path: root/doc/design/native.xml

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

<chapter id="chap-native"
         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/scons.xsd scons.xsd">
<title>Native Python Interface</title>

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<para>

 The "Native Python" interface is the interface
 that the actual &SCons; utility will present to users.
 Because it exposes the Python Build Engine API,
 &SCons; users will have direct access to the complete
 functionality of the Build Engine.
 In contrast, a different user interface such as a GUI
 may choose to only use, and present to the end-user,
 a subset of the Build Engine functionality.

</para>

<section id="sect-config">
 <title>Configuration files</title>

 <para>

  &SCons; configuration files are simply Python scripts that invoke
  methods to specify target files to be built, rules for building the
  target files, and dependencies.  Common build rules are available by
  default and need not be explicitly specified in the configuration
  files.

 </para>

 <para>

  By default, the &SCons; utility searches for a file named
  &SConstruct;, &Sconstruct; or &sconstruct; (in that order) in the
  current directory, and reads its configuration from the first file
  found.  A <option>-f</option> command-line option exists to read a
  different file name.

 </para>

</section>



<section id="sect-syntax">
 <title>Python syntax</title>

 <para>

  Because &SCons; configuration files are Python scripts, normal Python
  syntax can be used to generate or manipulate lists of targets or
  dependencies:

 </para>

	<programlisting>
	sources = ['aaa.c', 'bbb.c', 'ccc.c']
	env.Make('bar', sources)
	</programlisting>

 <para>

  Python flow-control can be used to iterate through invocations of
  build rules:

 </para>

	<programlisting>
	objects = ['aaa.o', 'bbb.o', 'ccc.o']
	for obj in objects:
	        src = replace(obj, '.o', '.c')
	        env.Make(obj, src)
	</programlisting>

 <para>

  or to handle more complicated conditional invocations:

 </para>

	<programlisting>
	# only build 'foo' on Linux systems
	if sys.platform == 'linux1':
	        env.Make('foo', 'foo.c')
	</programlisting>

 <para>

  Because &SCons; configuration files are Python scripts, syntax errors
  will be caught by the Python parser.  Target-building does not begin
  until after all configuration files are read, so a syntax error will
  not cause a build to fail half-way.

 </para>

</section>



<section id="sect-subsidiary">
 <title>Subsidiary configuration Files</title>

 <para>

  A configuration file can instruct &SCons; to read up subsidiary
  configuration files.  Subsidiary files are specified explicitly in a
  configuration file via the &SConscript; method.  As usual, multiple
  file names may be specified with white space separation, or in an
  array:

 </para>

	<programlisting>
	SConscript('other_file')
	SConscript('file1 file2')
	SConscript(['file3', 'file4'])
	SConscript(['file name with white space'])
	</programlisting>

 <para>

  An explicit <literal>sconscript</literal> keyword may be used:

 </para>

	<programlisting>
	SConscript(sconscript = 'other_file')
	</programlisting>

 <para>

  Including subsidiary configuration files is recursive: a configuration
  file included via &SConscript; may in turn &SConscript; other
  configuration files.

 </para>

</section>



<section id="sect-scoping">
 <title>Variable scoping in subsidiary files</title>

 <para>

  When a subsidiary configuration file is read, it is given its own
  namespace; it does not have automatic access to variables from the parent
  configuration file.

 </para>

 <para>

  Any variables (not just &SCons; objects) that are to be shared between configuration files must be
  explicitly passed in the &SConscript; call
  using the &Export; method:

 </para>

	<programlisting>
	env = Environment()
	debug = Environment(CCFLAGS = '-g')
	installdir = '/usr/bin'
	SConscript('src/SConscript', Export(env=env, debug=debug, installdir=installdir))
	</programlisting>

<!--
The <literal>env=env</literal> stuff bugs me
because it imposes extra work on the normal
case where you <emphasis>don't</emphasis> rename
the variables.
Can we simplify the &Export; method
so that a string
without a keyword assignment
is split into variables that are passed
through transparently?
Equivalent to the above example:
<literal>SConscript('src/SConscript', Export('env debug installdir'))</literal>
-->

 <para>

  Which may be specified explicitly using a keyword argument:

 </para>

	<programlisting>
	env = Environment()
	debug = Environment(CCFLAGS = '-g')
	installdir = '/usr/bin'
	SConscript(sconscript = 'src/SConscript',
	           export = Export(env=env, debug=debug, installdir=installdir))
	</programlisting>

 <para>

  Explicit variable-passing provides control over exactly what is available
  to a subsidiary file, and avoids unintended side effects of changes in
  one configuration file affecting other far-removed configuration files
  (a very hard-to-debug class of build problem).

 </para>

</section>



<section id="sect-hierarchy">
 <title>Hierarchical builds</title>

 <para>

  The &SConscript; method is so named because, by convention, subsidiary
  configuration files in subdirectories are named &SConscript;:

 </para>

	<programlisting>
	SConscript('src/SConscript')
	SConscript('lib/build_me')
	</programlisting>

 <para>

  When a subsidiary configuration file is read from a subdirectory, all
  of that configuration file's targets and build rules are interpreted
  relative to that directory (as if &SCons; had changed its working
  directory to that subdirectory).  This allows for easy support of
  hierarchical builds of directory trees for large projects.

 </para>

</section>



<section id="sect-sharing">
 <title>Sharing &consenvs;</title>

 <para>

  &SCons; will allow users to share &consenvs;, as well as other &SCons;
  objects and Python variables, by importing them from a central, shared
  repository using normal Python syntax:

 </para>

	<programlisting>
	from LocalEnvironments import optimized, debug

	optimized.Make('foo', 'foo.c')
	debug.Make('foo-d', 'foo.c')
	</programlisting>

 <para>

  The expectation is that some local tool-master, integrator or
  administrator will be responsible for assembling environments (creating
  the &Builder; objects that specify the tools, options, etc.) and make
  these available for sharing by all users.

 </para>

 <para>

  The modules containing shared &consenvs;
  (<literal>LocalEnvironments</literal> in the above example) can be
  checked in and controlled with the rest of the source files.  This
  allows a project to track the combinations of tools and command-line
  options that work on different platforms, at different times, and with
  different tool versions, by using already-familiar revision control
  tools.

 </para>

</section>



<section id="sect-help">
 <title>Help</title>

 <para>

  The &SCons; utility provides a &Help; function to allow the writer
  of a &SConstruct; file to provide help text that is specific to
  the local build tree:

 </para>

	<programlisting>
	Help("""
	Type:
	        scons .         build and test everything
	        scons test      build the software
	        scons src       run the tests
	        scons web       build the web pages
	""")
	</programlisting>

 <para>

  This help text is displayed in response to the <option>-h</option>
  command-line option.  Calling the &Help; function more than once is an
  error.

 </para>
  
</section>



<section id="sect-debug">
 <title>Debug</title>

 <para>

  &SCons; supports several command-line options for printing extra
  information with which to debug build problems.

 </para>

<!--
These need to be specified and explained
beyond what the man page will have.
-->

  <!-- BEGIN HTML -->

 <para>

  See the -d, -p, -pa, and -pw options
  in the  <!--<A HREF="#sccons_Man_page">man page</A>-->, below.
  All of these options make use of call-back functions to
  <!--<A HREF="reference.html#Customizing_output">control the output</A>-->
  printed by the Build Engine.

 </para>

  <!-- END HTML -->

</section>

</chapter>