path: root/doc/design/engine.xml

<?xml version='1.0'?>
<!DOCTYPE sconsdoc [
    <!ENTITY % scons SYSTEM "../scons.mod">
    %scons;
]>

<chapter id="chap-engine"
         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>Build Engine API</title>

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  a copy of this software and associated documentation files (the
  "Software"), to deal in the Software without restriction, including
  without limitation the rights to use, copy, modify, merge, publish,
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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
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<section id="sect-principles">
 <title>General Principles</title>

 <section>
  <title>Keyword arguments</title>
  
  <para>

   All methods and functions in this API will support the use of keyword
   arguments in calls, for the sake of explicitness and readability.
   For brevity in the hands of experts, most methods and functions
   will also support positional arguments for their most-commonly-used
   arguments.  As an explicit example, the following two lines will each
   arrange for an executable program named <filename>foo</filename> (or
   <filename>foo.exe</filename> on a Win32 system) to be compiled from
   the <filename>foo.c</filename> source file:

  </para>

	<programlisting>
	env.Program(target = 'foo', source = 'foo.c')

	env.Program('foo', 'foo.c')
	</programlisting>

 </section>

 <section>
  <title>Internal object representation</title>

  <para>

   All methods and functions use internal (Python) objects that
   represent the external objects (files, for example) for which they
   perform dependency analysis.

  </para>

  <para>

   All methods and functions in this API that accept an external object
   as an argument will accept <emphasis>either</emphasis> a string
   description or an object reference.  For example, the two following
   two-line examples are equivalent:

  </para>

	<programlisting>
	env.Object(target = 'foo.o', source = 'foo.c')
	env.Program(target = 'foo', 'foo.o')    # builds foo from foo.o

	foo_obj = env.Object(target = 'foo.o', source = 'foo.c')
	env.Program(target = 'foo', foo_obj)    # builds foo from foo.o
	</programlisting>

 </section>

</section>



<section id="sect-envs">
 <title>&ConsEnvs;</title>

 <para>

  A &consenv; is the basic means by which a software system interacts
  with the &SCons; Python API to control a build process.

 </para>

 <para>

  A &consenv; is an object with associated methods for generating target
  files of various types (&Builder; objects), other associated object
  methods for automatically determining dependencies from the contents
  of various types of source files (&Scanner; objects), and a dictionary
  of values used by these methods.

 </para>

 <para>

  Passing no arguments to the &Environment; instantiation creates a
  &consenv; with default values for the current platform:

 </para>

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

 <section>
  <title>&Consvars;</title>

  <para>

   A &consenv; has an associated dictionary of &consvars; that control how
   the build is performed.  By default, the &Environment; method creates
   a &consenv; with values that make most software build "out of the box"
   on the host system.  These default values will be generated at the
   time &SCons; is installed using functionality similar to that provided
   by GNU &Autoconf;.
   <footnote>
    <para>
     It would be nice if we could avoid re-inventing the wheel here by
     using some other Python-based tool &Autoconf; replacement--like what
     was supposed to come out of the Software Carpentry configuration
     tool contest.  It will probably be most efficient to roll our own
     logic initially and convert if something better does come along.
    </para>
   </footnote>
   At a minimum, there will be pre-configured sets of default values
   that will provide reasonable defaults for UNIX and Windows NT.

  </para>

  <para>

   The default &consenv; values may be overridden when a new &consenv; is
   created by specifying keyword arguments:

  </para>

	<programlisting>
	env = Environment(CC =          'gcc',
	                  CCFLAGS =    '-g',
	                  CPPPATH =    ['.', 'src', '/usr/include'],
	                  LIBPATH =    ['/usr/lib', '.'])
	</programlisting>

 </section>

 <section>
  <title>Fetching &consvars;</title>

  <para>

   A copy of the dictionary of &consvars; can be returned using
   the &Dictionary; method:

  </para>

	<programlisting>
	env = Environment()
	dict = env.Dictionary()
	</programlisting>

  <para>

   If any arguments are supplied, then just the corresponding value(s)
   are returned:

  </para>

	<programlisting>
	ccflags = env.Dictionary('CCFLAGS')
	cc, ld = env.Dictionary('CC', 'LD')
	</programlisting>

 </section>

 <section>
  <title>Copying a &consenv;</title>

  <para>

   A method exists to return a copy of an existing environment, with
   any overridden values specified as keyword arguments to the method:

  </para>

	<programlisting>
	env = Environment()
	debug = env.Copy(CCFLAGS = '-g')
	</programlisting>

 </section>

 <section>
  <title>Multiple &consenvs;</title>

  <para>

   Different external objects often require different build
   characteristics.  Multiple &consenvs; may be defined, each with
   different values:

  </para>

	<programlisting>
	env = Environment(CCFLAGS = '')
	debug = Environment(CCFLAGS = '-g')
	env.Make(target = 'hello', source = 'hello.c')
	debug.Make(target = 'hello-debug', source = 'hello.c')
	</programlisting>

  <para>

   Dictionaries of values from multiple &consenvs; may be passed to the
   &Environment; instantiation or the &Copy; method, in which case the
   last-specified dictionary value wins:

  </para>

	<programlisting>
	env1 = Environment(CCFLAGS = '-O', LDFLAGS = '-d')
	env2 = Environment(CCFLAGS = '-g')
	new = Environment(env1.Dictionary(), env2.Dictionary())
	</programlisting>

  <para>

   The <varname>new</varname> environment in the above example retains
   <literal>LDFLAGS = '-d'</literal> from the <varname>env1</varname>
   environment, and <literal>CCFLAGS = '-g'</literal> from the
   <varname>env2</varname> environment.

  </para>

  <!--

	hardware details
	current directory
	OS environment variables
	compilers and options,
	aliases for commands,
	versions of tools

	environment overrides a la Cons

	compilation options

	cross compilation via selection of tool+options

	paths for header files (specify alternate path)

	accomodate smart compilers that can tell you
	"I know how to turn .c or .ccp into .o",
	"I know how to turn .f into .o"

   -->

 </section>

 <section>
  <title>Variable substitution</title>

  <para>

   Within a construction command, any variable from the &consenv; may
   be interpolated by prefixing the name of the construction with
   <symbol>$</symbol>:

  </para>

	<programlisting>
	MyBuilder = Builder(command = "$XX $XXFLAGS -c $_INPUTS -o $target")

	env.Command(targets = 'bar.out', sources = 'bar.in',
	            command = "sed '1d' &lt; $source > $target")
	</programlisting>

  <para>

   Variable substitution is recursive:  the command line is expanded
   until no more substitutions can be made.

  </para>

  <para>

   Variable names following the <symbol>$</symbol> may be enclosed in
   braces.  This can be used to concatenate an interpolated value with an
   alphanumeric character:

  </para>

	<programlisting>
	VerboseBuilder = Builder(command = "$XX -${XXFLAGS}v > $target")
	</programlisting>

  <para>

   The variable within braces may contain a pair of parentheses
   after a Python function name to be evaluated (for example,
   <literal>${map()}</literal>).  &SCons; will interpolate the return
   value from the function (presumably a string):

  </para>

	<programlisting>
	env = Environment(FUNC = myfunc)
	env.Command(target = 'foo.out', source = 'foo.in',
	            command = "${FUNC($&lt;)}")
	</programlisting>

  <para>

   If a referenced variable is not defined in the &consenv;,
   the null string is interpolated.

  </para>

  <para>

   The following special variables can also be used:

  </para>

  <variablelist>

   <varlistentry>
    <term><literal>$targets</literal></term>
    <listitem>
     <para>

      All target file names.  If multiple targets are specified in an
      array, <literal>$targets</literal> expands to the entire list of
      targets, separated by a single space.

    </para>

    <para>

      Individual targets from a list may be extracted by enclosing
      the <literal>targets</literal> keyword in braces and using the
      appropriate Python array index or slice:

    </para>

	<programlisting>
	${targets[0]}     # expands to the first target

	${targets[1:]}    # expands to all but the first target

	${targets[1:-1]}  # expands to all but the first and last targets
	</programlisting>

    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>$target</literal></term>
    <listitem>
     <para>

      A synonym for <literal>${targets[0]}</literal>, the first target
      specified.

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

   <varlistentry>
    <term><literal>$sources</literal></term>
    <listitem>
     <para>

      All input file names.  Any input file names that
      are used anywhere else on the current command
      line (via <literal>${sources[0]}</literal>,
      <literal>${sources{[1]}</literal>, etc.) are removed from the
      expanded list.

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

  </variablelist>

  <para>

   Any of the above special variables may be enclosed in braces and
   followed immediately by one of the following attributes to select just
   a portion of the expanded path name:

  </para>

  <variablelist>

   <varlistentry>
    <term><literal>.base</literal></term>
    <listitem>
     <para>

      Basename: the directory plus the file name, minus any file suffix.

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

   <varlistentry>
    <term><literal>.dir</literal></term>
    <listitem>
     <para>

      The directory in which the file lives.  This is a relative path,
      where appropriate.

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

   <varlistentry>
    <term><literal>.file</literal></term>
    <listitem>
     <para>

      The file name, minus any directory portion.

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

   <varlistentry>
    <term><literal>.suffix</literal></term>
    <listitem>
     <para>

      The file name suffix (that is, the right-most dot in the file name,
      and all characters to the right of that).

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

   <varlistentry>
    <term><literal>.filebase</literal></term>
    <listitem>
     <para>

      The file name (no directory portion), minus any file suffix.

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

   <varlistentry>
    <term><literal>.abspath</literal></term>
    <listitem>
     <para>

      The absolute path to the file.

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

  </variablelist>

 </section>

</section>



<section id="sect-builders">
 <title>&Builder; Objects</title>

 <para>

  By default, &SCons; supplies (and uses) a number of pre-defined
  &Builder; objects:

 </para>

 <informaltable>
  <tgroup cols="2">
  <tbody>

   <row>
    <entry>&Object;</entry>
    <entry>compile or assemble an object file</entry>
   </row>

   <row>
    <entry>&Library;</entry>
    <entry>archive files into a library</entry>
   </row>

   <row>
    <entry>&SharedLibrary;</entry>
    <entry>archive files into a shared library</entry>
   </row>

   <row>
    <entry>&Program;</entry>
    <entry>link objects and/or libraries into an executable</entry>
   </row>

   <row>
    <entry>&MakeBuilder;</entry>
    <entry>build according to file suffixes; see below</entry>
   </row>

  </tbody>
  </tgroup>
 </informaltable>

<!--
&Library; and &SharedLibrary; have nearly identical
semantics, just different
tools and &consenvs (paths, etc.) that they use.
In other words, you can construct a shared library
using just the &Library; &Builder; object
with a different environment.
I think that's a better way to do it.
Feedback?
-->

 <para>

  A &consenv; can be explicitly initialized with associated &Builder;
  objects that will be bound to the &consenv; object:

 </para>

	<programlisting>
	env = Environment(BUILDERS = ['Object', 'Program'])
	</programlisting>

 <para>

  &Builder; objects bound to a &consenv; can be called directly as
  methods.  When invoked, a &Builder; object returns a (list of) objects
  that it will build:

 </para>

	<programlisting>
	obj = env.Object(target ='hello.o', source = 'hello.c')
	lib = env.Library(target ='libfoo.a',
	                  source = ['aaa.c', 'bbb.c'])
	slib = env.SharedLibrary(target ='libbar.so',
	                         source = ['xxx.c', 'yyy.c'])
	prog = env.Program(target ='hello',
	                   source = ['hello.o', 'libfoo.a', 'libbar.so'])
	</programlisting>

 <section>
  <title>Specifying multiple inputs</title>

  <para>

   Multiple input files that go into creating a target file may be passed
   in as a single string, with the individual file names separated by
   white space:

  </para>

	<programlisting>
	env.Library(target = 'foo.a', source = 'aaa.c bbb.c ccc.c')
	env.Object(target = 'yyy.o', source = 'yyy.c')
	env.Program(target = 'bar', source = 'xxx.c yyy.o foo.a')
	</programlisting>

  <para>

   Alternatively, multiple input files that go into creating a target
   file may be passed in as an array.  This allows input files to be
   specified using their object representation:

  </para>

	<programlisting>
	env.Library(target = 'foo.a', source = ['aaa.c', 'bbb.c', 'ccc.c'])
	yyy_obj = env.Object(target = 'yyy.o', source = 'yyy.c')
	env.Program(target = 'bar', source = ['xxx.c', yyy_obj, 'foo.a'])
	</programlisting>

   <para>

    Individual string elements within an array of input files are
    <emphasis>not</emphasis> further split into white-space separated
    file names.  This allows file names that contain white space to
    be specified by putting the value into an array:

	<programlisting>
	env.Program(target = 'foo', source = ['an input file.c'])
	</programlisting>

   </para>

 </section>

 <section>
  <title>Specifying multiple targets</title>

  <para>

   Conversely, the generated target may be a string listing multiple
   files separated by white space:

  </para>

	<programlisting>
	env.Object(target = 'grammar.o y.tab.h', source = 'grammar.y')
	</programlisting>

  <para>

   An array of multiple target files can be used to mix string and object
   representations, or to accomodate file names that contain white space:

  </para>

	<programlisting>
	env.Program(target = ['my program'], source = 'input.c')
	</programlisting>

 </section>

 <section>
  <title>File prefixes and suffixes</title>

  <para>

   For portability, if the target file name does not already have an
   appropriate file prefix or suffix, the &Builder; objects will
   append one appropriate for the file type on the current system:

  </para>

	<programlisting>
	# builds 'hello.o' on UNIX, 'hello.obj' on Windows NT:
	obj = env.Object(target ='hello', source = 'hello.c')

	# builds 'libfoo.a' on UNIX, 'foo.lib' on Windows NT:
	lib = env.Library(target ='foo', source = ['aaa.c', 'bbb.c'])

	# builds 'libbar.so' on UNIX, 'bar.dll' on Windows NT:
	slib = env.SharedLibrary(target ='bar', source = ['xxx.c', 'yyy.c'])

	# builds 'hello' on UNIX, 'hello.exe' on Windows NT:
	prog = env.Program(target ='hello',
	                   source = ['hello.o', 'libfoo.a', 'libbar.so'])
	</programlisting>

 </section>

 <section>
  <title>&Builder; object exceptions</title>

  <para>

   &Builder; objects raise the following exceptions on error:

 <!--
 LIST THESE ONCE WE FIGURE OUT WHAT THEY ARE FROM CODING THEM.
 -->

  </para>
 </section>

 <section>
  <title>User-defined &Builder; objects</title>

  <para>

   Users can define additional &Builder; objects for specific external
   object types unknown to &SCons;.  A &Builder; object may build its
   target by executing an external command:

  </para>

	<programlisting>
	WebPage = Builder(command = 'htmlgen $HTMLGENFLAGS $sources > $target',
	                  suffix = '.html',
	                  src_suffix = '.in')
	</programlisting>

  <para>

   Alternatively, a &Builder; object may also build its target by
   executing a Python function:

   </para>

	<programlisting>
	def update(dest):
	        # [code to update the object]
	        return 1

	OtherBuilder1 = Builder(function = update,
	                        src_suffix = ['.in', '.input'])
	</programlisting>

   <para>
   
   An optional argument to pass to the function may be specified:

  </para>

	<programlisting>
	def update_arg(dest, arg):
	        # [code to update the object]
	        return 1

	OtherBuilder2 = Builder(function = update_arg,
	                        function_arg = 'xyzzy',
	                        src_suffix = ['.in', '.input'])
	</programlisting>

  <para>

   Both an external command and an internal function may be specified,
   in which case the function will be called to build the object first,
   followed by the command line.

  </para>

 <!--
 NEED AN EXAMPLE HERE.
 -->

  <para>

   User-defined &Builder; objects can be used like the default &Builder;
   objects to initialize &consenvs;.

  </para>

	<programlisting>
	WebPage = Builder(command = 'htmlgen $HTMLGENFLAGS $sources > $target',
	                  suffix = '.html',
	                  src_suffix = '.in')
	env = Environment(BUILDERS = ['WebPage'])
	env.WebPage(target = 'foo.html', source = 'foo.in')
	# Builds 'bar.html' on UNIX, 'bar.htm' on Windows NT:
	env.WebPage(target = 'bar', source = 'bar.in')
	</programlisting>

  <para>

   The command-line specification can interpolate variables from the
   &consenv;; see "Variable substitution," above.

  </para>

  <para>

   A &Builder; object may optionally be initialized with a list of:

  </para>

   <itemizedlist>
     <listitem>
     <para>

       the prefix of the target file (e.g., 'lib' for libraries)

     </para>
     </listitem>

     <listitem>
     <para>

       the suffix of the target file (e.g., '.a' for libraries)

     </para>
     </listitem>

     <listitem>
     <para>

       the expected suffixes of the input files
       (e.g., '.o' for object files)

     </para>
     </listitem>
   </itemizedlist>

   <para>

    These arguments are used in automatic
    dependency analysis and to generate output file names that don't
    have suffixes supplied explicitly.

  </para>
 </section>

 <section>
  <title>Copying &Builder; Objects</title>

  <para>

   A &Copy; method exists to return a copy of an existing &Builder;
   object, with any overridden values specified as keyword arguments to
   the method:

  </para>

	<programlisting>
	build = Builder(function = my_build)
	build_out = build.Copy(suffix = '.out')
	</programlisting>

  <para>

   Typically, &Builder; objects will be supplied by a tool-master or
   administrator through a shared &consenv;.

  </para>
 </section>

 <section>
  <title>Special-purpose build rules</title>

  <para>

   A pre-defined &Command; builder exists to associate a target file with
   a specific command or list of commands for building the file:

  </para>

	<programlisting>
	env.Command(target = 'foo.out', source = 
	            command = 'foo.in', "foo.process $sources > $target")

	commands = [    "bar.process -o .tmpfile $sources",
	                "mv .tmpfile $target" ]
	env.Command(target = 'bar.out', source = 'bar.in', command = commands)
	</programlisting>

  <para>
   This is useful when it's too cumbersome to create a &Builder;
   object just to build a single file in a special way.

  </para>
 </section>

 <section>
  <title>The &MakeBuilder; &Builder;</title>

  <para>

   A pre-defined &Builder; object named &MakeBuilder; exists to make
   simple builds as easy as possible for users, at the expense of
   sacrificing some build portability.

  </para>

  <para>

   The following minimal example builds the 'hello' program from the
   'hello.c' source file:

  </para>

	<programlisting>
	Environment().Make('hello', 'hello.c')
	</programlisting>

  <para>

   Users of the &MakeBuilder; &Builder; object are not required to
   understand intermediate steps involved in generating a file--for
   example, the distinction between compiling source code into an object
   file, and then linking object files into an executable.  The details
   of intermediate steps are handled by the invoked method.  Users that
   need to, however, can specify intermediate steps explicitly:

  </para>

	<programlisting>
	env = Environment()
	env.Make(target = 'hello.o', source = 'hello.c')
	env.Make(target = 'hello', source = 'hello.o')
	</programlisting>

  <para>

   The &MakeBuilder; method understands the file suffixes specified and
   "does the right thing" to generate the target object and program
   files, respectively.  It does this by examining the specified output
   suffixes for the &Builder; objects bound to the environment.

  </para>

  <para>

   Because file name suffixes in the target and source file names
   must be specified, the &MakeBuilder; method can't be used
   portably across operating systems.  In other words, for the
   example above, the &MakeBuilder; builder will not generate
   <filename>hello.exe</filename> on Windows NT.

  </para>

 </section>

 <section>
  <title>&Builder; maps</title>

<!--
Do we even need this anymore?
Now that the individual builders
have specified <literal>suffix</literal>
and <literal>src_suffix</literal> values,
all of the information we need to support
the &MakeBuilder; builder is right there in the environment.
I think this is a holdover from before I
added the <literal>suffix</literal> arguments.
If you want &MakeBuilder; to do something different,
you set it up with another environment...
-->

  <para>

   The <function>env.Make</function> method "does the right thing" to
   build different file types because it uses a dictionary from the
   &consenv; that maps file suffixes to the appropriate &Builder; object.
   This &BUILDERMAP; can be initialized at instantiation:

  </para>

	<programlisting>
	env = Environment(BUILDERMAP = {
	                        '.o' : Object,
	                        '.a' : Library,
	                        '.html' : WebPage,
	                        '' : Program,
	                })
	</programlisting>

  <para>

   With the &BUILDERMAP; properly initialized, the
   <function>env.Make</function> method can be used to build additional
   file types:

  </para>

	<programlisting>
	env.Make(target = 'index.html', source = 'index.input')
	</programlisting>

  <para>

   &Builder; objects referenced in the &BUILDERMAP; do not need to be
   listed separately in the <literal>BUILDERS</literal> variable.  The &consenv; will
   bind the union of the &Builder; objects listed in both variables.

  </para>

 <!--

   YYY support scanners which detect files which haven't been generated yet

 -->

 </section>

</section>



<section id="sect-deps">
 <title>Dependencies</title>

 <section>
  <title>Automatic dependencies</title>

  <para>

   By default, &SCons; assumes that a target file has <literal>automatic
   dependencies</literal> on the:

  </para>

  <blockquote>
   <simplelist>

    <member>tool used to build the target file</member>

    <member>contents of the input files</member>

    <member>command line used to build the target file</member>

   </simplelist>
  </blockquote>

  <para>

   If any of these changes, the target file will be rebuilt.

  </para>
 </section>

 <section>
  <title>Implicit dependencies</title>

  <para>

   Additionally, &SCons; can scan the contents of files for
   <literal>implicit dependencies</literal> on other files.  For
   example, &SCons; will scan the contents of a <filename>.c</filename>
   file and determine that any object created from it is
   dependent on any <filename>.h</filename> files specified via
   <literal>#include</literal>.  &SCons;, therefore, "does the right
   thing" without needing to have these dependencies listed explicitly:

  </para>

	<programlisting>
	% cat Construct
	env = Environment()
	env.Program('hello', 'hello.c')
	% cat hello.c
	#include "hello_string.h"
	main()
	{
	        printf("%s\n", STRING);
	}
	% cat > hello_string.h
	#define STRING  "Hello, world!\n"
	% scons .
	gcc -c hello.c -o hello.o
	gcc -o hello hello.c
	% ./hello
	Hello, world!
	% cat > hello_string.h
	#define STRING  "Hello, world, hello!\n"
	% scons .
	gcc -c hello.c -o hello.o
	gcc -o hello hello.c
	% ./hello
	Hello, world, hello!
	%
	</programlisting>

 </section>

 <section>
  <title>Ignoring dependencies</title>

  <para>

   Undesirable <literal>automatic dependencies</literal> or
   <literal>implicit dependencies</literal> may be ignored:

  </para>

	<programlisting>
	env.Program(target = 'bar', source = 'bar.c')
	env.Ignore('bar', '/usr/bin/gcc', 'version.h')
	</programlisting>

  <para>

   In the above example, the <filename>bar</filename> program will not
   be rebuilt if the <filename>/usr/bin/gcc</filename> compiler or the
   <filename>version.h</filename> file change.

  </para>
 </section>

 <section>
  <title>Explicit dependencies</title>

  <para>

   Dependencies that are unknown to &SCons; may be specified explicitly
   in an &SCons; configuration file:

  </para>

	<programlisting>
	env.Dependency(target = 'output1', dependency = 'input_1 input_2')
	env.Dependency(target = 'output2', dependency = ['input_1', 'input_2'])
	env.Dependency(target = 'output3', dependency = ['white space input'])

	env.Dependency(target = 'output_a output_b', dependency = 'input_3')
	env.Dependency(target = ['output_c', 'output_d'], dependency = 'input_4')
	env.Dependency(target = ['white space output'], dependency = 'input_5')
	</programlisting>

  <para>

   Just like the <literal>target</literal> keyword argument, the
   <literal>dependency</literal> keyword argument may be specified as a
   string of white-space separated file names, or as an array.

  </para>

  <para>

   A dependency on an &SCons; configuration file itself may be specified
   explicitly to force a rebuild whenever the configuration file changes:

  </para>

	<programlisting>
	env.Dependency(target = 'archive.tar.gz', dependency = 'SConstruct')
	</programlisting>

 </section>

</section>



<section id="sect-scanners">
 <title>&Scanner; Objects</title>

 <para>

  Analagous to the previously-described &Builder; objects, &SCons;
  supplies (and uses) &Scanner; objects to search the contents of
  a file for implicit dependency files:

 </para>

 <informaltable>
  <tgroup cols="2">
  <tbody>

   <row>
    <entry>CScan</entry>
    <entry>scan .{c,C,cc,cxx,cpp} files for #include dependencies</entry>
   </row>

  </tbody>
  </tgroup>
 </informaltable>

 <para>

  A &consenv; can be explicitly initialized with
  associated &Scanner; objects:

 </para>

	<programlisting>
	env = Environment(SCANNERS = ['CScan', 'M4Scan'])
	</programlisting>

 <para>

  &Scanner; objects bound to a &consenv; can be
  associated directly with specified files:

 </para>

	<programlisting>
	env.CScan('foo.c', 'bar.c')
	env.M4Scan('input.m4')
	</programlisting>

 <section>
  <title>User-defined &Scanner; objects</title>

  <para>

   A user may define a &Scanner; object to scan a type of file for
   implicit dependencies:

  </para>

	<programlisting>
	def scanner1(file_contents):
	        # search for dependencies
	        return dependency_list

	FirstScan = Scanner(function = scanner1)
	</programlisting>

  <para>

   The scanner function must return a list of dependencies that its finds
   based on analyzing the file contents it is passed as an argument.

  </para>

  <para>

   The scanner function, when invoked, will be passed the calling
   environment.  The scanner function can use &consenvs; from the passed
   environment to affect how it performs its dependency scan--the
   canonical example being to use some sort of search-path construction
   variable to look for dependency files in other directories:

  </para>

	<programlisting>
	def scanner2(file_contents, env):
	        path = env.{'SCANNERPATH'}	# XXX
	        # search for dependencies using 'path'
	        return dependency_list

	SecondScan = Scanner(function = scanner2)
	</programlisting>

  <para>

   The user may specify an additional argument when the &Scanner; object
   is created.  When the scanner is invoked, the additional argument
   will be passed to the scanner funciton, which can be used in any way
   the scanner function sees fit:

  </para>

	<programlisting>
	def scanner3(file_contents, env, arg):
	        # skip 'arg' lines, then search for dependencies
	        return dependency_list

	Skip_3_Lines_Scan = Scanner(function = scanner2, argument = 3)
	Skip_6_Lines_Scan = Scanner(function = scanner2, argument = 6)
	</programlisting>

 </section>

 <section>
  <title>Copying &Scanner; Objects</title>

  <para>

   A method exists to return a copy of an existing &Scanner; object,
   with any overridden values specified as keyword arguments to the
   method:

  </para>

	<programlisting>
	scan = Scanner(function = my_scan)
	scan_path = scan.Copy(path = '%SCANNERPATH')
	</programlisting>

  <para>

   Typically, &Scanner; objects will be supplied by a tool-master or
   administrator through a shared &consenv;.

  </para>
 </section>

 <section>
  <title>&Scanner; maps</title>

<!--
If the &BUILDERMAP; proves unnecessary,
we could/should get rid of this one, too,
by adding a parallel <literal>src_suffix</literal>
argument to the &Scanner; factory...
Comments?
-->

  <para>

   Each &consenv; has a &SCANNERMAP;, a dictionary that associates
   different file suffixes with a scanner object that can be used to
   generate a list of dependencies from the contents of that file.  This
   &SCANNERMAP; can be initialized at instantiation:

  </para>

	<programlisting>
	env = Environment(SCANNERMAP = {
	                        '.c' : CScan,
	                        '.cc' : CScan,
	                        '.m4' : M4Scan,
	                })
	</programlisting>

  <para>

   &Scanner; objects referenced in the &SCANNERMAP; do not need to
   be listed separately in the <literal>SCANNERS</literal> variable.  The &consenv;
   will bind the union of the &Scanner; objects listed
   in both variables.

  </para>

 </section>

</section>



<section id="sect-targets">
 <title>Targets</title>

 <para>

  The methods in the build engine API described so far merely
  establish associations that describe file dependencies, how a
  file should be scanned, etc.  Since the real point is to actually
  <emphasis>build</emphasis> files, &SCons; also has methods that
  actually direct the build engine to build, or otherwise manipulate,
  target files.

 </para>

 <section>
  <title>Building targets</title>
  <para>

   One or more targets may be built as follows:

  </para>

	<programlisting>
	env.Build(target = ['foo', 'bar'])
	</programlisting>

  <para>

   Note that specifying a directory (or other collective object) will
   cause all subsidiary/dependent objects to be built as well:

  </para>

	<programlisting>
	env.Build(target = '.')

	env.Build(target = 'builddir')
	</programlisting>

  <para>

   By default, &SCons; explicitly removes a target file before
   invoking the underlying function or command(s) to build it.

  </para>
 </section>

 <section>
  <title>Removing targets</title>

  <para>

   A "cleanup" operation of removing generated (target) files is
   performed as follows:

  </para>

	<programlisting>
	env.Clean(target = ['foo', 'bar'])
	</programlisting>

  <para>

   Like the &Build; method, the &Clean; method may be passed a
   directory or other collective object, in which case the subsidiary
   target objects under the directory will be removed:

  </para>

	<programlisting>
	env.Clean(target = '.')

	env.Clean(target = 'builddir')
	</programlisting>

  <para>

   (The directories themselves are not removed.)

  </para>
 </section>
 
 <section>
  <title>Suppressing cleanup removal of build-targets</title>

  <para>
    
    By default, &SCons; explicitly removes all build-targets
    when invoked to perform "cleanup". Files that should not be 
    removed during "cleanup" can be specified via the
    &NoClean; method:
    
  </para>
  
  <programlisting>
env.Library(target = 'libfoo.a', source = ['aaa.c', 'bbb.c', 'ccc.c'])
env.NoClean('libfoo.a')
  </programlisting>
  
  <para>
    
    The NoClean operation has precedence over the Clean operation.
    A target that is specified as both Clean and NoClean, will not 
    be removed during a clean.
    
    In the following example, target 'foo' will not be removed 
    during "cleanup":
    
    <programlisting>
env.Clean(target = 'foo')
env.NoClean('foo')
    </programlisting>
    
    
  </para>
  
 </section>
 
 <section>
  <title>Suppressing build-target removal</title>

  <para>

   As mentioned, by default, &SCons; explicitly removes a target
   file before invoking the underlying function or command(s) to build
   it.  Files that should not be removed before rebuilding can be
   specified via the &Precious; method:

  </para>

	<programlisting>
	env.Library(target = 'libfoo.a', source = ['aaa.c', 'bbb.c', 'ccc.c'])
	env.Precious('libfoo.a')
	</programlisting>

 </section>

 <section>
  <title>Default targets</title>

  <para>

   The user may specify default targets that will be built if there are no
   targets supplied on the command line:

  </para>

	<programlisting>
	env.Default('install', 'src')
	</programlisting>

  <para>

   Multiple calls to the &Default; method (typically one per &SConscript;
   file) append their arguments to the list of default targets.

  </para>
 </section>

 <section>
  <title>File installation</title>

  <para>

   Files may be installed in a destination directory:

  </para>

	<programlisting>
	env.Install('/usr/bin', 'program1', 'program2')
	</programlisting>

  <para>

   Files may be renamed on installation:

  </para>

	<programlisting>
	env.InstallAs('/usr/bin/xyzzy', 'xyzzy.in')
	</programlisting>

  <para>

   Multiple files may be renamed on installation by specifying
   equal-length lists of target and source files:

  </para>

	<programlisting>
	env.InstallAs(['/usr/bin/foo', '/usr/bin/bar'],
	                ['foo.in', 'bar.in'])
	</programlisting>

 </section>

 <section>
  <title>Target aliases</title>

  <para>

   In order to provide convenient "shortcut" target names that expand to
   a specified list of targets, aliases may be established:

  </para>

	<programlisting>
	env.Alias(alias = 'install',
	          targets = ['/sbin', '/usr/lib', '/usr/share/man'])
	</programlisting>

  <para>

   In this example, specifying a target of <literal>install</literal>
   will cause all the files in the associated directories to be built
   (that is, installed).

  </para>

  <para>

   An &Alias; may include one or more other &Aliases; in its list:

  </para>

	<programlisting>
	env.Alias(alias = 'libraries', targets = ['lib'])
	env.Alias(alias = 'programs', targets = ['libraries', 'src'])
	</programlisting>

 </section>

</section>



<section id="sect-custom">
 <title>Customizing output</title>

<!--
Take this whole section with a grain of salt.
I whipped it up without a great deal of thought
to try to add a "competitive advantage"
for the second round of the Software Carpentry contest.
In particular, hard-coding the
analysis points and the keywords that specify them
feels inflexible,
but I can't think of another way it would be
done effectively.
I dunno, maybe this is fine as it is...
-->

 <para>

  The &SCons; API supports the ability to customize, redirect, or
  suppress its printed output through user-defined functions.
  &SCons; has several pre-defined points in its build process at
  which it calls a function to (potentially) print output.  User-defined
  functions can be specified for these call-back points when &Build;
  or &Clean;is invoked:

 </para>

	<programlisting>
	env.Build(target = '.',
	       on_analysis = dump_dependency,
	       pre_update = my_print_command,
	       post_update = my_error_handler)
	       on_error = my_error_handler)
	</programlisting>

 <para>

  The specific call-back points are:

 </para>

 <variablelist>

  <varlistentry>
   <term><literal>on_analysis</literal></term>
   <listitem>
    <para>

     Called for every object, immediately after the object has been
     analyzed to see if it's out-of-date.  Typically used to print a
     trace of considered objects for debugging of unexpected dependencies.

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

  <varlistentry>
   <term><literal>pre_update</literal></term>
   <listitem>
    <para>

     Called for every object that has been determined to be out-of-date
     before its update function or command is executed.  Typically used
     to print the command being called to update a target.

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

  <varlistentry>
   <term><literal>post_update</literal></term>
   <listitem>
    <para>

     Called for every object after its update function or command has
     been executed.  Typically used to report that a top-level specified
     target is up-to-date or was not remade.

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

  <varlistentry>
   <term><literal>on_error</literal></term>
   <listitem>
    <para>

     Called for every error returned by an update function or command.
     Typically used to report errors with some string that will be
     identifiable to build-analysis tools.

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

 </variablelist>

 <para>

  Functions for each of these call-back points all take the same
  arguments:

 </para>

	<programlisting>
	my_dump_dependency(target, level, status, update, dependencies)
	</programlisting>

 <para>

  where the arguments are:

 </para>

 <variablelist>

  <varlistentry>
   <term><literal>target</literal></term>
   <listitem>
    <para>

     The target object being considered.

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

  <varlistentry>
   <term><literal>level</literal></term>
   <listitem>
    <para>

     Specifies how many levels the dependency analysis has
     recursed in order to consider the <literal>target</literal>.
     A value of <literal>0</literal> specifies a top-level
     <literal>target</literal> (that is, one passed to the
     &Build; or &Clean; method).  Objects which a top-level
     <literal>target</literal> is directly dependent upon have a
     <literal>level</literal> of &lt;1>, their direct dependencies have a
     <literal>level</literal> of &lt;2>, etc.  Typically used to indent
     output to reflect the recursive levels.

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

  <varlistentry>
   <term><literal>status</literal></term>
   <listitem>
    <para>

     A string specifying the current status of the target
     (<literal>"unknown"</literal>, <literal>"built"</literal>,
     <literal>"error"</literal>, <literal>"analyzed"</literal>, etc.).  A
     complete list will be enumerated and described during implementation.

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

  <varlistentry>
   <term><literal>update</literal></term>
   <listitem>
    <para>

     The command line or function name that will be (or has been) executed
     to update the <literal>target</literal>.

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

  <varlistentry>
   <term><literal>dependencies</literal></term>
   <listitem>
    <para>

     A list of direct dependencies of the target.

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

 </variablelist>

</section>



<section id="separate">
 <title>Separate source and build trees</title>

<!--
I've never liked Cons' use of the name <literal>Link</literal>
for this functionality,
mainly because the term is overloaded
with linking object files into an executable.
Yet I've never come up with anything better.
Any suggestions?
-->

<!--
Also, I made this an &Environment; method because
it logically belongs in the API reference
(the build engine needs to know about it),
and I thought it was clean to have
everything in the build-engine API
be called through an &Environment; object.
But <literal>&Link</literal> isn't really
associated with a specific environment
(the &Cons; classic implementation just
leaves it as a bare function call),
so maybe we should just follow that example
and not call it through an environment...
-->

 <para>

  &SCons; allows target files to be built completely separately from
  the source files by "linking" a build directory to an underlying
  source directory:

 </para>

	<programlisting>
	env.Link('build', 'src')

	SConscript('build/SConscript')
	</programlisting>

 <para>

  &SCons; will copy (or hard link) necessary files (including the
  &SConscript; file) into the build directory hierarchy.  This allows the
  source directory to remain uncluttered by derived files.

 </para>

</section>



<section id="sect-variant">
 <title>Variant builds</title>

 <para>

  The &Link; method may be used in conjunction with multiple
  &consenvs; to support variant builds.  The following
  &SConstruct; and &SConscript; files would build separate debug and
  production versions of the same program side-by-side:

 </para>

	<programlisting>
	% cat SConstruct
	env = Environment()
	env.Link('build/debug', 'src')
	env.Link('build/production', 'src')
	flags = '-g'
	SConscript('build/debug/SConscript', Export(env))
	flags = '-O'
	SConscript('build/production/SConscript', Export(env))
	% cat src/SConscript
	env = Environment(CCFLAGS = flags)
	env.Program('hello', 'hello.c')
	</programlisting>

 <para>

  The following example would build the appropriate program for the current
  compilation platform, without having to clean any directories of object
  or executable files for other architectures:

 </para>

	<programlisting>
	% cat SConstruct
	build_platform = os.path.join('build', sys.platform)
	Link(build_platform, 'src')
	SConscript(os.path.join(build_platform, 'SConscript'))
	% cat src/SConscript
	env = Environment
	env.Program('hello', 'hello.c')
	</programlisting>

</section>



<section id="sect-repositories">
 <title>Code repositories</title>

<!--
Like &Link;, &Repository; and &Local; are part of the
API reference, but not really tied to any specific environment.
Is it better to be consistent about calling
everything in the API through an environment,
or to leave these independent so as
not to complicate their calling interface?
-->

 <para>

  &SCons; may use files from one or more shared code repositories in order
  to build local copies of changed target files.  A repository would
  typically be a central directory tree, maintained by an integrator,
  with known good libraries and executables.

 </para>

	<programlisting>
	Repository('/home/source/1.1', '/home/source/1.0')
	</programlisting>

 <para>

  Specified repositories will be searched in-order for any file
  (configuration file, input file, target file) that does not exist
  in the local directory tree.  When building a local target file,
  &SCons; will rewrite path names in the build command to use the
  necessary repository files.  This includes modifying lists of
  <option>-I</option> or <option>-L</option> flags to specify an
  appropriate set of include paths for dependency analysis.

 </para>
 <para>

  &SCons; will modify the Python <varname>sys.path</varname> variable to
  reflect the addition of repositories to the search path, so that any
  imported modules or packages necessary for the build can be found in a
  repository, as well.

 </para>
 <para>

  If an up-to-date target file is found in a code repository, the file
  will not be rebuilt or copied locally.  Files that must exist locally
  (for example, to run tests) may be specified:

 </para>

	<programlisting>
	Local('program', 'libfoo.a')
	</programlisting>

 <para>

  in which case &SCons; will copy or link an up-to-date copy of the
  file from the appropriate repository.

 </para>

</section>



<section id="sect-caching">
 <title>Derived-file caching</title>

<!--
There should be extensions to this part of the API for
auxiliary functions like cleaning the cache.
-->

 <para>

  &SCons; can maintain a cache directory of target files which may be
  shared among multiple builds.  This reduces build times by allowing
  developers working on a project together to share common target
  files:

 </para>

	<programlisting>
	Cache('/var/tmp/build.cache/i386')
	</programlisting>

 <para>

  When a target file is generated, a copy is added to the cache.
  When generating a target file, if &SCons; determines that a file
  that has been built with the exact same dependencies already exists
  in the specified cache, &SCons; will copy the cached file rather
  than re-building the target.

 </para>
 <para>

  Command-line options exist to modify the &SCons; caching behavior
  for a specific build, including disabling caching, building
  dependencies in random order, and displaying commands as if cached
  files were built.

 </para>

</section>



<section id="sect-jobs">
 <title>Job management</title>

<!--
This has been completely superseded by
the more sophisticated &Task; manager
that Anthony Roach has contributed.
I need to write that up...
-->

 <para>

  A simple API exists to inform the Build Engine how many jobs may
  be run simultaneously:

 </para>

	<programlisting>
	Jobs(limit = 4)
	</programlisting>

</section>

</chapter>