On Sat, Apr 10, 2010 at 11:37 PM, Jesus Sanchez <zexe...@gmail.com> wrote:
>
>> i don't think so. Use gdb and see where you are writing to.
>>
>> Based on your description of your a.c and b.c, I assume in a.c you
>> have "char foo;" (global), and in b.c you by mistake have "extern int
>> foo;".
>>
>> If your foo variable is at address, say 0x11111111, and you write two
>> bytes to that address, where foo is in fact a char (single byte), the
>> other byte will be written to address 0x11111112.
>>
>> You can test this with a simple example program, where you can declare
>> other char variables around your foo variable and print all of them
>> after the foo assignment and see which ones got clobbered.
>>
>> Then again, maybe I'm misunderstanding your initial problem, you
>> didn't show your code.
>>
>> --patrick
>>
>>
>> Breakpoint 1, main (argc=1, argv=0xfffc8c8c) at a.c:30
>> 30 bar();
>> (gdb) p&f
>> $1 = (int *) 0x1842116
>> (gdb) x/4b 0x1842116
>> 0x1842116<f>: 0x00 0x00 0x00 0x00
>> bar () at b.c:14
>> 14 f = 0x44332211;
>> (gdb) n
>> 16 }
>> (gdb) x/4b 0x1842116
>> 0x1842116<f>: 0x44 0x33 0x22 0x11
>> (gdb) p&g
>> $2 = 0x1842117 "3\"\021"
>> (gdb) p&k
>> $3 = 0x1842119 "\021"
>> (gdb) p&l
>> $4 = 0x1842118 "\"\021"
>> (gdb) p/x f
>> $13 = 0x44332211
>> (gdb) p/x g
>> $14 = 0x33
>> (gdb) p/x l
>> $15 = 0x22
>> (gdb) p/x k
>> $16 = 0x11
>>
>> Looks like the "extern int foo;" confused gdb.
>>
>>
>
> I'm not using extern for the variables,
I assumed you were.
Looking at the code you provided, my initial guess would have been
that foo in b.c and the one declared in a.c would have caused the
linker to complain, but I am wrong.
So, in fact, it looks like the section you found and quoted is the answer.
I learned something today :)
--patrick
> anyway
> here is the code (to a extremely basic level) using your hint about
> making more char variables around foo to show them later:
>
> all compiled with gcc -Wall -W without warning
>
> ---------- a.c ------------------------------------------
> #include <stdio.h>
>
> char foo;
> char foo1;
> char foo2;
> char foo3;
>
> void overfoo(); /* the func that will overflow the foo variable */
>
> int main(){
>
> overfoo(); /* after calling we have 0x2211 on foo variable */
>
> printf("%x\n",foo);
> printf("%x\n",foo1);
> printf("%x\n",foo2);
> printf("%x\n",foo3);
> return 0;
> }
>
> ---------- b.c ------------------------------------------
>
> int foo; /* here the problem begins */
>
> void overfoo(){
> foo=0x2211;
> }
>
> ---------------------------------------------------------------
>
> after compiling and executing , foo showed 0x11 and other 0x0.
>
> thanks for your time.
> -J
