c++ member arrays with variable size

c++ member arrays with variable size

[D Haley]

Hello,

I recently was doing some debugging, and came across
something new (to me). When debugging routines with
arrays that are declared with variable bounds i found
that GDB is not able to "see" them.

For example

#include <iostream>

void foo(unsigned int size);

int main()
{
	foo(3);
}

void foo(unsigned int size)
{
	char bar[size];
	int i=1;
	
	if(!size)
		return;

	std::cerr << bar[0] << std::endl;
}

upon placing a breakpoint on the std::cerr line, and
using "info locals" GDB wont show up the bar variable,
nor does trying to print bar work, it claims its not a
variable 'no symbol "bar" in current context'

Is it possible to identify these in GDB, or
alternatively how else can I view these arrays in the
debugger, short of rewriting it?

Thanks.


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Re: c++ member arrays with variable size

[Daniel Jacobowitz]

On Sat, Nov 03, 2007 at 06:52:51PM -0700, D Haley wrote:
> Is it possible to identify these in GDB, or
> alternatively how else can I view these arrays in the
> debugger, short of rewriting it?

Most of the time, if a variable does not show up in "info locals",
that means your compiler has not emitted debug info for it.  I tried
your example with G++ 4.2 on x86_64-linux, and did see the local
array.

-- 
Daniel Jacobowitz
CodeSourcery

Re: c++ member arrays with variable size

[D Haley]

Hi Daniel,

> Most of the time, if a variable does not show up in
> "info locals",
> that means your compiler has not emitted debug info
> for it.  I tried
> your example with G++ 4.2 on x86_64-linux, and did
> see the local
> array.

Hmm, I tried on another machine and yes, i can see it.
 For the record on the non-working version I am using
apple-darwin8-g++ (GCC) 4.0.1. Minus one point for
apple :( .

Thanks.


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Re: c++ member arrays with variable size

[Daniel Jacobowitz]

On Sun, Nov 04, 2007 at 03:26:50AM -0800, D Haley wrote:
> Hmm, I tried on another machine and yes, i can see it.
>  For the record on the non-working version I am using
> apple-darwin8-g++ (GCC) 4.0.1. Minus one point for
> apple :( .

This is probably because Apple's still using the older stabs debug
format.  I believe they either have switched, or are switching, to
DWARF-2; that will help a lot.

-- 
Daniel Jacobowitz
CodeSourcery

Re: c++ member arrays with variable size

[Jim Ingham]

Daniel's right, gcc generated stabs (regardless of platform) omits  
debug records for variable length arrays.  DWARF reports them as  
arrays of 0 size.  We've actually supported DWARF for a while -  
starting with Xcode 2.4.  If you are on Tiger, you might want to  
download the latest tools - Xcode 2.5 from Apple's developer site.   
There aren't any tools hosted on Panther or earlier that work with  
DWARF, so if you're still on Panther, you will have to upgrade to Tiger.

Jim

On Nov 4, 2007, at 6:34 AM, Daniel Jacobowitz wrote:

> On Sun, Nov 04, 2007 at 03:26:50AM -0800, D Haley wrote:
>> Hmm, I tried on another machine and yes, i can see it.
>> For the record on the non-working version I am using
>> apple-darwin8-g++ (GCC) 4.0.1. Minus one point for
>> apple :( .
>
> This is probably because Apple's still using the older stabs debug
> format.  I believe they either have switched, or are switching, to
> DWARF-2; that will help a lot.
>
> -- 
> Daniel Jacobowitz
> CodeSourcery

Re: c++ member arrays with variable size

[D Haley]

Hi Jim,

Thanks for the heads up, I am using Xcode 2.4.1; I
will try that update later, last XCode update (hoping
for GCC's new OpenMP support) -- which I did a few
weeks ago -- was ~1GB! I'm using OSX 10.4, which I
believe is tiger (according to wikipedia).

Thankyou.


--- Jim Ingham <jingham@apple.com> wrote:

> Daniel's right, gcc generated stabs (regardless of
> platform) omits  
> debug records for variable length arrays.  DWARF
> reports them as  
> arrays of 0 size.  We've actually supported DWARF
> for a while -  
> starting with Xcode 2.4.  If you are on Tiger, you
> might want to  
> download the latest tools - Xcode 2.5 from Apple's
> developer site.   
> There aren't any tools hosted on Panther or earlier
> that work with  
> DWARF, so if you're still on Panther, you will have
> to upgrade to Tiger.
> 
> Jim
> 
> On Nov 4, 2007, at 6:34 AM, Daniel Jacobowitz wrote:
> 
> > On Sun, Nov 04, 2007 at 03:26:50AM -0800, D Haley
> wrote:
> >> Hmm, I tried on another machine and yes, i can
> see it.
> >> For the record on the non-working version I am
> using
> >> apple-darwin8-g++ (GCC) 4.0.1. Minus one point
> for
> >> apple :( .
> >
> > This is probably because Apple's still using the
> older stabs debug
> > format.  I believe they either have switched, or
> are switching, to
> > DWARF-2; that will help a lot.
> >
> > -- 
> > Daniel Jacobowitz
> > CodeSourcery
> 
> 


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