Re: value of local variable stored in register

["Jim Blandy" <jimb@red-bean.com>]

On Feb 5, 2008 12:35 PM, Eli Zaretskii <eliz@gnu.org> wrote:
> > The output of 'info address' isn't very helpful if c has a non-trivial
> > DWARF location expression.
>
> Can you show an example?
>
> Also, is this problem relevant to the case in point (the OP presented
> a test program)?

Even simple programs can have this trouble.  In the example below, GDB
can't print the address of parameter 'x', even though its home is a
fixed offset from the base address of the frame.

$ cat recur.c
#include <stdio.h>

int
recur (int x)
{
  if (x > 0)
    return 2 * recur (x - 2);
  else
    return 1;
}

int
main (int argc, char **argv)
{
  printf ("%d\n", recur (7));
}
$ gcc -g recur.c -o recur
$ ~/gdb/pub/nat/gdb/gdb recur
GNU gdb 6.7.50.20080111-cvs
Copyright (C) 2008 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-pc-linux-gnu"...
(gdb) break recur
Breakpoint 1 at 0x804838a: file recur.c, line 6.
(gdb) run
Starting program: /home/jimb/play/recur

Breakpoint 1, recur (x=7) at recur.c:6
6         if (x > 0)
(gdb) info address x
Symbol "x" is a variable with complex or multiple locations (DWARF2).
(gdb) shell readelf -wi recur
The section .debug_info contains:

  Compilation Unit @ offset 0x0:
   Length:        369
   Version:       2
   Abbrev Offset: 0
   Pointer Size:  4
 <0><b>: Abbrev Number: 1 (DW_TAG_compile_unit)
     DW_AT_stmt_list   : 0
     DW_AT_high_pc     : 0x80483e7
     DW_AT_low_pc      : 0x8048384
     DW_AT_producer    : GNU C 4.1.2 20070626 (Red Hat 4.1.2-13)
     DW_AT_language    : 1      (ANSI C)
     DW_AT_name        : recur.c
     DW_AT_comp_dir    : /home/jimb/play
...
 <1><10b>: Abbrev Number: 5 (DW_TAG_subprogram)
     DW_AT_sibling     : <137>
     DW_AT_external    : 1
     DW_AT_name        : recur
     DW_AT_decl_file   : 1
     DW_AT_decl_line   : 5
     DW_AT_prototyped  : 1
     DW_AT_type        : <b8>
     DW_AT_low_pc      : 0x8048384
     DW_AT_high_pc     : 0x80483b1
     DW_AT_frame_base  : 0      (location list)
 <2><12a>: Abbrev Number: 6 (DW_TAG_formal_parameter)
     DW_AT_name        : x
     DW_AT_decl_file   : 1
     DW_AT_decl_line   : 4
     DW_AT_type        : <b8>
     DW_AT_location    : 2 byte block: 91 0     (DW_OP_fbreg: 0)
...
(gdb)