binutils/ld/ldint.7

.Dd 2015-03-02
.Dt LDINT 7
.Os
.Sh NAME
.Nm ldint
.Nd GNU Linker Internals
.Sh
This file documents the internals of the GNU linker
.Li 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
.Li ld
as you discover it (or as you design changes to
.Li ld ) .
.Pp
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".
.Pp
.Sh  The Pa README File
Check the
.Pa README
file; it often has useful information that does not appear anywhere else in
the directory.
.Pp
.Sh  How linker emulations are generated
Each linker target has an
.Em emulation .
The emulation includes the default linker script, and certain emulations also
modify certain types of linker behaviour.
.Pp
Emulations are created during the build process by the shell script
.Pa genscripts.sh .
.Pp
The
.Pa genscripts.sh
script starts by reading a file in the
.Pa emulparams
directory. This is a shell script which sets various shell variables used
by
.Pa genscripts.sh
and the other shell scripts it invokes.
.Pp
The
.Pa genscripts.sh
script will invoke a shell script in the
.Pa scripttempl
directory in order to create default linker scripts written in the linker
command language. The
.Pa 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.
.Pp
After creating the scripts,
.Pa genscripts.sh
will invoke yet another shell script, this time in the
.Pa 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
.Pa emultempl/generic.em .
.Pp
To summarize,
.Pa genscripts.sh
reads three shell scripts: an emulation parameters script in the
.Pa emulparams
directory, a linker script generation script in the
.Pa scripttempl
directory, and an emulation source file generation script in the
.Pa emultempl
directory.
.Pp
For example, the Sun 4 linker sets up variables in
.Pa emulparams/sun4.sh ,
creates linker scripts using
.Pa scripttempl/aout.sc ,
and creates the emulation code using
.Pa emultempl/sunos.em .
.Pp
Note that the linker can support several emulations simultaneously, depending
upon how it is configured. An emulation can be selected with the
.Li -m
option. The
.Li -V
option will list all supported emulations.
.Pp
.Ss  Pa emulparams scripts
Each target selects a particular file in the
.Pa emulparams
directory by setting the shell variable
.Li targ_emul
in
.Pa configure.tgt .
This shell variable is used by the
.Pa configure
script to control building an emulation source file.
.Pp
Certain conventions are enforced. Suppose the
.Li targ_emul
variable is set to
.Va emul
in
.Pa configure.tgt .
The name of the emulation shell script will be
.Pa emulparams/ Va emul.sh .
The
.Pa Makefile
must have a target named
.Pa e Va emul.c ;
this target must depend upon
.Pa emulparams/ Va emul.sh ,
as well as the appropriate scripts in the
.Pa scripttempl
and
.Pa emultempl
directories. The
.Pa Makefile
target must invoke
.Li GENSCRIPTS
with two arguments:
.Va emul ,
and the value of the make variable
.Li tdir_ Va emul .
The value of the latter variable will be set by the
.Pa configure
script, and is used to set the default target directory to search.
.Pp
By convention, the
.Pa emulparams/ Va emul.sh
shell script should only set shell variables. It may set shell variables which
are to be interpreted by the
.Pa scripttempl
and the
.Pa emultempl
scripts. Certain shell variables are interpreted directly by the
.Pa genscripts.sh
script.
.Pp
Here is a list of shell variables interpreted by
.Pa genscripts.sh ,
as well as some conventional shell variables interpreted by the
.Pa scripttempl
and
.Pa emultempl
scripts.
.Pp
.Bl -tag -width Ds
.It  SCRIPT_NAME
This is the name of the
.Pa scripttempl
script to use. If
.Li SCRIPT_NAME
is set to
.Va script ,
.Pa genscripts.sh
will use the script
.Pa scripttempl/ Va script.sc .
.Pp
.It  TEMPLATE_NAME
This is the name of the
.Pa emultempl
script to use. If
.Li TEMPLATE_NAME
is set to
.Va template ,
.Pa genscripts.sh
will use the script
.Pa emultempl/ Va template.em .
If this variable is not set, the default value is
.Li generic .
.Pp
.It  GENERATE_SHLIB_SCRIPT
If this is set to a nonempty string,
.Pa genscripts.sh
will invoke the
.Pa scripttempl
script an extra time to create a shared library script. linker scripts.
.Pp
.It  OUTPUT_FORMAT
This is normally set to indicate the BFD output format use (e.g.,
.Li "a.out-sunos-big" .
The
.Pa scripttempl
script will normally use it in an
.Li OUTPUT_FORMAT
expression in the linker script.
.Pp
.It  ARCH
This is normally set to indicate the architecture to use (e.g.,
.Li sparc ) .
The
.Pa scripttempl
script will normally use it in an
.Li OUTPUT_ARCH
expression in the linker script.
.Pp
.It  ENTRY
Some
.Pa scripttempl
scripts use this to set the entry address, in an
.Li ENTRY
expression in the linker script.
.Pp
.It  TEXT_START_ADDR
Some
.Pa scripttempl
scripts use this to set the start address of the
.Li .text
section.
.Pp
.It  NONPAGED_TEXT_START_ADDR
If this is defined, the
.Pa genscripts.sh
script sets
.Li TEXT_START_ADDR
to its value before running the
.Pa scripttempl
script for the
.Li -n
and
.Li -N
options (see Section
.Dq linker scripts ) .
.Pp
.It  SEGMENT_SIZE
The
.Pa genscripts.sh
script uses this to set the default value of
.Li DATA_ALIGNMENT
when running the
.Pa scripttempl
script.
.Pp
.It  TARGET_PAGE_SIZE
If
.Li SEGMENT_SIZE
is not defined, the
.Pa genscripts.sh
script uses this to define it.
.Pp
.It  ALIGNMENT
Some
.Pa scripttempl
scripts set this to a number to pass to
.Li ALIGN
to set the required alignment for the
.Li end
symbol.
.El
.Pp
.Ss  Pa scripttempl scripts
Each linker target uses a
.Pa scripttempl
script to generate the default linker scripts. The name of the
.Pa scripttempl
script is set by the
.Li SCRIPT_NAME
variable in the
.Pa emulparams
script. If
.Li SCRIPT_NAME
is set to
.Va script ,
.Li genscripts.sh
will invoke
.Pa scripttempl/ Va script.sc .
.Pp
The
.Pa genscripts.sh
script will invoke the
.Pa scripttempl
script 5 to 8 times. Each time it will set the shell variable
.Li 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
.Li get_script
emulation entry point; this describes the conventional behaviour).
.Pp
The
.Pa 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
.Pa emulparams
file without the
.Pa .sc
extension--is
.Va emul ,
then the output will be directed to
.Pa ldscripts/ Va emul. Va extension
in the build directory, where
.Va extension
changes each time the
.Pa scripttempl
script is invoked.
.Pp
Here is the list of values assigned to
.Li LD_FLAG .
.Pp
.Bl -tag -width Ds
.It  (empty)
The script generated is used by default (when none of the following cases
apply). The output has an extension of
.Pa .x .
.It  n
The script generated is used when the linker is invoked with the
.Li -n
option. The output has an extension of
.Pa .xn .
.It  N
The script generated is used when the linker is invoked with the
.Li -N
option. The output has an extension of
.Pa .xbn .
.It  r
The script generated is used when the linker is invoked with the
.Li -r
option. The output has an extension of
.Pa .xr .
.It  u
The script generated is used when the linker is invoked with the
.Li -Ur
option. The output has an extension of
.Pa .xu .
.It  shared
The
.Pa scripttempl
script is only invoked with
.Li LD_FLAG
set to this value if
.Li GENERATE_SHLIB_SCRIPT
is defined in the
.Pa emulparams
file. The
.Pa emultempl
script must arrange to use this script at the appropriate time, normally when
the linker is invoked with the
.Li -shared
option. The output has an extension of
.Pa .xs .
.It  c
The
.Pa scripttempl
script is only invoked with
.Li LD_FLAG
set to this value if
.Li GENERATE_COMBRELOC_SCRIPT
is defined in the
.Pa emulparams
file or if
.Li SCRIPT_NAME
is
.Li elf .
The
.Pa emultempl
script must arrange to use this script at the appropriate time, normally when
the linker is invoked with the
.Li -z combreloc
option. The output has an extension of
.Pa .xc .
.It  cshared
The
.Pa scripttempl
script is only invoked with
.Li LD_FLAG
set to this value if
.Li GENERATE_COMBRELOC_SCRIPT
is defined in the
.Pa emulparams
file or if
.Li SCRIPT_NAME
is
.Li elf
and
.Li GENERATE_SHLIB_SCRIPT
is defined in the
.Pa emulparams
file. The
.Pa emultempl
script must arrange to use this script at the appropriate time, normally when
the linker is invoked with the
.Li -shared -z combreloc
option. The output has an extension of
.Pa .xsc .
.El
.Pp
Besides the shell variables set by the
.Pa emulparams
script, and the
.Li LD_FLAG
variable, the
.Pa genscripts.sh
script will set certain variables for each run of the
.Pa scripttempl
script.
.Pp
.Bl -tag -width Ds
.It  RELOCATING
This will be set to a non-empty string when the linker is doing a final relocation
(e.g., all scripts other than
.Li -r
and
.Li -Ur ) .
.Pp
.It  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
.Li -r ) .
.Pp
.It  DATA_ALIGNMENT
This will be set to an
.Li ALIGN
expression when the output should be page aligned, or to
.Li .
when generating the
.Li -N
script.
.Pp
.It  CREATE_SHLIB
This will be set to a non-empty string when generating a
.Li -shared
script.
.Pp
.It  COMBRELOC
This will be set to a non-empty string when generating
.Li -z combreloc
scripts to a temporary file name which can be used during script generation.
.El
.Pp
The conventional way to write a
.Pa scripttempl
script is to first set a few shell variables, and then write out a linker
script using
.Li cat
with a here document. The linker script will use variable substitutions, based
on the above variables and those set in the
.Pa emulparams
script, to control its behaviour.
.Pp
When there are parts of the
.Pa scripttempl
script which should only be run when doing a final relocation, they should
be enclosed within a variable substitution based on
.Li RELOCATING .
For example, on many targets special symbols such as
.Li _end
should be defined when doing a final link. Naturally, those symbols should
not be defined when doing a relocatable link using
.Li -r .
The
.Pa scripttempl
script could use a construct like this to define those symbols:
.Bd -literal -offset indent
  ${RELOCATING+ _end = .;}
.Ed
This will do the symbol assignment only if the
.Li RELOCATING
variable is defined.
.Pp
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.
.Pp
For example, on most MIPS platforms, the linker is responsible for defining
the special symbol
.Li _gp ,
used to initialize the
.Li $gp
register. It must be set to the start of the small data section plus
.Li 0x8000 .
Naturally, it should only be defined when doing a final relocation. This will
typically be done like this:
.Bd -literal -offset indent
  ${RELOCATING+ _gp = ALIGN(16) + 0x8000;}
.Ed
This line would appear just before the sections which compose the small data
section (
.Li .sdata ,
.Li .sbss ) .
All those sections would be contiguous in memory.
.Pp
Many COFF systems build constructor tables in the linker script. The compiler
will arrange to output the address of each global constructor in a
.Li .ctor
section, and the address of each global destructor in a
.Li .dtor
section (this is done by defining
.Li ASM_OUTPUT_CONSTRUCTOR
and
.Li ASM_OUTPUT_DESTRUCTOR
in the
.Li gcc
configuration files). The
.Li gcc
runtime support routines expect the constructor table to be named
.Li __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
.Li 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
.Pa scripttempl
file.
.Bd -literal -offset indent
    ${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__ = .;}
.Ed
The use of
.Li 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.
.Pp
Embedded systems often need to set a stack address. This is normally best
done by using the
.Li PROVIDE
construct with a default stack address. This permits the user to easily override
the stack address using the
.Li --defsym
option. Here is an example:
.Bd -literal -offset indent
  ${RELOCATING+ PROVIDE (__stack = 0x80000000);}
.Ed
The value of the symbol
.Li __stack
would then be used in the startup code to initialize the stack pointer.
.Pp
.Ss  Pa emultempl scripts
Each linker target uses an
.Pa emultempl
script to generate the emulation code. The name of the
.Pa emultempl
script is set by the
.Li TEMPLATE_NAME
variable in the
.Pa emulparams
script. If the
.Li TEMPLATE_NAME
variable is not set, the default is
.Li generic .
If the value of
.Li TEMPLATE_NAME
is
.Va template ,
.Pa genscripts.sh
will use
.Pa emultempl/ Va template.em .
.Pp
Most targets use the generic
.Pa emultempl
script,
.Pa emultempl/generic.em .
A different
.Pa emultempl
script is only needed if the linker must support unusual actions, such as
linking against shared libraries.
.Pp
The
.Pa emultempl
script is normally written as a simple invocation of
.Li cat
with a here document. The document will use a few variable substitutions.
Typically each function names uses a substitution involving
.Li EMULATION_NAME ,
for ease of debugging when the linker supports multiple emulations.
.Pp
Every function and variable in the emitted file should be static. The only
globally visible object must be named
.Li ld_ Va EMULATION_NAME_emulation ,
where
.Va EMULATION_NAME
is the name of the emulation set in
.Pa configure.tgt
(this is also the name of the
.Pa emulparams
file without the
.Pa .sh
extension). The
.Pa genscripts.sh
script will set the shell variable
.Li EMULATION_NAME
before invoking the
.Pa emultempl
script.
.Pp
The
.Li ld_ Va EMULATION_NAME_emulation
variable must be a
.Li struct ld_emulation_xfer_struct ,
as defined in
.Pa 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
.Li EMULATION_NAME
and the default BFD target name (normally set from the shell variable
.Li OUTPUT_FORMAT
which is normally set by the
.Pa emulparams
file).
.Pp
The
.Pa genscripts.sh
script will set the shell variable
.Li COMPILE_IN
when it invokes the
.Pa emultempl
script for the default emulation. In this case, the
.Pa emultempl
script should include the linker scripts directly, and return them from the
.Li get_scripts
entry point. When the emulation is not the default, the
.Li get_scripts
entry point should just return a file name. See
.Pa emultempl/generic.em
for an example of how this is done.
.Pp
At some point, the linker emulation entry points should be documented.
.Pp
.Sh  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.
.Pp
The starting point for the linker is in
.Pa ldmain.c
where
.Li main
is defined. The bulk of the code that's emulation specific will initially
be in
.Li emultempl/ Va emulation.em
but will end up in
.Li e Va emulation.c
when the build is done. Most of the work to select and interface with emulations
is in
.Li ldemul.h
and
.Li ldemul.c .
Specifically,
.Li ldemul.h
defines the
.Li ld_emulation_xfer_struct
structure your emulation exports.
.Pp
Your emulation file exports a symbol
.Li ld_ Va EMULATION_NAME_emulation .
If your emulation is selected (it usually is, since usually there's only one),
.Li ldemul.c
sets the variable
.Va ld_emulation
to point to it.
.Li ldemul.c
also defines a number of API functions that interface to your emulation, like
.Li ldemul_after_parse
which simply calls your
.Li ld_ Va 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.
.Pp
We will also skip or gloss over parts of the link process that don't relate
to emulations, like setting up internationalization.
.Pp
After initialization,
.Li main
selects an emulation by pre-scanning the command line arguments. It calls
.Li ldemul_choose_target
to choose a target. If you set
.Li choose_target
to
.Li ldemul_default_target ,
it picks your
.Li target_name
by default.
.Pp
.Li main
calls
.Li ldemul_before_parse ,
then
.Li parse_args .
.Li parse_args
calls
.Li ldemul_parse_args
for each arg, which must update the
.Li getopt
globals if it recognizes the argument. If the emulation doesn't recognize
it, then parse_args checks to see if it recognizes it.
.Pp
Now that the emulation has had access to all its command-line options,
.Li main
calls
.Li 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.
.Pp
.Li main
now calls
.Li ldemul_get_script
to get the emulation script to use (based on arguments, no doubt,see Section
.Dq Emulations )
and runs it. While parsing,
.Li ldgram.y
may call
.Li ldemul_hll
or
.Li ldemul_syslib
to handle the
.Li HLL
or
.Li SYSLIB
commands. It may call
.Li ldemul_unrecognized_file
if you asked the linker to link a file it doesn't recognize. It will call
.Li 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
.Li ldemul_open_dynamic_archive )
and symbols and stuff. After it's done reading the script,
.Li main
calls
.Li 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.
.Pp
.Li main
next calls
.Li lang_process
in
.Li 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
.Li ldemul_set_output_arch ,
and calls
.Li 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.
.Pp
(*) 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.
.Pp
Next, the objects to be linked are opened and BFDs created for them, and
.Li 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.
.Pp
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,
.Li ldemul_place_orphan
will get called to figure out where it goes. Next it figures out the offsets
for each section, calling
.Li ldemul_before_allocation
before and
.Li ldemul_after_allocation
after deciding where each input section ends up in the output sections.
.Pp
The last part of
.Li lang_process
is to figure out all the symbols' values. After assigning final values to
the symbols,
.Li ldemul_finish
is called, and after that, any undefined symbols are turned into fatal errors.
.Pp
OK, back to
.Li main ,
which calls
.Li ldwrite
in
.Pa ldwrite.c .
.Li ldwrite
calls BFD's final_link, which does all the relocation fixups and writes the
output bfd to disk, and we're done.
.Pp
In summary,
.Pp
.Bl -bullet
.It
.Li main()
in
.Pa ldmain.c
.It
.Pa emultempl/ Va EMULATION.em
has your code
.It
.Li ldemul_choose_target
(defaults to your
.Li target_name )
.It
.Li ldemul_before_parse
.It
Parse argv, calls
.Li ldemul_parse_args
for each
.It
.Li ldemul_set_symbols
.It
.Li ldemul_get_script
.It
parse script
.Pp
.Bl -bullet
.It
may call
.Li ldemul_hll
or
.Li ldemul_syslib
.It
may call
.Li ldemul_open_dynamic_archive
.El
.Pp
.It
.Li ldemul_after_parse
.It
.Li lang_process()
in
.Pa ldlang.c
.Pp
.Bl -bullet
.It
create
.Li output_bfd
.It
.Li ldemul_set_output_arch
.It
.Li ldemul_create_output_section_statements
.It
read objects, create input bfds - all symbols exist, but have no values
.It
may call
.Li ldemul_unrecognized_file
.It
will call
.Li ldemul_recognized_file
.It
.Li ldemul_after_open
.It
map input sections to output sections
.It
may call
.Li ldemul_place_orphan
for remaining sections
.It
.Li ldemul_before_allocation
.It
gives input sections offsets into output sections, places output sections
.It
.Li ldemul_after_allocation
- section addresses valid
.It
assigns values to symbols
.It
.Li ldemul_finish
- symbol values valid
.El
.Pp
.It
output bfd is written to disk
.Pp
.El
.Sh  Some Architecture Specific Notes
This is the place for notes on the behavior of
.Li ld
on specific platforms. Currently, only Intel x86 is documented (and of that,
only the auto-import behavior for DLLs).
.Pp
.Ss  Intel x86
.Bl -tag -width Ds
.Li 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.
.Pp
.It  auto-import from DLLs
.Bl -enum
.It
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
.Li --enable-auto-import
flag, or disabled via the
.Li --disable-auto-import
flag. Auto-import is disabled by default.
.Pp
.It
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.
.Pp
.It
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.
.Pp
.It
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.
.El
.Pp
Motivation
.Pp
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
.Pp
.Li mov dll_var,%eax,
.Pp
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.
.Pp
Implementation
.Pp
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.
.Pp
.El
.Sh  GNU Free Documentation License
GNU Free Documentation License Version 1.1, March 2000
.Pp
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.
.Pp
0. PREAMBLE
.Pp
The purpose of this License is to make a manual, textbook, or other written
document "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.
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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.
.Pp
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.
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1. APPLICABILITY AND DEFINITIONS
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This License applies to any manual or other work that contains a notice placed
by the copyright holder saying it can be distributed under the terms of this
License. The "Document", below, refers to any such manual or work. Any member
of the public is a licensee, and is addressed as "you".
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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.
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A "Secondary Section" is a named appendix or a front-matter section of the
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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.
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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
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A "Transparent" copy of the Document means a machine-readable copy, represented
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2. VERBATIM COPYING
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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
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You may also lend copies, under the same conditions stated above, and you
may publicly display copies.
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3. COPYING IN QUANTITY
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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
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The front cover must present the full title with all words of the title equally
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Copying with changes limited to the covers, as long as they preserve the title
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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.
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If you publish or distribute Opaque copies of the Document numbering more
than 100, you must either include a machine-readable Transparent copy along
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If you use the latter option, you must take reasonably prudent steps, when
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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.
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4. MODIFICATIONS
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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:
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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.
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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.
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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.
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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.
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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.
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5. COMBINING DOCUMENTS
.Pp
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.
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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.
.Pp
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."
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6. COLLECTIONS OF DOCUMENTS
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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.
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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.
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7. AGGREGATION WITH INDEPENDENT WORKS
.Pp
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.
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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.
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8. TRANSLATION
.Pp
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.
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9. TERMINATION
.Pp
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.
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10. FUTURE REVISIONS OF THIS LICENSE
.Pp
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/.
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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.
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ADDENDUM: How to use this License for your documents
.Pp
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:
.Pp
.Bd -literal -offset indent
    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".
.Ed
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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.
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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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