Help about 'addend' of The ELF specification

[Anitha Boyapati]

On Tue, 14 Aug 2007, Brian Dessent wrote:

> PRC wrote:
> 
> > I'm studying the ELF specfication version 1.1 recently. 
> > I'm confused about the 'addend' which is used in relocation. There 
> > doesn't seems to be explict descriptions about 'addend' in the 
> > specification. Can somebody make detailed explainations about how to 
> > calculate the value of the 'addend' when relocating an executable.
> 
> 
> Each architecture defines its own set of R_<arch> relocations, each with
> its own definition, so you can't really deduce anything from the generic
> spec.

  True.

  Btwn, your question appears to me as the purpose of addend other than as 
  "constant" added to get the relocated value. It manifests itself during
  segment relocations - generally speaking. Incase thats not at all what 
  you meant, you can happily ignore the example.
                                                                                
  For e.g :
                                                                                
  R_386_PC32     2      word32  S + A - P
                                                                                
  This is a pc-relative relocation. S - symbol value, A - addend, P -
  PC value (location counter value)
                                                                                
  This says the efffective value is calculated by S + A - P
  Now as to A, this is computed by linker based on segment relocation
  
  If S is 100 in data section with base address as 1000 , P is 200 in text 
  section with base as 0, S - P suffices if there is no data segement 
  relocation(or text). However if data segment gets relocated to 2000, 
  A will be 1000.

  The example maynot be precise, but I hope you get some picture.

> 
> Brian
> 

-- 
Regards,
Anitha B
@S A N K H Y A