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@section coff backends

BFD supports a number of different flavours of coff format.

The major differences between formats are the sizes and

alignments of fields in structures on disk, and the occasional

extra field.

Coff in all its varieties is implemented with a few common

files and a number of implementation specific files. For

example, The 88k bcs coff format is implemented in the file

@file{coff-m88k.c}. This file @code{#include}s

@file{coff/m88k.h} which defines the external structure of the

coff format for the 88k, and @file{coff/internal.h} which

defines the internal structure. @file{coff-m88k.c} also

defines the relocations used by the 88k format

@xref{Relocations}.

The Intel i960 processor version of coff is implemented in

@file{coff-i960.c}. This file has the same structure as

@file{coff-m88k.c}, except that it includes @file{coff/i960.h}

rather than @file{coff-m88k.h}.

@subsection Porting to a new version of coff

The recommended method is to select from the existing

implementations the version of coff which is most like the one

you want to use.  For example, we'll say that i386 coff is

the one you select, and that your coff flavour is called foo.

Copy @file{i386coff.c} to @file{foocoff.c}, copy

@file{../include/coff/i386.h} to @file{../include/coff/foo.h},

and add the lines to @file{targets.c} and @file{Makefile.in}

so that your new back end is used. Alter the shapes of the

structures in @file{../include/coff/foo.h} so that they match

what you need. You will probably also have to add

@code{#ifdef}s to the code in @file{coff/internal.h} and

@file{coffcode.h} if your version of coff is too wild.

You can verify that your new BFD backend works quite simply by

building @file{objdump} from the @file{binutils} directory,

and making sure that its version of what's going on and your

host system's idea (assuming it has the pretty standard coff

dump utility, usually called @code{att-dump} or just

@code{dump}) are the same.  Then clean up your code, and send

what you've done to Cygnus. Then your stuff will be in the

next release, and you won't have to keep integrating it.

@subsection How the coff backend works

@subsubsection File layout

The Coff backend is split into generic routines that are

applicable to any Coff target and routines that are specific

to a particular target.  The target-specific routines are

further split into ones which are basically the same for all

Coff targets except that they use the external symbol format

or use different values for certain constants.

The generic routines are in @file{coffgen.c}.  These routines

work for any Coff target.  They use some hooks into the target

specific code; the hooks are in a @code{bfd_coff_backend_data}

structure, one of which exists for each target.

The essentially similar target-specific routines are in

@file{coffcode.h}.  This header file includes executable C code.

The various Coff targets first include the appropriate Coff

header file, make any special defines that are needed, and

then include @file{coffcode.h}.

Some of the Coff targets then also have additional routines in

the target source file itself.

For example, @file{coff-i960.c} includes

@file{coff/internal.h} and @file{coff/i960.h}.  It then

defines a few constants, such as @code{I960}, and includes

@file{coffcode.h}.  Since the i960 has complex relocation

types, @file{coff-i960.c} also includes some code to

manipulate the i960 relocs.  This code is not in

@file{coffcode.h} because it would not be used by any other

target.

@subsubsection Bit twiddling

Each flavour of coff supported in BFD has its own header file

describing the external layout of the structures. There is also

an internal description of the coff layout, in

@file{coff/internal.h}. A major function of the

coff backend is swapping the bytes and twiddling the bits to

translate the external form of the structures into the normal

internal form. This is all performed in the

@code{bfd_swap}_@i{thing}_@i{direction} routines. Some

elements are different sizes between different versions of

coff; it is the duty of the coff version specific include file

to override the definitions of various packing routines in

@file{coffcode.h}. E.g., the size of line number entry in coff is

sometimes 16 bits, and sometimes 32 bits. @code{#define}ing

@code{PUT_LNSZ_LNNO} and @code{GET_LNSZ_LNNO} will select the

correct one. No doubt, some day someone will find a version of

coff which has a varying field size not catered to at the

moment. To port BFD, that person will have to add more @code{#defines}.

Three of the bit twiddling routines are exported to

@code{gdb}; @code{coff_swap_aux_in}, @code{coff_swap_sym_in}

and @code{coff_swap_lineno_in}. @code{GDB} reads the symbol

table on its own, but uses BFD to fix things up.  More of the

bit twiddlers are exported for @code{gas};

@code{coff_swap_aux_out}, @code{coff_swap_sym_out},

@code{coff_swap_lineno_out}, @code{coff_swap_reloc_out},

@code{coff_swap_filehdr_out}, @code{coff_swap_aouthdr_out},

@code{coff_swap_scnhdr_out}. @code{Gas} currently keeps track

of all the symbol table and reloc drudgery itself, thereby

saving the internal BFD overhead, but uses BFD to swap things

on the way out, making cross ports much safer.  Doing so also

allows BFD (and thus the linker) to use the same header files

as @code{gas}, which makes one avenue to disaster disappear.

@subsubsection Symbol reading

The simple canonical form for symbols used by BFD is not rich

enough to keep all the information available in a coff symbol

table. The back end gets around this problem by keeping the original

symbol table around, "behind the scenes".

When a symbol table is requested (through a call to

@code{bfd_canonicalize_symtab}), a request gets through to

@code{coff_get_normalized_symtab}. This reads the symbol table from

the coff file and swaps all the structures inside into the

internal form. It also fixes up all the pointers in the table

(represented in the file by offsets from the first symbol in

the table) into physical pointers to elements in the new

internal table. This involves some work since the meanings of

fields change depending upon context: a field that is a

pointer to another structure in the symbol table at one moment

may be the size in bytes of a structure at the next.  Another

pass is made over the table. All symbols which mark file names

(@code{C_FILE} symbols) are modified so that the internal

string points to the value in the auxent (the real filename)

rather than the normal text associated with the symbol

(@code{".file"}).

At this time the symbol names are moved around. Coff stores

all symbols less than nine characters long physically

within the symbol table; longer strings are kept at the end of

the file in the string  table. This pass moves all strings

into memory and replaces them with pointers to the strings.

The symbol table is massaged once again, this time to create

the canonical table used by the BFD application. Each symbol

is inspected in turn, and a decision made (using the

@code{sclass} field) about the various flags to set in the

@code{asymbol}.  @xref{Symbols}. The generated canonical table

shares strings with the hidden internal symbol table.

Any linenumbers are read from the coff file too, and attached

to the symbols which own the functions the linenumbers belong to.

@subsubsection Symbol writing

Writing a symbol to a coff file which didn't come from a coff

file will lose any debugging information. The @code{asymbol}

structure remembers the BFD from which the symbol was taken, and on

output the back end makes sure that the same destination target as

source target is present.

When the symbols have come from a coff file then all the

debugging information is preserved.

Symbol tables are provided for writing to the back end in a

vector of pointers to pointers. This allows applications like

the linker to accumulate and output large symbol tables

without having to do too much byte copying.

This function runs through the provided symbol table and

patches each symbol marked as a file place holder

(@code{C_FILE}) to point to the next file place holder in the

list. It also marks each @code{offset} field in the list with

the offset from the first symbol of the current symbol.

Another function of this procedure is to turn the canonical

value form of BFD into the form used by coff. Internally, BFD

expects symbol values to be offsets from a section base; so a

symbol physically at 0x120, but in a section starting at

0x100, would have the value 0x20. Coff expects symbols to

contain their final value, so symbols have their values

changed at this point to reflect their sum with their owning

section.  This transformation uses the

@code{output_section} field of the @code{asymbol}'s

@code{asection} @xref{Sections}.

@itemize @bullet

@item

@code{coff_mangle_symbols}

@end itemize

This routine runs though the provided symbol table and uses

the offsets generated by the previous pass and the pointers

generated when the symbol table was read in to create the

structured hierarchy required by coff. It changes each pointer

to a symbol into the index into the symbol table of the asymbol.

@itemize @bullet

@item

@code{coff_write_symbols}

@end itemize

This routine runs through the symbol table and patches up the

symbols from their internal form into the coff way, calls the

bit twiddlers, and writes out the table to the file.

@findex coff_symbol_type

@subsubsection @code{coff_symbol_type}

@strong{Description}@*

The hidden information for an @code{asymbol} is described in a

@code{combined_entry_type}:

@example

typedef struct coff_ptr_struct

@{

  /* Remembers the offset from the first symbol in the file for

     this symbol. Generated by coff_renumber_symbols. */

  unsigned int offset;

  /* Should the value of this symbol be renumbered.  Used for

     XCOFF C_BSTAT symbols.  Set by coff_slurp_symbol_table.  */

  unsigned int fix_value : 1;

  /* Should the tag field of this symbol be renumbered.

     Created by coff_pointerize_aux. */

  unsigned int fix_tag : 1;

  /* Should the endidx field of this symbol be renumbered.

     Created by coff_pointerize_aux. */

  unsigned int fix_end : 1;

  /* Should the x_csect.x_scnlen field be renumbered.

     Created by coff_pointerize_aux. */

  unsigned int fix_scnlen : 1;

  /* Fix up an XCOFF C_BINCL/C_EINCL symbol.  The value is the

     index into the line number entries.  Set by coff_slurp_symbol_table.  */

  unsigned int fix_line : 1;

  /* The container for the symbol structure as read and translated

     from the file. */

  union

  @{

    union internal_auxent auxent;

    struct internal_syment syment;

  @} u;

@} combined_entry_type;

/* Each canonical asymbol really looks like this: */

typedef struct coff_symbol_struct

@{

  /* The actual symbol which the rest of BFD works with */

  asymbol symbol;

  /* A pointer to the hidden information for this symbol */

  combined_entry_type *native;

  /* A pointer to the linenumber information for this symbol */

  struct lineno_cache_entry *lineno;

  /* Have the line numbers been relocated yet ? */

  bfd_boolean done_lineno;

@} coff_symbol_type;

@end example

@findex bfd_coff_backend_data

@subsubsection @code{bfd_coff_backend_data}

@example

/* COFF symbol classifications.  */

enum coff_symbol_classification

@{

  /* Global symbol.  */

  COFF_SYMBOL_GLOBAL,

  /* Common symbol.  */

  COFF_SYMBOL_COMMON,

  /* Undefined symbol.  */

  COFF_SYMBOL_UNDEFINED,

  /* Local symbol.  */

  COFF_SYMBOL_LOCAL,

  /* PE section symbol.  */

  COFF_SYMBOL_PE_SECTION

@};

@end example

Special entry points for gdb to swap in coff symbol table parts:

@example

typedef struct

@{

  void (*_bfd_coff_swap_aux_in)

    (bfd *, void *, int, int, int, int, void *);

  void (*_bfd_coff_swap_sym_in)

    (bfd *, void *, void *);

  void (*_bfd_coff_swap_lineno_in)

    (bfd *, void *, void *);

  unsigned int (*_bfd_coff_swap_aux_out)

    (bfd *, void *, int, int, int, int, void *);

  unsigned int (*_bfd_coff_swap_sym_out)

    (bfd *, void *, void *);

  unsigned int (*_bfd_coff_swap_lineno_out)

    (bfd *, void *, void *);

  unsigned int (*_bfd_coff_swap_reloc_out)

    (bfd *, void *, void *);

  unsigned int (*_bfd_coff_swap_filehdr_out)

    (bfd *, void *, void *);

  unsigned int (*_bfd_coff_swap_aouthdr_out)

    (bfd *, void *, void *);

  unsigned int (*_bfd_coff_swap_scnhdr_out)

    (bfd *, void *, void *);

  unsigned int _bfd_filhsz;

  unsigned int _bfd_aoutsz;

  unsigned int _bfd_scnhsz;

  unsigned int _bfd_symesz;

  unsigned int _bfd_auxesz;

  unsigned int _bfd_relsz;

  unsigned int _bfd_linesz;

  unsigned int _bfd_filnmlen;

  bfd_boolean _bfd_coff_long_filenames;

  bfd_boolean _bfd_coff_long_section_names;

  unsigned int _bfd_coff_default_section_alignment_power;

  bfd_boolean _bfd_coff_force_symnames_in_strings;

  unsigned int _bfd_coff_debug_string_prefix_length;

  void (*_bfd_coff_swap_filehdr_in)

    (bfd *, void *, void *);

  void (*_bfd_coff_swap_aouthdr_in)

    (bfd *, void *, void *);

  void (*_bfd_coff_swap_scnhdr_in)

    (bfd *, void *, void *);

  void (*_bfd_coff_swap_reloc_in)

    (bfd *abfd, void *, void *);

  bfd_boolean (*_bfd_coff_bad_format_hook)

    (bfd *, void *);

  bfd_boolean (*_bfd_coff_set_arch_mach_hook)

    (bfd *, void *);

  void * (*_bfd_coff_mkobject_hook)

    (bfd *, void *, void *);

  bfd_boolean (*_bfd_styp_to_sec_flags_hook)

    (bfd *, void *, const char *, asection *, flagword *);

  void (*_bfd_set_alignment_hook)

    (bfd *, asection *, void *);

  bfd_boolean (*_bfd_coff_slurp_symbol_table)

    (bfd *);

  bfd_boolean (*_bfd_coff_symname_in_debug)

    (bfd *, struct internal_syment *);

  bfd_boolean (*_bfd_coff_pointerize_aux_hook)

    (bfd *, combined_entry_type *, combined_entry_type *,

            unsigned int, combined_entry_type *);

  bfd_boolean (*_bfd_coff_print_aux)

    (bfd *, FILE *, combined_entry_type *, combined_entry_type *,

            combined_entry_type *, unsigned int);

  void (*_bfd_coff_reloc16_extra_cases)

    (bfd *, struct bfd_link_info *, struct bfd_link_order *, arelent *,

           bfd_byte *, unsigned int *, unsigned int *);

  int (*_bfd_coff_reloc16_estimate)

    (bfd *, asection *, arelent *, unsigned int,

            struct bfd_link_info *);

  enum coff_symbol_classification (*_bfd_coff_classify_symbol)

    (bfd *, struct internal_syment *);

  bfd_boolean (*_bfd_coff_compute_section_file_positions)

    (bfd *);

  bfd_boolean (*_bfd_coff_start_final_link)

    (bfd *, struct bfd_link_info *);

  bfd_boolean (*_bfd_coff_relocate_section)

    (bfd *, struct bfd_link_info *, bfd *, asection *, bfd_byte *,

            struct internal_reloc *, struct internal_syment *, asection **);

  reloc_howto_type *(*_bfd_coff_rtype_to_howto)

    (bfd *, asection *, struct internal_reloc *,

            struct coff_link_hash_entry *, struct internal_syment *,

            bfd_vma *);

  bfd_boolean (*_bfd_coff_adjust_symndx)

    (bfd *, struct bfd_link_info *, bfd *, asection *,

            struct internal_reloc *, bfd_boolean *);

  bfd_boolean (*_bfd_coff_link_add_one_symbol)

    (struct bfd_link_info *, bfd *, const char *, flagword,

            asection *, bfd_vma, const char *, bfd_boolean, bfd_boolean,

            struct bfd_link_hash_entry **);

  bfd_boolean (*_bfd_coff_link_output_has_begun)

    (bfd *, struct coff_final_link_info *);

  bfd_boolean (*_bfd_coff_final_link_postscript)

    (bfd *, struct coff_final_link_info *);

@} bfd_coff_backend_data;

#define coff_backend_info(abfd) \

  ((bfd_coff_backend_data *) (abfd)->xvec->backend_data)

#define bfd_coff_swap_aux_in(a,e,t,c,ind,num,i) \

  ((coff_backend_info (a)->_bfd_coff_swap_aux_in) (a,e,t,c,ind,num,i))

#define bfd_coff_swap_sym_in(a,e,i) \

  ((coff_backend_info (a)->_bfd_coff_swap_sym_in) (a,e,i))

#define bfd_coff_swap_lineno_in(a,e,i) \

  ((coff_backend_info ( a)->_bfd_coff_swap_lineno_in) (a,e,i))

#define bfd_coff_swap_reloc_out(abfd, i, o) \

  ((coff_backend_info (abfd)->_bfd_coff_swap_reloc_out) (abfd, i, o))

#define bfd_coff_swap_lineno_out(abfd, i, o) \

  ((coff_backend_info (abfd)->_bfd_coff_swap_lineno_out) (abfd, i, o))

#define bfd_coff_swap_aux_out(a,i,t,c,ind,num,o) \

  ((coff_backend_info (a)->_bfd_coff_swap_aux_out) (a,i,t,c,ind,num,o))

#define bfd_coff_swap_sym_out(abfd, i,o) \

  ((coff_backend_info (abfd)->_bfd_coff_swap_sym_out) (abfd, i, o))

#define bfd_coff_swap_scnhdr_out(abfd, i,o) \

  ((coff_backend_info (abfd)->_bfd_coff_swap_scnhdr_out) (abfd, i, o))

#define bfd_coff_swap_filehdr_out(abfd, i,o) \

  ((coff_backend_info (abfd)->_bfd_coff_swap_filehdr_out) (abfd, i, o))

#define bfd_coff_swap_aouthdr_out(abfd, i,o) \

  ((coff_backend_info (abfd)->_bfd_coff_swap_aouthdr_out) (abfd, i, o))

#define bfd_coff_filhsz(abfd) (coff_backend_info (abfd)->_bfd_filhsz)

#define bfd_coff_aoutsz(abfd) (coff_backend_info (abfd)->_bfd_aoutsz)

#define bfd_coff_scnhsz(abfd) (coff_backend_info (abfd)->_bfd_scnhsz)

#define bfd_coff_symesz(abfd) (coff_backend_info (abfd)->_bfd_symesz)

#define bfd_coff_auxesz(abfd) (coff_backend_info (abfd)->_bfd_auxesz)

#define bfd_coff_relsz(abfd)  (coff_backend_info (abfd)->_bfd_relsz)

#define bfd_coff_linesz(abfd) (coff_backend_info (abfd)->_bfd_linesz)

#define bfd_coff_filnmlen(abfd) (coff_backend_info (abfd)->_bfd_filnmlen)

#define bfd_coff_long_filenames(abfd) \

  (coff_backend_info (abfd)->_bfd_coff_long_filenames)

#define bfd_coff_long_section_names(abfd) \

  (coff_backend_info (abfd)->_bfd_coff_long_section_names)

#define bfd_coff_default_section_alignment_power(abfd) \

  (coff_backend_info (abfd)->_bfd_coff_default_section_alignment_power)

#define bfd_coff_swap_filehdr_in(abfd, i,o) \

  ((coff_backend_info (abfd)->_bfd_coff_swap_filehdr_in) (abfd, i, o))

#define bfd_coff_swap_aouthdr_in(abfd, i,o) \

  ((coff_backend_info (abfd)->_bfd_coff_swap_aouthdr_in) (abfd, i, o))

#define bfd_coff_swap_scnhdr_in(abfd, i,o) \

  ((coff_backend_info (abfd)->_bfd_coff_swap_scnhdr_in) (abfd, i, o))

#define bfd_coff_swap_reloc_in(abfd, i, o) \

  ((coff_backend_info (abfd)->_bfd_coff_swap_reloc_in) (abfd, i, o))

#define bfd_coff_bad_format_hook(abfd, filehdr) \

  ((coff_backend_info (abfd)->_bfd_coff_bad_format_hook) (abfd, filehdr))

#define bfd_coff_set_arch_mach_hook(abfd, filehdr)\

  ((coff_backend_info (abfd)->_bfd_coff_set_arch_mach_hook) (abfd, filehdr))

#define bfd_coff_mkobject_hook(abfd, filehdr, aouthdr)\

  ((coff_backend_info (abfd)->_bfd_coff_mkobject_hook)\

   (abfd, filehdr, aouthdr))

#define bfd_coff_styp_to_sec_flags_hook(abfd, scnhdr, name, section, flags_ptr)\

  ((coff_backend_info (abfd)->_bfd_styp_to_sec_flags_hook)\

   (abfd, scnhdr, name, section, flags_ptr))

#define bfd_coff_set_alignment_hook(abfd, sec, scnhdr)\

  ((coff_backend_info (abfd)->_bfd_set_alignment_hook) (abfd, sec, scnhdr))

#define bfd_coff_slurp_symbol_table(abfd)\

  ((coff_backend_info (abfd)->_bfd_coff_slurp_symbol_table) (abfd))

#define bfd_coff_symname_in_debug(abfd, sym)\

  ((coff_backend_info (abfd)->_bfd_coff_symname_in_debug) (abfd, sym))

#define bfd_coff_force_symnames_in_strings(abfd)\

  (coff_backend_info (abfd)->_bfd_coff_force_symnames_in_strings)

#define bfd_coff_debug_string_prefix_length(abfd)\

  (coff_backend_info (abfd)->_bfd_coff_debug_string_prefix_length)

#define bfd_coff_print_aux(abfd, file, base, symbol, aux, indaux)\

  ((coff_backend_info (abfd)->_bfd_coff_print_aux)\

   (abfd, file, base, symbol, aux, indaux))

#define bfd_coff_reloc16_extra_cases(abfd, link_info, link_order,\

                                     reloc, data, src_ptr, dst_ptr)\

  ((coff_backend_info (abfd)->_bfd_coff_reloc16_extra_cases)\

   (abfd, link_info, link_order, reloc, data, src_ptr, dst_ptr))

#define bfd_coff_reloc16_estimate(abfd, section, reloc, shrink, link_info)\

  ((coff_backend_info (abfd)->_bfd_coff_reloc16_estimate)\

   (abfd, section, reloc, shrink, link_info))

#define bfd_coff_classify_symbol(abfd, sym)\

  ((coff_backend_info (abfd)->_bfd_coff_classify_symbol)\

   (abfd, sym))

#define bfd_coff_compute_section_file_positions(abfd)\

  ((coff_backend_info (abfd)->_bfd_coff_compute_section_file_positions)\

   (abfd))

#define bfd_coff_start_final_link(obfd, info)\

  ((coff_backend_info (obfd)->_bfd_coff_start_final_link)\

   (obfd, info))

#define bfd_coff_relocate_section(obfd,info,ibfd,o,con,rel,isyms,secs)\

  ((coff_backend_info (ibfd)->_bfd_coff_relocate_section)\

   (obfd, info, ibfd, o, con, rel, isyms, secs))

#define bfd_coff_rtype_to_howto(abfd, sec, rel, h, sym, addendp)\

  ((coff_backend_info (abfd)->_bfd_coff_rtype_to_howto)\

   (abfd, sec, rel, h, sym, addendp))

#define bfd_coff_adjust_symndx(obfd, info, ibfd, sec, rel, adjustedp)\

  ((coff_backend_info (abfd)->_bfd_coff_adjust_symndx)\

   (obfd, info, ibfd, sec, rel, adjustedp))

#define bfd_coff_link_add_one_symbol(info, abfd, name, flags, section,\

                                     value, string, cp, coll, hashp)\

  ((coff_backend_info (abfd)->_bfd_coff_link_add_one_symbol)\

   (info, abfd, name, flags, section, value, string, cp, coll, hashp))

#define bfd_coff_link_output_has_begun(a,p) \

  ((coff_backend_info (a)->_bfd_coff_link_output_has_begun) (a, p))

#define bfd_coff_final_link_postscript(a,p) \

  ((coff_backend_info (a)->_bfd_coff_final_link_postscript) (a, p))

@end example

@subsubsection Writing relocations

To write relocations, the back end steps though the

canonical relocation table and create an

@code{internal_reloc}. The symbol index to use is removed from

the @code{offset} field in the symbol table supplied.  The

address comes directly from the sum of the section base

address and the relocation offset; the type is dug directly

from the howto field.  Then the @code{internal_reloc} is

swapped into the shape of an @code{external_reloc} and written

out to disk.

@subsubsection Reading linenumbers

Creating the linenumber table is done by reading in the entire

coff linenumber table, and creating another table for internal use.

A coff linenumber table is structured so that each function

is marked as having a line number of 0. Each line within the

function is an offset from the first line in the function. The

base of the line number information for the table is stored in

the symbol associated with the function.

Note: The PE format uses line number 0 for a flag indicating a

new source file.

The information is copied from the external to the internal

table, and each symbol which marks a function is marked by

pointing its...

How does this work ?

@subsubsection Reading relocations

Coff relocations are easily transformed into the internal BFD form

(@code{arelent}).

Reading a coff relocation table is done in the following stages:

@itemize @bullet

@item

Read the entire coff relocation table into memory.

@item

Process each relocation in turn; first swap it from the

external to the internal form.

@item

Turn the symbol referenced in the relocation's symbol index

into a pointer into the canonical symbol table.

This table is the same as the one returned by a call to

@code{bfd_canonicalize_symtab}. The back end will call that

routine and save the result if a canonicalization hasn't been done.

@item

The reloc index is turned into a pointer to a howto

structure, in a back end specific way. For instance, the 386

and 960 use the @code{r_type} to directly produce an index

into a howto table vector; the 88k subtracts a number from the

@code{r_type} field and creates an addend field.

@end itemize