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1 Relocations

BFD maintains relocations in much the same was as it maintains symbols; they are left alone until required, then read in en-mass and traslated into an internal form. There is a common routine bfd_perform_relocation which acts upon the canonical form to to the actual fixup. Note that relocations are maintained on a per section basis, whilst symbols are maintained on a per BFD basis. All a back end has to do to fit the BFD interface is to create as many struct reloc_cache_entry as there are relocations in a particuar section, and fill in the right bits:



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1.1 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,

       	/* Unused */
  bfd_reloc_notsupported,

       	/* Unsupported relocation size requested.  */
  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. */
  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 */
  rawdata_offset address;

       	/* addend for relocation value */
  bfd_vma addend;    

       	/* Pointer to how to perform the required relocation */
  CONST struct reloc_howto_struct *howto;

} arelent;

Description
Here is a description of each of the fields within a relent:



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1.1.1 reloc_howto_type

The reloc_howto_type is a structure which contains all the information that BFD needs to know to tie up a back end’s data.
.struct symbol_cache_entry; /* Forward declaration */

typedef CONST struct reloc_howto_struct 
{ 
       	/*  The type field has mainly a documetary use - the back end can
           to what it wants with it, though the normally the back end's
           external idea of what a reloc number would be would be stored
           in this field. For example, the a PC relative word relocation
           in a coff environment would have 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 - 0, is one byte, 1 is 2
           bytes, 3 is four bytes. */
  unsigned int size;

       	/*  Now obsolete */
  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. */
  boolean pc_relative;

       	/*  Now obsolete */
  unsigned int bitpos;

       	/*  Now obsolete */
  boolean absolute;

       	/* Causes the relocation routine to return an error if overflow
          is detected when relocating. */
  boolean 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 (eg, i960 callj
          instructions). */
  bfd_reloc_status_type EXFUN ((*special_function), 
					    (bfd *abfd,
					     arelent *reloc_entry,
                                            struct symbol_cache_entry *symbol,
                                            PTR data,
                                            asection *input_section));

       	/* The textual name of the relocation type. */
  char *name;

       	/* When performing a partial link, some formats must modify the
          relocations rather than the data - this flag signals this.*/
  boolean partial_inplace;

       	/* The src_mask is used to select what parts of the read in data
          are to be used in the relocation sum. Eg, if this was an 8 bit
          bit of data which we read and relocated, this would be
          0x000000ff. When we have relocs which have an addend, such as
          sun4 extended relocs, the value in the offset part of a
          relocating field is garbage so we never use it. In this case
          the mask would be 0x00000000. */
  bfd_word src_mask;

       	/* The dst_mask is what parts of the instruction are replaced
          into the instruction. In most cases src_mask == dst_mask,
          except in the above special case, where dst_mask would be
          0x000000ff, and src_mask would be 0x00000000.   */
  bfd_word 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 (eg sun3 a.out).
          Some formats leave the displacement part of an instruction
          empty (eg m88k bcs), this flag signals the fact.*/
  boolean pcrel_offset;

} reloc_howto_type;

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1.1.2 the HOWTO macro

Description
The HOWTO define is horrible and will go away.

#define HOWTO(C, R,S,B, P, BI, ABS, O, SF, NAME, INPLACE, MASKSRC, MASKDST, PC) \
  {(unsigned)C,R,S,B, P, BI, ABS,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,false,false,FUNCTION, NAME,false,0,0,IN)


Description
Helper routine to turn a symbol into a relocation value.

#define HOWTO_PREPARE(relocation, symbol)      \
  {                                            \
  if (symbol != (asymbol *)NULL) {             \
    if (symbol->section == &bfd_com_section) { \
      relocation = 0;                          \
    }                                          \
    else {                                     \
      relocation = symbol->value;              \
    }                                          \
  }                                            \
}


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1.1.3 reloc_chain

Description
How relocs are tied together

typedef unsigned char bfd_byte;

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


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1.1.4 bfd_perform_relocation

Synopsis

bfd_reloc_status_type
bfd_perform_relocation
   (bfd * abfd,
    arelent *reloc_entry,
    PTR data,
    asection *input_section,
    bfd *output_bfd);

Description
If an 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 (eg 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.


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2 The howto manager

When an application wants to create a relocation, but doesn’t know what the target machine might call it, it can find out by using this bit of code.


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2.0.1 bfd_reloc_code_type

Description
The insides of a reloc code
.

typedef enum bfd_reloc_code_real 
{
       	/* 16 bits wide, simple reloc */
  BFD_RELOC_16,        

       	/* 8 bits wide, but used to form an address like 0xffnn */
  BFD_RELOC_8_FFnn,

       	/* 8 bits wide, simple */
  BFD_RELOC_8,

       	/* 8 bits wide, pc relative */
  BFD_RELOC_8_PCREL,

       	/* The type of reloc used to build a contructor table - at the
          moment probably a 32 bit wide abs address, but the cpu can
          choose. */

  BFD_RELOC_CTOR
 } bfd_reloc_code_real_type;

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3 bfd_reloc_type_lookup


Synopsis

CONST struct reloc_howto_struct *
bfd_reloc_type_lookup
   (CONST bfd_arch_info_type *arch, bfd_reloc_code_type code);

Description
This routine returns a pointer to a howto struct which when invoked, will perform the supplied relocation on data from the architecture noted.


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3.0.1 bfd_default_reloc_type_lookup

Synopsis

CONST struct reloc_howto_struct *bfd_default_reloc_type_lookup
   (CONST struct bfd_arch_info *,
    bfd_reloc_code_type  code);

Description
Provides a default relocation lookuperer for any architectue


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3.0.2 bfd_generic_relax_section

Synopsis

boolean bfd_generic_relax_section
   (bfd *abfd,
    asection *section,
    asymbol **symbols);

Description
Provides default handling for relaxing for back ends which don’t do relaxing - ie does nothing


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3.0.3 bfd_generic_get_relocated_section_contents

Synopsis

bfd_byte *
bfd_generic_get_relocated_section_contents(bfd *abfd,
    struct bfd_seclet_struct  *seclet)

Description
Provides default handling of relocation effort for back ends which can’t be bothered to do it efficiently.


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