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BFD back ends

All of BFD lives in one directory.

a.out backends

BFD supports a number of different flavours of a.out format, though the major differences are only the sizes of the structures on disk, and the shape of the relocation information. The support is split into a basic support file `aoutx.h' and other files which derive functions from the base. One derivation file is `aoutf1.h' (for a.out flavour 1), and adds to the basic a.out functions support for sun3, sun4, 386 and 29k a.out files, to create a target jump vector for a specific target. This information is further split out into more specific files for each machine, including `sunos.c' for sun3 and sun4, `newsos3.c' for the Sony NEWS, and `demo64.c' for a demonstration of a 64 bit a.out format. The base file `aoutx.h' defines general mechanisms for reading and writing records to and from disk and various other methods which BFD requires. It is included by `aout32.c' and `aout64.c' to form the names aout_32_swap_exec_header_in, aout_64_swap_exec_header_in, etc. As an example, this is what goes on to make the back end for a sun4, from `aout32.c':

	#define ARCH_SIZE 32
	#include "aoutx.h"

Which exports names:


from `sunos.c':

	#define TARGET_NAME "a.out-sunos-big"
	#define VECNAME    sunos_big_vec
	#include "aoutf1.h"

requires all the names from `aout32.c', and produces the jump vector


The file `host-aout.c' is a special case. It is for a large set of hosts that use "more or less standard" a.out files, and for which cross-debugging is not interesting. It uses the standard 32-bit a.out support routines, but determines the file offsets and addresses of the text, data, and BSS sections, the machine architecture and machine type, and the entry point address, in a host-dependent manner. Once these values have been determined, generic code is used to handle the object file. When porting it to run on a new system, you must supply:

        HOST_MACHINE_ARCH       (optional)
        HOST_MACHINE_MACHINE    (optional)

in the file `../include/sys/h-XXX.h' (for your host). These values, plus the structures and macros defined in `a.out.h' on your host system, will produce a BFD target that will access ordinary a.out files on your host. To configure a new machine to use `host-aout.c', specify:

	TDEFAULTS = -DDEFAULT_VECTOR=host_aout_big_vec
	TDEPFILES= host-aout.o trad-core.o

in the `config/XXX.mt' file, and modify `configure.in' to use the `XXX.mt' file (by setting "bfd_target=XXX") when your configuration is selected.


The file `aoutx.h' provides for both the standard and extended forms of a.out relocation records. The standard records contain only an address, a symbol index, and a type field. The extended records (used on 29ks and sparcs) also have a full integer for an addend.

Internal entry points

`aoutx.h' exports several routines for accessing the contents of an a.out file, which are gathered and exported in turn by various format specific files (eg sunos.c).



void aout_size_swap_exec_header_in,
   (bfd *abfd,
    struct external_exec *raw_bytes,
    struct internal_exec *execp);

Swap the information in an executable header raw_bytes taken from a raw byte stream memory image into the internal exec header structure execp.



void aout_size_swap_exec_header_out
   (bfd *abfd,
    struct internal_exec *execp,
    struct external_exec *raw_bytes);

Swap the information in an internal exec header structure execp into the buffer raw_bytes ready for writing to disk.



const bfd_target *aout_size_some_aout_object_p
   (bfd *abfd,
    const bfd_target *(*callback_to_real_object_p)());

Some a.out variant thinks that the file open in abfd checking is an a.out file. Do some more checking, and set up for access if it really is. Call back to the calling environment's "finish up" function just before returning, to handle any last-minute setup.



boolean aout_size_mkobject, (bfd *abfd);

Initialize BFD abfd for use with a.out files.



enum machine_type  aout_size_machine_type
   (enum bfd_architecture arch,
    unsigned long machine));
    @end example
Keep track of machine architecture and machine type for a.out's. Return the machine_type for a particular architecture and machine, or M_UNKNOWN if that exact architecture and machine can't be represented in a.out format. If the architecture is understood, machine type 0 (default) is always understood.



boolean aout_size_set_arch_mach,
   (bfd *,
    enum bfd_architecture arch,
    unsigned long machine));
    @end example
Set the architecture and the machine of the BFD abfd to the values arch and machine. Verify that abfd's format can support the architecture required.



boolean aout_size_new_section_hook,
   (bfd *abfd,
    asection *newsect));
    @end example
Called by the BFD in response to a bfd_make_section request.

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 `coff-m88k.c'. This file #includes `coff/m88k.h' which defines the external structure of the coff format for the 88k, and `coff/internal.h' which defines the internal structure. `coff-m88k.c' also defines the relocations used by the 88k format See section Relocations. The Intel i960 processor version of coff is implemented in `coff-i960.c'. This file has the same structure as `coff-m88k.c', except that it includes `coff/i960.h' rather than `coff-m88k.h'.

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 `i386coff.c' to `foocoff.c', copy `../include/coff/i386.h' to `../include/coff/foo.h', and add the lines to `targets.c' and `Makefile.in' so that your new back end is used. Alter the shapes of the structures in `../include/coff/foo.h' so that they match what you need. You will probably also have to add #ifdefs to the code in `coff/internal.h' and `coffcode.h' if your version of coff is too wild. You can verify that your new BFD backend works quite simply by building `objdump' from the `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 att-dump or just 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.

How the coff backend works

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 `coffgen.c'. These routines work for any Coff target. They use some hooks into the target specific code; the hooks are in a bfd_coff_backend_data structure, one of which exists for each target. The essentially similar target-specific routines are in `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 `coffcode.h'. Some of the Coff targets then also have additional routines in the target source file itself. For example, `coff-i960.c' includes `coff/internal.h' and `coff/i960.h'. It then defines a few constants, such as I960, and includes `coffcode.h'. Since the i960 has complex relocation types, `coff-i960.c' also includes some code to manipulate the i960 relocs. This code is not in `coffcode.h' because it would not be used by any other target.

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 `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 bfd_swap_thing_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 `coffcode.h'. E.g., the size of line number entry in coff is sometimes 16 bits, and sometimes 32 bits. #defineing PUT_LNSZ_LNNO and 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 #defines. Three of the bit twiddling routines are exported to gdb; coff_swap_aux_in, coff_swap_sym_in and coff_swap_linno_in. GDB reads the symbol table on its own, but uses BFD to fix things up. More of the bit twiddlers are exported for gas; coff_swap_aux_out, coff_swap_sym_out, coff_swap_lineno_out, coff_swap_reloc_out, coff_swap_filehdr_out, coff_swap_aouthdr_out, coff_swap_scnhdr_out. 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 gas, which makes one avenue to disaster disappear.

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 bfd_canonicalize_symtab), a request gets through to 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 (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 (".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 sclass field) about the various flags to set in the asymbol. See section 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.

Symbol writing

Writing a symbol to a coff file which didn't come from a coff file will lose any debugging information. The 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 (C_FILE) to point to the next file place holder in the list. It also marks each 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 output_section field of the asymbol's asection See section Sections.

  • coff_mangle_symbols

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 hierachy required by coff. It changes each pointer to a symbol into the index into the symbol table of the asymbol.

  • coff_write_symbols

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.


The hidden information for an asymbol is described in a combined_entry_type:

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 ? */
boolean done_lineno;
} coff_symbol_type;


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

typedef struct
  void (*_bfd_coff_swap_aux_in) PARAMS ((
       bfd            *abfd,
       PTR             ext,
       int             type,
       int             class,
       int             indaux,
       int             numaux,
       PTR             in));

  void (*_bfd_coff_swap_sym_in) PARAMS ((
       bfd            *abfd ,
       PTR             ext,
       PTR             in));

  void (*_bfd_coff_swap_lineno_in) PARAMS ((
       bfd            *abfd,
       PTR            ext,
       PTR             in));

Special entry points for gas to swap out coff parts:

 unsigned int (*_bfd_coff_swap_aux_out) PARAMS ((
       bfd   	*abfd,
       PTR	in,
       int    	type,
       int    	class,
       int     indaux,
       int     numaux,
       PTR    	ext));

 unsigned int (*_bfd_coff_swap_sym_out) PARAMS ((
      bfd      *abfd,
      PTR	in,
      PTR	ext));

 unsigned int (*_bfd_coff_swap_lineno_out) PARAMS ((
      	bfd   	*abfd,
      	PTR	in,
	PTR	ext));

 unsigned int (*_bfd_coff_swap_reloc_out) PARAMS ((
      	bfd     *abfd,
     	PTR	src,
	PTR	dst));

 unsigned int (*_bfd_coff_swap_filehdr_out) PARAMS ((
      	bfd  	*abfd,
	PTR 	in,
	PTR 	out));

 unsigned int (*_bfd_coff_swap_aouthdr_out) PARAMS ((
      	bfd 	*abfd,
	PTR 	in,
	PTR	out));

 unsigned int (*_bfd_coff_swap_scnhdr_out) PARAMS ((
      	bfd  	*abfd,
      	PTR	in,
	PTR	out));

Special entry points for generic COFF routines to call target dependent COFF routines:

 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;
 boolean _bfd_coff_long_filenames;
 void (*_bfd_coff_swap_filehdr_in) PARAMS ((
       bfd     *abfd,
       PTR     ext,
       PTR     in));
 void (*_bfd_coff_swap_aouthdr_in) PARAMS ((
       bfd     *abfd,
       PTR     ext,
       PTR     in));
 void (*_bfd_coff_swap_scnhdr_in) PARAMS ((
       bfd     *abfd,
       PTR     ext,
       PTR     in));
 void (*_bfd_coff_swap_reloc_in) PARAMS ((
       bfd     *abfd,
       PTR     ext,
       PTR     in));
 boolean (*_bfd_coff_bad_format_hook) PARAMS ((
       bfd     *abfd,
       PTR     internal_filehdr));
 boolean (*_bfd_coff_set_arch_mach_hook) PARAMS ((
       bfd     *abfd,
       PTR     internal_filehdr));
 PTR (*_bfd_coff_mkobject_hook) PARAMS ((
       bfd     *abfd,
       PTR     internal_filehdr,
       PTR     internal_aouthdr));
 flagword (*_bfd_styp_to_sec_flags_hook) PARAMS ((
       bfd     *abfd,
       PTR     internal_scnhdr,
       const char *name));
 void (*_bfd_set_alignment_hook) PARAMS ((
       bfd     *abfd,
       asection *sec,
       PTR     internal_scnhdr));
 boolean (*_bfd_coff_slurp_symbol_table) PARAMS ((
       bfd     *abfd));
 boolean (*_bfd_coff_symname_in_debug) PARAMS ((
       bfd     *abfd,
       struct internal_syment *sym));
 boolean (*_bfd_coff_pointerize_aux_hook) PARAMS ((
       bfd *abfd,
       combined_entry_type *table_base,
       combined_entry_type *symbol,
       unsigned int indaux,
       combined_entry_type *aux));
 boolean (*_bfd_coff_print_aux) PARAMS ((
       bfd *abfd,
       FILE *file,
       combined_entry_type *table_base,
       combined_entry_type *symbol,
       combined_entry_type *aux,
       unsigned int indaux));
 void (*_bfd_coff_reloc16_extra_cases) PARAMS ((
       bfd     *abfd,
       struct bfd_link_info *link_info,
       struct bfd_link_order *link_order,
       arelent *reloc,
       bfd_byte *data,
       unsigned int *src_ptr,
       unsigned int *dst_ptr));
 int (*_bfd_coff_reloc16_estimate) PARAMS ((
       bfd *abfd,
       asection *input_section,
       arelent *r,
       unsigned int shrink,
       struct bfd_link_info *link_info));
 boolean (*_bfd_coff_sym_is_global) PARAMS ((
       bfd *abfd,
       struct internal_syment *));
 void (*_bfd_coff_compute_section_file_positions) PARAMS ((
       bfd *abfd));
 boolean (*_bfd_coff_start_final_link) PARAMS ((
       bfd *output_bfd,
       struct bfd_link_info *info));
 boolean (*_bfd_coff_relocate_section) PARAMS ((
       bfd *output_bfd,
       struct bfd_link_info *info,
       bfd *input_bfd,
       asection *input_section,
       bfd_byte *contents,
       struct internal_reloc *relocs,
       struct internal_syment *syms,
       asection **sections));
 reloc_howto_type *(*_bfd_coff_rtype_to_howto) PARAMS ((
       bfd *abfd,
       asection *sec,
       struct internal_reloc *rel,
       struct coff_link_hash_entry *h,
       struct internal_syment *sym,
       bfd_vma *addendp));
 boolean (*_bfd_coff_adjust_symndx) PARAMS ((
       bfd *obfd,
       struct bfd_link_info *info,
       bfd *ibfd,
       asection *sec,
       struct internal_reloc *reloc,
       boolean *adjustedp));
 boolean (*_bfd_coff_link_add_one_symbol) PARAMS ((
       struct bfd_link_info *info,
       bfd *abfd,
       const char *name,
       flagword flags, 
       asection *section,
       bfd_vma value,
       const char *string,
       boolean copy,
       boolean collect, 
       struct bfd_link_hash_entry **hashp));

} 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_long_filenames(abfd) (coff_backend_info (abfd)->_bfd_coff_long_filenames)
#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)\
        ((coff_backend_info (abfd)->_bfd_styp_to_sec_flags_hook) (abfd, scnhdr, name))

#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_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_sym_is_global(abfd, sym)\
        ((coff_backend_info (abfd)->_bfd_coff_sym_is_global)\
         (abfd, sym))

#define bfd_coff_compute_section_file_positions(abfd)\
        ((coff_backend_info (abfd)->_bfd_coff_compute_section_file_positions)\

#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))

Writing relocations

To write relocations, the back end steps though the canonical relocation table and create an internal_reloc. The symbol index to use is removed from the 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 internal_reloc is swapped into the shape of an external_reloc and written out to disk.

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. 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 ?

Reading relocations

Coff relocations are easily transformed into the internal BFD form (arelent). Reading a coff relocation table is done in the following stages:

  • Read the entire coff relocation table into memory.
  • Process each relocation in turn; first swap it from the external to the internal form.
  • 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 bfd_canonicalize_symtab. The back end will call that routine and save the result if a canonicalization hasn't been done.
  • 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 r_type to directly produce an index into a howto table vector; the 88k subtracts a number from the r_type field and creates an addend field.

ELF backends

BFD support for ELF formats is being worked on. Currently, the best supported back ends are for sparc and i386 (running svr4 or Solaris 2). Documentation of the internals of the support code still needs to be written. The code is changing quickly enough that we haven't bothered yet.



struct elf_internal_shdr *bfd_elf_find_section (bfd *abfd, char *name);

Helper functions for GDB to locate the string tables. Since BFD hides string tables from callers, GDB needs to use an internal hook to find them. Sun's .stabstr, in particular, isn't even pointed to by the .stab section, so ordinary mechanisms wouldn't work to find it, even if we had some.

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