Unwinding the stack for PA-RISC and HP-UX 10.20/ backtrace

[Roland Egger <egger at tarantel dot rz dot fh-muenchen dot de>]

Hi,
I've not a problem with gdb but I'm wondering how unwinding the
stack for PA-RISC with HP-UX work (I want to write a backtrace 
function).It would be great if somebody can give me some advice
and hints.
The next lines show my function and the problem to go from the
stack pointer to the old stack pointer and from the there to the
stack pointer before ...
###########################################################################
Example programm:

#include <stdio.h>
#include <iostream.h>

void backtrace();

int main()
{
   backtrace();
   return 0;
}

void backtrace()
{
// At first I fetch the interesting registers ...
// I'll write sp for stack pointer and fp for fp
   register long unsigned *plFp asm("r3");   // fp or old sp
   register long unsigned *plSp asm("r30");  // sp
   register long unsigned *r2 asm("r2");     // rp

// To get the return pointer rp I could use r2 but I want to
// walk from sp because I can't get r2 and rp of the functions that called
// backtrace

  unsigned long *plStack, *saved_rp,  *pc;
  saved_rp = (unsigned long*)((long)(plSp)- 4*5); // or // saved_rp = plSp - 5;
  // At http://devrsrc1.external.hp.com/STK/partner/rad_11_0_32.pdf
  // p. 72 I've read that at the position sp - 20 Byte
  // the saved rp can be fetched. This works.
  cout << "The adresses: saved_rp = " << saved_rp << ", plSp = "  << plSp << endl;

  pc = (unsigned long*)(*saved_rp + 5); //

  cout << "r2  = " << r2 << "  pc = " << pc << " *saved_rp  = " << *saved_rp << endl;

  // But at the same page can be read that at sp - 4 the previous
  // sp can be fetched. Sadly that doesn't work ;(
  // Hmm, rad_11_0_32.pdf is written for HP-UX 11, maybe that's the 
  // problem ?
  plStack = (unsigned long*)((long)(plSp) - 4);
  cout << "sp  = " << plSp  <<" fp  = " << plFp  << " plStack  = " << plStack << endl
       << "*sp = " << *plSp <<" *fp = " << *plFp << " *plStack = " << *plStack << endl;
  // also the program shows *plSp != 1 gdb shows 1 here

}
###########################################################################
Program results:
***************
The adresses: saved_rp = 0x7b03a7bc, plSp = 0x7b03a7d0
r2  = 0x6207  pc = 0x61e4 *saved_rp  = 25183
sp  = 0x7b03a7d0 fp  = 0x7b03a750 plStack  = 0x7b03a7cc
*sp = 2063836880 *fp = 2063836944 *plStack = 0
or run in gdb
The adresses: saved_rp = 0x7b03a7f4, plSp = 0x7b03a808
r2  = 0x6207  pc = 0x61e4 *saved_rp  = 25183
sp  = 0x7b03a808 fp  = 0x7b03a788 plStack  = 0x7b03a804
*sp = 2063836936 *fp = 2063837000 *plStack = 0
###########################################################################


In gdb:
*******
(gdb) f
#0  main () at backtrace_hp.cpp:8
8          backtrace()
(gdb) i r sp
sp 7b03a788
...
(gdb) f
#0  backtrace () at backtrace_hp.cpp:25
25        saved_rp = (unsigned long*)((long)(plSp)- 4*5); // or // saved_rp = plSp - 5;
(gdb) print plSp
$1 = (long unsigned int *) 0x7b03a808
(gdb) print plFp
$2 = (long unsigned int *) 0x7b03a788
(gdb) i r fp
r3 7b03a788
(gdb) i r sp
sp 7b03a808
(gdb) f
#0  backtrace () at backtrace_hp.cpp:31
31        pc = (unsigned long*)(*saved_rp + 5); //
(gdb) print *saved_rp
$4 = 25055
(gdb) i r rp
rp 61df

=> so far everything is ok

(gdb) print plSp
$2 = (long unsigned int *) 0x7b03a808
(gdb) print *plSp
$1 = 1

Here *plSp should be 2063836936 or 0x7B03A708
But my real problem is that I don't find anything near the
stackpointer that points to the old stackpointer and make a
backtrace possible. 
##########################################################################
Does anybody now anything about the stack on PA-RISC (HP-UX 10.20)
and where I can find information how to unwind the stack there
(howto find/calculate the distance from the stack pointer to the 
frame pointer....?)
What's the problem with sp-4 which is in my case != previous sp?
My next step will probably be to study the sources of gdb but it
would be great if anybody who knows anything about the stack problematic
could help me.

Thank you for any advice 
                      Roland

Attachment: backtrace_hp.cpp
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