source: trunk/binutils/bfd/doc/bfd.info-3@ 3232

This is bfd.info, produced by makeinfo version 4.3 from bfd.texinfo.

START-INFO-DIR-ENTRY
* Bfd: (bfd).                   The Binary File Descriptor library.
END-INFO-DIR-ENTRY

   This file documents the BFD library.

   Copyright (C) 1991, 2000, 2001, 2003 Free Software Foundation, Inc.

   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 no Invariant Sections, with no Front-Cover Texts, and with no
    Back-Cover Texts.  A copy of the license is included in the
section entitled "GNU Free Documentation License".


File: bfd.info,  Node: typedef arelent,  Next: howto manager,  Prev: Relocations,  Up: Relocations

typedef arelent
---------------

   This is the structure of a relocation entry:


     typedef enum bfd_reloc_status
     {
       /* No errors detected.  */
       bfd_reloc_ok,
     
       /* The relocation was performed, but there was an overflow.  */
       bfd_reloc_overflow,
     
       /* The address to relocate was not within the section supplied.  */
       bfd_reloc_outofrange,
     
       /* Used by special functions.  */
       bfd_reloc_continue,
     
       /* Unsupported relocation size requested.  */
       bfd_reloc_notsupported,
     
       /* Unused.  */
       bfd_reloc_other,
     
       /* The symbol to relocate against was undefined.  */
       bfd_reloc_undefined,
     
       /* The relocation was performed, but may not be ok - presently
          generated only when linking i960 coff files with i960 b.out
          symbols.  If this type is returned, the error_message argument
          to bfd_perform_relocation will be set.  */
       bfd_reloc_dangerous
      }
      bfd_reloc_status_type;
     
     
     typedef struct reloc_cache_entry
     {
       /* A pointer into the canonical table of pointers.  */
       struct symbol_cache_entry **sym_ptr_ptr;
     
       /* offset in section.  */
       bfd_size_type address;
     
       /* addend for relocation value.  */
       bfd_vma addend;
     
       /* Pointer to how to perform the required relocation.  */
       reloc_howto_type *howto;
     
     }
     arelent;
   *Description*
Here is a description of each of the fields within an `arelent':

   * `sym_ptr_ptr'
   The symbol table pointer points to a pointer to the symbol
associated with the relocation request.  It is the pointer into the
table returned by the back end's `get_symtab' action. *Note Symbols::.
The symbol is referenced through a pointer to a pointer so that tools
like the linker can fix up all the symbols of the same name by
modifying only one pointer. The relocation routine looks in the symbol
and uses the base of the section the symbol is attached to and the
value of the symbol as the initial relocation offset. If the symbol
pointer is zero, then the section provided is looked up.

   * `address'
   The `address' field gives the offset in bytes from the base of the
section data which owns the relocation record to the first byte of
relocatable information. The actual data relocated will be relative to
this point; for example, a relocation type which modifies the bottom
two bytes of a four byte word would not touch the first byte pointed to
in a big endian world.

   * `addend'
   The `addend' is a value provided by the back end to be added (!)  to
the relocation offset. Its interpretation is dependent upon the howto.
For example, on the 68k the code:

             char foo[];
             main()
                     {
                     return foo[0x12345678];
                     }

   Could be compiled into:

             linkw fp,#-4
             moveb @#12345678,d0
             extbl d0
             unlk fp
             rts

   This could create a reloc pointing to `foo', but leave the offset in
the data, something like:

     RELOCATION RECORDS FOR [.text]:
     offset   type      value
     00000006 32        _foo
     
     00000000 4e56 fffc          ; linkw fp,#-4
     00000004 1039 1234 5678     ; moveb @#12345678,d0
     0000000a 49c0               ; extbl d0
     0000000c 4e5e               ; unlk fp
     0000000e 4e75               ; rts

   Using coff and an 88k, some instructions don't have enough space in
them to represent the full address range, and pointers have to be
loaded in two parts. So you'd get something like:

             or.u     r13,r0,hi16(_foo+0x12345678)
             ld.b     r2,r13,lo16(_foo+0x12345678)
             jmp      r1

   This should create two relocs, both pointing to `_foo', and with
0x12340000 in their addend field. The data would consist of:

     RELOCATION RECORDS FOR [.text]:
     offset   type      value
     00000002 HVRT16    _foo+0x12340000
     00000006 LVRT16    _foo+0x12340000
     
     00000000 5da05678           ; or.u r13,r0,0x5678
     00000004 1c4d5678           ; ld.b r2,r13,0x5678
     00000008 f400c001           ; jmp r1

   The relocation routine digs out the value from the data, adds it to
the addend to get the original offset, and then adds the value of
`_foo'. Note that all 32 bits have to be kept around somewhere, to cope
with carry from bit 15 to bit 16.

   One further example is the sparc and the a.out format. The sparc has
a similar problem to the 88k, in that some instructions don't have room
for an entire offset, but on the sparc the parts are created in odd
sized lumps. The designers of the a.out format chose to not use the
data within the section for storing part of the offset; all the offset
is kept within the reloc. Anything in the data should be ignored.

             save %sp,-112,%sp
             sethi %hi(_foo+0x12345678),%g2
             ldsb [%g2+%lo(_foo+0x12345678)],%i0
             ret
             restore

   Both relocs contain a pointer to `foo', and the offsets contain junk.

     RELOCATION RECORDS FOR [.text]:
     offset   type      value
     00000004 HI22      _foo+0x12345678
     00000008 LO10      _foo+0x12345678
     
     00000000 9de3bf90     ; save %sp,-112,%sp
     00000004 05000000     ; sethi %hi(_foo+0),%g2
     00000008 f048a000     ; ldsb [%g2+%lo(_foo+0)],%i0
     0000000c 81c7e008     ; ret
     00000010 81e80000     ; restore

   * `howto'
   The `howto' field can be imagined as a relocation instruction. It is
a pointer to a structure which contains information on what to do with
all of the other information in the reloc record and data section. A
back end would normally have a relocation instruction set and turn
relocations into pointers to the correct structure on input - but it
would be possible to create each howto field on demand.

`enum complain_overflow'
........................

   Indicates what sort of overflow checking should be done when
performing a relocation.


     enum complain_overflow
     {
       /* Do not complain on overflow.  */
       complain_overflow_dont,
     
       /* Complain if the bitfield overflows, whether it is considered
          as signed or unsigned.  */
       complain_overflow_bitfield,
     
       /* Complain if the value overflows when considered as signed
          number.  */
       complain_overflow_signed,
     
       /* Complain if the value overflows when considered as an
          unsigned number.  */
       complain_overflow_unsigned
     };

`reloc_howto_type'
..................

   The `reloc_howto_type' is a structure which contains all the
information that libbfd needs to know to tie up a back end's data.

     struct symbol_cache_entry;             /* Forward declaration.  */
     
     struct reloc_howto_struct
     {
       /*  The type field has mainly a documentary use - the back end can
           do what it wants with it, though normally the back end's
           external idea of what a reloc number is stored
           in this field.  For example, a PC relative word relocation
           in a coff environment has the type 023 - because that's
           what the outside world calls a R_PCRWORD reloc.  */
       unsigned int type;
     
       /*  The value the final relocation is shifted right by.  This drops
           unwanted data from the relocation.  */
       unsigned int rightshift;
     
       /*  The size of the item to be relocated.  This is *not* a
           power-of-two measure.  To get the number of bytes operated
           on by a type of relocation, use bfd_get_reloc_size.  */
       int size;
     
       /*  The number of bits in the item to be relocated.  This is used
           when doing overflow checking.  */
       unsigned int bitsize;
     
       /*  Notes that the relocation is relative to the location in the
           data section of the addend.  The relocation function will
           subtract from the relocation value the address of the location
           being relocated.  */
       bfd_boolean pc_relative;
     
       /*  The bit position of the reloc value in the destination.
           The relocated value is left shifted by this amount.  */
       unsigned int bitpos;
     
       /* What type of overflow error should be checked for when
          relocating.  */
       enum complain_overflow complain_on_overflow;
     
       /* If this field is non null, then the supplied function is
          called rather than the normal function.  This allows really
          strange relocation methods to be accomodated (e.g., i960 callj
          instructions).  */
       bfd_reloc_status_type (*special_function)
         PARAMS ((bfd *, arelent *, struct symbol_cache_entry *, PTR, asection *,
                  bfd *, char **));
     
       /* The textual name of the relocation type.  */
       char *name;
     
       /* Some formats record a relocation addend in the section contents
          rather than with the relocation.  For ELF formats this is the
          distinction between USE_REL and USE_RELA (though the code checks
          for USE_REL == 1/0).  The value of this field is TRUE if the
          addend is recorded with the section contents; when performing a
          partial link (ld -r) the section contents (the data) will be
          modified.  The value of this field is FALSE if addends are
          recorded with the relocation (in arelent.addend); when performing
          a partial link the relocation will be modified.
          All relocations for all ELF USE_RELA targets should set this field
          to FALSE (values of TRUE should be looked on with suspicion).
          However, the converse is not true: not all relocations of all ELF
          USE_REL targets set this field to TRUE.  Why this is so is peculiar
          to each particular target.  For relocs that aren't used in partial
          links (e.g. GOT stuff) it doesn't matter what this is set to.  */
       bfd_boolean partial_inplace;
     
       /* src_mask selects the part of the instruction (or data) to be used
          in the relocation sum.  If the target relocations don't have an
          addend in the reloc, eg. ELF USE_REL, src_mask will normally equal
          dst_mask to extract the addend from the section contents.  If
          relocations do have an addend in the reloc, eg. ELF USE_RELA, this
          field should be zero.  Non-zero values for ELF USE_RELA targets are
          bogus as in those cases the value in the dst_mask part of the
          section contents should be treated as garbage.  */
       bfd_vma src_mask;
     
       /* dst_mask selects which parts of the instruction (or data) are
          replaced with a relocated value.  */
       bfd_vma dst_mask;
     
       /* When some formats create PC relative instructions, they leave
          the value of the pc of the place being relocated in the offset
          slot of the instruction, so that a PC relative relocation can
          be made just by adding in an ordinary offset (e.g., sun3 a.out).
          Some formats leave the displacement part of an instruction
          empty (e.g., m88k bcs); this flag signals the fact.  */
       bfd_boolean pcrel_offset;
     };

`The HOWTO Macro'
.................

   *Description*
The HOWTO define is horrible and will go away.
     #define HOWTO(C, R, S, B, P, BI, O, SF, NAME, INPLACE, MASKSRC, MASKDST, PC) \
       { (unsigned) C, R, S, B, P, BI, O, SF, NAME, INPLACE, MASKSRC, MASKDST, PC }

   *Description*
And will be replaced with the totally magic way. But for the moment, we
are compatible, so do it this way.
     #define NEWHOWTO(FUNCTION, NAME, SIZE, REL, IN) \
       HOWTO (0, 0, SIZE, 0, REL, 0, complain_overflow_dont, FUNCTION, \
              NAME, FALSE, 0, 0, IN)

   *Description*
This is used to fill in an empty howto entry in an array.
     #define EMPTY_HOWTO(C) \
       HOWTO ((C), 0, 0, 0, FALSE, 0, complain_overflow_dont, NULL, \
              NULL, FALSE, 0, 0, FALSE)

   *Description*
Helper routine to turn a symbol into a relocation value.
     #define HOWTO_PREPARE(relocation, symbol)               \
       {                                                     \
         if (symbol != (asymbol *) NULL)                     \
           {                                                 \
             if (bfd_is_com_section (symbol->section))       \
               {                                             \
                 relocation = 0;                             \
               }                                             \
             else                                            \
               {                                             \
                 relocation = symbol->value;                 \
               }                                             \
           }                                                 \
       }

`bfd_get_reloc_size'
....................

   *Synopsis*
     unsigned int bfd_get_reloc_size (reloc_howto_type *);
   *Description*
For a reloc_howto_type that operates on a fixed number of bytes, this
returns the number of bytes operated on.

`arelent_chain'
...............

   *Description*
How relocs are tied together in an `asection':
     typedef struct relent_chain
     {
       arelent relent;
       struct relent_chain *next;
     }
     arelent_chain;

`bfd_check_overflow'
....................

   *Synopsis*
     bfd_reloc_status_type
     bfd_check_overflow
        (enum complain_overflow how,
         unsigned int bitsize,
         unsigned int rightshift,
         unsigned int addrsize,
         bfd_vma relocation);
   *Description*
Perform overflow checking on RELOCATION which has BITSIZE significant
bits and will be shifted right by RIGHTSHIFT bits, on a machine with
addresses containing ADDRSIZE significant bits.  The result is either of
`bfd_reloc_ok' or `bfd_reloc_overflow'.

`bfd_perform_relocation'
........................

   *Synopsis*
     bfd_reloc_status_type
     bfd_perform_relocation
        (bfd *abfd,
         arelent *reloc_entry,
         PTR data,
         asection *input_section,
         bfd *output_bfd,
         char **error_message);
   *Description*
If OUTPUT_BFD is supplied to this function, the generated image will be
relocatable; the relocations are copied to the output file after they
have been changed to reflect the new state of the world. There are two
ways of reflecting the results of partial linkage in an output file: by
modifying the output data in place, and by modifying the relocation
record.  Some native formats (e.g., basic a.out and basic coff) have no
way of specifying an addend in the relocation type, so the addend has
to go in the output data.  This is no big deal since in these formats
the output data slot will always be big enough for the addend. Complex
reloc types with addends were invented to solve just this problem.  The
ERROR_MESSAGE argument is set to an error message if this return
`bfd_reloc_dangerous'.

`bfd_install_relocation'
........................

   *Synopsis*
     bfd_reloc_status_type
     bfd_install_relocation
        (bfd *abfd,
         arelent *reloc_entry,
         PTR data, bfd_vma data_start,
         asection *input_section,
         char **error_message);
   *Description*
This looks remarkably like `bfd_perform_relocation', except it does not
expect that the section contents have been filled in.  I.e., it's
suitable for use when creating, rather than applying a relocation.

   For now, this function should be considered reserved for the
assembler.