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<!--

=head1 More on construction environments

As previously mentioned, a B<construction environment> is an object that
has a set of keyword/value pairs and a set of methods, and which is used
to tell Cons how target files should be built.  This section describes
how Cons uses and expands construction environment values to control its
build behavior.

=head2 Construction variable expansion

Construction variables from a construction environment are expanded
by preceding the keyword with a C<%> (percent sign):

     Construction variables:
	XYZZY => 'abracadabra',

     The string:  "The magic word is:  %XYZZY!"
     expands to:  "The magic word is:  abracadabra!"

A construction variable name may be surrounded by C<{> and C<}> (curly
braces), which are stripped as part of the expansion.  This can
sometimes be necessary to separate a variable expansion from trailing
alphanumeric characters:

     Construction variables:
	OPT    => 'value1',
	OPTION => 'value2',

     The string:  "%OPT %{OPT}ION %OPTION %{OPTION}"
     expands to:  "value1 value1ION value2 value2"

Construction variable expansion is recursive, that is, a string
containing C<%->expansions after substitution will be re-expanded until
no further substitutions can be made:

     Construction variables:
	STRING => 'The result is:  %FOO',
	FOO    => '%BAR',
	BAR    => 'final value',

     The string:  "The string says:  %STRING"
     expands to:  "The string says:  The result is:  final value"

If a construction variable is not defined in an environment, then the
null string is substituted:

     Construction variables:
	FOO => 'value1',
	BAR => 'value2',

     The string:  "%FOO <%NO_VARIABLE> %BAR"
     expands to:  "value1 <> value2"

A doubled C<%%> will be replaced by a single C<%>:

     The string:  "Here is a percent sign:  %%"
     expands to:  "Here is a percent sign: %"

=head2 Default construction variables

When you specify no arguments when creating a new construction
environment:

     $env = new cons();

Cons creates a reference to a new, default construction
environment. This contains a number of construction variables and some
methods. At the present writing, the default construction variables on a
UNIX system are:

     CC            => 'cc',
     CFLAGS        => '',
     CCCOM         => '%CC %CFLAGS %_IFLAGS -c %< -o %>',
     CXX           => '%CC',
     CXXFLAGS      => '%CFLAGS',
     CXXCOM        => '%CXX %CXXFLAGS %_IFLAGS -c %< -o %>',
     INCDIRPREFIX  => '-I',
     INCDIRSUFFIX  => '',
     LINK          => '%CXX',
     LINKCOM       => '%LINK %LDFLAGS -o %> %< %_LDIRS %LIBS',
     LINKMODULECOM => '%LD -r -o %> %<',
     LIBDIRPREFIX  => '-L',
     LIBDIRSUFFIX  => '',
     AR		=> 'ar',
     ARFLAGS	=> 'r',
     ARCOM		=> ['%AR %ARFLAGS %> %<', '%RANLIB %>'],
     RANLIB	=> 'ranlib',
     AS		=> 'as',
     ASFLAGS	=> '',
     ASCOM		=> '%AS %ASFLAGS %< -o %>',
     LD		=> 'ld',
     LDFLAGS	=> '',
     PREFLIB	=> 'lib',
     SUFLIB	=> '.a',
     SUFLIBS	=> '.so:.a',
     SUFOBJ	=> '.o',
     SIGNATURE     => [ '*' => 'build' ],
     ENV		=> { 'PATH' => '/bin:/usr/bin' },


And on a Windows system (Windows NT), the default construction variables
are (unless the default rule style is set using the B<DefaultRules>
method):

     CC		=> 'cl',
     CFLAGS	=> '/nologo',
     CCCOM		=> '%CC %CFLAGS %_IFLAGS /c %< /Fo%>',
     CXXCOM        => '%CXX %CXXFLAGS %_IFLAGS /c %< /Fo%>',
     INCDIRPREFIX  => '/I',
     INCDIRSUFFIX  => '',
     LINK          => 'link',
     LINKCOM       => '%LINK %LDFLAGS /out:%> %< %_LDIRS %LIBS',
     LINKMODULECOM => '%LD /r /o %> %<',
     LIBDIRPREFIX  => '/LIBPATH:',
     LIBDIRSUFFIX  => '',
     AR            => 'lib',
     ARFLAGS       => '/nologo ',
     ARCOM         => "%AR %ARFLAGS /out:%> %<",
     RANLIB        => '',
     LD            => 'link',
     LDFLAGS       => '/nologo ',
     PREFLIB       => '',
     SUFEXE	=> '.exe',
     SUFLIB	=> '.lib',
     SUFLIBS	=> '.dll:.lib',
     SUFOBJ	=> '.obj',
     SIGNATURE     => [ '*' => 'build' ],

These variables are used by the various methods associated with the
environment. In particular, any method that ultimately invokes an external
command will substitute these variables into the final command, as
appropriate. For example, the C<Objects> method takes a number of source
files and arranges to derive, if necessary, the corresponding object
files:

     Objects $env 'foo.c', 'bar.c';

This will arrange to produce, if necessary, F<foo.o> and F<bar.o>. The
command invoked is simply C<%CCCOM>, which expands, through substitution,
to the appropriate external command required to build each object. The
substitution rules will be discussed in detail in the next section.

The construction variables are also used for other purposes. For example,
C<CPPPATH> is used to specify a colon-separated path of include
directories. These are intended to be passed to the C preprocessor and are
also used by the C-file scanning machinery to determine the dependencies
involved in a C Compilation.

Variables beginning with underscore are created by various methods,
and should normally be considered ``internal'' variables. For example,
when a method is called which calls for the creation of an object from
a C source, the variable C<_IFLAGS> is created: this corresponds to the
C<-I> switches required by the C compiler to represent the directories
specified by C<CPPPATH>.

Note that, for any particular environment, the value of a variable is set
once, and then never reset (to change a variable, you must create a new
environment. Methods are provided for copying existing environments for this
purpose). Some internal variables, such as C<_IFLAGS> are created on demand,
but once set, they remain fixed for the life of the environment.

The C<CFLAGS>, C<LDFLAGS>, and C<ARFLAGS> variables all supply a place
for passing options to the compiler, loader, and archiver, respectively.

The C<INCDIRPREFIX> and C<INCDIRSUFFIX> variables specify option
strings to be appended to the beginning and end, respectively, of each
include directory so that the compiler knows where to find F<.h> files.
Similarly, the C<LIBDIRPREFIX> and C<LIBDIRSUFFIX> variables specify the
option string to be appended to the beginning of and end, respectively,
of each directory that the linker should search for libraries.

Another variable, C<ENV>, is used to determine the system environment during
the execution of an external command. By default, the only environment
variable that is set is C<PATH>, which is the execution path for a UNIX
command. For the utmost reproducibility, you should really arrange to set
your own execution path, in your top-level F<Construct> file (or perhaps by
importing an appropriate construction package with the Perl C<use>
command). The default variables are intended to get you off the ground.

=head2 Expanding variables in construction commands

Within a construction command, construction variables will be expanded
according to the rules described above.  In addition to normal variable
expansion from the construction environment, construction commands also
expand the following pseudo-variables to insert the specific input and
output files in the command line that will be executed:

=over 10

=item %>

The target file name.  In a multi-target command, this expands to the
first target mentioned.)

=item %0

Same as C<%E<gt>>.

=item %1, %2, ..., %9

These refer to the first through ninth input file, respectively.

=item %E<lt>

The full set of input file names. If any of these have been used
anywhere else in the current command line (via C<%1>, C<%2>, etc.), then
those will be deleted from the list provided by C<%E<lt>>. Consider the
following command found in a F<Conscript> file in the F<test> directory:

     Command $env 'tgt', qw(foo bar baz), qq(
	echo %< -i %1 > %>
	echo %< -i %2 >> %>
	echo %< -i %3 >> %>
     );

If F<tgt> needed to be updated, then this would result in the execution of
the following commands, assuming that no remapping has been established for
the F<test> directory:

     echo test/bar test/baz -i test/foo > test/tgt
     echo test/foo test/baz -i test/bar >> test/tgt
     echo test/foo test/bar -i test/baz >> test/tgt

=back

Any of the above pseudo-variables may be followed immediately by one of
the following suffixes to select a portion of the expanded path name:

     :a    the absolute path to the file name
     :b    the directory plus the file name stripped of any suffix
     :d    the directory
     :f    the file name
     :s    the file name suffix
     :F    the file name stripped of any suffix
     :S    the absolute path path to a Linked source file

Continuing with the above example, C<%E<lt>:f> would expand to C<foo bar baz>,
and C<%E<gt>:d> would expand to C<test>.

There are additional C<%> elements which affect the command line(s):

=over 10

=item %[ %]

It is possible to programmatically rewrite part of the command by
enclosing part of it between C<%[> and C<%]>.  This will call the
construction variable named as the first word enclosed in the brackets
as a Perl code reference; the results of this call will be used to
replace the contents of the brackets in the command line.  For example,
given an existing input file named F<tgt.in>:

     @keywords = qw(foo bar baz);
     $env = new cons(X_COMMA => sub { join(",", @_) });
     Command $env 'tgt', 'tgt.in', qq(
	echo '# Keywords: %[X_COMMA @keywords %]' > %>
	cat %< >> %>
     );

This will execute:

     echo '# Keywords: foo,bar,baz' > tgt
     cat tgt.in >> tgt

=item %( %)

Cons includes the text of the command line in the MD5 signature for a
build, so that targets get rebuilt if you change the command line (to
add or remove an option, for example).  Command-line text in between
C<%(> and C<%)>, however, will be ignored for MD5 signature calculation.

Internally, Cons uses C<%(> and C<%)> around include and library
directory options (C<-I> and C<-L> on UNIX systems, C</I> and
C</LIBPATH> on Windows NT) to avoid rebuilds just because the directory
list changes.  Rebuilds occur only if the changed directory list causes
any included I<files> to change, and a changed include file is detected
by the MD5 signature calculation on the actual file contents.

=back

XXX DESCRIBE THE Literal() FUNCTION, TOO XXX

=head2 Expanding construction variables in file names

Cons expands construction variables in the source and target file names
passed to the various construction methods according to the expansion
rules described above:

     $env = new cons(
	DESTDIR	=>	'programs',
	SRCDIR	=>	'src',
     );
     Program $env '%DESTDIR/hello', '%SRCDIR/hello.c';

This allows for flexible configuration, through the construction
environment, of directory names, suffixes, etc.

-->

  <para>

  An <literal>environment</literal>
  is a collection of values that
  can affect how a program executes.
  &SCons; distinguishes between three
  different types of environments
  that can affect the behavior of &SCons; itself
  (subject to the configuration in the &SConscript; files),
  as well as the compilers and other tools it executes:

  </para>

  <variablelist>

    <varlistentry>
    <term>External Environment</term>

    <listitem>
    <para>

    The <literal>external environment</literal>
    is the set of variables in the user's environment
    at the time the user runs &SCons;.
    These variables are available within the &SConscript; files
    through the Python <literal>os.environ</literal> dictionary.
    See <xref linkend="sect-external-environments"></xref>, below.

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

    <varlistentry>
    <term>&ConsEnv;</term>

    <listitem>
    <para>

    A &consenv;
    is a distinct object creating within
    a &SConscript; file and
    and which contains values that
    affect how &SCons; decides
    what action to use to build a target,
    and even to define which targets
    should be built from which sources.
    One of the most powerful features of &SCons;
    is the ability to create multiple &consenvs;,
    including the ability to clone a new, customized
    &consenv; from an existing &consenv;.
    See <xref linkend="sect-construction-environments"></xref>, below.

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

    <varlistentry>
    <term>Execution Environment</term>

    <listitem>
    <para>

    An <literal>execution environment</literal>
    is the values that &SCons; sets
    when executing an external
    command (such as a compiler or linker)
    to build one or more targets.
    Note that this is not the same as
    the <literal>external environment</literal>
    (see above).
    See <xref linkend="sect-execution-environments"></xref>, below.

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

  </variablelist>

  <para>

  Unlike &Make;,  &SCons; does not automatically
  copy or import values between different environments
  (with the exception of explicit clones of &consenvs;,
  which inherit values from their parent).
  This is a deliberate design choice
  to make sure that builds are,
  by default, repeatable regardless of
  the values in the user's external environment.
  This avoids a whole class of problems with builds
  where a developer's local build works
  because a custom variable setting
  causes a different compiler or build option to be used,
  but the checked-in change breaks the official build
  because it uses different environment variable settings.

  </para>

  <para>

  Note that the &SConscript; writer can
  easily arrange for variables to be
  copied or imported between environments,
  and this is often very useful
  (or even downright necessary)
  to make it easy for developers
  to customize the build in appropriate ways.
  The point is <emphasis>not</emphasis>
  that copying variables between different environments
  is evil and must always be avoided.
  Instead, it should be up to the
  implementer of the build system
  to make conscious choices
  about how and when to import
  a variable from one environment to another,
  making informed decisions about
  striking the right balance
  between making the build
  repeatable on the one hand
  and convenient to use on the other.

  </para>

  <section id="sect-external-environments">
  <title>Using Values From the External Environment</title>

  <para>

  The <literal>external environment</literal>
  variable settings that
  the user has in force
  when executing &SCons;
  are available through the normal Python
  <envar>os.environ</envar>
  dictionary.
  This means that you must add an
  <literal>import os</literal> statement
  to any &SConscript; file
  in which you want to use
  values from the user's external environment.

  </para>

   <programlisting>
     import os
   </programlisting>

  <para>

  More usefully, you can use the
  <envar>os.environ</envar>
  dictionary in your &SConscript;
  files to initialize &consenvs;
  with values from the user's external environment.
  See the next section,
  <xref linkend="sect-construction-environments"></xref>,
  for information on how to do this.

  </para>

  </section>

  <section id="sect-construction-environments">
  <title>Construction Environments</title>

    <para>

      It is rare that all of the software in a large,
      complicated system needs to be built the same way.
      For example, different source files may need different options
      enabled on the command line,
      or different executable programs need to be linked
      with different libraries.
      &SCons; accommodates these different build
      requirements by allowing you to create and
      configure multiple &consenvs;
      that control how the software is built.
      A &consenv; is an object
      that has a number of associated
      &consvars;, each with a name and a value.
      (A construction environment also has an attached
      set of &Builder; methods,
      about which we'll learn more later.)

    </para>

    <section>
    <title>Creating a &ConsEnv;:  the &Environment; Function</title>

      <para>

        A &consenv; is created by the &Environment; method:

      </para>

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

      <para>

        By default, &SCons; initializes every
        new construction environment
        with a set of &consvars;
        based on the tools that it finds on your system,
        plus the default set of builder methods
        necessary for using those tools.
        The construction variables
        are initialized with values describing
        the C compiler,
        the Fortran compiler,
        the linker,
        etc.,
        as well as the command lines to invoke them.

      </para>

      <para>

        When you initialize a construction environment
        you can set the values of the
        environment's &consvars;
        to control how a program is built.
        For example:

      </para>

       <programlisting>
     import os

         env = Environment(CC = 'gcc',
                           CCFLAGS = '-O2')

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

      <para>

        The construction environment in this example
        is still initialized with the same default
        construction variable values,
        except that the user has explicitly specified use of the
        GNU C compiler &gcc;,
        and further specifies that the <literal>-O2</literal>
        (optimization level two)
        flag should be used when compiling the object file.
        In other words, the explicit initializations of
        &cv-link-CC; and &cv-link-CCFLAGS;
        override the default values in the newly-created
        construction environment.
        So a run from this example would look like:

      </para>

      <screen>
         % <userinput>scons -Q</userinput>
         gcc -o foo.o -c -O2 foo.c
         gcc -o foo foo.o
      </screen>

    </section>

    <section>
    <title>Fetching Values From a &ConsEnv;</title>

      <para>

      You can fetch individual construction variables
      using the normal syntax
      for accessing individual named items in a Python dictionary:

      </para>

      <programlisting>
         env = Environment()
         print "CC is:", env['CC']
      </programlisting>

      <para>

      This example &SConstruct; file doesn't build anything,
      but because it's actually a Python script,
      it will print the value of &cv-link-CC; for us:

      </para>

      <screen>
         % <userinput>scons -Q</userinput>
         CC is: cc
         scons: `.' is up to date.
      </screen>

      <para>

      A construction environment, however,
      is actually an object with associated methods, etc.
      If you want to have direct access to only the
      dictionary of construction variables,
      you can fetch this using the &Dictionary; method:

      </para>

      <programlisting>
         env = Environment(FOO = 'foo', BAR = 'bar')
         dict = env.Dictionary()
         for key in ['OBJSUFFIX', 'LIBSUFFIX', 'PROGSUFFIX']:
             print "key = %s, value = %s" % (key, dict[key])
      </programlisting>

      <para>

      This &SConstruct; file
      will print the specified dictionary items for us on POSIX
      systems as follows:

      </para>

      <screen>
         % <userinput>scons -Q</userinput>
         key = OBJSUFFIX, value = .o
         key = LIBSUFFIX, value = .a
         key = PROGSUFFIX, value = 
         scons: `.' is up to date.
      </screen>

      <para>

      And on Windows:

      </para>

      <screen>
         C:\><userinput>scons -Q</userinput>
         key = OBJSUFFIX, value = .obj
         key = LIBSUFFIX, value = .lib
         key = PROGSUFFIX, value = .exe
         scons: `.' is up to date.
      </screen>

      <para>

      If you want to loop and print the values of
      all of the construction variables in a construction environment,
      the Python code to do that in sorted order might look something like:

      </para>

      <programlisting>
         env = Environment()
         for item in sorted(env.Dictionary().items()):
             print "construction variable = '%s', value = '%s'" % item
      </programlisting>

    </section>

    <section>
    <title>Expanding Values From a &ConsEnv;:  the &subst; Method</title>

      <para>

      Another way to get information from
      a construction environment.
      is to use the &subst; method
      on a string containing <literal>$</literal> expansions
      of construction variable names.
      As a simple example,
      the example from the previous
      section that used
      <literal>env['CC']</literal>
      to fetch the value of &cv-link-CC;
      could also be written as:

      </para>

      <programlisting>
        env = Environment()
        print "CC is:", env.subst('$CC')
      </programlisting>

      <para>

      One advantage of using
      &subst; to expand strings is
      that construction variables
      in the result get re-expanded until
      there are no expansions left in the string.
      So a simple fetch of a value like
      &cv-link-CCCOM;:

      </para>

      <programlisting>
        env = Environment(CCFLAGS = '-DFOO')
        print "CCCOM is:", env['CCCOM']
      </programlisting>

      <para>

      Will print the unexpanded value of &cv-CCCOM;,
      showing us the construction
      variables that still need to be expanded:

      </para>

      <screen>
        % <userinput>scons -Q</userinput>
        CCCOM is: $CC $CCFLAGS $CPPFLAGS $_CPPDEFFLAGS $_CPPINCFLAGS -c -o $TARGET $SOURCES
        scons: `.' is up to date.
      </screen>

      <para>

      Calling the &subst; method on <varname>$CCOM</varname>,
      however:

      </para>

      <programlisting>
        env = Environment(CCFLAGS = '-DFOO')
        print "CCCOM is:", env.subst('$CCCOM')
      </programlisting>

      <para>

      Will recursively expand all of
      the construction variables prefixed
      with <literal>$</literal> (dollar signs),
      showing us the final output:

      </para>

      <screen>
        % <userinput>scons -Q</userinput>
        CCCOM is: gcc -DFOO -c -o
        scons: `.' is up to date.
      </screen>

      <para>

      Note that because we're not expanding this
      in the context of building something
      there are no target or source files
      for &cv-link-TARGET; and &cv-link-SOURCES; to expand.

      </para>

    </section>

    <section>
    <title>Handling Problems With Value Expansion</title>

      <para>

      If a problem occurs when expanding a construction variable,
      by default it is expanded to <literal>''</literal>
      (a null string), and will not cause scons to fail. 
      </para>

       <programlisting>
          env = Environment()
          print "value is:", env.subst( '-&gt;$MISSING&lt;-' )
      </programlisting>

       <screen>
          % <userinput>scons -Q</userinput>
          value is: -&gt;&lt;-
          scons: `.' is up to date.
       </screen>

      <para>
      This default behaviour can be changed using the &AllowSubstExceptions;
      function.
      When a problem occurs with a variable expansion it generates
      an exception, and the &AllowSubstExceptions; function controls
      which of these exceptions are actually fatal and which are
      allowed to occur safely.   By default, &NameError; and &IndexError;
      are the two exceptions that are allowed to occur: so instead of
      causing scons to fail, these are caught, the variable expanded to
      <literal>''</literal>
      and scons execution continues.
      To require that all construction variable names exist, and that
      indexes out of range are not allowed, call &AllowSubstExceptions;
      with no extra arguments.
      </para>

       <programlisting>
          AllowSubstExceptions()
          env = Environment()
          print "value is:", env.subst( '-&gt;$MISSING&lt;-' )
      </programlisting>

       <screen>
          % <userinput>scons -Q</userinput>
          value is:
          scons: *** NameError `MISSING' trying to evaluate `$MISSING'
          File "/home/my/project/SConstruct", line 3, in &lt;module&gt;
       </screen>

      <para>
      This can also be used to allow other exceptions that might occur,
      most usefully with the <literal>${...}</literal> construction
      variable syntax.  For example, this would allow zero-division to
      occur in a variable expansion in addition to the default exceptions
      allowed
      </para>

       <programlisting>
          AllowSubstExceptions(IndexError, NameError, ZeroDivisionError)
          env = Environment()
          print "value is:", env.subst( '-&gt;${1 / 0}&lt;-' )
      </programlisting>

       <screen>
          % <userinput>scons -Q</userinput>
          value is: -&gt;&lt;-
          scons: `.' is up to date.
       </screen>
      <programlisting>
      </programlisting>

      <para>
      If &AllowSubstExceptions; is called multiple times, each call
      completely overwrites the previous list of allowed exceptions.
      </para>

    </section>

    <section>
    <title>Controlling the Default &ConsEnv;:  the &DefaultEnvironment; Function</title>

      <para>

      All of the &Builder; functions that we've introduced so far,
      like &Program; and &Library;,
      actually use a default &consenv;
      that contains settings
      for the various compilers
      and other tools that
      &SCons; configures by default,
      or otherwise knows about
      and has discovered on your system.
      The goal of the default construction environment
      is to make many configurations to "just work"
      to build software using
      readily available tools
      with a minimum of configuration changes.

      </para>

      <para>

      You can, however, control the settings
      in the default contstruction environment
      by using the &DefaultEnvironment; function
      to initialize various settings:

      </para>

      <programlisting>

      DefaultEnvironment(CC = '/usr/local/bin/gcc')

      </programlisting>

      <para>

      When configured as above,
      all calls to the &Program;
      or &Object; Builder
      will build object files with the
      <filename>/usr/local/bin/gcc</filename>
      compiler.

      </para>

      <para>

      Note that the &DefaultEnvironment; function
      returns the initialized
      default construction environment object,
      which can then be manipulated like any
      other construction environment.
      So the following
      would be equivalent to the
      previous example,
      setting the &cv-CC;
      variable to <filename>/usr/local/bin/gcc</filename>
      but as a separate step after
      the default construction environment has been initialized:

      </para>

      <programlisting>

      env = DefaultEnvironment()
      env['CC'] = '/usr/local/bin/gcc'

      </programlisting>

      <para>

      One very common use of the &DefaultEnvironment; function
      is to speed up &SCons; initialization.
      As part of trying to make most default
      configurations "just work,"
      &SCons; will actually
      search the local system for installed
      compilers and other utilities.
      This search can take time,
      especially on systems with
      slow or networked file systems.
      If you know which compiler(s) and/or
      other utilities you want to configure,
      you can control the search
      that &SCons; performs
      by specifying some specific
      tool modules with which to
      initialize the default construction environment:

      </para>

      <programlisting>

      env = DefaultEnvironment(tools = ['gcc', 'gnulink'],
                               CC = '/usr/local/bin/gcc')

      </programlisting>

      <para>

      So the above example would tell &SCons;
      to explicitly configure the default environment
      to use its normal GNU Compiler and GNU Linker settings
      (without having to search for them,
      or any other utilities for that matter),
      and specifically to use the compiler found at
      <filename>/usr/local/bin/gcc</filename>.

      </para>

    </section>

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

      <para>

      The real advantage of construction environments
      is that you can create as many different construction
      environments as you need,
      each tailored to a different way to build
      some piece of software or other file.
      If, for example, we need to build
      one program with the <literal>-O2</literal> flag
      and another with the <literal>-g</literal> (debug) flag,
      we would do this like so:

      </para>

      <programlisting>
         opt = Environment(CCFLAGS = '-O2')
         dbg = Environment(CCFLAGS = '-g')

         opt.Program('foo', 'foo.c')

         dbg.Program('bar', 'bar.c')
      </programlisting>

      <screen>
         % <userinput>scons -Q</userinput>
         cc -o bar.o -c -g bar.c
         cc -o bar bar.o
         cc -o foo.o -c -O2 foo.c
         cc -o foo foo.o
      </screen>

      <para>

      We can even use multiple construction environments to build
      multiple versions of a single program.
      If you do this by simply trying to use the
      &b-link-Program; builder with both environments, though,
      like this:

      </para>

      <programlisting>
         opt = Environment(CCFLAGS = '-O2')
         dbg = Environment(CCFLAGS = '-g')

         opt.Program('foo', 'foo.c')

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

      <para>

      Then &SCons; generates the following error:

      </para>

      <screen>
         % <userinput>scons -Q</userinput>
         
         scons: *** Two environments with different actions were specified for the same target: foo.o
         File "/home/my/project/SConstruct", line 6, in &lt;module&gt;
      </screen>

      <para>

      This is because the two &b-Program; calls have
      each implicitly told &SCons; to generate an object file named
      <filename>foo.o</filename>,
      one with a &cv-link-CCFLAGS; value of
      <literal>-O2</literal>
      and one with a &cv-link-CCFLAGS; value of
      <literal>-g</literal>.
      &SCons; can't just decide that one of them
      should take precedence over the other,
      so it generates the error.
      To avoid this problem,
      we must explicitly specify
      that each environment compile
      <filename>foo.c</filename>
      to a separately-named object file
      using the &b-link-Object; builder, like so:

      </para>

      <programlisting>
         opt = Environment(CCFLAGS = '-O2')
         dbg = Environment(CCFLAGS = '-g')

         o = opt.Object('foo-opt', 'foo.c')
         opt.Program(o)

         d = dbg.Object('foo-dbg', 'foo.c')
         dbg.Program(d)
      </programlisting>

      <para>

      Notice that each call to the &b-Object; builder
      returns a value,
      an internal &SCons; object that
      represents the object file that will be built.
      We then use that object
      as input to the &b-Program; builder.
      This avoids having to specify explicitly
      the object file name in multiple places,
      and makes for a compact, readable
      &SConstruct; file.
      Our &SCons; output then looks like:

      </para>

      <screen>
         % <userinput>scons -Q</userinput>
         cc -o foo-dbg.o -c -g foo.c
         cc -o foo-dbg foo-dbg.o
         cc -o foo-opt.o -c -O2 foo.c
         cc -o foo-opt foo-opt.o
      </screen>

    </section>

    <section id="sect-clone-environments">
    <title>Making Copies of &ConsEnvs;:  the &Clone; Method</title>

      <para>

      Sometimes you want more than one construction environment
      to share the same values for one or more variables.
      Rather than always having to repeat all of the common
      variables when you create each construction environment,
      you can use the &Clone; method
      to create a copy of a construction environment.

      </para>

      <para>

      Like the &Environment; call that creates a construction environment,
      the &Clone; method takes &consvar; assignments,
      which will override the values in the copied construction environment.
      For example, suppose we want to use &gcc;
      to create three versions of a program,
      one optimized, one debug, and one with neither.
      We could do this by creating a "base" construction environment
      that sets &cv-link-CC; to &gcc;,
      and then creating two copies,
      one which sets &cv-link-CCFLAGS; for optimization
      and the other which sets &cv-CCFLAGS; for debugging:

      </para>

      <programlisting>
         env = Environment(CC = 'gcc')
         opt = env.Clone(CCFLAGS = '-O2')
         dbg = env.Clone(CCFLAGS = '-g')

         env.Program('foo', 'foo.c')

         o = opt.Object('foo-opt', 'foo.c')
         opt.Program(o)

         d = dbg.Object('foo-dbg', 'foo.c')
         dbg.Program(d)
      </programlisting>

      <para>

      Then our output would look like:

      </para>

      <screen>
         % <userinput>scons -Q</userinput>
         gcc -o foo.o -c foo.c
         gcc -o foo foo.o
         gcc -o foo-dbg.o -c -g foo.c
         gcc -o foo-dbg foo-dbg.o
         gcc -o foo-opt.o -c -O2 foo.c
         gcc -o foo-opt foo-opt.o
      </screen>

    </section>

    <section>
    <title>Replacing Values:  the &Replace; Method</title>

      <para>

      You can replace existing construction variable values
      using the &Replace; method:

      </para>

      <programlisting>
         env = Environment(CCFLAGS = '-DDEFINE1')
         env.Replace(CCFLAGS = '-DDEFINE2')
         env.Program('foo.c')
      </programlisting>

      <para>

      The replacing value
      (<literal>-DDEFINE2</literal> in the above example)
      completely replaces the value in the
      construction environment:

      </para>

      <screen>
         % <userinput>scons -Q</userinput>
         cc -o foo.o -c -DDEFINE2 foo.c
         cc -o foo foo.o
      </screen>

      <para>

      You can safely call &Replace;
      for construction variables that
      don't exist in the construction environment:

      </para>

      <programlisting>
         env = Environment()
         env.Replace(NEW_VARIABLE = 'xyzzy')
         print "NEW_VARIABLE =", env['NEW_VARIABLE']
      </programlisting>

      <para>

      In this case,
      the construction variable simply
      gets added to the construction environment:

      </para>

      <screen>
         % <userinput>scons -Q</userinput>
         NEW_VARIABLE = xyzzy
         scons: `.' is up to date.
      </screen>

      <para>

      Because the variables
      aren't expanded until the construction environment
      is actually used to build the targets,
      and because &SCons; function and method calls
      are order-independent,
      the last replacement "wins"
      and is used to build all targets,
      regardless of the order in which
      the calls to Replace() are
      interspersed with calls to
      builder methods:

      </para>

      <programlisting>
         env = Environment(CCFLAGS = '-DDEFINE1')
         print "CCFLAGS =", env['CCFLAGS']
         env.Program('foo.c')

         env.Replace(CCFLAGS = '-DDEFINE2')
         print "CCFLAGS =", env['CCFLAGS']
         env.Program('bar.c')
      </programlisting>

      <para>

      The timing of when the replacement
      actually occurs relative
      to when the targets get built
      becomes apparent
      if we run &scons; without the <literal>-Q</literal>
      option:

      </para>

      <screen>
         % <userinput>scons</userinput>
         scons: Reading SConscript files ...
         CCFLAGS = -DDEFINE1
         CCFLAGS = -DDEFINE2
         scons: done reading SConscript files.
         scons: Building targets ...
         cc -o bar.o -c -DDEFINE2 bar.c
         cc -o bar bar.o
         cc -o foo.o -c -DDEFINE2 foo.c
         cc -o foo foo.o
         scons: done building targets.
      </screen>

      <para>

      Because the replacement occurs while
      the &SConscript; files are being read,
      the &cv-link-CCFLAGS;
      variable has already been set to
      <literal>-DDEFINE2</literal>
      by the time the &foo_o; target is built,
      even though the call to the &Replace;
      method does not occur until later in
      the &SConscript; file.

      </para>

    </section>

    <section>
    <title>Setting Values Only If They're Not Already Defined:  the &SetDefault; Method</title>

      <para>

      Sometimes it's useful to be able to specify
      that a construction variable should be
      set to a value only if the construction environment
      does not already have that variable defined
      You can do this with the &SetDefault; method,
      which behaves similarly to the <function>set_default</function>
      method of Python dictionary objects:

      </para>

      <programlisting>
      env.SetDefault(SPECIAL_FLAG = '-extra-option')
      </programlisting>

      <para>

      This is especially useful
      when writing your own <literal>Tool</literal> modules
      to apply variables to construction environments.
      <!--
      See <xref linkend="chap-tool-modules"></xref>
      for more information about writing
      Tool modules.
      -->

      </para>

    </section>

    <section>
    <title>Appending to the End of Values:  the &Append; Method</title>

      <para>

      You can append a value to
      an existing construction variable
      using the &Append; method:

      </para>

      <programlisting>
         env = Environment(CCFLAGS = ['-DMY_VALUE'])
         env.Append(CCFLAGS = ['-DLAST'])
         env.Program('foo.c')
      </programlisting>

      <para>

      &SCons; then supplies both the <literal>-DMY_VALUE</literal> and
      <literal>-DLAST</literal> flags when compiling the object file:

      </para>

      <screen>
         % <userinput>scons -Q</userinput>
         cc -o foo.o -c -DMY_VALUE -DLAST foo.c
         cc -o foo foo.o
      </screen>

      <para>

      If the construction variable doesn't already exist,
      the &Append; method will create it:

      </para>

      <programlisting>
         env = Environment()
         env.Append(NEW_VARIABLE = 'added')
         print "NEW_VARIABLE =", env['NEW_VARIABLE']
      </programlisting>

      <para>

      Which yields:

      </para>

      <screen>
         % <userinput>scons -Q</userinput>
         NEW_VARIABLE = added
         scons: `.' is up to date.
      </screen>

      <para>

      Note that the &Append; function tries to be "smart"
      about how the new value is appended to the old value.
      If both are strings, the previous and new strings
      are simply concatenated.
      Similarly, if both are lists,
      the lists are concatenated.
      If, however, one is a string and the other is a list,
      the string is added as a new element to the list.

      </para>

    </section>

    <section>
    <title>Appending Unique Values:  the &AppendUnique; Method</title>

      <para>

      Some times it's useful to add a new value
      only if the existing construction variable
      doesn't already contain the value.
      This can be done using the &AppendUnique; method:

      </para>

      <programlisting>
      env.AppendUnique(CCFLAGS=['-g'])
      </programlisting>

      <para>

      In the above example,
      the <literal>-g</literal> would be added
      only if the &cv-CCFLAGS; variable
      does not already contain a <literal>-g</literal> value.

      </para>

    </section>

    <section>
    <title>Appending to the Beginning of Values:  the &Prepend; Method</title>

      <para>

      You can append a value to the beginning of
      an existing construction variable
      using the &Prepend; method:

      </para>

      <programlisting>
         env = Environment(CCFLAGS = ['-DMY_VALUE'])
         env.Prepend(CCFLAGS = ['-DFIRST'])
         env.Program('foo.c')
      </programlisting>

      <para>

      &SCons; then supplies both the <literal>-DFIRST</literal> and
      <literal>-DMY_VALUE</literal> flags when compiling the object file:

      </para>

      <screen>
         % <userinput>scons -Q</userinput>
         cc -o foo.o -c -DFIRST -DMY_VALUE foo.c
         cc -o foo foo.o
      </screen>

      <para>

      If the construction variable doesn't already exist,
      the &Prepend; method will create it:

      </para>

      <programlisting>
         env = Environment()
         env.Prepend(NEW_VARIABLE = 'added')
         print "NEW_VARIABLE =", env['NEW_VARIABLE']
      </programlisting>

      <para>

      Which yields:

      </para>

      <screen>
         % <userinput>scons -Q</userinput>
         NEW_VARIABLE = added
         scons: `.' is up to date.
      </screen>

      <para>

      Like the &Append; function,
      the &Prepend; function tries to be "smart"
      about how the new value is appended to the old value.
      If both are strings, the previous and new strings
      are simply concatenated.
      Similarly, if both are lists,
      the lists are concatenated.
      If, however, one is a string and the other is a list,
      the string is added as a new element to the list.

      </para>

    </section>

    <section>
    <title>Prepending Unique Values:  the &PrependUnique; Method</title>

      <para>

      Some times it's useful to add a new value
      to the beginning of a construction variable
      only if the existing value
      doesn't already contain the to-be-added value.
      This can be done using the &PrependUnique; method:

      </para>

      <programlisting>
      env.PrependUnique(CCFLAGS=['-g'])
      </programlisting>

      <para>

      In the above example,
      the <literal>-g</literal> would be added
      only if the &cv-CCFLAGS; variable
      does not already contain a <literal>-g</literal> value.

      </para>

    </section>

  </section>

  <section id="sect-execution-environments">
  <title>Controlling the Execution Environment for Issued Commands</title>

    <para>

      When &SCons; builds a target file,
      it does not execute the commands with
      the same external environment
      that you used to execute &SCons;.
      Instead, it uses the dictionary
      stored in the &cv-link-ENV; construction variable
      as the external environment
      for executing commands.

    </para>

    <para>

      The most important ramification of this behavior
      is that the &PATH; environment variable,
      which controls where the operating system
      will look for commands and utilities,
      is not the same as in the external environment
      from which you called &SCons;.
      This means that &SCons; will not, by default,
      necessarily find all of the tools
      that you can execute from the command line.

    </para>

    <para>

      The default value of the &PATH; environment variable
      on a POSIX system
      is <literal>/usr/local/bin:/bin:/usr/bin</literal>.
      The default value of the &PATH; environment variable
      on a Windows system comes from the Windows registry
      value for the command interpreter.
      If you want to execute any commands--compilers, linkers, etc.--that
      are not in these default locations,
      you need to set the &PATH; value
      in the &cv-ENV; dictionary
      in your construction environment.

    </para>

    <para>

      The simplest way to do this is to initialize explicitly
      the value when you create the construction environment;
      this is one way to do that:

    </para>

    <programlisting>
      path = ['/usr/local/bin', '/bin', '/usr/bin']
      env = Environment(ENV = {'PATH' : path})
    </programlisting>

    <para>

    Assign a dictionary to the &cv-ENV;
    construction variable in this way
    completely resets the external environment
    so that the only variable that will be
    set when external commands are executed
    will be the &PATH; value.
    If you want to use the rest of
    the values in &cv-ENV; and only
    set the value of &PATH;,
    the most straightforward way is probably:

    </para>

    <programlisting>
      env['ENV']['PATH'] = ['/usr/local/bin', '/bin', '/usr/bin']
    </programlisting>

    <para>

    Note that &SCons; does allow you to define
    the directories in the &PATH; in a string,
    separated by the pathname-separator character
    for your system (':' on POSIX systems, ';' on Windows):

    </para>

    <programlisting>
      env['ENV']['PATH'] = '/usr/local/bin:/bin:/usr/bin'
    </programlisting>

    <para>

    But doing so makes your &SConscript; file less portable,
    (although in this case that may not be a huge concern
    since the directories you list are likley system-specific, anyway).

    </para>

    <!--

    <scons_example name="ex1">
      <file name="SConstruct" printme="1">
      env = Environment()
      env.Command('foo', [], '__ROOT__/usr/bin/printenv.py')
      </file>
      <file name="__ROOT__/usr/bin/printenv.py" chmod="0755">
      #!/usr/bin/env python
      import os
      import sys
      if len(sys.argv) &gt; 1:
          keys = sys.argv[1:]
      else:
          keys = sorted(os.environ.keys())
      for key in keys:
          print "    " + key + "=" + os.environ[key]
      </file>
    </scons_example>

    <para>

    </para>

    <scons_output example="ex1">
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

    -->

    <section>
    <title>Propagating &PATH; From the External Environment</title>

      <para>

      You may want to propagate the external &PATH;
      to the execution environment for commands.
      You do this by initializing the &PATH;
      variable with the &PATH; value from
      the <literal>os.environ</literal>
      dictionary,
      which is Python's way of letting you
      get at the external environment:

      </para>

      <programlisting>
        import os
        env = Environment(ENV = {'PATH' : os.environ['PATH']})
      </programlisting>

      <para>

      Alternatively, you may find it easier
      to just propagate the entire external
      environment to the execution environment
      for commands.
      This is simpler to code than explicity
      selecting the &PATH; value:

      </para>

      <programlisting>
        import os
        env = Environment(ENV = os.environ)
      </programlisting>

      <para>

      Either of these will guarantee that
      &SCons; will be able to execute
      any command that you can execute from the command line.
      The drawback is that the build can behave
      differently if it's run by people with
      different &PATH; values in their environment--for example,
      if both the <literal>/bin</literal> and
      <literal>/usr/local/bin</literal> directories
      have different &cc; commands,
      then which one will be used to compile programs
      will depend on which directory is listed
      first in the user's &PATH; variable.

      </para>

    </section>

    <section>
    <title>Adding to <varname>PATH</varname> Values in the Execution Environment</title>

      <para>

      One of the most common requirements
      for manipulating a variable in the execution environment
      is to add one or more custom directories to a search
      like the <envar>$PATH</envar> variable on Linux or POSIX systems,
      or the <envar>%PATH%</envar> variable on Windows,
      so that a locally-installed compiler or other utility
      can be found when &SCons; tries to execute it to update a target.
      &SCons; provides &PrependENVPath; and &AppendENVPath; functions
      to make adding things to execution variables convenient.
      You call these functions by specifying the variable
      to which you want the value added,
      and then value itself.
      So to add some <filename>/usr/local</filename> directories
      to the <envar>$PATH</envar> and <envar>$LIB</envar> variables,
      you might:

      </para>

      <programlisting>
        env = Environment(ENV = os.environ)
        env.PrependENVPath('PATH', '/usr/local/bin')
        env.AppendENVPath('LIB', '/usr/local/lib')
      </programlisting>

      <para>

      Note that the added values are strings,
      and if you want to add multiple directories to
      a variable like <envar>$PATH</envar>,
      you must include the path separate character
      (<literal>:</literal> on Linux or POSIX,
      <literal>;</literal> on Windows)
      in the string.

      </para>

    </section>

  </section>