GDB syntax error when casting to base class

GDB syntax error when casting to base class

[Marc Khouzam]

Hi,

I got an eclipse bug report that led me to a bizarre behavior from GDB.
In the very short session below, I'm getting a syntax error when casting
a variable to its base class.

If I remove the constructor (see comment in the code below)
the problem goes away.  Or if I add the word 'struct', it also goes away.

This is causing problems in Eclipse because -var-info-path-expression
returns the expression that causes a syntax error.

I tried to debug it but I ended up in c-exp.y and had to give up.

I was going to propose a patch that adds 'struct' to the 
-var-info-path-expression output, but I didn't think that was the 
greatest of solutions.  Currently, this is what we are planning to 
do in Eclipse to work around this problem.

I can open a PR if it makes sense?

Thanks

Marc

> gdb.7.2 a.out
GNU gdb (GDB) 7.2
(gdb) l 1
1       class bar{};
2
3       class foo : public bar {
4         public:
5           foo() {}  // Comment this line to make things work
6           void test() { return; }
7       };
8
9       int main(void)
10      {
11          foo f;
12          f.test();
13          return 0;
14      }

(gdb) b 6
Breakpoint 1 at 0x8048457: file base.cc, line 6.
(gdb) r
Starting program: /local/lmckhou/testing/a.out 
Breakpoint 1, foo::test (this=0xbfffbfa3) at base.cc:6
6           void test() { return; }

(gdb) p (*(bar*) this)
A syntax error in expression, near `) this)'.

(gdb) p (*(struct bar*) this)
$1 = {<No data fields>}

Re: GDB syntax error when casting to base class

[André Pönitz]

On Thursday 09 September 2010 14:50:07 ext Marc Khouzam wrote:
> Hi,
> 
> I got an eclipse bug report that led me to a bizarre behavior from GDB.
> In the very short session below, I'm getting a syntax error when casting
> a variable to its base class.
> If I remove the constructor (see comment in the code below)
> the problem goes away.  Or if I add the word 'struct', it also goes away.
> 
> This is causing problems in Eclipse because -var-info-path-expression
> returns the expression that causes a syntax error.
> 
> I tried to debug it but I ended up in c-exp.y and had to give up.
> 
> I was going to propose a patch that adds 'struct' to the 
> -var-info-path-expression output, but I didn't think that was the 
> greatest of solutions.  Currently, this is what we are planning to 
> do in Eclipse to work around this problem.
>
> I can open a PR if it makes sense?

Tom mentioned the 'struct' workaround in comment #4 to
http://sourceware.org/bugzilla/show_bug.cgi?id=11912, so
maybe the issue is related.

Andre'

RE: GDB syntax error when casting to base class

[Marc Khouzam]

> -----Original Message-----
> From: André Pönitz [mailto:andre.poenitz@nokia.com] 
> Sent: Thursday, September 09, 2010 10:00 AM
> To: gdb@sourceware.org
> Cc: Marc Khouzam
> Subject: Re: GDB syntax error when casting to base class
> 
> On Thursday 09 September 2010 14:50:07 ext Marc Khouzam wrote:
> > Hi,
> > 
> > I got an eclipse bug report that led me to a bizarre 
> behavior from GDB.
> > In the very short session below, I'm getting a syntax error 
> when casting
> > a variable to its base class.
> > If I remove the constructor (see comment in the code below)
> > the problem goes away.  Or if I add the word 'struct', it 
> also goes away.
> > 
> > This is causing problems in Eclipse because 
> -var-info-path-expression
> > returns the expression that causes a syntax error.
> > 
> > I tried to debug it but I ended up in c-exp.y and had to give up.
> > 
> > I was going to propose a patch that adds 'struct' to the 
> > -var-info-path-expression output, but I didn't think that was the 
> > greatest of solutions.  Currently, this is what we are planning to 
> > do in Eclipse to work around this problem.
> >
> > I can open a PR if it makes sense?
> 
> Tom mentioned the 'struct' workaround in comment #4 to
> http://sourceware.org/bugzilla/show_bug.cgi?id=11912, so
> maybe the issue is related.

Yes, it looks like the same issue except that it also happens with 6.8.
I've added my info to the bug.

Thanks!

Marc