reloc.5

<B>Digital的Unix操作系统VAX 4.2源码</B>

.TH reloc 5 RISC
.SH Name
reloc \- relocation information for a MIPS object file
.SH Syntax
.B #include  <reloc.h>
.SH Description
Object files have one relocation entry for each relocatable
reference in the text or data.  If relocation information is
present, it will be in the following format:
.PP
.EX
struct   reloc
{
   long      r_vaddr ;          /* (virtual) address of reference */
   long      r_symndx ;         /* index into symbol table */
   ushort    r_type ;           /* relocation type */
   unsigned  r_symndx:24,       /* index into symbol table */
             r_reserved:3,
             r_type:4,          /* relocation type */
             r_extern:1;        /* if 1 symndx is an index into the 
                                   external symbol table, else symndx 
                                   is a section # */
} ;
.EE
.PP
.EX
/* Relocation types */

#define  R_ABS       0
#define  R_REFHALF   1
#define  R_REFWORD   2
#define  R_JMPADDR   3
#define  R_REFHI     4
#define  R_REFLO     5
#define  R_GPREL     6
#define  R_LITERAL   7
.EE
.PP
.EX
/* Section numbers */

#define  R_SN_NULL   0
#define  R_SN_TEXT   1
#define  R_SN_RDATA  2
#define  R_SN_DATA   3
#define  R_SN_SDATA  4
#define  R_SN_SBSS   5
#define  R_SN_BSS    6
#define  R_SN_INIT   7
#define  R_SN_LIT8   8
#define  R_SN_LIT4   9
.EE
.PP
The link editor reads each input section and performs
relocation.  The relocation entries direct how references
found within the input section are treated.
.PP
If 
.PN r_extern 
is zero, it is a local relocation entry and
then 
.PN r_symndex 
is a section number (R_SN_*).  For these
entries, the starting address for the section referenced by
the section number is used in place of an external symbol
table entry's value.  The assembler and loader always use
local relocation entries if the item to be relocated is
defined in the object file.
.PP
For every external relocation (except R_ABS) a signed
constant is added to the symbol's virtual address that the
relocation entry refers to.  This constant is assembled at
the address being relocated.
.TP 15
R_ABS
The reference is absolute and no
relocation is necessary.  The entry will
be ignored.
.TP 15
R_REFHALF
A 16-bit reference to the symbol's virtual
address.
.TP 15
R_REFWORD
A 32-bit reference to the symbol's virtual
address.
.TP 15
R_JMPADDR
A 26-bit jump instruction reference to the
symbol's virtual address.
.TP 15
R_REFHI
A reference to the high 16 bits of the
symbol's virtual address.  The next
relocation entry must be the corresponding
R_REFLO entry, so the proper value of the
constant to be added to the symbol's
virtual address can be reconstructed.
.TP 15
R_REFLO
A reference to low 16 bits to the symbol's
virtual address.
.TP 15
R_GPREL
A 16-bit offset to the symbol's virtual
address from the global pointer register.
.TP 15
R_LITERAL
A 16-bit offset to the literal's virtual
address from the global pointer register.
.PP
Relocation entries are generated automatically by the
assembler and automatically used by the link editor.  Link
editor options exist for both preserving and removing the
relocation entries from object files.
.PP
The number of relocation entries for a section is found in
the 
.PN s_nreloc 
field of the section header.  This field is a
C language short and can overflow with large objects.  If
this field overflows, the section header 
.PN s_flags 
field has
the S_NRELOC_OVFL bit set.  In this case, the true number of
relocation entries is found in the 
.PN r_vaddr 
field of the
first relocation entry for that section.  That relocation
entry has a type of R_ABS, so it is ignored when the
relocation takes place. 
.SH See Also
as(1), ld(1), a.out(5), syms(5), scnhdr(5)