Help needed with browsing GDB code

Nityananda j.nityananda@gmail.com
Thu Feb 19 05:17:00 GMT 2009


Hi Everyone,
Thanks for the information. Can you please help me out with more  
questions?
I am not sure how I can find the starting offset from the stack frame  
pointer for the local variables on the frame. Can you please tell me  
how I can find it in GDB code for the i386 architecture.
Also how can i find the same information about the location of the  
local variables when using fomit-frame-pointer compiler flag. Since  
the frame pointers are no longer going to be in the stack frames.

Thanks and regards,
Nityananda

On Feb 7, 2009, at 6:06 AM, Ramana Radhakrishnan wrote:

> Hi Nityananda,
>
>
> On Sat, Feb 7, 2009 at 1:52 AM, Nityananda <j.nityananda@gmail.com>  
> wrote:
>> HI Thiago,
>> Thanks for the information. I am reading the code to deal with the  
>> stack
>> frame information without the debug information. Can you please  
>> point me to
>> the code with the debug information? You mentioned that it uses  
>> DWARF2.
>
> Look at gdb/dwarf2-frame.c for DWARF2 frame reading .
>
>> So are the local variables always at the same offset of the frame  
>> base address
>> or there is a possibility of these addresses changing from one  
>> process to
>> another?
>
> Local variables will always be at the same offset from the frame base
> address for the same program unless you have self modifying code .
> Operating Systems 101 - A process can be multiple instantiations of
> the same program.
>
> HTH
>
> cheers
> Ramana
>
>>
>> Thank you very much in advance,
>> Nityananda
>>
>> On Feb 6, 2009, at 4:01 AM, Thiago Jung Bauermann wrote:
>>
>>> Hi Nityananda,
>>>
>>> El jue, 05-02-2009 a las 18:26 -0800, Nityananda escribió:
>>>>
>>>> I am looking for how
>>>> GDB obtains the address of stack local variables. I am seeing some
>>>> code related to frame_info but do not know how it actually works.
>>>
>>> Well, there are two situations: with debug information available,  
>>> and
>>> without. For the first case it's simple: the DWARF2 format  
>>> includes the
>>> frame base address as part of the unwind information, and  
>>> addresses of
>>> local variables in the debuginfo are relative to that base address.
>>>
>>> When there's no debuginfo available, GDB uses its knowledge of the  
>>> OS
>>> ABI for the given architecture. For example, for ppc64-linux, the  
>>> stack
>>> frame layout is given here:
>>>
>>> http://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi-1.9.html#STACK
>>>
>>> And the code which uses that knowledge is in
>>> rs6000-tdep.c:rs6000_frame_cache. It's kinda hairy...
>>> --
>>> []'s
>>> Thiago Jung Bauermann
>>> IBM Linux Technology Center
>>>
>>
>>



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