source: downloads/boost_1_34_1/tools/build/v2/util/option.jam @ 45

#  Copyright (c) 2005 Vladimir Prus.
#
#  Use, modification and distribution is subject to the Boost Software
#  License Version 1.0. (See accompanying file LICENSE_1_0.txt or
#  http://www.boost.org/LICENSE_1_0.txt)

import modules ;

rule get ( name : default-value ? )
{
    local m = [ MATCH --$(name)=(.*) : [ modules.peek : ARGV ] ] ;
    if $(m)
    {
        return $(m[1]) ;
    }
    else
    {
        return $(default-value) ;
    }        
}


rule process ( )
{ 
    # Check command-line args as soon as possible.  For each option try
    # to load module named after option. Is that succeeds, invoke 'process'
    # rule in the module. The rule may return "true" to indicate that the
    # regular built process should not be attempted.
    #
    # Options take the general form of: --<name>[=<value>] [<value>]
    #
    
    local ARGV = [ modules.peek : ARGV ] ;
    local BOOST_BUILD_PATH = [ modules.peek : BOOST_BUILD_PATH ] ;
    
    local dont-build ;
    local args = $(ARGV) ;
    while $(args)
    {
        local arg = [ MATCH ^--(.*) : $(args[1]) ] ;
        while $(args[2-]) && ! $(arg)
        {
            args = $(args[2-]) ;
            arg = [ MATCH ^--(.*) : $(args[1]) ] ;
        }
        args = $(args[2-]) ;

        if $(arg)
        {
            local split = [ MATCH ^(([^-=]+)[^=]*)(=?)(.*)$ : $(arg) ] ;
            local full-name = $(split[1]) ;
            local prefix = $(split[2]) ;
            local values ;

            if $(split[3])
            {
                values = $(split[4]) ;
            }
            if $(args) && ! [ MATCH ^(--).* : $(args[1]) ]
            {
                values += $(args[1]) ;
                args = $(args[2-]) ;
            }

            # look in options subdirectories of BOOST_BUILD_PATH for modules
            # matching the full option name and then its prefix.
            local plugin-dir = options ;
            local option-files = [
              GLOB $(plugin-dir:D=$(BOOST_BUILD_PATH)) : $(full-name).jam $(prefix).jam
            ] ;

            if $(option-files)
            {
                # load the file into a module named for the option
                local f = $(option-files[1]) ;
                local module-name = --$(f:D=:S=) ;
                modules.load $(module-name) : $(f:D=) : $(f:D) ;

                # if there's a process rule, call it with the full option name
                # and its value (if any).  If there was no "=" in the option,
                # the value will be empty.
                if process in [ RULENAMES $(module-name) ]
                {
                    dont-build +=
                      [ modules.call-in $(module-name) : process --$(full-name) : $(values) ] ;
                }
            }
        }
    }

    return $(dont-build) ;
}