Re: value of local variable stored in register

["Rohit Arul Raj" <rohitarulraj@gmail.com>]

Hi,

Then, the register value is getting corrupted due to the segmentation fault.
Try to set the break-point before the *a = 0 statement, and try to get
the register info and local variable info.

Regards,
Rohit

On Feb 4, 2008 12:09 PM, Neeraj kushwaha <kushneeraj@gmail.com> wrote:
>
> Hi Rohit,
>
> thanks for the reply.
>
> segmentation occurs due to unreferencing null pointer. (this was done
> intensionally)
> I am running gdb on core dump.
>
> My main intention to get the value of the local variable stored in register.
> From the dwarf dump information i can say it was stored in DW_OP_reg2.
>
> Also if for the arithmetic operations, data is moved to register from
> memory, but when the printf function returns all the saved register
> value should be restored.
>
> this is the dwarf information extracted from a.out
>
> <1><  276>      DW_TAG_subprogram
>                DW_AT_sibling               <341>
>                DW_AT_external              yes(1)
>                DW_AT_name                  main
>                DW_AT_decl_file             1
> /home/neeraj/SSG/Release2/gdblocal/2.c
>                DW_AT_decl_line             5
>                DW_AT_type                  <199>
>                DW_AT_low_pc                0x80483a4
>                DW_AT_high_pc               0x80483f5
>                DW_AT_frame_base            [
> 0]<lowpc=0x0><highpc=0x4>DW_OP_breg4+4
>                                        [ 1]<lowpc=0x4><highpc=0xa>DW_OP_reg1
>                                        [ 2]<lowpc=0xa><highpc=0xb>DW_OP_breg4+4
>                                        [ 3]<lowpc=0xb><highpc=0xd>DW_OP_breg4+8
>                                        [
> 4]<lowpc=0xd><highpc=0x51>DW_OP_breg5+8
> <2><  305>      DW_TAG_variable
>                DW_AT_name                  a
>                DW_AT_decl_file             1
> /home/neeraj/SSG/Release2/gdblocal/2.c
>                DW_AT_decl_line             6
>                DW_AT_type                  <341>
>                DW_AT_location              DW_OP_fbreg -16
> <2><  317>      DW_TAG_variable
>                DW_AT_name                  b
>                DW_AT_decl_file             1
> /home/neeraj/SSG/Release2/gdblocal/2.c
>                DW_AT_decl_line             7
>                DW_AT_type                  <347>
>                DW_AT_location              DW_OP_fbreg -20
> <2><  329>      DW_TAG_variable
>                DW_AT_name                  c
>                DW_AT_decl_file             1
> /home/neeraj/SSG/Release2/gdblocal/2.c
>                DW_AT_decl_line             8
>                DW_AT_type                  <199>
>                DW_AT_location              DW_OP_reg2
> <1><  341>      DW_TAG_pointer_type
>                DW_AT_byte_size             4
>                DW_AT_type                  <199>
> <1><  347>      DW_TAG_volatile_type
>                DW_AT_type                  <199>
>
> Regards
> Neeraj
>
> On Feb 4, 2008 10:58 AM, Rohit Arul Raj <rohitarulraj@gmail.com> wrote:
>
> > Hi,
> >
> > 1. You are getting segmentation fault because of trying to write to a
> > invalid memory location (0x0) through pointer a;
> > 2. It is not always true that if u give the storage class specifier as
> > "Register", a register will be allocated for the local variable. It
> > always depends on the compiler.
> > 3. It is possible that for arithmetic operations, the compiler may
> > move your data to register from memory (operation: b*c & your
> > arithmetic operations does not support memory operands). In that case
> > u can look at the assembly generated.
> >
> > Regards,
> > Rohit
> >
> >
> > On Feb 4, 2008 10:23 AM, Neeraj kushwaha <kushneeraj@gmail.com> wrote:
> > > Hi All,
> > >
> > > I am trying to find the value of local variable stored in register.
> > > But I am getting wrong value from gdb.
> > > Here what i am trying
> > >
> > > #include<stdio.h>
> > >
> > > int main()
> > > {
> > > int *a=0;
> > > volatile register int b=0xabcd;
> > >  register int c=20;
> > > printf("%x\n",b*c);
> > > *a=0;
> > > return 0;
> > > }
> > >
> > >
> > > ------------------------------
> > > -----------------------
> > > GDB info:
> > >
> > > Core was generated by `./a.out'.
> > > Program terminated with signal 11, Segmentation fault.
> > >  #0  0x080483e1 in main () at 1.c:9
> > > 9       *a=0;
> > > (gdb) info local
> > > a = (int *) 0x0
> > > b = 43981
> > > c = 11305136
> > > (gdb) info reg
> > > eax            0x0      0
> > > ecx            0x0      0
> > > edx            0xac80b0 11305136
> > >  ebx            0xac6ff4 11300852
> > > esp            0xbfc65a00       0xbfc65a00
> > > ebp            0xbfc65a28       0xbfc65a28
> > > esi            0x973ca0 9911456
> > > edi            0x0      0
> > > eip            0x80483e1        0x80483e1 <main+61>
> > >  eflags         0x210282 [ SF IF RF ID ]
> > > cs             0x73     115
> > > ss             0x7b     123
> > > ds             0x7b     123
> > > es             0x7b     123
> > > fs             0x0      0
> > > gs             0x33     51
> > >  (gdb)
> > >
> > >
> > > The value of local variable is shown as c = 11305136, but actual value is 20.
> > >
> > > How to get the correct value of variables which are stored in register.
> > >
> > > Regards
> > > Neeraj
> > >
> >
>