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LDINT(7)           FreeBSD Miscellaneous Information Manual           LDINT(7)

NAME
     ldint - GNU Linker Internals This file documents the internals of the GNU
     linker ld.  It is a collection of miscellaneous information with little
     form at this point.  Mostly, it is a repository into which you can put
     information about GNU ld as you discover it (or as you design changes to
     ld).

     This document is distributed under the terms of the GNU Free
     Documentation License. A copy of the license is included in the section
     entitled "GNU Free Documentation License".

The README File
     Check the README file; it often has useful information that does not
     appear anywhere else in the directory.

How linker emulations are generated
     Each linker target has an emulation.  The emulation includes the default
     linker script, and certain emulations also modify certain types of linker
     behaviour.

     Emulations are created during the build process by the shell script
     genscripts.sh.

     The genscripts.sh script starts by reading a file in the emulparams
     directory. This is a shell script which sets various shell variables used
     by genscripts.sh and the other shell scripts it invokes.

     The genscripts.sh script will invoke a shell script in the scripttempl
     directory in order to create default linker scripts written in the linker
     command language. The scripttempl script will be invoked 5 (or, in some
     cases, 6) times, with different assignments to shell variables, to create
     different default scripts. The choice of script is made based on the
     command line options.

     After creating the scripts, genscripts.sh will invoke yet another shell
     script, this time in the emultempl directory. That shell script will
     create the emulation source file, which contains C code. This C code
     permits the linker emulation to override various linker behaviours. Most
     targets use the generic emulation code, which is in emultempl/generic.em.

     To summarize, genscripts.sh reads three shell scripts: an emulation
     parameters script in the emulparams directory, a linker script generation
     script in the scripttempl directory, and an emulation source file
     generation script in the emultempl directory.

     For example, the Sun 4 linker sets up variables in emulparams/sun4.sh,
     creates linker scripts using scripttempl/aout.sc, and creates the
     emulation code using emultempl/sunos.em.

     Note that the linker can support several emulations simultaneously,
     depending upon how it is configured. An emulation can be selected with
     the -m option. The -V option will list all supported emulations.

   emulparams scripts
     Each target selects a particular file in the emulparams directory by
     setting the shell variable targ_emul in configure.tgt.  This shell
     variable is used by the configure script to control building an emulation
     source file.

     Certain conventions are enforced. Suppose the targ_emul variable is set
     to emul in configure.tgt.  The name of the emulation shell script will be
     emulparams/ emul.sh.  The Makefile must have a target named e emul.c;
     this target must depend upon emulparams/ emul.sh, as well as the
     appropriate scripts in the scripttempl and emultempl directories. The
     Makefile target must invoke GENSCRIPTS with two arguments: emul, and the
     value of the make variable tdir_ emul.  The value of the latter variable
     will be set by the configure script, and is used to set the default
     target directory to search.

     By convention, the emulparams/ emul.sh shell script should only set shell
     variables. It may set shell variables which are to be interpreted by the
     scripttempl and the emultempl scripts. Certain shell variables are
     interpreted directly by the genscripts.sh script.

     Here is a list of shell variables interpreted by genscripts.sh, as well
     as some conventional shell variables interpreted by the scripttempl and
     emultempl scripts.

     SCRIPT_NAME
             This is the name of the scripttempl script to use. If SCRIPT_NAME
             is set to script, genscripts.sh will use the script scripttempl/
             script.sc.

     TEMPLATE_NAME
             This is the name of the emultempl script to use. If TEMPLATE_NAME
             is set to template, genscripts.sh will use the script emultempl/
             template.em.  If this variable is not set, the default value is
             generic.

     GENERATE_SHLIB_SCRIPT
             If this is set to a nonempty string, genscripts.sh will invoke
             the scripttempl script an extra time to create a shared library
             script. linker scripts.

     OUTPUT_FORMAT
             This is normally set to indicate the BFD output format use (e.g.,
             a.out-sunos-big.  The scripttempl script will normally use it in
             an OUTPUT_FORMAT expression in the linker script.

     ARCH    This is normally set to indicate the architecture to use (e.g.,
             sparc).  The scripttempl script will normally use it in an
             OUTPUT_ARCH expression in the linker script.

     ENTRY   Some scripttempl scripts use this to set the entry address, in an
             ENTRY expression in the linker script.

     TEXT_START_ADDR
             Some scripttempl scripts use this to set the start address of the
             .text section.

     NONPAGED_TEXT_START_ADDR
             If this is defined, the genscripts.sh script sets TEXT_START_ADDR
             to its value before running the scripttempl script for the -n and
             -N options (see Section ``linker scripts'').

     SEGMENT_SIZE
             The genscripts.sh script uses this to set the default value of
             DATA_ALIGNMENT when running the scripttempl script.

     TARGET_PAGE_SIZE
             If SEGMENT_SIZE is not defined, the genscripts.sh script uses
             this to define it.

     ALIGNMENT
             Some scripttempl scripts set this to a number to pass to ALIGN to
             set the required alignment for the end symbol.

   scripttempl scripts
     Each linker target uses a scripttempl script to generate the default
     linker scripts. The name of the scripttempl script is set by the
     SCRIPT_NAME variable in the emulparams script. If SCRIPT_NAME is set to
     script, genscripts.sh will invoke scripttempl/ script.sc.

     The genscripts.sh script will invoke the scripttempl script 5 to 8 times.
     Each time it will set the shell variable LD_FLAG to a different value.
     When the linker is run, the options used will direct it to select a
     particular script. (Script selection is controlled by the get_script
     emulation entry point; this describes the conventional behaviour).

     The scripttempl script should just write a linker script, written in the
     linker command language, to standard output. If the emulation name--the
     name of the emulparams file without the .sc extension--is emul, then the
     output will be directed to ldscripts/ emul. extension in the build
     directory, where extension changes each time the scripttempl script is
     invoked.

     Here is the list of values assigned to LD_FLAG.

     (empty)
             The script generated is used by default (when none of the
             following cases apply). The output has an extension of .x.

     n       The script generated is used when the linker is invoked with the
             -n option. The output has an extension of .xn.

     N       The script generated is used when the linker is invoked with the
             -N option. The output has an extension of .xbn.

     r       The script generated is used when the linker is invoked with the
             -r option. The output has an extension of .xr.

     u       The script generated is used when the linker is invoked with the
             -Ur option. The output has an extension of .xu.

     shared  The scripttempl script is only invoked with LD_FLAG set to this
             value if GENERATE_SHLIB_SCRIPT is defined in the emulparams file.
             The emultempl script must arrange to use this script at the
             appropriate time, normally when the linker is invoked with the
             -shared option. The output has an extension of .xs.

     c       The scripttempl script is only invoked with LD_FLAG set to this
             value if GENERATE_COMBRELOC_SCRIPT is defined in the emulparams
             file or if SCRIPT_NAME is elf.  The emultempl script must arrange
             to use this script at the appropriate time, normally when the
             linker is invoked with the -z combreloc option. The output has an
             extension of .xc.

     cshared
             The scripttempl script is only invoked with LD_FLAG set to this
             value if GENERATE_COMBRELOC_SCRIPT is defined in the emulparams
             file or if SCRIPT_NAME is elf and GENERATE_SHLIB_SCRIPT is
             defined in the emulparams file. The emultempl script must arrange
             to use this script at the appropriate time, normally when the
             linker is invoked with the -shared -z combreloc option. The
             output has an extension of .xsc.

     Besides the shell variables set by the emulparams script, and the LD_FLAG
     variable, the genscripts.sh script will set certain variables for each
     run of the scripttempl script.

     RELOCATING
             This will be set to a non-empty string when the linker is doing a
             final relocation (e.g., all scripts other than -r and -Ur).

     CONSTRUCTING
             This will be set to a non-empty string when the linker is
             building global constructor and destructor tables (e.g., all
             scripts other than -r).

     DATA_ALIGNMENT
             This will be set to an ALIGN expression when the output should be
             page aligned, or to .  when generating the -N script.

     CREATE_SHLIB
             This will be set to a non-empty string when generating a -shared
             script.

     COMBRELOC
             This will be set to a non-empty string when generating -z
             combreloc scripts to a temporary file name which can be used
             during script generation.

     The conventional way to write a scripttempl script is to first set a few
     shell variables, and then write out a linker script using cat with a here
     document. The linker script will use variable substitutions, based on the
     above variables and those set in the emulparams script, to control its
     behaviour.

     When there are parts of the scripttempl script which should only be run
     when doing a final relocation, they should be enclosed within a variable
     substitution based on RELOCATING.  For example, on many targets special
     symbols such as _end should be defined when doing a final link.
     Naturally, those symbols should not be defined when doing a relocatable
     link using -r.  The scripttempl script could use a construct like this to
     define those symbols:

             ${RELOCATING+ _end = .;}
     This will do the symbol assignment only if the RELOCATING variable is
     defined.

     The basic job of the linker script is to put the sections in the correct
     order, and at the correct memory addresses. For some targets, the linker
     script may have to do some other operations.

     For example, on most MIPS platforms, the linker is responsible for
     defining the special symbol _gp, used to initialize the $gp register. It
     must be set to the start of the small data section plus 0x8000.
     Naturally, it should only be defined when doing a final relocation. This
     will typically be done like this:

             ${RELOCATING+ _gp = ALIGN(16) + 0x8000;}
     This line would appear just before the sections which compose the small
     data section ( .sdata, .sbss).  All those sections would be contiguous in
     memory.

     Many COFF systems build constructor tables in the linker script. The
     compiler will arrange to output the address of each global constructor in
     a .ctor section, and the address of each global destructor in a .dtor
     section (this is done by defining ASM_OUTPUT_CONSTRUCTOR and
     ASM_OUTPUT_DESTRUCTOR in the gcc configuration files). The gcc runtime
     support routines expect the constructor table to be named __CTOR_LIST__.
     They expect it to be a list of words, with the first word being the count
     of the number of entries. There should be a trailing zero word.
     (Actually, the count may be -1 if the trailing word is present, and the
     trailing word may be omitted if the count is correct, but, as the gcc
     behaviour has changed slightly over the years, it is safest to provide
     both).  Here is a typical way that might be handled in a scripttempl
     file.

               ${CONSTRUCTING+ __CTOR_LIST__ = .;}
               ${CONSTRUCTING+ LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2)}
               ${CONSTRUCTING+ *(.ctors)}
               ${CONSTRUCTING+ LONG(0)}
               ${CONSTRUCTING+ __CTOR_END__ = .;}
               ${CONSTRUCTING+ __DTOR_LIST__ = .;}
               ${CONSTRUCTING+ LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2)}
               ${CONSTRUCTING+ *(.dtors)}
               ${CONSTRUCTING+ LONG(0)}
               ${CONSTRUCTING+ __DTOR_END__ = .;}
     The use of CONSTRUCTING ensures that these linker script commands will
     only appear when the linker is supposed to be building the constructor
     and destructor tables. This example is written for a target which uses 4
     byte pointers.

     Embedded systems often need to set a stack address. This is normally best
     done by using the PROVIDE construct with a default stack address. This
     permits the user to easily override the stack address using the --defsym
     option. Here is an example:

             ${RELOCATING+ PROVIDE (__stack = 0x80000000);}
     The value of the symbol __stack would then be used in the startup code to
     initialize the stack pointer.

   emultempl scripts
     Each linker target uses an emultempl script to generate the emulation
     code. The name of the emultempl script is set by the TEMPLATE_NAME
     variable in the emulparams script. If the TEMPLATE_NAME variable is not
     set, the default is generic.  If the value of TEMPLATE_NAME is template,
     genscripts.sh will use emultempl/ template.em.

     Most targets use the generic emultempl script, emultempl/generic.em.  A
     different emultempl script is only needed if the linker must support
     unusual actions, such as linking against shared libraries.

     The emultempl script is normally written as a simple invocation of cat
     with a here document. The document will use a few variable substitutions.
     Typically each function names uses a substitution involving
     EMULATION_NAME, for ease of debugging when the linker supports multiple
     emulations.

     Every function and variable in the emitted file should be static. The
     only globally visible object must be named ld_ EMULATION_NAME_emulation,
     where EMULATION_NAME is the name of the emulation set in configure.tgt
     (this is also the name of the emulparams file without the .sh extension).
     The genscripts.sh script will set the shell variable EMULATION_NAME
     before invoking the emultempl script.

     The ld_ EMULATION_NAME_emulation variable must be a struct
     ld_emulation_xfer_struct, as defined in ldemul.h.  It defines a set of
     function pointers which are invoked by the linker, as well as strings for
     the emulation name (normally set from the shell variable EMULATION_NAME
     and the default BFD target name (normally set from the shell variable
     OUTPUT_FORMAT which is normally set by the emulparams file).

     The genscripts.sh script will set the shell variable COMPILE_IN when it
     invokes the emultempl script for the default emulation. In this case, the
     emultempl script should include the linker scripts directly, and return
     them from the get_scripts entry point. When the emulation is not the
     default, the get_scripts entry point should just return a file name. See
     emultempl/generic.em for an example of how this is done.

     At some point, the linker emulation entry points should be documented.

A Walkthrough of a Typical Emulation
     This chapter is to help people who are new to the way emulations interact
     with the linker, or who are suddenly thrust into the position of having
     to work with existing emulations. It will discuss the files you need to
     be aware of. It will tell you when the given "hooks" in the emulation
     will be called.  It will, hopefully, give you enough information about
     when and how things happen that you'll be able to get by. As always, the
     source is the definitive reference to this.

     The starting point for the linker is in ldmain.c where main is defined.
     The bulk of the code that's emulation specific will initially be in
     emultempl/ emulation.em but will end up in e emulation.c when the build
     is done. Most of the work to select and interface with emulations is in
     ldemul.h and ldemul.c.  Specifically, ldemul.h defines the
     ld_emulation_xfer_struct structure your emulation exports.

     Your emulation file exports a symbol ld_ EMULATION_NAME_emulation.  If
     your emulation is selected (it usually is, since usually there's only
     one), ldemul.c sets the variable ld_emulation to point to it.  ldemul.c
     also defines a number of API functions that interface to your emulation,
     like ldemul_after_parse which simply calls your ld_
     EMULATION_emulation.after_parse function. For the rest of this section,
     the functions will be mentioned, but you should assume the indirect
     reference to your emulation also.

     We will also skip or gloss over parts of the link process that don't
     relate to emulations, like setting up internationalization.

     After initialization, main selects an emulation by pre-scanning the
     command line arguments. It calls ldemul_choose_target to choose a target.
     If you set choose_target to ldemul_default_target, it picks your
     target_name by default.

     main calls ldemul_before_parse, then parse_args.  parse_args calls
     ldemul_parse_args for each arg, which must update the getopt globals if
     it recognizes the argument. If the emulation doesn't recognize it, then
     parse_args checks to see if it recognizes it.

     Now that the emulation has had access to all its command-line options,
     main calls ldemul_set_symbols.  This can be used for any initialization
     that may be affected by options. It is also supposed to set up any
     variables needed by the emulation script.

     main now calls ldemul_get_script to get the emulation script to use
     (based on arguments, no doubt,see Section ``Emulations'') and runs it.
     While parsing, ldgram.y may call ldemul_hll or ldemul_syslib to handle
     the HLL or SYSLIB commands. It may call ldemul_unrecognized_file if you
     asked the linker to link a file it doesn't recognize. It will call
     ldemul_recognized_file for each file it does recognize, in case the
     emulation wants to handle some files specially. All the while, it's
     loading the files (possibly calling ldemul_open_dynamic_archive) and
     symbols and stuff. After it's done reading the script, main calls
     ldemul_after_parse.  Use the after-parse hook to set up anything that
     depends on stuff the script might have set up, like the entry point.

     main next calls lang_process in ldlang.c.  This appears to be the main
     core of the linking itself, as far as emulation hooks are concerned(*).
     It first opens the output file's BFD, calling ldemul_set_output_arch, and
     calls ldemul_create_output_section_statements in case you need to use
     other means to find or create object files (i.e. shared libraries found
     on a path, or fake stub objects). Despite the name, nobody creates output
     sections here.

     (*) In most cases, the BFD library does the bulk of the actual linking,
     handling symbol tables, symbol resolution, relocations, and building the
     final output file. See the BFD reference for all the details. Your
     emulation is usually concerned more with managing things at the file and
     section level, like "put this here, add this section", etc.

     Next, the objects to be linked are opened and BFDs created for them, and
     ldemul_after_open is called. At this point, you have all the objects and
     symbols loaded, but none of the data has been placed yet.

     Next comes the Big Linking Thingy (except for the parts BFD does). All
     input sections are mapped to output sections according to the script. If
     a section doesn't get mapped by default, ldemul_place_orphan will get
     called to figure out where it goes. Next it figures out the offsets for
     each section, calling ldemul_before_allocation before and
     ldemul_after_allocation after deciding where each input section ends up
     in the output sections.

     The last part of lang_process is to figure out all the symbols' values.
     After assigning final values to the symbols, ldemul_finish is called, and
     after that, any undefined symbols are turned into fatal errors.

     OK, back to main, which calls ldwrite in ldwrite.c.  ldwrite calls BFD's
     final_link, which does all the relocation fixups and writes the output
     bfd to disk, and we're done.

     In summary,

        main() in ldmain.c

        emultempl/ EMULATION.em has your code

        ldemul_choose_target (defaults to your target_name)

        ldemul_before_parse

        Parse argv, calls ldemul_parse_args for each

        ldemul_set_symbols

        ldemul_get_script

        parse script

            may call ldemul_hll or ldemul_syslib

            may call ldemul_open_dynamic_archive

        ldemul_after_parse

        lang_process() in ldlang.c

            create output_bfd

            ldemul_set_output_arch

            ldemul_create_output_section_statements

            read objects, create input bfds - all symbols exist, but have no
             values

            may call ldemul_unrecognized_file

            will call ldemul_recognized_file

            ldemul_after_open

            map input sections to output sections

            may call ldemul_place_orphan for remaining sections

            ldemul_before_allocation

            gives input sections offsets into output sections, places output
             sections

            ldemul_after_allocation - section addresses valid

            assigns values to symbols

            ldemul_finish - symbol values valid

        output bfd is written to disk

Some Architecture Specific Notes
     This is the place for notes on the behavior of ld on specific platforms.
     Currently, only Intel x86 is documented (and of that, only the auto-
     import behavior for DLLs).

   Intel x86
     ld can create DLLs that operate with various runtimes available on a
     common x86 operating system. These runtimes include native (using the
     mingw "platform"), cygwin, and pw.

     auto-import from DLLs

             1.   With this feature on, DLL clients can import variables from
                  DLL without any concern from their side (for example,
                  without any source code modifications).  Auto-import can be
                  enabled using the --enable-auto-import flag, or disabled via
                  the --disable-auto-import flag. Auto-import is disabled by
                  default.

             2.   This is done completely in bounds of the PE specification
                  (to be fair, there's a minor violation of the spec at one
                  point, but in practice auto-import works on all known
                  variants of that common x86 operating system) So, the
                  resulting DLL can be used with any other PE compiler/linker.

             3.   Auto-import is fully compatible with standard import method,
                  in which variables are decorated using attribute modifiers.
                  Libraries of either type may be mixed together.

             4.   Overhead (space): 8 bytes per imported symbol, plus 20 for
                  each reference to it; Overhead (load time): negligible;
                  Overhead (virtual/physical memory): should be less than
                  effect of DLL relocation.

             Motivation

             The obvious and only way to get rid of dllimport insanity is to
             make client access variable directly in the DLL, bypassing the
             extra dereference imposed by ordinary DLL runtime linking. I.e.,
             whenever client contains something like

             mov dll_var,%eax,

             address of dll_var in the command should be relocated to point
             into loaded DLL. The aim is to make OS loader do so, and than
             make ld help with that.  Import section of PE made following way:
             there's a vector of structures each describing imports from
             particular DLL. Each such structure points to two other parallel
             vectors: one holding imported names, and one which will hold
             address of corresponding imported name. So, the solution is de-
             vectorize these structures, making import locations be sparse and
             pointing directly into code.

             Implementation

             For each reference of data symbol to be imported from DLL (to set
             of which belong symbols with name <sym>, if __imp_<sym> is found
             in implib), the import fixup entry is generated. That entry is of
             type IMAGE_IMPORT_DESCRIPTOR and stored in .idata$3 subsection.
             Each fixup entry contains pointer to symbol's address within
             .text section (marked with __fuN_<sym> symbol, where N is
             integer), pointer to DLL name (so, DLL name is referenced by
             multiple entries), and pointer to symbol name thunk. Symbol name
             thunk is singleton vector (__nm_th_<symbol>) pointing to
             IMAGE_IMPORT_BY_NAME structure (__nm_<symbol>) directly
             containing imported name. Here comes that "om the edge" problem
             mentioned above: PE specification rambles that name vector
             (OriginalFirstThunk) should run in parallel with addresses vector
             (FirstThunk), i.e. that they should have same number of elements
             and terminated with zero. We violate this, since FirstThunk
             points directly into machine code. But in practice, OS loader
             implemented the sane way: it goes thru OriginalFirstThunk and
             puts addresses to FirstThunk, not something else. It once again
             should be noted that dll and symbol name structures are reused
             across fixup entries and should be there anyway to support
             standard import stuff, so sustained overhead is 20 bytes per
             reference. Other question is whether having several
             IMAGE_IMPORT_DESCRIPTORS for the same DLL is possible.  Answer is
             yes, it is done even by native compiler/linker (libth32's
             functions are in fact resident in windows9x kernel32.dll, so if
             you use it, you have two IMAGE_IMPORT_DESCRIPTORS for
             kernel32.dll). Yet other question is whether referencing the same
             PE structures several times is valid. The answer is why not,
             prohibiting that (detecting violation) would require more work on
             behalf of loader than not doing it.

GNU Free Documentation License
     GNU Free Documentation License Version 1.1, March 2000

     Copyright (C) 2000 Free Software Foundation, Inc. 51 Franklin Street,
     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.

     0. PREAMBLE

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     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.

     2. 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.

     3. COPYING IN QUANTITY

     If you publish printed copies 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
     publicly-accessible computer-network location containing a complete
     Transparent copy of the Document, free of added material, which the
     general network-using public has access to download anonymously at no
     charge using public-standard network protocols.  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.

     4. 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:

     A. 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. B. 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 less than five). C.  State on the Title page the name
     of the publisher of the Modified Version, as the publisher. D. Preserve
     all the copyright notices of the Document. E.  Add an appropriate
     copyright notice for your modifications adjacent to the other copyright
     notices. F. 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.  G.
     Preserve in that license notice the full lists of Invariant Sections and
     required Cover Texts given in the Document's license notice. H. Include
     an unaltered copy of this License. I. Preserve the section entitled
     "History", and 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. J. 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.  K. In any section entitled "Acknowledgements" or
     "Dedications", preserve the section's title, and preserve in the section
     all the substance and tone of each of the contributor acknowledgements
     and/or dedications given therein.  L. 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.
     M. Delete any section entitled "Endorsements".  Such a section may not be
     included in the Modified Version. N. Do not retitle any existing section
     as "Endorsements" or to conflict in title with any Invariant Section.

     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.

     5. 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.

     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."

     6. 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.

     7. 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, does not as a whole count as a Modified Version of
     the Document, provided no compilation copyright is claimed for the
     compilation. Such a compilation is called an "aggregate", and this
     License does not apply to the other self-contained works thus compiled
     with the Document, on account of their being thus compiled, if they 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 quarter of the
     entire aggregate, the Document's Cover Texts may be placed on covers that
     surround only the Document within the aggregate. Otherwise they must
     appear on covers around the whole aggregate.

     8. 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
     provided that you also include the original English version of this
     License. In case of a disagreement between the translation and the
     original English version of this License, the original English version
     will prevail.

     9. 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.

     10. 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 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.

     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:

               Copyright (c)  YEAR  YOUR NAME.
               Permission is granted to copy, distribute and/or modify this document
               under the terms of the GNU Free Documentation License, Version 1.1
               or any later version published by the Free Software Foundation;
               with the Invariant Sections being LIST THEIR TITLES, with the
               Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST.
               A copy of the license is included in the section entitled "GNU
               Free Documentation License".

     If you have no Invariant Sections, write "with no Invariant Sections"
     instead of saying which ones are invariant. If you have no Front-Cover
     Texts, write "no Front-Cover Texts" instead of "Front-Cover Texts being
     LIST"; likewise for Back-Cover Texts.

     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.

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