fact, the &SCons; interfaces for creating
  &Builder; objects are flexible enough and easy enough to use
  that all of the the &SCons; built-in &Builder; objects
  are created using the mechanisms described in this section.)

  </para>

  <section>
  <title>Writing Builders That Execute External Commands</title>

    <para>

    The simplest &Builder; to create is
    one that executes an external command.
    For example, if we want to build
    an output file by running the contents
    of the input file through a command named
    <literal>foobuild</literal>,
    creating that &Builder; might look like:

    </para>

    <programlisting>
bld = Builder(action = 'foobuild &lt; $SOURCE &gt; $TARGET')
    </programlisting>

    <para>

    All the above line does is create a free-standing
    &Builder; object.
    The next section will show us how to actually use it.

    </para>

  </section>

  <section>
  <title>Attaching a Builder to a &ConsEnv;</title>

    <para>

    A &Builder; object isn't useful
    until it's attached to a &consenv;
    so that we can call it to arrange
    for files to be built.
    This is done through the &cv-link-BUILDERS;
    &consvar; in an environment.
    The &cv-BUILDERS; variable is a Python dictionary
    that maps the names by which you want to call
    various &Builder; objects to the objects themselves.
    For example, if we want to call the
    &Builder; we just defined by the name
    <function>Foo</function>,
    our &SConstruct; file might look like:

    </para>

    <scons_example name="builderswriting_ex1">
       <file name="SConstruct">
bld = Builder(action = 'foobuild &lt; $SOURCE &gt; $TARGET')
env = Environment(BUILDERS = {'Foo' : bld})
import os
env['ENV']['PATH'] = env['ENV']['PATH'] + os.pathsep + os.getcwd()
env.Foo('file.foo', 'file.input')
       </file>
       <file name="file.input">
file.input
       </file>
       <file name="foobuild" chmod="0755">
cat
       </file>
    </scons_example>

    <sconstruct>
bld = Builder(action = 'foobuild &lt; $SOURCE &gt; $TARGET')
env = Environment(BUILDERS = {'Foo' : bld})
    </sconstruct>

    <para>

    With the &Builder; attached to our &consenv;
    with the name <function>Foo</function>,
    we can now actually call it like so:

    </para>

    <programlisting>
env.Foo('file.foo', 'file.input')
    </programlisting>

    <para>

    Then when we run &SCons; it looks like:

    </para>

    <scons_output example="builderswriting_ex1" suffix="1">
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

    <para>

    Note, however, that the default &cv-BUILDERS;
    variable in a &consenv;
    comes with a default set of &Builder; objects
    already defined:
    &b-link-Program;, &b-link-Library;, etc.
    And when we explicitly set the &cv-BUILDERS; variable
    when we create the &consenv;,
    the default &Builder;s are no longer part of
    the environment:

    </para>

   <!--
   The ToolSurrogate stuff that's used to capture output initializes
   SCons.Defaults.ConstructionEnvironment with its own list of TOOLS.
   In this next example, we want to show the user that when they
   set the BUILDERS explicitly, the call to env.Program() generates
   an AttributeError.  This won't happen with all of the default
   ToolSurrogates in the default construction environment.  To make the
   AttributeError show up, we have to overwite the default construction
   environment's TOOLS variable so Program() builder doesn't show up.

   We do this by executing a slightly different SConstruct file than the
   one we print in the guide, with two extra statements at the front
   that overwrite the TOOLS variable as described.  Note that we have
   to jam those statements on to the first line to keep the line number
   in the generated error consistent with what the user will see in the
   User's Guide.
   -->
    <scons_example name="builderswriting_ex2">
       <file name="SConstruct">
import SCons.Defaults; SCons.Defaults.ConstructionEnvironment['TOOLS'] = {}; bld = Builder(action = 'foobuild &lt; $SOURCE &gt; $TARGET')
env = Environment(BUILDERS = {'Foo' : bld})
env.Foo('file.foo', 'file.input')
env.Program('hello.c')
       </file>
       <file name="SConstruct.printme" printme="1">
bld = Builder(action = 'foobuild &lt; $SOURCE &gt; $TARGET')
env = Environment(BUILDERS = {'Foo' : bld})
env.Foo('file.foo', 'file.input')
env.Program('hello.c')
       </file>
       <file name="file.input">
file.input
       </file>
       <file name="hello.c">
hello.c
       </file>
    </scons_example>

    <scons_output example="builderswriting_ex2" suffix="1">
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

    <para>

    To be able to use both our own defined &Builder; objects
    and the default &Builder; objects in the same &consenv;,
    you can either add to the &cv-BUILDERS; variable
    using the &Append; function:

    </para>

    <scons_example name="builderswriting_ex3">
       <file name="SConstruct">
env = Environment()
import os
env['ENV']['PATH'] = env['ENV']['PATH'] + os.pathsep + os.getcwd()
bld = Builder(action = 'foobuild &lt; $SOURCE &gt; $TARGET')
env.Append(BUILDERS = {'Foo' : bld})
env.Foo('file.foo', 'file.input')
env.Program('hello.c')
       </file>
       <file name="file.input">
file.input
       </file>
       <file name="hello.c">
hello.c
       </file>
       <file name="foobuild" chmod="0755">
cat
       </file>
    </scons_example>

    <sconstruct>
env = Environment()
bld = Builder(action = 'foobuild &lt; $SOURCE &gt; $TARGET')
env.Append(BUILDERS = {'Foo' : bld})
env.Foo('file.foo', 'file.input')
env.Program('hello.c')
    </sconstruct>

    <para>

    Or you can explicitly set the appropriately-named
    key in the &cv-BUILDERS; dictionary:

    </para>

    <sconstruct>
env = Environment()
bld = Builder(action = 'foobuild &lt; $SOURCE &gt; $TARGET')
env['BUILDERS']['Foo'] = bld
env.Foo('file.foo', 'file.input')
env.Program('hello.c')
    </sconstruct>

    <para>

    Either way, the same &consenv;
    can then use both the newly-defined
    <function>Foo</function> &Builder;
    and the default &b-link-Program; &Builder;:

    </para>

    <scons_output example="builderswriting_ex3" suffix="1">
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

  </section>

  <section>
  <title>Letting &SCons; Handle The File Suffixes</title>

    <para>

    By supplying additional information
    when you create a &Builder;,
    you can let &SCons; add appropriate file
    suffixes to the target and/or the source file.
    For example, rather than having to specify
    explicitly that you want the <literal>Foo</literal>
    &Builder; to build the <literal>file.foo</literal>
    target file from the <literal>file.input</literal> source file,
    you can give the <literal>.foo</literal>
    and <literal>.input</literal> suffixes to the &Builder;,
    making for more compact and readable calls to
    the <literal>Foo</literal> &Builder;:

    </para>

    <scons_example name="builderswriting_ex4">
       <file name="SConstruct">
bld = Builder(action = 'foobuild &lt; $SOURCE &gt; $TARGET',
              suffix = '.foo',
              src_suffix = '.input')
env = Environment(BUILDERS = {'Foo' : bld})
import os
env['ENV']['PATH'] = env['ENV']['PATH'] + os.pathsep + os.getcwd()
env.Foo('file1')
env.Foo('file2')
       </file>
       <file name="file1.input">
file1.input
       </file>
       <file name="file2.input">
file2.input
       </file>
       <file name="foobuild" chmod="0755">
cat
       </file>
    </scons_example>

    <sconstruct>
bld = Builder(action = 'foobuild &lt; $SOURCE &gt; $TARGET',
              suffix = '.foo',
              src_suffix = '.input')
env = Environment(BUILDERS = {'Foo' : bld})
env.Foo('file1')
env.Foo('file2')
    </sconstruct>

    <scons_output example="builderswriting_ex4" suffix="1">
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

    <para>

    You can also supply a <literal>prefix</literal> keyword argument
    if it's appropriate to have &SCons; append a prefix
    to the beginning of target file names.

    </para>

  </section>

  <section>
  <title>Builders That Execute Python Functions</title>

    <para>

    In &SCons;, you don't have to call an external command
    to build a file.
    You can, instead, define a Python function
    that a &Builder; object can invoke
    to build your target file (or files).
    Such a &buildfunc; definition looks like:

    </para>

    <programlisting>
def build_function(target, source, env):
    # Code to build "target" from "source"
    return None
    </programlisting>

    <para>

    The arguments of a &buildfunc; are:

    </para>

    <variablelist>

      <varlistentry>
      <term>target</term>

      <listitem>
      <para>

      A list of Node objects representing
      the target or targets to be
      built by this builder function.
      The file names of these target(s)
      may be extracted using the Python &str; function.

      </para>
      </listitem>
      </varlistentry>

      <varlistentry>
      <term>source</term>

      <listitem>
      <para>

      A list of Node objects representing
      the sources to be
      used by this builder function to build the targets.
      The file names of these source(s)
      may be extracted using the Python &str; function.

      </para>
      </listitem>
      </varlistentry>

      <varlistentry>
      <term>env</term>

      <listitem>
      <para>

      The &consenv; used for building the target(s).
      The builder function may use any of the
      environment's construction variables
      in any way to affect how it builds the targets.

      </para>
      </listitem>
      </varlistentry>

    </variablelist>

    <para>

    The builder function must
    return a <literal>0</literal> or <literal>None</literal> value
    if the target(s) are built successfully.
    The builder function
    may raise an exception
    or return any non-zero value
    to indicate that the build is unsuccessful.

    </para>

    <para>

    Once you've defined the Python function
    that will build your target file,
    defining a &Builder; object for it is as
    simple as specifying the name of the function,
    instead of an external command,
    as the &Builder;'s
    <literal>action</literal>
    argument:

    </para>

    <scons_example name="builderswriting_ex5">
       <file name="SConstruct" printme="1">
def build_function(target, source, env):
    # Code to build "target" from "source"
    return None
bld = Builder(action = build_function,
              suffix = '.foo',
              src_suffix = '.input')
env = Environment(BUILDERS = {'Foo' : bld})
env.Foo('file')
       </file>
       <file name="file.input">
file.input
       </file>
    </scons_example>

    <para>

    And notice that the output changes slightly,
    reflecting the fact that a Python function,
    not an external command,
    is now called to build the target file:

    </para>

    <scons_output example="builderswriting_ex5" suffix="1">
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

  </section>

  <section>
  <title>Builders That Create Actions Using a &Generator;</title>

    <para>

    &SCons; Builder objects can create an action "on the fly"
    by using a function called a &generator;.
    This provides a great deal of flexibility to
    construct just the right list of commands
    to build your target.
    A &generator; looks like:

    </para>

    <programlisting>
def generate_actions(source, target, env, for_signature):
    return 'foobuild &lt; %s &gt; %s' % (target[0], source[0])
    </programlisting>

    <para>

    The arguments of a &generator; are:

    </para>

    <variablelist>

      <varlistentry>
      <term>source</term>

      <listitem>
      <para>

      A list of Node objects representing
      the sources to be built
      by the command or other action
      generated by this function.
      The file names of these source(s)
      may be extracted using the Python &str; function.

      </para>
      </listitem>

      </varlistentry>

      <varlistentry>
      <term>target</term>

      <listitem>
      <para>

      A list of Node objects representing
      the target or targets to be built
      by the command or other action
      generated by this function.
      The file names of these target(s)
      may be extracted using the Python &str; function.

      </para>
      </listitem>

      </varlistentry>

      <varlistentry>
      <term>env</term>

      <listitem>
      <para>

      The &consenv; used for building the target(s).
      The generator may use any of the
      environment's construction variables
      in any way to determine what command
      or other action to return.

      </para>
      </listitem>

      </varlistentry>

      <varlistentry>
      <term>for_signature</term>

      <listitem>
      <para>

      A flag that specifies whether the
      generator is being called to contribute to a build signature,
      as opposed to actually executing the command.

      <!-- XXX NEED MORE HERE, describe generators use in signatures -->

      </para>
      </listitem>

      </varlistentry>

    </variablelist>

    <para>

    The &generator; must return a
    command string or other action that will be used to
    build the specified target(s) from the specified source(s).

    </para>

    <para>

    Once you've defined a &generator;,
    you create a &Builder; to use it
    by specifying the generator keyword argument
    instead of <literal>action</literal>.

    </para>

    <scons_example name="builderswriting_ex6">
       <file name="SConstruct">
def generate_actions(source, target, env, for_signature):
    return 'foobuild &lt; %s &gt; %s' % (source[0], target[0])
bld = Builder(generator = generate_actions,
              suffix = '.foo',
              src_suffix = '.input')
env = Environment(BUILDERS = {'Foo' : bld})
import os
env['ENV']['PATH'] = env['ENV']['PATH'] + os.pathsep + os.getcwd()
env.Foo('file')
       </file>
       <file name="file.input">
file.input
       </file>
       <file name="foobuild" chmod="0755">
cat
       </file>
    </scons_example>

    <sconstruct>
def generate_actions(source, target, env, for_signature):
    return 'foobuild &lt; %s &gt; %s' % (source[0], target[0])
bld = Builder(generator = generate_actions,
              suffix = '.foo',
              src_suffix = '.input')
env = Environment(BUILDERS = {'Foo' : bld})
env.Foo('file')
    </sconstruct>

    <scons_output example="builderswriting_ex6" suffix="1">
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

    <para>

    Note that it's illegal to specify both an
    <literal>action</literal>
    and a
    <literal>generator</literal>
    for a &Builder;.

    </para>

  </section>

  <section>
  <title>Builders That Modify the Target or Source Lists Using an &Emitter;</title>

    <para>

    &SCons; supports the ability for a Builder to modify the
    lists of target(s) from the specified source(s).
    You do this by defining an &emitter; function
    that takes as its arguments
    the list of the targets passed to the builder,
    the list of the sources passed to the builder,
    and the construction environment.
    The emitter function should return the modified
    lists of targets that should be built
    and sources from which the targets will be built.

    </para>

    <para>

    For example, suppose you want to define a Builder
    that always calls a <filename>foobuild</filename> program,
    and you want to automatically add
    a new target file named
    <filename>new_target</filename>
    and a new source file named
    <filename>new_source</filename>
    whenever it's called.
    The &SConstruct; file might look like this:

    </para>

    <scons_example name="builderswriting_ex7">
       <file name="SConstruct">
def modify_targets(target, source, env):
    target.append('new_target')
    source.append('new_source')
    return target, source
bld = Builder(action = 'foobuild $TARGETS - $SOURCES',
              suffix = '.foo',
              src_suffix = '.input',
              emitter = modify_targets)
env = Environment(BUILDERS = {'Foo' : bld})
import os
env['ENV']['PATH'] = env['ENV']['PATH'] + os.pathsep + os.getcwd()
env.Foo('file')
       </file>
       <file name="file.input">
file.input
       </file>
       <file name="new_source">
new_source
       </file>
       <file name="foobuild" chmod="0755">
cat
       </file>
    </scons_example>

    <sconstruct>
def modify_targets(target, source, env):
    target.append('new_target')
    source.append('new_source')
    return target, source
bld = Builder(action = 'foobuild $TARGETS - $SOURCES',
              suffix = '.foo',
              src_suffix = '.input',
              emitter = modify_targets)
env = Environment(BUILDERS = {'Foo' : bld})
env.Foo('file')
    </sconstruct>

    <para>
    
    And would yield the following output:

    </para>

    <scons_output example="builderswriting_ex7" suffix="1">
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

    <para>

    One very flexible thing that you can do is
    use a construction variable to specify
    different emitter functions for different
    construction variable.
    To do this, specify a string
    containing a construction variable
    expansion as the emitter when you call
    the &Builder; function,
    and set that construction variable to
    the desired emitter function
    in different construction environments:

    </para>

    <scons_example name="builderswriting_MY_EMITTER">

      <file name="SConstruct" printme="1">
bld = Builder(action = './my_command $SOURCES &gt; $TARGET',
              suffix = '.foo',
              src_suffix = '.input',
              emitter = '$MY_EMITTER')
def modify1(target, source, env):
    return target, source + ['modify1.in']
def modify2(target, source, env):
    return target, source + ['modify2.in']
env1 = Environment(BUILDERS = {'Foo' : bld},
                   MY_EMITTER = modify1)
env2 = Environment(BUILDERS = {'Foo' : bld},
                   MY_EMITTER = modify2)
env1.Foo('file1')
env2.Foo('file2')
        </file>
        <file name="file1.input">
file1.input
        </file>
        <file name="file2.input">
file2.input
        </file>
        <file name="modify1.in">
modify1.input
        </file>
        <file name="modify2.in">
modify2.input
        </file>
        <file name="my_command" chmod="0755">
cat
        </file>

    </scons_example>

    <para>

    In this example, the <filename>modify1.in</filename>
    and <filename>modify2.in</filename> files
    get added to the source lists
    of the different commands:

    </para>

    <scons_output example="builderswriting_MY_EMITTER" suffix="1">
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

  </section>

  <!--

  <section>
  <title>target_factor=, source_factory=</title>

  </section>

  <section>
  <title>target_scanner=, source_scanner=</title>

  </section>

  <section>
  <title>multi=</title>

  </section>

  <section>
  <title>single_source=</title>

  </section>

  <section>
  <title>src_builder=</title>

  </section>

  <section>
  <title>ensure_suffix=</title>

  </section>

  -->

  <section>
  <title>Where To Put Your Custom Builders and Tools</title>

    <para>

    The <filename>site_scons</filename> directories give you a place to
    put Python modules and packages that you can import into your &SConscript; files
    (<filename>site_scons</filename>),
    add-on tools that can integrate into &SCons;
    (<filename>site_scons/site_tools</filename>),
    and a <filename>site_scons/site_init.py</filename> file that
    gets read before any &SConstruct; or &SConscript; file,
    allowing you to change &SCons;'s default behavior.

    </para>

    <para>

    Each system type (Windows, Mac, Linux, etc.) searches a canonical
    set of directories for site_scons; see the man page for details.
    The top-level SConstruct's site_scons dir is always searched last,
    and its dir is placed first in the tool path so it overrides all
    others.

    </para>

    <para>

    If you get a tool from somewhere (the &SCons; wiki or a third party,
    for instance) and you'd like to use it in your project, a
    <filename>site_scons</filename> dir is the simplest place to put it.
    Tools come in two flavors; either a Python function that operates on
    an &Environment; or a Python module or package  containing two functions,
    <function>exists()</function> and <function>generate()</function>.

    </para>

    <para>

    A single-function Tool can just be included in your
    <filename>site_scons/site_init.py</filename> file where it will be
    parsed and made available for use.  For instance, you could have a
    <filename>site_scons/site_init.py</filename> file like this:

    </para>

    <scons_example name="builderswriting_site1">
      <file name="site_scons/site_init.py" printme="1">
def TOOL_ADD_HEADER(env):
   """A Tool to add a header from $HEADER to the source file"""
   add_header = Builder(action=['echo "$HEADER" &gt; $TARGET',
                                'cat $SOURCE &gt;&gt; $TARGET'])
   env.Append(BUILDERS = {'AddHeader' : add_header})
   env['HEADER'] = '' # set default value
      </file>
      <file name="SConstruct">
env=Environment(tools=['default', TOOL_ADD_HEADER], HEADER="=====")
env.AddHeader('tgt', 'src')
      </file>
      <file name="src">
hi there
      </file>
    </scons_example>

    <para>

    and a &SConstruct; like this:

    </para>

    <sconstruct>
# Use TOOL_ADD_HEADER from site_scons/site_init.py
env=Environment(tools=['default', TOOL_ADD_HEADER], HEADER="=====")
env.AddHeader('tgt', 'src')
    </sconstruct>

    <para>

      The <function>TOOL_ADD_HEADER</function> tool method will be
      called to add the <function>AddHeader</function> tool to the
      environment.

    </para>

    <!-- 
    <scons_output example="builderswriting_site1" os="posix" suffix="1">
       <scons_output_command>scons -Q</scons_output_command>
    </scons_output>
    -->

    <para>
      A more full-fledged tool with
      <function>exists()</function> and <function>generate()</function>
      methods can be installed either as a module in the file
      <filename>site_scons/site_tools/toolname.py</filename> or as a
      package in the
      directory <filename>site_scons/site_tools/toolname</filename>.  In
      the case of using a package, the <function>exists()</function>
      and <function>generate()</function> are in the
      file <filename>site_scons/site_tools/toolname/__init__.py</filename>.
      (In all the above case <filename>toolname</filename> is replaced
      by the name of the tool.)
      Since <filename>site_scons/site_tools</filename> is automatically
      added to the head of the tool search path, any tool found there
      will be available to all environments.  Furthermore, a tool found
      there will override a built-in tool of the same name, so if you
      need to change the behavior of a built-in
      tool, <filename>site_scons</filename> gives you the hook you need.
    </para>

    <para>
      Many people have a library of utility Python functions they'd like
      to include in &SConscript;s; just put that module in
      <filename>site_scons/my_utils.py</filename> or any valid Python module name of your
      choice.  For instance you can do something like this in
      <filename>site_scons/my_utils.py</filename> to add
      <function>build_id</function> and <function>MakeWorkDir</function>
      functions:
    </para>
      
    <scons_example name="builderswriting_site2">
      <file name="site_scons/my_utils.py" printme="1">
from SCons.Script import *   # for Execute and Mkdir
def build_id():
   """Return a build ID (stub version)"""
   return "100"
def MakeWorkDir(workdir):
   """Create the specified dir immediately"""
   Execute(Mkdir(workdir))
      </file>
      <file name="SConscript">
import my_utils
MakeWorkDir('/tmp/work')
print("build_id=" + my_utils.build_id())
      </file>
    </scons_example>

    <para>

    And then in your &SConscript; or any sub-&SConscript; anywhere in
    your build, you can import <filename>my_utils</filename> and use it:

    </para>

    <sconstruct>
import my_utils
print("build_id=" + my_utils.build_id())
my_utils.MakeWorkDir('/tmp/work')
    </sconstruct>

    <para>
      Note that although you can put this library in
      <filename>site_scons/site_init.py</filename>,
      it is no better there than <filename>site_scons/my_utils.py</filename>
      since you still have to import that module into your &SConscript;.
      Also note that in order to refer to objects in the SCons namespace
      such as &Environment; or &Mkdir; or &Execute; in any file other
      than a &SConstruct; or &SConscript; you always need to do
    </para>
    <sconstruct>
from SCons.Script import *
    </sconstruct>

    <para>
      This is true in modules in <filename>site_scons</filename> such as
      <filename>site_scons/site_init.py</filename> as well.
    </para>

    <para>

      You can use any of the user- or machine-wide site dirs such as
      <filename>~/.scons/site_scons</filename> instead of
      <filename>./site_scons</filename>, or use the
      <literal>--site-dir</literal> option to point to your own dir.
      <filename>site_init.py</filename> and
      <filename>site_tools</filename> will be located under that dir.
      To avoid using a <filename>site_scons</filename> dir at all,
      even if it exists, use the <literal>--no-site-dir</literal>
      option.

    </para>

  </section>


  <!--

  <section>
  <title>Builders That Use Other Builders</title>

    <para>

    XXX para

    </para>

    <scons_example name="builderswriting_ex8">
       <file name="SConstruct" printme="1">
env = Environment()
#env.SourceCode('.', env.BitKeeper('my_command'))
env.Program('hello.c')
       </file>
       <file name="hello.c">
hello.c
       </file>
    </scons_example>

    <scons_output example="builderswriting_ex8" suffix="1">
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

  </section>

  -->

</chapter>
