<?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>Extending &SCons;: Writing Your Own Builders</title>

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-->

  <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 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-link-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">
import os
bld=Builder(action = 'foobuild &lt; $SOURCE &gt; $TARGET')
env = Environment(BUILDERS={'Foo': bld})
env.AppendENVPath('PATH', os.getcwd())
env.Foo('file.foo', 'file.input')
       </file>
       <file name="file.input">
file.input
       </file>
       <file name="foobuild" chmod="0o755">
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-link-BUILDERS; variable
    using the &Append; function:

    </para>

    <scons_example name="builderswriting_ex3">
       <file name="SConstruct">
import os
env = Environment()
env.AppendENVPath('PATH', 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="0o755">
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 <filename>file.foo</filename>
    target file from the <filename>file.input</filename> 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">
import os
bld = Builder(
    action='foobuild &lt; $SOURCE &gt; $TARGET',
    suffix='.foo',
    src_suffix='.input',
)
env = Environment(BUILDERS={'Foo': bld})
env.AppendENVPath('PATH', 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="0o755">
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><parameter>target</parameter></term>

      <listitem>
      <para>

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

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

      <varlistentry>
      <term><parameter>source</parameter></term>

      <listitem>
      <para>

      A list of Node objects representing
      the sources to be
      used by this 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><parameter>env</parameter></term>

      <listitem>
      <para>

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

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

    </variablelist>

    <para>

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

    For more information on Actions see the Action Objects section of
    the man page.

    </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 <firstterm>&Generator;</firstterm>.
    (Note: this is not the same thing as a &Python; generator function
    described in <ulink url="https://www.python.org/dev/peps/pep-0255/">PEP 255</ulink>)
    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><parameter>source</parameter></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><parameter>target</parameter></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><parameter>env</parameter></term>

      <listitem>
      <para>

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

      </para>
      </listitem>

      </varlistentry>

      <varlistentry>
      <term><parameter>for_signature</parameter></term>

      <listitem>
      <para>

      A flag that specifies whether the
      &generator; is being called to contribute to a &buildsig;,
      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 <parameter>generator</parameter> keyword argument
    instead of <parameter>action</parameter>.

    </para>

    <scons_example name="builderswriting_ex6">
       <file name="SConstruct">
import os
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.AppendENVPath('PATH', os.getcwd())
env.Foo('file')
       </file>
       <file name="file.input">
file.input
       </file>
       <file name="foobuild" chmod="0o755">
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
    <parameter>action</parameter>
    and a
    <parameter>generator</parameter>
    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 &consenv;.
    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 <command>foobuild</command> 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">
import os
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.AppendENVPath('PATH', os.getcwd())
env.Foo('file')
       </file>
       <file name="file.input">
file.input
       </file>
       <file name="new_source">
new_source
       </file>
       <file name="foobuild" chmod="0o755">
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 &consvar; to specify
    different emitter functions for different &consenvs;.
    To do this, specify a string
    containing a &consvar;
    expansion as the emitter when you call
    the &f-link-Builder; function,
    and set that &consvar; to
    the desired emitter function
    in different &consenvs;:

    </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="0o755">
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>Modifying a Builder by adding an Emitter</title>

    <para>

    Defining an emitter to work with a custom Builder
    is a powerful concept, but sometimes all you really want
    is to be able to use an existing builder but change its
    concept of what targets are created.
    In this case,
    trying to recreate the logic of an existing Builder to
    supply a special emitter can be a lot of work.
    The typical case for this is when you want to use a compiler flag
    that causes additional files to be generated.
    For example the GNU linker accepts an option
    <option>-Map</option> which outputs a link map
    to the file specified by the option's argument.
    If this option is just supplied to the build,
    &SCons; will not consider the link map file a tracked target,
    which has various undesirable efffects.

    </para>

    <para>

    To help with this, &SCons; provides &consvars; which correspond
    to a few standard builders:
    &cv-link-PROGEMITTER; for &b-link-Program;;
    &cv-link-LIBEMITTER; for &b-link-Library;;
    &cv-link-SHLIBEMITTER; for &b-link-SharedLibrary; and
    &cv-link-LDMODULEEMITTER; for &b-link-LoadableModule;;.
    Adding an emitter to one of these will cause it to be
    invoked in addition to any existing emitter for the
    corresponding builder.

    </para>

    <para>

    This example adds map creation as a linker flag,
    and modifies the standard &b-link-Program;
    emitter to know that map generation is a side-effect:

    </para>

    <scons_example name="builders_adding_emitter">

      <file name="SConstruct" printme="1">
env = Environment()
map_filename = "${TARGET.name}.map"

def map_emitter(target, source, env):
    target.append(map_filename)
    return target, source

env.Append(LINKFLAGS="-Wl,-Map={},--cref".format(map_filename))
env.Append(PROGEMITTER=map_emitter)
env.Program('hello.c')
      </file>
      <file name="hello.c">
#include &lt;stdio.h&gt;

int
main()
{
    printf("Hello, world!\n");
}
      </file>
    </scons_example>

    <para>

    If you run this example, adding an option to tell &SCons; to
    dump some information about the dependencies it knows,
    it shows the map file option in use, and that &SCons; indeed
    knows about the map file,
    it's not just a silent side effect of the compiler:

    </para>

    <scons_output example="builders_adding_emitter" suffix="1">
      <scons_output_command>scons -Q --tree=prune</scons_output_command>
    </scons_output>

  </section>

  <!-- Placeholder for describing other keyword args to Builder

  <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 (at the top level)
    add-on tools that can integrate into &SCons;
    (in a <filename>site_tools</filename> subdirectory),
    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 <filename>site_scons</filename>;
    see the man page for details.
    The top-level SConstruct's <filename>site_scons</filename> dir
    (that is, the one in the project) 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 collection of utility &Python; functions they'd like
      to include in their &SConscript; files: just put them 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>
      You can put this collection in its own module in a
      <filename>site_scons</filename> and import it as in the example,
      or you can include it in
      <filename>site_scons/site_init.py</filename>,
      which is automatically imported (unless you disable site directories).
      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 of 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 directories such as
      <filename>~/.scons/site_scons</filename> instead of
      <filename>./site_scons</filename>, or use the
      <option>--site-dir</option> option to point to your own directory.
      <filename>site_init.py</filename> and
      <filename>site_tools</filename> will be located under that directory.
      To avoid using a <filename>site_scons</filename> directory at all,
      even if it exists, use the <option>--no-site-dir</option>
      option.
    </para>

  </section>

</chapter>
