path: root/Configurations/README

Intro
=====

This directory contains a few sets of files that are used for
configuration in diverse ways:

    *.conf      Target platform configurations, please read
                'Configurations of OpenSSL target platforms' for more
                information.
    *.tmpl      Build file templates, please read 'Build-file
                programming with the "unified" build system' as well
                as 'Build info files' for more information.
    *.pm        Helper scripts / modules for the main `Configure`
                script.  See 'Configure helper scripts for more
                information.


Configurations of OpenSSL target platforms
==========================================

Configuration targets are a collection of facts that we know about
different platforms and their capabilities.  We organise them in a
hash table, where each entry represent a specific target.

Note that configuration target names must be unique across all config
files.  The Configure script does check that a config file doesn't
have config targets that shadow config targets from other files.

In each table entry, the following keys are significant:

        inherit_from    => Other targets to inherit values from.
                           Explained further below. [1]
        template        => Set to 1 if this isn't really a platform
                           target.  Instead, this target is a template
                           upon which other targets can be built.
                           Explained further below.  [1]

        sys_id          => System identity for systems where that
                           is difficult to determine automatically.

        enable          => Enable specific configuration features.
                           This MUST be an array of words.
        disable         => Disable specific configuration features.
                           This MUST be an array of words.
                           Note: if the same feature is both enabled
                           and disabled, disable wins.

        as              => The assembler command.  This is not always
                           used (for example on Unix, where the C
                           compiler is used instead).
        asflags         => Default assembler command flags [4].
        cpp             => The C preprocessor command, normally not
                           given, as the build file defaults are
                           usually good enough.
        cppflags        => Default C preprocessor flags [4].
        defines         => As an alternative, macro definitions may be
                           given here instead of in `cppflags' [4].
                           If given here, they MUST be as an array of
                           the string such as "MACRO=value", or just
                           "MACRO" for definitions without value.
        includes        => As an alternative, inclusion directories
                           may be given here instead of in `cppflags'
                           [4].  If given here, the MUST be an array
                           of strings, one directory specification
                           each.
        cc              => The C compiler command, usually one of "cc",
                           "gcc" or "clang".  This command is normally
                           also used to link object files and
                           libraries into the final program.
        cxx             => The C++ compiler command, usually one of
                           "c++", "g++" or "clang++".  This command is
                           also used when linking a program where at
                           least one of the object file is made from
                           C++ source.
        cflags          => Defaults C compiler flags [4].
        cxxflags        => Default  C++ compiler flags [4].  If unset,
                           it gets the same value as cflags.

        (linking is a complex thing, see [3] below)
        ld              => Linker command, usually not defined
                           (meaning the compiler command is used
                           instead).
                           (NOTE: this is here for future use, it's
                           not implemented yet)
        lflags          => Default flags used when linking apps,
                           shared libraries or DSOs [4].
        ex_libs         => Extra libraries that are needed when
                           linking shared libraries, DSOs or programs.
                           The value is also assigned to Libs.private
                           in $(libdir)/pkgconfig/libcrypto.pc.

        shared_cppflags => Extra C preprocessor flags used when
                           processing C files for shared libraries.
        shared_cflag    => Extra C compiler flags used when compiling
                           for shared libraries, typically something
                           like "-fPIC".
        shared_ldflag   => Extra linking flags used when linking
                           shared libraries.
        module_cppflags
        module_cflags
        module_ldflags  => Has the same function as the corresponding
                           `shared_' attributes, but for building DSOs.
                           When unset, they get the same values as the
                           corresponding `shared_' attributes.

        ar              => The library archive command, the default is
                           "ar".
                           (NOTE: this is here for future use, it's
                           not implemented yet)
        arflags         => Flags to be used with the library archive
                           command.  On Unix, this includes the
                           command letter, 'r' by default.

        ranlib          => The library archive indexing command, the
                           default is 'ranlib' it it exists.

        unistd          => An alternative header to the typical
                           '<unistd.h>'.  This is very rarely needed.

        shared_extension => File name extension used for shared
                            libraries. 
        obj_extension   => File name extension used for object files.
                           On unix, this defaults to ".o" (NOTE: this
                           is here for future use, it's not
                           implemented yet)
        exe_extension   => File name extension used for executable
                           files.  On unix, this defaults to "" (NOTE:
                           this is here for future use, it's not
                           implemented yet)
        shlib_variant   => A "variant" identifier inserted between the base
                           shared library name and the extension.  On "unixy"
                           platforms (BSD, Linux, Solaris, MacOS/X, ...) this
                           supports installation of custom OpenSSL libraries
                           that don't conflict with other builds of OpenSSL
                           installed on the system.  The variant identifier
                           becomes part of the SONAME of the library and also
                           any symbol versions (symbol versions are not used or
                           needed with MacOS/X).  For example, on a system
                           where a default build would normally create the SSL
                           shared library as 'libssl.so -> libssl.so.1.1' with
                           the value of the symlink as the SONAME, a target
                           definition that sets 'shlib_variant => "-abc"' will
                           create 'libssl.so -> libssl-abc.so.1.1', again with
                           an SONAME equal to the value of the symlink.  The
                           symbol versions associated with the variant library
                           would then be 'OPENSSL_ABC_<version>' rather than
                           the default 'OPENSSL_<version>'. The string inserted
                           into symbol versions is obtained by mapping all
                           letters in the "variant" identifier to upper case
                           and all non-alphanumeric characters to '_'.

        thread_scheme   => The type of threads is used on the
                           configured platform.  Currently known
                           values are "(unknown)", "pthreads",
                           "uithreads" (a.k.a solaris threads) and
                           "winthreads".  Except for "(unknown)", the
                           actual value is currently ignored but may
                           be used in the future.  See further notes
                           below [2].
        dso_scheme      => The type of dynamic shared objects to build
                           for.  This mostly comes into play with
                           engines, but can be used for other purposes
                           as well.  Valid values are "DLFCN"
                           (dlopen() et al), "DLFCN_NO_H" (for systems
                           that use dlopen() et al but do not have
                           fcntl.h), "DL" (shl_load() et al), "WIN32"
                           and "VMS".
        perlasm_scheme  => The perlasm method used to create the
                           assembler files used when compiling with
                           assembler implementations.
        shared_target   => The shared library building method used.
                           This is a target found in Makefile.shared.
        build_scheme    => The scheme used to build up a Makefile.
                           In its simplest form, the value is a string
                           with the name of the build scheme.
                           The value may also take the form of a list
                           of strings, if the build_scheme is to have
                           some options.  In this case, the first
                           string in the list is the name of the build
                           scheme.
                           Currently recognised build scheme is "unified".
                           For the "unified" build scheme, this item
                           *must* be an array with the first being the
                           word "unified" and the second being a word
                           to identify the platform family.

        multilib        => On systems that support having multiple
                           implementations of a library (typically a
                           32-bit and a 64-bit variant), this is used
                           to have the different variants in different
                           directories.

        bn_ops          => Building options (was just bignum options in
                           the earlier history of this option, hence the
                           name). This is a string of words that describe
                           algorithms' implementation parameters that
                           are optimal for the designated target platform,
                           such as the type of integers used to build up
                           the bignum, different ways to implement certain
                           ciphers and so on. To fully comprehend the
                           meaning, the best is to read the affected
                           source.
                           The valid words are:

                           THIRTY_TWO_BIT       bignum limbs are 32 bits,
                                                this is default if no
                                                option is specified, it
                                                works on any supported
                                                system [unless "wider"
                                                limb size is implied in
                                                assembly code];
                           BN_LLONG             bignum limbs are 32 bits,
                                                but 64-bit 'unsigned long
                                                long' is used internally
                                                in calculations;
                           SIXTY_FOUR_BIT_LONG  bignum limbs are 64 bits
                                                and sizeof(long) is 8;
                           SIXTY_FOUR_BIT       bignums limbs are 64 bits,
                                                but execution environment
                                                is ILP32;
                           RC4_CHAR             RC4 key schedule is made
                                                up of 'unsigned char's;
                           RC4_INT              RC4 key schedule is made
                                                up of 'unsigned int's;
                           EXPORT_VAR_AS_FN     for shared libraries,
                                                export vars as
                                                accessor functions.

        apps_aux_src    => Extra source to build apps/openssl and other
                           apps, as needed by the target and that can be
                           collected in a library.
        apps_init_src   => Init source to build apps/openssl and other
                           apps, as needed by the target.  This code
                           cannot be placed in a library, as the rest
                           of the code isn't expected to link to it
                           explicitely.
        cpuid_asm_src   => assembler implementation of cpuid code as
                           well as OPENSSL_cleanse().
                           Default to mem_clr.c
        bn_asm_src      => Assembler implementation of core bignum
                           functions.
                           Defaults to bn_asm.c
        ec_asm_src      => Assembler implementation of core EC
                           functions.
        des_asm_src     => Assembler implementation of core DES
                           encryption functions.
                           Defaults to 'des_enc.c fcrypt_b.c'
        aes_asm_src     => Assembler implementation of core AES
                           functions.
                           Defaults to 'aes_core.c aes_cbc.c'
        bf_asm_src      => Assembler implementation of core BlowFish
                           functions.
                           Defaults to 'bf_enc.c'
        md5_asm_src     => Assembler implementation of core MD5
                           functions.
        sha1_asm_src    => Assembler implementation of core SHA1,
                           functions, and also possibly SHA256 and
                           SHA512 ones.
        cast_asm_src    => Assembler implementation of core CAST
                           functions.
                           Defaults to 'c_enc.c'
        rc4_asm_src     => Assembler implementation of core RC4
                           functions.
                           Defaults to 'rc4_enc.c rc4_skey.c'
        rmd160_asm_src  => Assembler implementation of core RMD160
                           functions.
        rc5_asm_src     => Assembler implementation of core RC5
                           functions.
                           Defaults to 'rc5_enc.c'
        wp_asm_src      => Assembler implementation of core WHIRLPOOL
                           functions.
        cmll_asm_src    => Assembler implementation of core CAMELLIA
                           functions.
                           Defaults to 'camellia.c cmll_misc.c cmll_cbc.c'
        modes_asm_src   => Assembler implementation of cipher modes,
                           currently the functions gcm_gmult_4bit and
                           gcm_ghash_4bit.
        padlock_asm_src => Assembler implementation of core parts of
                           the padlock engine.  This is mandatory on
                           any platform where the padlock engine might
                           actually be built.


[1] as part of the target configuration, one can have a key called
    'inherit_from' that indicate what other configurations to inherit
    data from.  These are resolved recursively.

    Inheritance works as a set of default values that can be overridden
    by corresponding key values in the inheriting configuration.

    Note 1: any configuration table can be used as a template.
    Note 2: pure templates have the attribute 'template => 1' and
            cannot be used as build targets.

    If several configurations are given in the 'inherit_from' array,
    the values of same attribute are concatenated with space
    separation.  With this, it's possible to have several smaller
    templates for different configuration aspects that can be combined
    into a complete configuration.

    instead of a scalar value or an array, a value can be a code block
    of the form 'sub { /* your code here */ }'.  This code block will
    be called with the list of inherited values for that key as
    arguments.  In fact, the concatenation of strings is really done
    by using 'sub { join(" ",@_) }' on the list of inherited values.

    An example:

        "foo" => {
                template => 1,
                haha => "ha ha",
                hoho => "ho",
                ignored => "This should not appear in the end result",
        },
        "bar" => {
                template => 1,
                haha => "ah",
                hoho => "haho",
                hehe => "hehe"
        },
        "laughter" => {
                inherit_from => [ "foo", "bar" ],
                hehe => sub { join(" ",(@_,"!!!")) },
                ignored => "",
        }

        The entry for "laughter" will become as follows after processing:

        "laughter" => {
                haha => "ha ha ah",
                hoho => "ho haho",
                hehe => "hehe !!!",
                ignored => ""
        }

[2] OpenSSL is built with threading capabilities unless the user
    specifies 'no-threads'.  The value of the key 'thread_scheme' may
    be "(unknown)", in which case the user MUST give some compilation
    flags to Configure.

[3] OpenSSL has three types of things to link from object files or
    static libraries:

    - shared libraries; that would be libcrypto and libssl.
    - shared objects (sometimes called dynamic libraries);  that would
      be the engines.
    - applications; those are apps/openssl and all the test apps.

    Very roughly speaking, linking is done like this (words in braces
    represent the configuration settings documented at the beginning
    of this file):

    shared libraries:
        {ld} $(CFLAGS) {lflags} {shared_ldflag} -o libfoo.so \
            foo/something.o foo/somethingelse.o {ex_libs}

    shared objects:
        {ld} $(CFLAGS) {lflags} {module_ldflags} -o libeng.so \
            blah1.o blah2.o -lcrypto {ex_libs}

    applications:
        {ld} $(CFLAGS) {lflags} -o app \
            app1.o utils.o -lssl -lcrypto {ex_libs}

[4] There are variants of these attribute, prefixed with `lib_',
    `dso_' or `bin_'.  Those variants replace the unprefixed attribute
    when building library, DSO or program modules specifically.

Historically, the target configurations came in form of a string with
values separated by colons.  This use is deprecated.  The string form
looked like this:

   "target" => "{cc}:{cflags}:{unistd}:{thread_cflag}:{sys_id}:{lflags}:{bn_ops}:{cpuid_obj}:{bn_obj}:{ec_obj}:{des_obj}:{aes_obj}:{bf_obj}:{md5_obj}:{sha1_obj}:{cast_obj}:{rc4_obj}:{rmd160_obj}:{rc5_obj}:{wp_obj}:{cmll_obj}:{modes_obj}:{padlock_obj}:{perlasm_scheme}:{dso_scheme}:{shared_target}:{shared_cflag}:{shared_ldflag}:{shared_extension}:{ranlib}:{arflags}:{multilib}"


Build info files
================

The build.info files that are spread over the source tree contain the
minimum information needed to build and distribute OpenSSL.  It uses a
simple and yet fairly powerful language to determine what needs to be
built, from what sources, and other relationships between files.

For every build.info file, all file references are relative to the
directory of the build.info file for source files, and the
corresponding build directory for built files if the build tree
differs from the source tree.

When processed, every line is processed with the perl module
Text::Template, using the delimiters "{-" and "-}".  The hashes
%config and %target are passed to the perl fragments, along with
$sourcedir and $builddir, which are the locations of the source
directory for the current build.info file and the corresponding build
directory, all relative to the top of the build tree.

To begin with, things to be built are declared by setting specific
variables:

    PROGRAMS=foo bar
    LIBS=libsomething
    ENGINES=libeng
    SCRIPTS=myhack
    EXTRA=file1 file2

Note that the files mentioned for PROGRAMS, LIBS and ENGINES *must* be
without extensions.  The build file templates will figure them out.

For each thing to be built, it is then possible to say what sources
they are built from:

    PROGRAMS=foo bar
    SOURCE[foo]=foo.c common.c
    SOURCE[bar]=bar.c extra.c common.c

It's also possible to tell some other dependencies:

    DEPEND[foo]=libsomething
    DEPEND[libbar]=libsomethingelse

(it could be argued that 'libsomething' and 'libsomethingelse' are
source as well.  However, the files given through SOURCE are expected
to be located in the source tree while files given through DEPEND are
expected to be located in the build tree)

It's also possible to depend on static libraries explicitly:

    DEPEND[foo]=libsomething.a
    DEPEND[libbar]=libsomethingelse.a

This should be rarely used, and care should be taken to make sure it's
only used when supported.  For example, native Windows build doesn't
support building static libraries and DLLs at the same time, so using
static libraries on Windows can only be done when configured
'no-shared'.

One some platforms, shared libraries come with a name that's different
from their static counterpart.  That's declared as follows:

    SHARED_NAME[libfoo]=cygfoo-{- $config{shlibver} -}

The example is from Cygwin, which has a required naming convention.

Sometimes, it makes sense to rename an output file, for example a
library:

    RENAME[libfoo]=libbar

That line has "libfoo" renamed to "libbar".  While it makes no
sense at all to just have a rename like that (why not just use
"libbar" everywhere?), it does make sense when it can be used
conditionally.  See a little further below for an example.

In some cases, it's desirable to include some source files in the
shared form of a library only:

    SHARED_SOURCE[libfoo]=dllmain.c

For any file to be built, it's also possible to tell what extra
include paths the build of their source files should use:

    INCLUDE[foo]=include

In some cases, one might want to generate some source files from
others, that's done as follows:

    GENERATE[foo.s]=asm/something.pl $(CFLAGS)
    GENERATE[bar.s]=asm/bar.S

The value of each GENERATE line is a command line or part of it.
Configure places no rules on the command line, except that the first
item must be the generator file.  It is, however, entirely up to the
build file template to define exactly how those command lines should
be handled, how the output is captured and so on.

Sometimes, the generator file itself depends on other files, for
example if it is a perl script that depends on other perl modules.
This can be expressed using DEPEND like this:

    DEPEND[asm/something.pl]=../perlasm/Foo.pm

There may also be cases where the exact file isn't easily specified,
but an inclusion directory still needs to be specified.  INCLUDE can
be used in that case:

    INCLUDE[asm/something.pl]=../perlasm

NOTE: GENERATE lines are limited to one command only per GENERATE.

As a last resort, it's possible to have raw build file lines, between
BEGINRAW and ENDRAW lines as follows:

    BEGINRAW[Makefile(unix)]
    haha.h: {- $builddir -}/Makefile
        echo "/* haha */" > haha.h
    ENDRAW[Makefile(unix)]

The word within square brackets is the build_file configuration item
or the build_file configuration item followed by the second word in the
build_scheme configuration item for the configured target within
parenthesis as shown above.  For example, with the following relevant
configuration items:

   build_file   => "build.ninja"
   build_scheme => [ "unified", "unix" ]

... these lines will be considered:

   BEGINRAW[build.ninja]
   build haha.h: echo "/* haha */" > haha.h
   ENDRAW[build.ninja]

   BEGINRAW[build.ninja(unix)]
   build hoho.h: echo "/* hoho */" > hoho.h
   ENDRAW[build.ninja(unix)]

Should it be needed because the recipes within a RAW section might
clash with those generated by Configure, it's possible to tell it
not to generate them with the use of OVERRIDES, for example:

    SOURCE[libfoo]=foo.c bar.c
    
    OVERRIDES=bar.o
    BEGINRAW[Makefile(unix)]
    bar.o: bar.c
    	$(CC) $(CFLAGS) -DSPECIAL -c -o $@ $<
    ENDRAW[Makefile(unix)]

See the documentation further up for more information on configuration
items.

Finally, you can have some simple conditional use of the build.info
information, looking like this:

    IF[1]
     something
    ELSIF[2]
     something other
    ELSE
     something else
    ENDIF

The expression in square brackets is interpreted as a string in perl,
and will be seen as true if perl thinks it is, otherwise false.  For
example, the above would have "something" used, since 1 is true.

Together with the use of Text::Template, this can be used as
conditions based on something in the passed variables, for example:

    IF[{- $disabled{shared} -}]
      LIBS=libcrypto
      SOURCE[libcrypto]=...
    ELSE
      LIBS=libfoo
      SOURCE[libfoo]=...
    ENDIF

or:

    # VMS has a cultural standard where all libraries are prefixed.
    # For OpenSSL, the choice is 'ossl_'
    IF[{- $config{target} =~ /^vms/ -}]
     RENAME[libcrypto]=ossl_libcrypto
     RENAME[libssl]=ossl_libssl
    ENDIF


Build-file programming with the "unified" build system
======================================================

"Build files" are called "Makefile" on Unix-like operating systems,
"descrip.mms" for MMS on VMS, "makefile" for nmake on Windows, etc.

To use the "unified" build system, the target configuration needs to
set the three items 'build_scheme', 'build_file' and 'build_command'.
In the rest of this section, we will assume that 'build_scheme' is set
to "unified" (see the configurations documentation above for the
details).

For any name given by 'build_file', the "unified" system expects a
template file in Configurations/ named like the build file, with
".tmpl" appended, or in case of possible ambiguity, a combination of
the second 'build_scheme' list item and the 'build_file' name.  For
example, if 'build_file' is set to "Makefile", the template could be
Configurations/Makefile.tmpl or Configurations/unix-Makefile.tmpl.
In case both Configurations/unix-Makefile.tmpl and
Configurations/Makefile.tmpl are present, the former takes
precedence.

The build-file template is processed with the perl module
Text::Template, using "{-" and "-}" as delimiters that enclose the
perl code fragments that generate configuration-dependent content.
Those perl fragments have access to all the hash variables from
configdata.pem.

The build-file template is expected to define at least the following
perl functions in a perl code fragment enclosed with "{-" and "-}".
They are all expected to return a string with the lines they produce.

    generatesrc - function that produces build file lines to generate
                  a source file from some input.

                  It's called like this:

                        generatesrc(src => "PATH/TO/tobegenerated",
                                    generator => [ "generatingfile", ... ]
                                    generator_incs => [ "INCL/PATH", ... ]
                                    generator_deps => [ "dep1", ... ]
                                    generator => [ "generatingfile", ... ]
                                    incs => [ "INCL/PATH", ... ],
                                    deps => [ "dep1", ... ],
                                    intent => one of "libs", "dso", "bin" );

                  'src' has the name of the file to be generated.
                  'generator' is the command or part of command to
                  generate the file, of which the first item is
                  expected to be the file to generate from.
                  generatesrc() is expected to analyse and figure out
                  exactly how to apply that file and how to capture
                  the result.  'generator_incs' and 'generator_deps'
                  are include directories and files that the generator
                  file itself depends on.  'incs' and 'deps' are
                  include directories and files that are used if $(CC)
                  is used as an intermediary step when generating the
                  end product (the file indicated by 'src').  'intent'
                  indicates what the generated file is going to be
                  used for.

    src2obj     - function that produces build file lines to build an
                  object file from source files and associated data.

                  It's called like this:

                        src2obj(obj => "PATH/TO/objectfile",
                                srcs => [ "PATH/TO/sourcefile", ... ],
                                deps => [ "dep1", ... ],
                                incs => [ "INCL/PATH", ... ]
                                intent => one of "lib", "dso", "bin" );

                  'obj' has the intended object file with '.o'
                  extension, src2obj() is expected to change it to
                  something more suitable for the platform.
                  'srcs' has the list of source files to build the
                  object file, with the first item being the source
                  file that directly corresponds to the object file.
                  'deps' is a list of explicit dependencies.  'incs'
                  is a list of include file directories.  Finally,
                  'intent' indicates what this object file is going
                  to be used for.

    obj2lib     - function that produces build file lines to build a
                  static library file ("libfoo.a" in Unix terms) from
                  object files.

                  called like this:

                        obj2lib(lib => "PATH/TO/libfile",
                                objs => [ "PATH/TO/objectfile", ... ]);

                  'lib' has the intended library file name *without*
                  extension, obj2lib is expected to add that.  'objs'
                  has the list of object files to build this library.

    libobj2shlib - backward compatibility function that's used the
                  same way as obj2shlib (described next), and was
                  expected to build the shared library from the
                  corresponding static library when that was suitable.
                  NOTE: building a shared library from a static
                  library is now DEPRECATED, as they no longer share
                  object files.  Attempting to do this will fail.

    obj2shlib   - function that produces build file lines to build a
                  shareable object library file ("libfoo.so" in Unix
                  terms) from the corresponding object files.

                  called like this:

                        obj2shlib(shlib => "PATH/TO/shlibfile",
                                  lib => "PATH/TO/libfile",
                                  objs => [ "PATH/TO/objectfile", ... ],
                                  deps => [ "PATH/TO/otherlibfile", ... ]);

                  'lib' has the base (static) library ffile name
                  *without* extension.  This is useful in case
                  supporting files are needed (such as import
                  libraries on Windows).
                  'shlib' has the corresponding shared library name
                  *without* extension.  'deps' has the list of other
                  libraries (also *without* extension) this library
                  needs to be linked with.  'objs' has the list of
                  object files to build this library.

    obj2dso     - function that produces build file lines to build a
                  dynamic shared object file from object files.

                  called like this:

                        obj2dso(lib => "PATH/TO/libfile",
                                objs => [ "PATH/TO/objectfile", ... ],
                                deps => [ "PATH/TO/otherlibfile",
                                ... ]);

                  This is almost the same as obj2shlib, but the
                  intent is to build a shareable library that can be
                  loaded in runtime (a "plugin"...).

    obj2bin     - function that produces build file lines to build an
                  executable file from object files.

                  called like this:

                        obj2bin(bin => "PATH/TO/binfile",
                                objs => [ "PATH/TO/objectfile", ... ],
                                deps => [ "PATH/TO/libfile", ... ]);

                  'bin' has the intended executable file name
                  *without* extension, obj2bin is expected to add
                  that.  'objs' has the list of object files to build
                  this library.  'deps' has the list of library files
                  (also *without* extension) that the programs needs
                  to be linked with.

    in2script   - function that produces build file lines to build a
                  script file from some input.

                  called like this:

                        in2script(script => "PATH/TO/scriptfile",
                                  sources => [ "PATH/TO/infile", ... ]);

                  'script' has the intended script file name.
                  'sources' has the list of source files to build the
                  resulting script from.

In all cases, file file paths are relative to the build tree top, and
the build file actions run with the build tree top as current working
directory.

Make sure to end the section with these functions with a string that
you thing is appropriate for the resulting build file.  If nothing
else, end it like this:

      "";       # Make sure no lingering values end up in the Makefile
    -}


Configure helper scripts
========================

Configure uses helper scripts in this directory:

Checker scripts
---------------

These scripts are per platform family, to check the integrity of the
tools used for configuration and building.  The checker script used is
either {build_platform}-{build_file}-checker.pm or
{build_platform}-checker.pm, where {build_platform} is the second
'build_scheme' list element from the configuration target data, and
{build_file} is 'build_file' from the same target data.

If the check succeeds, the script is expected to end with a non-zero
expression.  If the check fails, the script can end with a zero, or
with a `die`.