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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.  */

  /* The relocation was performed, but there was an overflow.  */

  /* The address to relocate was not within the section supplied.  */

  /* Used by special functions.  */

  /* Unsupported relocation size requested.  */

  /* Unused.  */

  /* The symbol to relocate against was 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.  */

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;


Here is a description of each of the fields within an arelent:

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. See section 2.7 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.

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.

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[];
                return foo[0x12345678];

Could be compiled into:

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

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

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:

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

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

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

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.

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