Relocation information

Object files have one relocation entry for each relocatable reference in the text or data. The relocation information consists of entries with the format described in Table 7-8, ``Relocation section contents''.

Table 7-8 Relocation section contents

 Bytes   Declaration      Name       Description
 --------------------------------------------------------
  0-3    long int         r_vaddr    (Virtual) address of
                                     reference
  4-7    long int         r_symndx   Symbol table index
  8-9    unsigned short   r_type     Relocation type

The first 4 bytes of the entry are the virtual address of the text or data to which this entry applies. The next field is the index, counted from 0, of the symbol table entry that is being referenced. The type field indicates the type of relocation to be applied.

As the link editor reads each input section and performs relocation, the relocation entries are read. They direct how references found within the input section are treated. The currently recognized relocation types are given in Table 7-9, ``Relocation types''.

Table 7-9 Relocation types

 Mnemonic      Flag   Meaning
 ----------------------------------------------
 R_ABS           0    Reference is absolute; no
                      relocation is necessary.
                      The entry will be
                      ignored.
 R_DIR16 ∗      01    Direct, 16-bit reference
                      to a symbol's virtual
                      address.
 R_REL16 ∗      02    ``PC-relative'', 16-bit
                      reference to a symbol's
                      virtual address.
                      Relative references occur
                      in instructions such as
                      jumps and calls.
 R_DIR32        06    Direct 32-bit reference
                      to the symbol's virtual
                      address.
 R_SEG12 ∗     011    Direct, 16-bit reference
                      to the segment-selector
                      bits of a 32-bit virtual
                      address.
 R_PCRLONG +   024    ``PC-relative'', 32-bit
                      reference to a symbol's
                      virtual address.
 ----------------------------------------------
 ∗  80286 Computer only.
 +  80386 Computer only.

Relocation entry declaration

The structure declaration for relocation entries is shown below. This declaration may be found in the header file reloc.h.

   struct reloc
   {
     long            r_vaddr;   /* virtual address of reference */
     long            r_symndx;  /* index into symbol table */
     unsigned short  r_type;    /* relocation type */
   };
   

   #define RELOC    struct reloc
   #define RELSZ    10