offset of a struct field (qSymbol)

offset of a struct field (qSymbol)

[Mukesh Rathor]

Hi,
 I've written a gdbserver (gdbsx) for xen to debug guests. I'm trying to figure out if there's a way to figure offset of a field from gdbserver. 

Thus, init_mm is a struct, and I can do qSymbol on init_mm, but not init_mm.pgd. I am wondering if there's anyway to do that at present, or get gdb to do "print &(init_mm.pgd) somehow and send me the value.

thanks a lot,
Mukesh

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Re: offset of a struct field (qSymbol)

[Daniel Jacobowitz]

On Wed, Oct 01, 2008 at 01:50:48AM +0000, Mukesh Rathor wrote:
> 
> Hi,
>  I've written a gdbserver (gdbsx) for xen to debug guests. I'm trying to figure out if there's a way to figure offset of a field from gdbserver. 
> 
> Thus, init_mm is a struct, and I can do qSymbol on init_mm, but not
> init_mm.pgd. I am wondering if there's anyway to do that at present,
> or get gdb to do "print &(init_mm.pgd) somehow and send me the
> value.

No, not presently.

-- 
Daniel Jacobowitz
CodeSourcery

Re: offset of a struct field (qSymbol)

[Michael Snyder]

Mukesh Rathor wrote:
> Hi,
>  I've written a gdbserver (gdbsx) for xen to debug guests. I'm trying to figure out if there's a way to figure offset of a field from gdbserver.
> 
> Thus, init_mm is a struct, and I can do qSymbol on init_mm, but not init_mm.pgd. I am wondering if there's anyway to do that at present, or get gdb to do "print &(init_mm.pgd) somehow and send me the value.
> 
> thanks a lot,
> Mukesh

OK, if I understand you, you want to know the offset of
a field within a struct.  Or perhaps you want the address
of a field within a struct, but you can get that by adding
the offset to the base address.

No, there's nothing presently, but...

There is a syntax for gdb to send an expression to the
server, and have the server evaluate it.  Perhaps we could
simply reverse that syntax, so that the server could send
an expression to gdb, and have gdb evaluate it and send
back the answer?

The kind of  expression that I usually use to compute
a field offset is something like:

   &(((struct init_mm *) 0).pgd)

Re: offset of a struct field (qSymbol)

[Daniel Jacobowitz]

On Wed, Oct 01, 2008 at 03:19:04PM -0700, Michael Snyder wrote:
> There is a syntax for gdb to send an expression to the
> server, and have the server evaluate it.  Perhaps we could
> simply reverse that syntax, so that the server could send
> an expression to gdb, and have gdb evaluate it and send
> back the answer?

IMO this is a bad idea.  It's too ambiguous, and it will breed remote
servers that rely on quirks in a particular version of the GDB
expression parser.

I've worked with some other debug tools that worked this way and it
was a terrible headache.

-- 
Daniel Jacobowitz
CodeSourcery

RE: offset of a struct field (qSymbol)

[Mukesh Rathor]

----------------------------------------
> Date: Wed, 1 Oct 2008 18:25:10 -0400
> From: drow@false.org
> To: msnyder@vmware.com
> CC: todfod@hotmail.com; gdb@sourceware.org
> Subject: Re: offset of a struct field (qSymbol)
> 
> On Wed, Oct 01, 2008 at 03:19:04PM -0700, Michael Snyder wrote:
>> There is a syntax for gdb to send an expression to the
>> server, and have the server evaluate it.  Perhaps we could
>> simply reverse that syntax, so that the server could send
>> an expression to gdb, and have gdb evaluate it and send
>> back the answer?
> 
> IMO this is a bad idea.  It's too ambiguous, and it will breed remote
> servers that rely on quirks in a particular version of the GDB
> expression parser.
> 
> I've worked with some other debug tools that worked this way and it
> was a terrible headache.

I managed to get by. Basically, I ask the user to lookup the value manually, and then use monitor to send the info to gdbserver. The monitor cmd really helped simplify things.

> 
> -- 
> Daniel Jacobowitz
> CodeSourcery

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