From gdb  Fri Apr 15 03:09:29 2016
From: Yichao Yu <yyc1992 () gmail ! com>
Date: Fri, 15 Apr 2016 03:09:29 +0000
To: gdb
Subject: Re: Casting an object to another type while debugging?
Message-Id: <CAMvDr+T9bEOn5D01FFpkr0v5EGXy9ys2KUwmbBVdunL1g9hL-g () mail ! gmail ! com>
X-MARC-Message: https://marc.info/?l=gdb&m=149901967461503

On Thu, Apr 14, 2016 at 11:02 PM, Thomas Nyberg <tomnyberg@gmail.com> wrote:
> Hello,
>
> Please yell and scream if I'm posting this to the wrong list (and then
> point me in the right direction :) ).
>
> To make my situation specific, I'm stepping through the cpython
> implementation of the python interpreter using gdb. The implementation
> tends to pass around all objects as type (PyObject *). I know that a
> certain object can be cast to (PyUnicode_Type *) and so I've tried to do
> this to print out the structure, but without success. Here are some
> examples:
>
> ```
> (gdb) print name
> $10 = (PyObject *) 0x7ffff7ee3298
> (gdb) print *name
> $11 = {ob_refcnt = 2, ob_type = 0x8a1a00 <PyUnicode_Type>}
> (gdb) print name
> $12 = (PyObject *) 0x7ffff7ee3298
> (gdb) print *name
> $13 = {ob_refcnt = 2, ob_type = 0x8a1a00 <PyUnicode_Type>}
> (gdb) print (void *) name
> $14 = (void *) 0x7ffff7ee3298
> (gdb) print * (void *) name
> Attempt to dereference a generic pointer.
> (gdb) print * (PyUnicode_Type *) name
> A syntax error in expression, near `) name'.
> (gdb) print * (PyUnicode_Typ *) name
> No symbol "PyUnicode_Typ" in current context.
> (gdb) print (PyUnicode_Type *) name
> A syntax error in expression, near `) name'.
> (gdb) print ((PyUnicode_Type *) name)
> A syntax error in expression, near `) name)'.
> (gdb) print ((PyUnicode_Type *) name);
> A syntax error in expression, near `) name);'.

PyUnicode_Type is not the name of the C type it's a variable/address
that's holding the python type/vtable. I believe PyUnicodObject is the
C type name.


> ```
>
> So from this it seems to me like it's recognizing the PyUnicode_Type
> fine, but I just get this weird syntax error. My google-foo hasn't
> worked out that well so far in resolving this. What am I doing wrong? Am
> I misunderstanding how casting works?
>
> Thanks for any help!
>
> Cheers,
> Thomas