Untitled Document: typedef arelent

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#### 2.10.1 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 bfd_symbol **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 `canonicalize_symtab` action. See 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.

#### 2.10.1.1 `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 value overflows when considered as a signed
         number one bit larger than the field.  ie. A bitfield of N bits
         is allowed to represent -2**n to 2**n-1.  */
      complain_overflow_bitfield,

      /* Complain if the value overflows when considered as a signed
         number.  */
      complain_overflow_signed,

      /* Complain if the value overflows when considered as an
         unsigned number.  */
      complain_overflow_unsigned
    };

#### 2.10.1.2 `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 bfd_symbol;             /* 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;

      /*  The relocation is relative to the field 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 accommodated (e.g., i960 callj
         instructions).  */
      bfd_reloc_status_type (*special_function)
        (bfd *, arelent *, struct bfd_symbol *, void *, 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;
    };

#### 2.10.1.3 `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 != NULL)                                 \
          {                                                 \
            if (bfd_is_com_section (symbol->section))       \
              {                                             \
                relocation = 0;                             \
              }                                             \
            else                                            \
              {                                             \
                relocation = symbol->value;                 \
              }                                             \
          }                                                 \
      }

#### 2.10.1.4 `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.

#### 2.10.1.5 `arelent_chain`

Description
How relocs are tied together in an `asection`:

    typedef struct relent_chain
    {
      arelent relent;
      struct relent_chain *next;
    }
    arelent_chain;

#### 2.10.1.6 `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`.

#### 2.10.1.7 `bfd_perform_relocation`

Synopsis

    bfd_reloc_status_type bfd_perform_relocation
       (bfd *abfd,
        arelent *reloc_entry,
        void *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`.

#### 2.10.1.8 `bfd_install_relocation`

Synopsis

    bfd_reloc_status_type bfd_install_relocation
       (bfd *abfd,
        arelent *reloc_entry,
        void *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.

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