path: root/doc/user/command-line.sgml

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  <para>

  &SCons; provides a number of ways that
  allow the writer of the &SConscript; files
  to give users a great deal of control over how to run the builds.

  </para>

  <section>
  <title>Not Having to Specify Command-Line Options Each Time:  the &SCONSFLAGS; Environment Variable</title>

    <para>

    Users may find themselves supplying
    the same command-line options every time
    they run &SCons;.
    For example, a user might find that it saves time
    to always specify a value of <literal>-j 2</literal>
    to run the builds in parallel.

    </para>

    

    <literallayout>
      % <userinput>scons</userinput>
      scons: Reading SConscript files ...
          ...
      scons: done reading SConscript files.
      scons: Building targets ...
      scons: `.' is up to date.
      scons: done building targets.
      % <userinput>export SCONSFLAGS="-Q"</userinput>
      % <userinput>scons</userinput>
      scons: Reading SConscript files ...
          ...
      scons: done reading SConscript files.
      scons: Building targets ...
      scons: `.' is up to date.
      scons: done building targets.
    </literallayout>

    <para>

    </para>

  </section>

  <section>
  <title>Controlling the Default Targets</title>

    <para>

    One of the most basic things you can control
    is which targets &SCons; will build by default.
    As mentioned previously,
    &SCons; will normally build every target
    in or below the current directory
    by default--that is, when you don't
    explicitly specify one or more targets
    on the command line.
    Sometimes, however, you may want
    to specify explicitly that only
    certain programs, or programs in certain directories,
    should be built by default.
    You do this with the &Default; function:

    </para>

    <programlisting>
       env = Environment()
       hello = env.Program('hello.c')
       env.Program('goodbye.c')
       Default(hello)
    </programlisting>

    <para>

    This &SConstruct; file knows how to build two programs,
    &hello; and &goodbye;,
    but only builds the
    &hello; program by default:

    </para>

    <literallayout>
       % <userinput>scons -Q</userinput>
       cc -c -o hello.o hello.c
       cc -o hello hello.o
       % <userinput>scons -Q</userinput>
       scons: `hello' is up to date.
       % <userinput>scons -Q goodbye</userinput>
       cc -c -o goodbye.o goodbye.c
       cc -o goodbye goodbye.o
    </literallayout>

    <para>

    Note that, even when you use the &Default;
    function in your &SConstruct; file,
    you can still explicitly specify the current directory
    (<literal>.</literal>) on the command line
    to tell &SCons; to build
    everything in (or below) the current directory:

    </para>

    <literallayout>
       % <userinput>scons -Q .</userinput>
       cc -c -o goodbye.o goodbye.c
       cc -o goodbye goodbye.o
       cc -c -o hello.o hello.c
       cc -o hello hello.o
    </literallayout>

    <para>

    You can also call the &Default;
    function more than once,
    in which case each call
    adds to the list of targets to be built by default:

    </para>

    <programlisting>
       env = Environment()
       prog1 = env.Program('prog1.c')
       Default(prog1)
       prog2 = env.Program('prog2.c')
       prog3 = env.Program('prog3.c')
       Default(prog3)
    </programlisting>

    <para>

    Or you can specify more than one target
    in a single call to the &Default; function:

    </para>

    <programlisting>
       env = Environment()
       prog1 = env.Program('prog1.c')
       prog2 = env.Program('prog2.c')
       prog3 = env.Program('prog3.c')
       Default(prog1, prog3)
    </programlisting>

    <para>

    Either of these last two examples
    will build only the
    <application>prog1</application>
    and
    <application>prog3</application>
    programs by default:

    </para>

    <literallayout>
       % <userinput>scons -Q</userinput>
       cc -c -o prog1.o prog1.c
       cc -o prog1 prog1.o
       cc -c -o prog3.o prog3.c
       cc -o prog3 prog3.o
       % <userinput>scons -Q .</userinput>
       cc -c -o prog2.o prog2.c
       cc -o prog2 prog2.o
    </literallayout>

    <para>

    You can list a directory as
    an argument to &Default;:

    </para>

    <programlisting>
       env = Environment()
       env.Program(['prog1/main.c', 'prog1/foo.c'])
       env.Program(['prog2/main.c', 'prog2/bar.c'])
       Default('prog1')
    </programlisting>

    <para>

    In which case only the target(s) in that
    directory will be built by default:

    </para>

    <literallayout>
       % <userinput>scons -Q</userinput>
       cc -c -o prog1/foo.o prog1/foo.c
       cc -c -o prog1/main.o prog1/main.c
       cc -o prog1/main prog1/main.o prog1/foo.o
       % <userinput>scons -Q</userinput>
       scons: `prog1' is up to date.
       % <userinput>scons -Q .</userinput>
       cc -c -o prog2/bar.o prog2/bar.c
       cc -c -o prog2/main.o prog2/main.c
       cc -o prog2/main prog2/main.o prog2/bar.o
    </literallayout>

    <para>

    Lastly, if for some reason you don't want
    any targets built by default,
    you can use the Python <literal>None</literal>
    variable:

    </para>

    <programlisting>
       env = Environment()
       prog1 = env.Program('prog1.c')
       prog2 = env.Program('prog2.c')
       Default(None)
    </programlisting>

    <para>

    Which would produce build output like:

    </para>

    <literallayout>
       % <userinput>scons -Q</userinput>
       scons: *** No targets specified and no Default() targets found.  Stop.
       % <userinput>scons -Q .</userinput>
       cc -c -o prog1.o prog1.c
       cc -o prog1 prog1.o
       cc -c -o prog2.o prog2.c
       cc -o prog2 prog2.o
    </literallayout>

  </section>

  <!--

  <section>
  <title>Getting at Command-Line Targets</title>

    <para>

    XXX

    </para>

  </section>

  -->

  <section>
  <title>Command-Line <varname>variable</varname>=<varname>value</varname> Build Options</title>

    <para>

    You may want to control various aspects
    of your build by allowing the user
    to specify <varname>variable</varname>=<varname>value</varname>
    values on the command line.
    For example, suppose you
    want users to be able to
    build a debug version of a program
    by running &SCons; as follows:

    </para>

    <literallayout>
      % scons -Q debug=1
    </literallayout>

    <para>

    &SCons; provides an &ARGUMENTS; dictionary
    that stores all of the
    <varname>variable</varname>=<varname>value</varname>
    assignments from the command line.
    This allows you to modify
    aspects of your build in response
    to specifications on the command line.
    (Note that unless you want to require
    that users <emphasis>always</emphasis>
    specify an option,
    you probably want to use
    the Python
    <literal>ARGUMENTS.get()</literal> function,
    which allows you to specify a default value
    to be used if there is no specification
    on the command line.)

    </para>

    <para>

    The following code sets the &CCFLAGS; construction
    variable in response to the <varname>debug</varname>
    flag being set in the &ARGUMENTS; dictionary:

    </para>

    <programlisting>
       env = Environment()
       debug = ARGUMENTS.get('debug', 0)
       if int(debug):
           env.Append(CCFLAGS = '-g')
       env.Program('prog.c')
    </programlisting>

    <para>

    This results in the <varname>-g</varname>
    compiler option being used when
    <literal>debug=1</literal>
    is used on the command line:

    </para>

    <literallayout>
       % <userinput>scons -Q debug=0</userinput>
       cc -c -o prog.o prog.c
       cc -o prog prog.o
       % <userinput>scons -Q debug=0</userinput>
       scons: `.' is up to date.
       % <userinput>scons -Q debug=1</userinput>
       cc -g -c -o prog.o prog.c
       cc -o prog prog.o
       % <userinput>scons -Q debug=1</userinput>
       scons: `.' is up to date.
    </literallayout>

    <para>

    Notice that &SCons; keeps track of
    the last values used to build the object files,
    and as a result correctly rebuilds
    the object and executable files
    only when the value of the <literal>debug</literal>
    argument has changed.

    </para>

  </section>

  <section>
  <title>Controlling Command-Line Build Options</title>

    <para>

    Being able to use a command-line build option like
    <literal>debug=1</literal> is handy,
    but it can be a chore to write specific Python code
    to recognize each such option
    and apply the values to a construction variable.
    To help with this,
    &SCons; supports a class to
    define such build options easily,
    and a mechanism to apply the
    build options to a construction environment.
    This allows you to control how the build options affect
    construction environments.

    </para>

    <para>

    For example, suppose that you want users to set
    a &RELEASE; construction variable on the
    command line whenever the time comes to build
    a program for release,
    and that the value of this variable
    should be added to the command line
    with the appropriate <literal>-D</literal> option
    (or other command line option)
    to pass the value to the C compiler.
    Here's how you might do that by setting
    the appropriate value in a dictionary for the
    &CPPDEFINES; construction variable:

    </para>

    <programlisting>
         opts = Options()
         opts.Add('RELEASE', 'Set to 1 to build for release', 0)
         env = Environment(options = opts,
                           CPPDEFINES={'RELEASE_BUILD' : '${RELEASE}'})
         env.Program(['foo.c', 'bar.c'])
    </programlisting>

    <para>

    This &SConstruct; file first creates an
    &Options; object
    (the <literal>opts = Options()</literal> call),
    and then uses the object's &Add;
    method to indicate that the &RELEASE;
    option can be set on the command line,
    and that it's default value will be <literal>0</literal>
    (the third argument to the &Add; method).
    The second argument is a line of help text;
    we'll learn how to use it in the next section.

    </para>

    <para>

    We then pass the created &Options;
    object as an &options; keyword argument
    to the &Environment; call
    used to create the construction environment.
    This then allows a user to set the
    &RELEASE; build option on the command line
    and have the variable show up in
    the command line used to build each object from
    a C source file:

    </para>

    <literallayout>
      % <userinput>scons -Q RELEASE=1</userinput>
      cc -DRELEASE_BUILD=1 -c -o bar.o bar.c
      cc -DRELEASE_BUILD=1 -c -o foo.o foo.c
      cc -o foo foo.o bar.o
    </literallayout>

  </section>

  <section>
  <title>Providing Help for Command-Line Build Options</title>

    <para>

    To make command-line build options most useful,
    you ideally want to provide
    some help text that will describe
    the available options
    when the user runs <literal>scons -h</literal>.
    You could write this text by hand,
    but &SCons; provides an easier way.
    &Options; objects support a
    &GenerateHelpText; method
    that will, as its name indicates,
    generate text that describes
    the various options that
    have been added to it.
    You then pass the output from this method to
    the &Help; function:

    </para>

    <programlisting>
         opts = Options('custom.py')
         opts.Add('RELEASE', 'Set to 1 to build for release', 0)
         env = Environment(options = opts)
         Help(opts.GenerateHelpText(env))
    </programlisting>

    <para>

    &SCons; will now display some useful text
    when the <literal>-h</literal> option is used:

    </para>

    <literallayout>
      % <userinput>scons -Q -h</userinput>
      
      RELEASE: Set to 1 to build for release
          default: 0
          actual: 0
      
      Use scons -H for help about command-line options.
    </literallayout>

    <para>

    Notice that the help output shows the default value,
    and the current actual value of the build option.

    </para>

  </section>

  <section>
  <title>Reading Build Options From a File</title>

    <para>

    Being able to use a command-line build option like
    <literal>debug=1</literal> is handy,
    but it can be a chore to write specific Python code
    to recognize each such option
    and apply the values to a construction variable.
    To help with this,
    &SCons; supports a class to
    define such build options easily
    and to read build option values from a file.
    This allows you to control how the build options affect
    construction environments.
    The way you do this is by specifying
    a file name when you call &Options;,
    like &custom_py; in the following example:

    </para>

    <programlisting>
         opts = Options('custom.py')
         opts.Add('RELEASE', 'Set to 1 to build for release', 0)
         env = Environment(options = opts,
                           CPPDEFINES={'RELEASE_BUILD' : '${RELEASE}'})
         env.Program(['foo.c', 'bar.c'])
         Help(opts.GenerateHelpText(env))
    </programlisting>

    <para>

    This then allows us to control the &RELEASE;
    variable by setting it in the &custom_py; file:

    </para>

    <programlisting>
      RELEASE = 1
      </programlisting>

    <para>

    Note that this file is actually executed
    like a Python script.
    Now when we run &SCons;:

    </para>

    <literallayout>
      % <userinput>scons -Q</userinput>
      cc -DRELEASE_BUILD=1 -c -o bar.o bar.c
      cc -DRELEASE_BUILD=1 -c -o foo.o foo.c
      cc -o foo foo.o bar.o
    </literallayout>

    <para>

    And if we change the contents of &custom_py; to:

    </para>

    <programlisting>
      RELEASE = 0
    </programlisting>

    <para>

    The object files are rebuilt appropriately
    with the new option:

    </para>

    <literallayout>
      % <userinput>scons -Q</userinput>
      cc -DRELEASE_BUILD=0 -c -o bar.o bar.c
      cc -DRELEASE_BUILD=0 -c -o foo.o foo.c
      cc -o foo foo.o bar.o
    </literallayout>

  </section>

  <section>
  <title>Canned Build Options</title>

    <para>

    &SCons; provides a number of functions
    that provide ready-made behaviors
    for various types of command-line build options.

    </para>

    <section>
    <title>True/False Values:  the &BoolOption; Build Option</title>

      <para>

      It's often handy to be able to specify an
      option that controls a simple Boolean variable
      with a &true; or &false; value.
      It would be even more handy to accomodate
      users who have different preferences for how to represent
      &true; or &false; values.
      The &BoolOption; function
      makes it easy to accomodate a variety of
      common values that represent
      &true; or &false;.

      </para>

      <para>

      The &BoolOption; function takes three arguments:
      the name of the build option,
      the default value of the build option,
      and the help string for the option.
      It then returns appropriate information for
      passing to the &Add; method of an &Options; object, like so:

      </para>

      <programlisting>
           opts = Options('custom.py')
           opts.Add(BoolOption('RELEASE', 0, 'Set to build for release'))
           env = Environment(options = opts,
                             CPPDEFINES={'RELEASE_BUILD' : '${RELEASE}'})
           env.Program('foo.c')
      </programlisting>

      <para>

      With this build option,
      the &RELEASE; variable can now be enabled by
      setting it to the value <literal>yes</literal>
      or <literal>t</literal>:

      </para>

      <literallayout>
        % <userinput>scons -Q RELEASE=yes foo.o</userinput>
        cc -DRELEASE_BUILD=1 -c -o foo.o foo.c
      </literallayout>

      <literallayout>
        % <userinput>scons -Q RELEASE=t foo.o</userinput>
        cc -DRELEASE_BUILD=1 -c -o foo.o foo.c
      </literallayout>

      <para>

      Other values that equate to &true; include
      <literal>y</literal>,
      <literal>1</literal>,
      <literal>on</literal>
      and
      <literal>all</literal>.

      </para>

      <para>

      Conversely, &RELEASE; may now be given a &false;
      value by setting it to
      <literal>no</literal>
      or
      <literal>f</literal>:

      </para>

      <literallayout>
        % <userinput>scons -Q RELEASE=no foo.o</userinput>
        cc -DRELEASE_BUILD=0 -c -o foo.o foo.c
      </literallayout>

      <literallayout>
        % <userinput>scons -Q RELEASE=f foo.o</userinput>
        cc -DRELEASE_BUILD=0 -c -o foo.o foo.c
      </literallayout>

      <para>

      Other values that equate to &true; include
      <literal>n</literal>,
      <literal>0</literal>,
      <literal>off</literal>
      and
      <literal>none</literal>.

      </para>

      <para>

      Lastly, if a user tries to specify
      any other value,
      &SCons; supplies an appropriate error message:

      </para>

      <literallayout>
        % <userinput>scons -Q RELEASE=bad_value foo.o</userinput>
        
        scons: *** Error converting option: RELEASE
        Invalid value for boolean option: bad_value
        File "SConstruct", line 4, in ?
      </literallayout>

    </section>

    <section>
    <title>Single Value From a List:  the &EnumOption; Build Option</title>

      <para>

      Suppose that we want a user to be able to
      set a &COLOR; option
      that selects a background color to be
      displayed by an application,
      but that we want to restrict the
      choices to a specific set of allowed colors.
      This can be set up quite easily
      using the &EnumOption;,
      which takes a list of &allowed_values
      in addition to the variable name,
      default value,
      and help text arguments:

      </para>

      <programlisting>
           opts = Options('custom.py')
           opts.Add(EnumOption('COLOR', 'red', 'Set background color',
                               allowed_values=('red', 'green', 'blue')))
           env = Environment(options = opts,
                             CPPDEFINES={'COLOR' : '"${COLOR}"'})
           env.Program('foo.c')
      </programlisting>

      <para>

      The user can now explicity set the &COLOR; build option
      to any of the specified allowed values:

      </para>

      <literallayout>
        % <userinput>scons -Q COLOR=red foo.o</userinput>
        cc -DCOLOR="red" -c -o foo.o foo.c
        % <userinput>scons -Q COLOR=blue foo.o</userinput>
        cc -DCOLOR="blue" -c -o foo.o foo.c
        % <userinput>scons -Q COLOR=green foo.o</userinput>
        cc -DCOLOR="green" -c -o foo.o foo.c
      </literallayout>

      <para>

      But, almost more importantly,
      an attempt to set &COLOR;
      to a value that's not in the list
      generates an error message:

      </para>

      <literallayout>
        % <userinput>scons -Q COLOR=magenta foo.o</userinput>
        
        scons: *** Invalid value for option COLOR: magenta
        File "SConstruct", line 5, in ?
      </literallayout>

      <para>

      The &EnumOption; function also supports a way
      to map alternate names to allowed values.
      Suppose, for example,
      that we want to allow the user
      to use the word <literal>navy</literal> as a synonym for
      <literal>blue</literal>.
      We do this by adding a &map; dictionary
      that will map its key values
      to the desired legal value:

      </para>

      <programlisting>
           opts = Options('custom.py')
           opts.Add(EnumOption('COLOR', 'red', 'Set background color',
                               allowed_values=('red', 'green', 'blue'),
                               map={'navy':'blue'}))
           env = Environment(options = opts,
                             CPPDEFINES={'COLOR' : '"${COLOR}"'})
           env.Program('foo.c')
      </programlisting>

      <para>

      As desired, the user can then use
      <literal>navy</literal> on the command line,
      and &SCons; will translate it into <literal>blue</literal>
      when it comes time to use the &COLOR;
      option to build a target:

      </para>

      <literallayout>
        % <userinput>scons -Q COLOR=navy foo.o</userinput>
        cc -DCOLOR="blue" -c -o foo.o foo.c
      </literallayout>

      <para>

      By default, when using the &EnumOption; function,
      arguments that differ
      from the legal values
      only in case
      are treated as illegal values:

      </para>

      <literallayout>
        % <userinput>scons -Q COLOR=Red foo.o</userinput>
        
        scons: *** Invalid value for option COLOR: Red
        File "SConstruct", line 5, in ?
        % <userinput>scons -Q COLOR=BLUE foo.o</userinput>
        
        scons: *** Invalid value for option COLOR: BLUE
        File "SConstruct", line 5, in ?
        % <userinput>scons -Q COLOR=nAvY foo.o</userinput>
        
        scons: *** Invalid value for option COLOR: nAvY
        File "SConstruct", line 5, in ?
      </literallayout>

      <para>

      The &EnumOption; function can take an additional
      &ignorecase; keyword argument that,
      when set to <literal>1</literal>,
      tells &SCons; to allow case differences
      when the values are specified:

      </para>

      <programlisting>
           opts = Options('custom.py')
           opts.Add(EnumOption('COLOR', 'red', 'Set background color',
                               allowed_values=('red', 'green', 'blue'),
                               map={'navy':'blue'},
                               ignorecase=1))
           env = Environment(options = opts,
                             CPPDEFINES={'COLOR' : '"${COLOR}"'})
           env.Program('foo.c')
      </programlisting>

      <para>

      Which yields the output:

      </para>

      <literallayout>
        % <userinput>scons -Q COLOR=Red foo.o</userinput>
        cc -DCOLOR="Red" -c -o foo.o foo.c
        % <userinput>scons -Q COLOR=BLUE foo.o</userinput>
        cc -DCOLOR="BLUE" -c -o foo.o foo.c
        % <userinput>scons -Q COLOR=nAvY foo.o</userinput>
        cc -DCOLOR="blue" -c -o foo.o foo.c
        % <userinput>scons -Q COLOR=green foo.o</userinput>
        cc -DCOLOR="green" -c -o foo.o foo.c
      </literallayout>

      <para>

      Notice that an &ignorecase; value of <literal>1</literal>
      preserves the case-spelling that the user supplied.
      If you want &SCons; to translate the names
      into lower-case,
      regardless of the case used by the user,
      specify an &ignorecase; value of <literal>2</literal>:

      </para>

      <programlisting>
           opts = Options('custom.py')
           opts.Add(EnumOption('COLOR', 'red', 'Set background color',
                               allowed_values=('red', 'green', 'blue'),
                               map={'navy':'blue'},
                               ignorecase=2))
           env = Environment(options = opts,
                             CPPDEFINES={'COLOR' : '"${COLOR}"'})
           env.Program('foo.c')
      </programlisting>

      <para>

      Now &SCons; will use values of
      <literal>red</literal>,
      <literal>green</literal> or
      <literal>blue</literal>
      regardless of how the user spells
      those values on the command line:

      </para>

      <literallayout>
        % <userinput>scons -Q COLOR=Red foo.o</userinput>
        cc -DCOLOR="red" -c -o foo.o foo.c
        % <userinput>scons -Q COLOR=nAvY foo.o</userinput>
        cc -DCOLOR="blue" -c -o foo.o foo.c
        % <userinput>scons -Q COLOR=GREEN foo.o</userinput>
        cc -DCOLOR="green" -c -o foo.o foo.c
      </literallayout>

    </section>

    <section>
    <title>Multiple Values From a List:  the &ListOption; Build Option</title>

      <para>

      Another way in which you might want to allow users
      to control build option is to
      specify a list of one or more legal values.
      &SCons; supports this through the &ListOption; function.
      If, for example, we want a user to be able to set a
      &COLORS; option to one or more of the legal list of values:

      </para>

      <programlisting>
           opts = Options('custom.py')
           opts.Add(ListOption('COLORS', 0, 'List of colors',
                               ['red', 'green', 'blue']))
           env = Environment(options = opts,
                             CPPDEFINES={'COLORS' : '"${COLORS}"'})
           env.Program('foo.c')
      </programlisting>

      <para>

      A user can now specify a comma-separated list
      of legal values,
      which will get translated into a space-separated
      list for passing to the any build commands:

      </para>

      <literallayout>
        % <userinput>scons -Q COLORS=red,blue foo.o</userinput>
        TypeError: sequence item 0: expected string, int found:
        % <userinput>scons -Q COLORS=blue,green,red foo.o</userinput>
        TypeError: sequence item 0: expected string, int found:
      </literallayout>

      <para>

      In addition, the &ListOption; function
      allows the user to specify explicit keywords of
      &all; or &none;
      to select all of the legal values,
      or none of them, respectively:

      </para>

      <literallayout>
        % <userinput>scons -Q COLORS=all foo.o</userinput>
        TypeError: sequence item 0: expected string, int found:
        % <userinput>scons -Q COLORS=none foo.o</userinput>
        TypeError: sequence item 0: expected string, int found:
      </literallayout>

      <para>

      And, of course, an illegal value
      still generates an error message:

      </para>

      <literallayout>
        % <userinput>scons -Q COLORS=magenta foo.o</userinput>
        TypeError: sequence item 0: expected string, int found:
      </literallayout>

    </section>

    <section>
    <title>Path Names:  the &PathOption; Build Option</title>

      <para>

      &SCons; supports a &PathOption; function
      to make it easy to create a build option
      to control an expected path name.
      If, for example, you need to
      define a variable in the preprocessor
      that control the location of a
      configuration file:

      </para>

      <programlisting>
           opts = Options('custom.py')
           opts.Add(PathOption('CONFIG', '/etc/my_config', 'Path to configuration file'))
           env = Environment(options = opts,
                             CPPDEFINES={'CONFIG_FILE' : '"$CONFIG"'})
           env.Program('foo.c')
      </programlisting>

      <para>

      This then allows the user to
      override the &CONFIG; build option
      on the command line as necessary:

      </para>

      <literallayout>
        % <userinput>scons -Q foo.o</userinput>
        
        scons: *** Path does not exist for option CONFIG: Path to configuration file
        File "SConstruct", line 4, in ?
        % <userinput>scons -Q CONFIG=/usr/local/etc/other_config foo.o</userinput>
        cc -DCONFIG_FILE="/usr/local/etc/other_config" -c -o foo.o foo.c
      </literallayout>

    </section>

    <section>
    <title>Enabled/Disabled Path Names: the &PackageOption; Build Option</title>

      <para>

      Sometimes you want to give users
      even more control over a path name variable,
      allowing them to explicitly enable or
      disable the path name
      by using <literal>yes</literal> or <literal>no</literal> keywords,
      in addition to allow them
      to supply an explicit path name.
      &SCons; supports the &PackageOption;
      function to support this:

      </para>

      <programlisting>
           opts = Options('custom.py')
           opts.Add(PackageOption('PACKAGE', '/opt/location', 'Location package'))
           env = Environment(options = opts,
                             CPPDEFINES={'PACKAGE' : '"$PACKAGE"'})
           env.Program('foo.c')
      </programlisting>

      <para>

      When the &SConscript; file uses the &PackageOption; funciton,
      user can now still use the default
      or supply an overriding path name,
      but can now explicitly set the
      specified variable to a value
      that indicates the package should be enabled
      (in which case the default should be used)
      or disabled:

      </para>

      <literallayout>
        % <userinput>scons -Q foo.o</userinput>
        
        scons: *** Path does not exist for option PACKAGE: Location package
        File "SConstruct", line 4, in ?
        % <userinput>scons -Q PACKAGE=/usr/local/location foo.o</userinput>
        cc -DPACKAGE="/usr/local/location" -c -o foo.o foo.c
        % <userinput>scons -Q PACKAGE=yes foo.o</userinput>
        cc -DPACKAGE="1" -c -o foo.o foo.c
        % <userinput>scons -Q PACKAGE=no foo.o</userinput>
        cc -DPACKAGE="0" -c -o foo.o foo.c
      </literallayout>

    </section>

  </section>

  <section>
  <title>Adding Multiple Command-Line Build Options at Once</title>

    <para>

    Lastly, &SCons; provides a way to add
    multiple build options to an &Options; object at once.
    Instead of having to call the &Add; method
    multiple times,
    you can call the &AddOptions;
    method with a list of build options
    to be added to the object.
    Each build option is specified
    as either a tuple of arguments,
    just like you'd pass to the &Add; method itself,
    or as a call to one of the canned
    functions for pre-packaged command-line build options.
    in any order:

    </para>

    <programlisting>
        opts = Options()
        opts.AddOptions(
            ('RELEASE', 'Set to 1 to build for release', 0),
            ('CONFIG', 'Configuration file', '/etc/my_config'),
            BoolOption('warnings', 'compilation with -Wall and similiar', 1),
            EnumOption('debug', 'debug output and symbols', 'no',
                       allowed_values=('yes', 'no', 'full'),
                       map={}, ignorecase=0),  # case sensitive
            ListOption('shared',
                       'libraries to build as shared libraries',
                       'all',
                       names = list_of_libs),
            PackageOption('x11',
                          'use X11 installed here (yes = search some places)',
                          'yes'),
            PathOption('qtdir', 'where the root of Qt is installed', qtdir),
        )
    </programlisting>

    <para>
    </para>

  </section>