bfd/doc/coffcode.texi

@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_boolean (*_bfd_coff_print_pdata)
    (bfd *, void *);

@} 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))

#define bfd_coff_have_print_pdata(a) \
  (coff_backend_info (a)->_bfd_coff_print_pdata)
#define bfd_coff_print_pdata(a,p) \
  ((coff_backend_info (a)->_bfd_coff_print_pdata) (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