\input texinfo @c -*-Texinfo-*-
@comment %**start of header
@setfilename mit-scheme-ffi
@set VERSION 1.0
@set UPDATED 2018-06-10
@settitle FFI @value{VERSION}
@comment %**end of header

@ifhtml
@macro myresult{}
 =>
@end macro
@end ifhtml
@ifnothtml
@alias myresult = result
@end ifnothtml

@copying
This manual documents @acronym{FFI} @value{VERSION}.

Copyright @copyright{} 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993,
    1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004,
    2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015,
    2016, 2017, 2018, 2019, 2020 Massachusetts Institute of Technology

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with no Front-Cover Texts and no Back-Cover Texts.
A copy of the license is included in the section entitled ``GNU Free
Documentation License.''
@end quotation
@end copying

@dircategory Programming Languages
@direntry
* MIT/GNU Scheme FFI: (mit-scheme-ffi).
                                Foreign Function Interface
@end direntry

@titlepage
@title The FFI Reference Manual
@subtitle a Foreign Function Interface (version @value{VERSION})
@subtitle for MIT/GNU Scheme version 9.0.1
@subtitle @value{UPDATED}
@author by Matt Birkholz
@end titlepage
@page
@iftex
@contents
@vskip 0pt plus 1filll
@insertcopying
@end iftex

@ifnottex
@node Top, Introduction, (dir), (dir)
@top FFI

@insertcopying
@end ifnottex

@menu
* Introduction:: A synopsis and quick summary.
* C Declarations:: Declare C types and functions.
* Alien Data:: Manipulate C data.
* Alien Functions:: Generate callout trampolines and call them.
* Callbacks:: Generate callback trampolines and register handlers for them.
* Compiling and Linking:: Build and install the shim.
* Hello World:: A short example.
* GNU Free Documentation License::
@end menu


@node Introduction, C Declarations, Top, Top
@chapter Introduction

This FFI provides Scheme syntax for calling C functions and accessing
C data.  The functions and data structures are declared in a case
sensitive @file{.cdecl} file, which is used by a shim generator to
produce callout and callback trampoline functions.  The trampolines
are compiled and linked to the C toolkit, producing a shared object
that Scheme can dynamically load.

@heading Synopsis

Examples of the new syntax:

@example
(C-include "prhello")

@group
(malloc (C-sizeof "GdkEvent"))
@myresult{} #[alien 42 0x081afc60]
@end group

(C->= @verb{"#@42"} "GdkEvent any type" 14)

@group
(C-> @verb{"#@42"} "GdkEvent any type")
@myresult{} 14
@end group

@group
(C-enum "GdkEventType" 14)
@myresult{} |GDK_MAP|
@end group

@group
(C-enum "GDK_MAP")
@myresult{} 14
@end group

@group
(C-sizeof "GdkColor")
@myresult{} 12
@end group

@group
(C-offset "GdkColor blue")
@myresult{} 8
@end group

@group
(C-array-loc @verb{"#@43"} "GdkColor" (C-enum "GTK_STATE_NORMAL"))
@myresult{} #[alien 44 0x081afc60] @r{; New alien.}
@end group

@group
(C-array-loc! @verb{"#@43"} "GdkColor" (C-enum "GTK_STATE_PRELIGHT"))
@myresult{} #[alien 43 0x081afc78] @r{; Modified alien.}
@end group

@group
(C-call "gtk_window_new" retval args @dots{})
@myresult{} #!unspecific
@end group

@group
(C-callback "delete_event")
@myresult{} #[alien-function 44 Scm_delete_event]
@end group

@group
(C-callback (lambda (window event) @dots{}))
@myresult{} 13                     @r{; A fixnum registration ID.}
@end group
@end example
@comment The C-array-loc! example assumes 2 GdkColors are 6 words, #x18bytes.
@comment 0x081afc78 - 0x081afc60 = 0x18

@heading Summary

A Scheme-like declaration of a toolkit's C functions, constants, and
data types is given in a case sensitive @file{.cdecl} file.  The C
declarations look like this:

@smallexample
(extern (* GtkWidget)
        gtk_window_new
        (type GtkWindowType))

(typedef GtkWindowType
         (enum
          (GTK_WINDOW_TOPLEVEL)
          (GTK_WINDOW_POPUP)))
@end smallexample

The @strong{@code{c-generate}} procedure reads these declarations and
writes three files: @file{@i{library}-types.bin} (a fasdump of the
parsed declarations), @file{@i{library}-const.c} (a C program that
prints C constants and struct offsets), and @file{@i{library}-shim.c}
(trampoline functions adapting Scheme procedure application to C
function call).  The @file{-const.c} program generates a
@file{-const.scm} file, which can be expanded into a
@file{-const.bin} file.

@smallexample
(load-option 'FFI)
(c-generate "prhello" "#include <gtk/gtk.h>")
@end smallexample

The @file{-types.bin} and @file{-const.bin} files together provide
the information needed to expand @code{C-...} syntax, and are only
needed at syntax time.  The compiled @file{-shim.so} file is used at
run time, dynamically loaded into the Scheme machine.  @xref{Compiling
and Linking}, which describes these files in more detail, and shows
how they might be built and installed.

@smallexample
(C-include "prhello")
@end smallexample

The @strong{@code{C-include}} syntax loads the @file{-types.bin} and
@file{-const.bin} files @emph{at syntax time}.  It should appear
at the top level of any file containing @code{C-...} syntax, or be
expanded in the syntax environment of such code.

The @strong{@code{C-call}} syntax arranges to invoke a callout
trampoline.  Arguments to the trampoline can be integers, floats,
strings (or bytevectors) or aliens (non-heap pointers, to C data
structures, @pxref{Alien Data}).  If a string argument might contain
non-ASCII characters (code points U+0080 and larger), it should be
converted to a bytevector e.g.@: by @code{string->utf8}, else an error
could be signaled.

@smallexample
(let ((alien (make-alien '|GtkWidget|)))
  (C-call "gtk_window_new" alien type)
  (if (alien-null? alien) (error "could not open new window"))
  alien)
@end smallexample

The @strong{@code{C-callback}} syntax is used when registering a
Scheme callback trampoline.  The two forms of the syntax provide two
arguments for the registration function: the callback trampoline's
address, and a ``user data'' argument.  When the toolkit calls the
trampoline, it must provide the fixnum-sized user data as an argument.

@smallexample
(C-call "g_signal_connect" window "delete_event"
        (C-callback "delete_event")     ; @r{@i{e.g. &Scm_delete_event}}
        (C-callback                     ; @r{@i{e.g. 314}}
          (lambda (window event)
            (C-call "gtk_widget_destroy" window)
            0)))
@end smallexample

The first use of @code{C-callback} (above) expands into a callback
trampoline address --- an alien function.  The second use evaluates to
a fixnum, which is associated with the given Scheme procedure.

The @strong{@code{C->}} and @strong{@code{C->=}} syntaxes peek and
poke values into alien data structures.  They take an alien and a
constant string specifying the alien data type and the member to be
accessed (if any).

@smallexample
@group
(C-> alien "GdkRectangle y")
@expansion{}
(#[primitive c-peek-int] alien 4)
@end group
@c (param:pp-primitives-by-name? #f) to display primitives thuswise.

@group
(C->= alien "GdkRectangle width" 0)
@expansion{}
(#[primitive c-poke-int] alien 8 0)
@end group
@end smallexample

@smallexample
@group
(C-> alien "GdkEvent any type")
@expansion{}
(#[primitive c-peek-int] alien 0)
@end group
@end smallexample

@smallexample
@group
(C-> alien "gfloat")
@expansion{}
(#[primitive c-peek-float] alien 0)
@end group
@end smallexample

A three argument form of the syntax provides an alien to receive a
peeked pointer.  This avoids consing a new alien.

@smallexample
(C-> alien "GtkWidget style" alien)
@end smallexample

The above syntax is understood to say ``The data at this @code{alien}
address is a GtkWidget.  Load its @code{style} member into
@code{alien} (clobbering @code{alien}'s old address).''

The @strong{@code{C-enum}}, @strong{@code{C-sizeof}} and
@strong{@code{C-offset}} syntaxes all
transform into integer constants.  The last two transform into a padded
byte size and a byte offset respectively.

@smallexample
@group
(C-enum "GTK_WINDOW_POPUP")
@expansion{}
1
@end group

@group
(C-sizeof "GdkColor")
@expansion{}
12
@end group

@group
(C-offset "GdkColor blue")
@expansion{}
8
@end group
@end smallexample

The two element form of the @code{C-enum} syntax can be used to find
the name of a constant given its runtime value.  It expects the name
of an enum type in a constant string.  If the runtime (second)
argument is not one of the constants declared by that type, the
returned value is @code{#f}.

@smallexample
@group
(C-enum "GdkEventType" (C-> @verb{"#@42"} "GdkEvent any type"))
@myresult{} |GDK_MAP|
@end group
@end smallexample

The @strong{@code{c-array-loc}} and @strong{@code{c-array-loc!}}
syntaxes compute the locations of C array elements.  They can be used
to advance a scan pointer or locate an element by its index.  The
examples in the synopsis might expand as shown here.

@smallexample
(C-array-loc @verb{"#@43"} "GdkColor" (C-enum "GTK_STATE_NORMAL"))
@expansion{}
(alien-byte-increment @verb{"#@43"} (* (C-sizeof "GdkColor")
                              (C-enum "GTK_STATE_NORMAL")))
@expansion{}
(alien-byte-increment @verb{"#@43"} (* 12 0))
@myresult{} @verb{"#@44"}

(C-array-loc! @verb{"#@43"} "GdkColor" (C-enum "GTK_STATE_PRELIGHT"))
@expansion{}
(alien-byte-increment! @verb{"#@43"} (* (C-sizeof "GdkColor")
                               (C-enum "GTK_STATE_PRELIGHT")))
@expansion{}
(alien-byte-increment! @verb{"#@43"} (* 12 2))
@myresult{} @verb{"#@43"}
@end smallexample

A simple scan of characters in the wide string @code{alien} might
look like this.

@smallexample
(let ((len (C-> alien "toolkit_string_type int_member"))
      (scan (C-> alien "toolkit_string_type array_member")))
  (let loop ((n 0))
    (if (< n len)
        (let ((wchar (C-> scan "wchar")))
          (process wchar)
          (C-array-loc! scan "wchar" 1)
          (loop (1+ n))))))
@end smallexample

That is a quick look at the facilities.  The next section describes
the C declaration language, and the following sections examine the FFI's
syntax and runtime facilities in detail.  Final sections provide an
example program and show how its dynamically loaded shim is built.


@node C Declarations, Alien Data, Introduction, Top
@chapter C Declarations

A shim between Scheme and a C toolkit is specified by a case sensitive
@file{.cdecl} file containing Scheme-like declarations of all relevant
toolkit types, constants, and functions.  Callback functions to be
passed to the toolkit are also specified here.

Each top-level form in the C declaration file must look like one of
these:

@smallexample
(include "filename")
(typedef Name @var{any})
(struct Name (Member @var{type}) @dots{})
(union Name (Member @var{type}) @dots{})
(enum @i{Name} (Member) @dots{})
(extern @var{function-type} Name (param1 @var{arg-type}) @dots{})
(callback @var{callback-type} Name (param1 @var{callback-arg-type}) @dots{})
@end smallexample

The @code{include} expression includes another @file{.cdecl} file in
the current @file{.cdecl} file.  The string argument is interpreted
relative to the current file's directory.

@var{any} can be a @var{type} or the word @code{void}.

@var{arg-type} can be any @var{type} @emph{except} anonymous structs
and unions.

@var{function-type} can be any @var{arg-type} or @code{void}.

@var{callback-arg-type} can be any @var{type} @emph{except} struct and
union types.

@var{callback-type} can be any @var{callback-arg-type} or @code{void}.

@var{type} can look like any of these:

@smallexample
Name
@var{basics}
(* @var{any})
(enum Name)
(enum @i{Name} (Member) @dots{})
(struct Name)
(struct @i{Name} (Member @var{type}) @dots{})
(union Name)
(union @i{Name} (Member @var{type}) @dots{})
@end smallexample

@var{Name} should be defined via a @code{typedef} form somewhere in
the (included) file(s).  It does not have to be defined before it is
referenced.  It does not have to be defined @emph{at all} if it is
only the target of a pointer type.

@var{basics} can be any of the words: @code{char}, @code{uchar},
@code{short}, @code{ushort}, @code{int}, @code{uint}, @code{long},
@code{ulong}, @code{float}, or @code{double} (all lowercase).

While the informal grammar above allows anonymous structs to be member
types, they are useless outside a named type declaration.  The peek
and poke (@code{C->} and @code{C->=}) syntaxes require a type name
(e.g.@: @code{"GdkEventAny"} or @code{"struct _GdkEventAny"}) before
any member names.

@smallexample
(C-include "prhello")
@end smallexample

The @strong{@code{C-include}} syntax takes a library name and loads
the corresponding @file{-types} and @file{-const} files at syntax
time.  This makes the C types and constants available to the other
@code{C-...} syntax expanders.  The form binds @code{c-includes} in
the syntax environment @i{unless} it is already defined there.  Thus a
@code{(C-include "library")} form can be placed at the top of every
file with @code{C-...} syntax, @emph{or} loaded into the syntax-time
environment of those files.


@node Alien Data, Alien Functions, C Declarations, Top
@chapter Alien Data

A C data structure is represented by an alien containing the data
structure's memory address.  ``Peek'' primitives are available to read
pointers and the basic C types (e.g. ints, floats) at small (fixnum)
offsets from an alien's address.  They return to Scheme an alien
address, integer or flonum as appropriate.  ``Poke'' primitives
do the reverse, storing pointers, integers or floats at fixnum offsets
from alien addresses.
Other procedures on aliens are @code{alien?},
@code{alien-null?}, @code{alien-null!}, @code{copy-alien}, @code{alien=?},
@code{alien-byte-increment}, and @code{c-peek-cstring}.  Refer to
@file{ffi.pkg} in The Source for a complete list.

The @code{C->} and @code{C->=} syntaxes apply the peek and poke
primitives to constant offsets.  They expect their first argument
subform to be a constant string --- space-separated words naming a C
type and any member to be accessed.  A member within a struct or union
member is specified by appending its name.  For example @code{"struct
_GdkEvent any window"} would specify a peek at the @code{window}
member of the @code{any} member of the @code{struct _GdkEvent} data at
some alien address.  Note that the final member's type must be a basic
C type, pointer type, or enum type.  Otherwise, an error is signaled
at syntax time.

@smallexample
@group
(C-> alien "struct _GdkEvent any window" window-alien)
@expansion{}
(#[primitive c-peek-pointer] alien 0 window-alien)
@myresult{} #[alien 44 (* GdkWindow) 0x081afc60]
@end group
@end smallexample

Note that in the example above, the final member has a pointer type.
In this case an extra alien argument can be provided to receive the
peeked pointer.  Otherwise a new alien is created and returned.

@heading Malloc

The @code{malloc} procedure returns an alien that will automatically
free the malloced memory when it is garbage collected.
It can also be explicitly freed with the @code{free} procedure.  The alien
address can be incremented to scan the malloced memory, then freed
(without returning it to the original, malloced address).  A band
restore marks all malloced aliens as though they have been freed.

@smallexample
(free (malloc '|GdkRectangle|))
@end smallexample


@node Alien Functions, Callbacks, Alien Data, Top
@chapter Alien Functions

The @code{C-call} syntax produces code that applies @code{call-alien}
to an alien function structure --- a cache for the callout
trampoline's entry address.

@smallexample
@group
(C-call "gtk_button_new" (make-alien '(* |GtkWidget|)))
@expansion{}
(call-alien '#[alien-function gtk_button_new] (make-alien @dots{}))
@end group
@end smallexample

The alien function contains all the information needed to load the
callout trampoline on demand (i.e. its name and library).  Once the
alien function has cached the entry address, @code{call-alien} can
invoke the trampoline (via @code{#[primitive c-call]}).  The
trampoline gets its arguments off the Scheme stack, converts them to C
values, calls the C function, conses a result, and returns it to
Scheme.

A function returning a pointer type is treated specially.
Its trampoline expects an extra (first) argument.
If the argument is @code{#f}, the return value is ignored.
If the argument is an alien, the function's return value clobbers the
alien's address.  This makes it easy to grab pointers to toolkit
resources without dropping them, and to avoid unnecessary consing of
aliens.

A function returning a struct or union type is treated similarly.
Its trampoline expects an extra (first) argument.
If the argument is @code{#f}, the return value is ignored.
If the argument is an alien, the returned struct or union is copied to
that address.

Struct and union type parameters of a function are treated similarly.
The function's trampoline expects an alien argument for each such
parameter and copies the struct or union from the argument address
into a local variable.  Callbacks currently cannot receive struct or
union type arguments, though they @emph{can} receive pointer type
arguments (consing an alien for each).

The @code{alien-function} structures are fasdumpable.  The caching
mechanism invalidates the cache when a band is restored, or a
fasdumped object is fasloaded.  The alien function will lookup the
trampoline entry point again on demand.


@node Callbacks, Compiling and Linking, Alien Functions, Top
@chapter Callbacks

A callback declaration must include a parameter named ``ID''.  The ID
argument will be used to find the Scheme callback procedure.  It must
be the same ``user data'' value provided to the toolkit when the
callback was registered.  For example, a callback trampoline named
@code{Scm_delete_event} might be declared like this:

@smallexample
(callback gint
          delete_event
          (window (* GtkWidget))
          (event (* GdkEventAny))
          (ID gpointer))
@end smallexample

The callback might be registered with the toolkit like this:

@smallexample
(C-call "g_signal_connect" window "delete_event"
        (C-callback "delete_event")     ; @r{@i{e.g. &Scm_delete_event}}
        (C-callback                     ; @r{@i{e.g. 314}}
          (lambda (window event)
            (C-call "gtk_widget_destroy" window)
            0)))
@end smallexample

The toolkit's registration function, @code{g_signal_connect}, would be
declared like this:

@smallexample
(extern void
        g_signal_connect
        (object (* GtkObject))
        (name (* gchar))
        (CALLBACK GtkSignalFunc)
        (ID gpointer))
@end smallexample

This function should have parameters named @code{CALLBACK} and
@code{ID}.  The callout trampoline will convert the callback argument
from a Scheme alien function to an entry address.  The @code{ID} argument
will be converted to a C integer and then cast to its declared type
(in this example, @code{gpointer}).

Note that the registered callback procedures are effectively pinned.
They cannot be garbage collected.  They are ``on call'' to handle
callbacks from the toolkit until they are explicitly de-registered.  A
band restore automatically de-registers all callbacks.

Callback procedures are executed with thread preemption suspended.
Thus Scheme will not switch to another thread, especially not one
preempted in an earlier callback.  Such a thread could finish its
callback and return from the later callback, not to its original
caller.

Scheme will not preempt a callback, but if the callback calls
@code{suspend-thread} it will switch to a running thread.  If the
callback does IO (and blocks), suspends, yields, sleeps, or grabs
(waits for) a mutex, the runtime system will switch to another thread,
possibly a thread that blocked for IO during an earlier callback but
is now recently unblocked and determined to finish and return to the
wrong caller.  Thus callback procedures should be written as if they
were interrupt handlers.  They should be short and simple because they
must not wait.

@c Something clever with a Scheme-side model of the C stack and some
@c thread-join operations might provide the necessary guarantee (LIFO
@c calls/returns across threads) so that callbacks can wait (for a
@c mutex or IO or just for the C stack to unwind enough).

@c Doing that for a multi-processing world "just" requires threads to
@c wait for (and be scheduled for) a particular processor (a
@c particular C stack).  Such an exercise is left to the poor soul who
@c could not figure out how to avoid the problem in the first place.

The @code{outf-error} procedure is provided for debugging purposes.
It writes one or more argument strings (and @code{write}s any
non-strings) to the Unix ``stderr'' channel, atomically, via a machine
primitive, bypassing the runtime's IO buffering and thread switching.
Thus trace messages from multiple threads will appear on stderr
intact, uninterrupted.


@node Compiling and Linking, Hello World, Callbacks, Top
@chapter Compiling and Linking

The @strong{@code{c-generate}} procedure takes a library name and an
optional preamble.  It reads the @file{@i{library}.cdecl} file and
writes two @file{.c} files.  The preamble is included at the top of
both.  It typically contains @code{#include} C pre-processor
directives required by the C library, but could include additional
shim code.  Here is a short script that generates a shim for the
example ``Hello, World!'' program.

@smallexample
(load-option 'FFI)
(c-generate "prhello" "#include <gtk/gtk.h>")
@end smallexample

This script will produce three files:

@table @file

@item prhello-shim.c
This file contains the trampoline functions --- one for each declared
C extern or callback.  It includes the @file{mit-scheme.h} header
file, found in the @code{AUXDIR} directory ---
e.g. @file{/usr/local/lib/mit-scheme-i386/}.

@item prhello-const.c
This file contains a C program that creates
@file{prhello-const.scm}.  It is compiled and linked
as normal for programs using the toolkit, and does not depend on the
Scheme machine.  It does not actually call any
toolkit functions.  It just collects information from the compiler
about the declared C types and constants.

@item prhello-types.bin
This file is a fasdumped @code{c-includes} structure containing all of
the types, constants and functions declared in the @file{.cdecl} file.

@end table

The following Makefile rules describe the process of building and
installing a shim for the example ``Hello, World!'' program.

@example
@verbatiminclude prhello-Makefile
@end example

The FFI also supports libraries created by GNU automake (libtool).
The source distribution includes several simple plugins.  Each uses a
portable Makefile.am to build and install its shared object.


@node Hello World, GNU Free Documentation License, Compiling and Linking, Top
@chapter Hello World

This
@iftex
chapter
@end iftex
@ifnottex
node
@end ifnottex
includes the C declarations and Scheme code required to implement
Havoc Pennington's Hello World example from
@uref{http://developer.gnome.org/doc/GGAD/, GGAD}.  For an extra,
Schemely treat, its @code{delete_event} callback is a Scheme procedure
closed over a binding of @code{counter} that is used to implement some
impertinent behavior.
@example
@verbatiminclude prhello.scm
@end example

Here are the C declarations.
@example
@verbatiminclude prhello.cdecl
@end example


@node GNU Free Documentation License, , Hello World, Top
@appendix GNU Free Documentation License

@center Version 1.2, November 2002

@display
Copyright @copyright{} 2000,2001,2002 Free Software Foundation, Inc.
51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
@end display

@enumerate 0
@item
PREAMBLE

The purpose of this License is to make a manual, textbook, or other
functional and useful document @dfn{free} in the sense of freedom: to
assure everyone the effective freedom to copy and redistribute it,
with or without modifying it, either commercially or noncommercially.
Secondarily, this License preserves for the author and publisher a way
to get credit for their work, while not being considered responsible
for modifications made by others.

This License is a kind of ``copyleft'', which means that derivative
works of the document must themselves be free in the same sense.  It
complements the GNU General Public License, which is a copyleft
license designed for free software.

We have designed this License in order to use it for manuals for free
software, because free software needs free documentation: a free
program should come with manuals providing the same freedoms that the
software does.  But this License is not limited to software manuals;
it can be used for any textual work, regardless of subject matter or
whether it is published as a printed book.  We recommend this License
principally for works whose purpose is instruction or reference.

@item
APPLICABILITY AND DEFINITIONS

This License applies to any manual or other work, in any medium, that
contains a notice placed by the copyright holder saying it can be
distributed under the terms of this License.  Such a notice grants a
world-wide, royalty-free license, unlimited in duration, to use that
work under the conditions stated herein.  The ``Document'', below,
refers to any such manual or work.  Any member of the public is a
licensee, and is addressed as ``you''.  You accept the license if you
copy, modify or distribute the work in a way requiring permission
under copyright law.

A ``Modified Version'' of the Document means any work containing the
Document or a portion of it, either copied verbatim, or with
modifications and/or translated into another language.

A ``Secondary Section'' is a named appendix or a front-matter section
of the Document that deals exclusively with the relationship of the
publishers or authors of the Document to the Document's overall
subject (or to related matters) and contains nothing that could fall
directly within that overall subject.  (Thus, if the Document is in
part a textbook of mathematics, a Secondary Section may not explain
any mathematics.)  The relationship could be a matter of historical
connection with the subject or with related matters, or of legal,
commercial, philosophical, ethical or political position regarding
them.

The ``Invariant Sections'' are certain Secondary Sections whose titles
are designated, as being those of Invariant Sections, in the notice
that says that the Document is released under this License.  If a
section does not fit the above definition of Secondary then it is not
allowed to be designated as Invariant.  The Document may contain zero
Invariant Sections.  If the Document does not identify any Invariant
Sections then there are none.

The ``Cover Texts'' are certain short passages of text that are listed,
as Front-Cover Texts or Back-Cover Texts, in the notice that says that
the Document is released under this License.  A Front-Cover Text may
be at most 5 words, and a Back-Cover Text may be at most 25 words.

A ``Transparent'' copy of the Document means a machine-readable copy,
represented in a format whose specification is available to the
general public, that is suitable for revising the document
straightforwardly with generic text editors or (for images composed of
pixels) generic paint programs or (for drawings) some widely available
drawing editor, and that is suitable for input to text formatters or
for automatic translation to a variety of formats suitable for input
to text formatters.  A copy made in an otherwise Transparent file
format whose markup, or absence of markup, has been arranged to thwart
or discourage subsequent modification by readers is not Transparent.
An image format is not Transparent if used for any substantial amount
of text.  A copy that is not ``Transparent'' is called ``Opaque''.

Examples of suitable formats for Transparent copies include plain
@sc{ascii} without markup, Texinfo input format, La@TeX{} input
format, @acronym{SGML} or @acronym{XML} using a publicly available
@acronym{DTD}, and standard-conforming simple @acronym{HTML},
PostScript or @acronym{PDF} designed for human modification.  Examples
of transparent image formats include @acronym{PNG}, @acronym{XCF} and
@acronym{JPG}.  Opaque formats include proprietary formats that can be
read and edited only by proprietary word processors, @acronym{SGML} or
@acronym{XML} for which the @acronym{DTD} and/or processing tools are
not generally available, and the machine-generated @acronym{HTML},
PostScript or @acronym{PDF} produced by some word processors for
output purposes only.

The ``Title Page'' means, for a printed book, the title page itself,
plus such following pages as are needed to hold, legibly, the material
this License requires to appear in the title page.  For works in
formats which do not have any title page as such, ``Title Page'' means
the text near the most prominent appearance of the work's title,
preceding the beginning of the body of the text.

A section ``Entitled XYZ'' means a named subunit of the Document whose
title either is precisely XYZ or contains XYZ in parentheses following
text that translates XYZ in another language.  (Here XYZ stands for a
specific section name mentioned below, such as ``Acknowledgements'',
``Dedications'', ``Endorsements'', or ``History''.)  To ``Preserve the Title''
of such a section when you modify the Document means that it remains a
section ``Entitled XYZ'' according to this definition.

The Document may include Warranty Disclaimers next to the notice which
states that this License applies to the Document.  These Warranty
Disclaimers are considered to be included by reference in this
License, but only as regards disclaiming warranties: any other
implication that these Warranty Disclaimers may have is void and has
no effect on the meaning of this License.

@item
VERBATIM COPYING

You may copy and distribute the Document in any medium, either
commercially or noncommercially, provided that this License, the
copyright notices, and the license notice saying this License applies
to the Document are reproduced in all copies, and that you add no other
conditions whatsoever to those of this License.  You may not use
technical measures to obstruct or control the reading or further
copying of the copies you make or distribute.  However, you may accept
compensation in exchange for copies.  If you distribute a large enough
number of copies you must also follow the conditions in section 3.

You may also lend copies, under the same conditions stated above, and
you may publicly display copies.

@item
COPYING IN QUANTITY

If you publish printed copies (or copies in media that commonly have
printed covers) of the Document, numbering more than 100, and the
Document's license notice requires Cover Texts, you must enclose the
copies in covers that carry, clearly and legibly, all these Cover
Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
the back cover.  Both covers must also clearly and legibly identify
you as the publisher of these copies.  The front cover must present
the full title with all words of the title equally prominent and
visible.  You may add other material on the covers in addition.
Copying with changes limited to the covers, as long as they preserve
the title of the Document and satisfy these conditions, can be treated
as verbatim copying in other respects.

If the required texts for either cover are too voluminous to fit
legibly, you should put the first ones listed (as many as fit
reasonably) on the actual cover, and continue the rest onto adjacent
pages.

If you publish or distribute Opaque copies of the Document numbering
more than 100, you must either include a machine-readable Transparent
copy along with each Opaque copy, or state in or with each Opaque copy
a computer-network location from which the general network-using
public has access to download using public-standard network protocols
a complete Transparent copy of the Document, free of added material.
If you use the latter option, you must take reasonably prudent steps,
when you begin distribution of Opaque copies in quantity, to ensure
that this Transparent copy will remain thus accessible at the stated
location until at least one year after the last time you distribute an
Opaque copy (directly or through your agents or retailers) of that
edition to the public.

It is requested, but not required, that you contact the authors of the
Document well before redistributing any large number of copies, to give
them a chance to provide you with an updated version of the Document.

@item
MODIFICATIONS

You may copy and distribute a Modified Version of the Document under
the conditions of sections 2 and 3 above, provided that you release
the Modified Version under precisely this License, with the Modified
Version filling the role of the Document, thus licensing distribution
and modification of the Modified Version to whoever possesses a copy
of it.  In addition, you must do these things in the Modified Version:

@enumerate A
@item
Use in the Title Page (and on the covers, if any) a title distinct
from that of the Document, and from those of previous versions
(which should, if there were any, be listed in the History section
of the Document).  You may use the same title as a previous version
if the original publisher of that version gives permission.

@item
List on the Title Page, as authors, one or more persons or entities
responsible for authorship of the modifications in the Modified
Version, together with at least five of the principal authors of the
Document (all of its principal authors, if it has fewer than five),
unless they release you from this requirement.

@item
State on the Title page the name of the publisher of the
Modified Version, as the publisher.

@item
Preserve all the copyright notices of the Document.

@item
Add an appropriate copyright notice for your modifications
adjacent to the other copyright notices.

@item
Include, immediately after the copyright notices, a license notice
giving the public permission to use the Modified Version under the
terms of this License, in the form shown in the Addendum below.

@item
Preserve in that license notice the full lists of Invariant Sections
and required Cover Texts given in the Document's license notice.

@item
Include an unaltered copy of this License.

@item
Preserve the section Entitled ``History'', Preserve its Title, and add
to it an item stating at least the title, year, new authors, and
publisher of the Modified Version as given on the Title Page.  If
there is no section Entitled ``History'' in the Document, create one
stating the title, year, authors, and publisher of the Document as
given on its Title Page, then add an item describing the Modified
Version as stated in the previous sentence.

@item
Preserve the network location, if any, given in the Document for
public access to a Transparent copy of the Document, and likewise
the network locations given in the Document for previous versions
it was based on.  These may be placed in the ``History'' section.
You may omit a network location for a work that was published at
least four years before the Document itself, or if the original
publisher of the version it refers to gives permission.

@item
For any section Entitled ``Acknowledgements'' or ``Dedications'', Preserve
the Title of the section, and preserve in the section all the
substance and tone of each of the contributor acknowledgements and/or
dedications given therein.

@item
Preserve all the Invariant Sections of the Document,
unaltered in their text and in their titles.  Section numbers
or the equivalent are not considered part of the section titles.

@item
Delete any section Entitled ``Endorsements''.  Such a section
may not be included in the Modified Version.

@item
Do not retitle any existing section to be Entitled ``Endorsements'' or
to conflict in title with any Invariant Section.

@item
Preserve any Warranty Disclaimers.
@end enumerate

If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no material
copied from the Document, you may at your option designate some or all
of these sections as invariant.  To do this, add their titles to the
list of Invariant Sections in the Modified Version's license notice.
These titles must be distinct from any other section titles.

You may add a section Entitled ``Endorsements'', provided it contains
nothing but endorsements of your Modified Version by various
parties---for example, statements of peer review or that the text has
been approved by an organization as the authoritative definition of a
standard.

You may add a passage of up to five words as a Front-Cover Text, and a
passage of up to 25 words as a Back-Cover Text, to the end of the list
of Cover Texts in the Modified Version.  Only one passage of
Front-Cover Text and one of Back-Cover Text may be added by (or
through arrangements made by) any one entity.  If the Document already
includes a cover text for the same cover, previously added by you or
by arrangement made by the same entity you are acting on behalf of,
you may not add another; but you may replace the old one, on explicit
permission from the previous publisher that added the old one.

The author(s) and publisher(s) of the Document do not by this License
give permission to use their names for publicity for or to assert or
imply endorsement of any Modified Version.

@item
COMBINING DOCUMENTS

You may combine the Document with other documents released under this
License, under the terms defined in section 4 above for modified
versions, provided that you include in the combination all of the
Invariant Sections of all of the original documents, unmodified, and
list them all as Invariant Sections of your combined work in its
license notice, and that you preserve all their Warranty Disclaimers.

The combined work need only contain one copy of this License, and
multiple identical Invariant Sections may be replaced with a single
copy.  If there are multiple Invariant Sections with the same name but
different contents, make the title of each such section unique by
adding at the end of it, in parentheses, the name of the original
author or publisher of that section if known, or else a unique number.
Make the same adjustment to the section titles in the list of
Invariant Sections in the license notice of the combined work.

In the combination, you must combine any sections Entitled ``History''
in the various original documents, forming one section Entitled
``History''; likewise combine any sections Entitled ``Acknowledgements'',
and any sections Entitled ``Dedications''.  You must delete all
sections Entitled ``Endorsements.''

@item
COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other documents
released under this License, and replace the individual copies of this
License in the various documents with a single copy that is included in
the collection, provided that you follow the rules of this License for
verbatim copying of each of the documents in all other respects.

You may extract a single document from such a collection, and distribute
it individually under this License, provided you insert a copy of this
License into the extracted document, and follow this License in all
other respects regarding verbatim copying of that document.

@item
AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other separate
and independent documents or works, in or on a volume of a storage or
distribution medium, is called an ``aggregate'' if the copyright
resulting from the compilation is not used to limit the legal rights
of the compilation's users beyond what the individual works permit.
When the Document is included an aggregate, this License does not
apply to the other works in the aggregate which are not themselves
derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these
copies of the Document, then if the Document is less than one half of
the entire aggregate, the Document's Cover Texts may be placed on
covers that bracket the Document within the aggregate, or the
electronic equivalent of covers if the Document is in electronic form.
Otherwise they must appear on printed covers that bracket the whole
aggregate.

@item
TRANSLATION

Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section 4.
Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
translations of some or all Invariant Sections in addition to the
original versions of these Invariant Sections.  You may include a
translation of this License, and all the license notices in the
Document, and any Warrany Disclaimers, provided that you also include
the original English version of this License and the original versions
of those notices and disclaimers.  In case of a disagreement between
the translation and the original version of this License or a notice
or disclaimer, the original version will prevail.

If a section in the Document is Entitled ``Acknowledgements'',
``Dedications'', or ``History'', the requirement (section 4) to Preserve
its Title (section 1) will typically require changing the actual
title.

@item
TERMINATION

You may not copy, modify, sublicense, or distribute the Document except
as expressly provided for under this License.  Any other attempt to
copy, modify, sublicense or distribute the Document is void, and will
automatically terminate your rights under this License.  However,
parties who have received copies, or rights, from you under this
License will not have their licenses terminated so long as such
parties remain in full compliance.

@item
FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions
of the GNU Free Documentation License from time to time.  Such new
versions will be similar in spirit to the present version, but may
differ in detail to address new problems or concerns.  See
@uref{http://www.gnu.org/copyleft/}.

Each version of the License is given a distinguishing version number.
If the Document specifies that a particular numbered version of this
License ``or any later version'' applies to it, you have the option of
following the terms and conditions either of that specified version or
of any later version that has been published (not as a draft) by the
Free Software Foundation.  If the Document does not specify a version
number of this License, you may choose any version ever published (not
as a draft) by the Free Software Foundation.
@end enumerate

@page
@appendixsec ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and
license notices just after the title page:

@smallexample
@group
  Copyright (C)  @var{year}  @var{your name}.
  Permission is granted to copy, distribute and/or modify this document
  under the terms of the GNU Free Documentation License, Version 1.2
  or any later version published by the Free Software Foundation;
  with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
  A copy of the license is included in the section entitled ``GNU
  Free Documentation License''.
@end group
@end smallexample

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts,
replace the ``with...Texts.'' line with this:

@smallexample
@group
    with the Invariant Sections being @var{list their titles}, with
    the Front-Cover Texts being @var{list}, and with the Back-Cover Texts
    being @var{list}.
@end group
@end smallexample

If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of
free software license, such as the GNU General Public License,
to permit their use in free software.
@bye
