+
+Relocation is the process of connecting symbolic references +with symbolic definitions. +For example, when a program calls a function, the associated call +instruction must transfer control to the proper destination address +at execution. +Relocatable files must have +``relocation entries'' which +are necessary because they contain information that +describes how to modify their section contents, thus allowing +executable and shared object files to hold +the right information for a process's program image. +
+
+
+typedef struct {
+ Elf32_Addr r_offset;
+ Elf32_Word r_info;
+} Elf32_Rel;
+
+typedef struct {
+ Elf32_Addr r_offset;
+ Elf32_Word r_info;
+ Elf32_Sword r_addend;
+} Elf32_Rela;
+
+typedef struct {
+ Elf64_Addr r_offset;
+ Elf64_Xword r_info;
+} Elf64_Rel;
+
+typedef struct {
+ Elf64_Addr r_offset;
+ Elf64_Xword r_info;
+ Elf64_Sxword r_addend;
+} Elf64_Rela;
+
+
+r_offsetr_infoSTN_UNDEF,
+the undefined symbol index,
+the relocation uses 0 as the ``symbol value''.
+Relocation types are processor-specific;
+descriptions of their behavior appear in the processor
+supplement.
+When the text below refers to a relocation entry's
+relocation type or symbol table index, it means the result of applying
+ELF32_R_TYPE (or ELF64_R_TYPE) or ELF32_R_SYM (or ELF64_R_SYM),
+respectively, to the entry's r_info member.
++ #define ELF32_R_SYM(i) ((i)>>8) + #define ELF32_R_TYPE(i) ((unsigned char)(i)) + #define ELF32_R_INFO(s,t) (((s)<<8)+(unsigned char)(t)) + + #define ELF64_R_SYM(i) ((i)>>32) + #define ELF64_R_TYPE(i) ((i)&0xffffffffL) + #define ELF64_R_INFO(s,t) (((s)<<32)+((t)&0xffffffffL)) ++
r_addend
+As specified previously, only
+Elf32_Rela and Elf64_Rela
+entries contain an explicit addend.
+Entries of type Elf32_Rel and Elf64_Rel
+store an implicit addend in the location to be modified.
+Depending on the processor architecture, one form or the other
+might be necessary or more convenient.
+Consequently, an implementation for a particular machine
+may use one form exclusively or either form depending on context.
+
+A relocation section references two other sections:
+a symbol table and a section to modify.
+The section header's sh_info and sh_link
+members, described in
+``Sections''
+above, specify these relationships.
+Relocation entries for different object files have
+slightly different interpretations for the
+r_offset member.
+
+
r_offset
+holds a section offset.
+The relocation section itself describes how to
+modify another section in the file; relocation offsets
+designate a storage unit within the second section.
+r_offset holds a virtual address.
+To make these files' relocation entries more useful
+for the dynamic linker, the section offset (file interpretation)
+gives way to a virtual address (memory interpretation).
+r_offset
+changes for different object files to
+allow efficient access by the relevant programs,
+the relocation types' meanings stay the same.
++ +The typical application of an ELF relocation is to determine the +referenced symbol value, extract the addend (either from the +field to be relocated or from the addend field contained in +the relocation record, as appropriate for the type of relocation +record), apply the expression implied by the relocation type +to the symbol and addend, extract the desired part of the expression +result, and place it in the field to be relocated. +
+If multiple consecutive relocation records are applied
+to the same relocation location (r_offset),
+they are composed instead
+of being applied independently, as described above.
+By consecutive, we mean that the relocation records are
+contiguous within a single relocation section. By composed,
+we mean that the standard application described above is modified
+as follows:
+
+Note that a consequence of the above rules is that the location specified +by a relocation type is relevant for the +first element of a composed sequence (and then only for relocation +records that do not contain an explicit addend field) and for the +last element, where the location determines where the relocated value +will be placed. For all other relocation operands in a composed +sequence, the location specified is ignored. +
+An ABI processor supplement may specify individual relocation types +that always stop a composition sequence, or always start a new one. + +
+This section requires processor-specific information. The ABI
+supplement for the desired processor describes the details.
+
+
+
+