gdb break address is different when break (function name) / break *(function name)

Question

#include <stdio.h>

int main(void){
   int sum = 0;
   sum += 0xabcd;
   printf(“%x”, sum);
   return 0;
}

This is my code and when I use gdb I can find different address when break main / break *main.

When I just type disassemble main it shows like this:

Dump of assembler code for function main:

0x080483c4 <+0>: push %ebp
0x080483c5 <+1>: mov %esp,%ebp
0x080483c7 <+3>: and $0xfffffff0,%esp
0x080483ca <+6>: sub $0x20,%esp
0x080483cd <+9>: movl $0x0,0x1c(%esp)
0x080483d5 <+17>:addl $0xabcd,0x1c(%esp)
0x080483dd <+25>:mov $0x80484c0,%eax
0x080483e2 <+30>:mov 0x1c(%esp),%edx
0x080483e6 <+34>:mov %edx,0x4(%esp)
0x080483ea <+38>:mov %eax,(%esp)
0x080483ed <+41>:call 0x80482f4 <printf@plt>
0x080483f2 <+46>:mov $0x0,%eax
0x080483f7 <+51>:leave
0x080483f8 <+52>:ret

End of assembler dump.

So when I type [break *main] it starts 0x080483c4 but type [break main] it start 0x080483cd

Why is start address is different?

chyx · Accepted Answer · 2012-09-27 16:55:44Z

5

Why is the address different.

Because break function and break *address are not the same thing(*address specifies the address of the function's first instruction, before the stack frame and arguments have been set up).

In the first case, GDB skips function prolog (setting up the current frame).

edited Sep 27, 2012 at 16:55

chyx

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answered Sep 26, 2012 at 6:56

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Comments

Adrian Cornish · Accepted Answer · 2012-09-26 02:23:18Z

0

Total guess - and prepared to be totally wrong.

*main if address of the function

Breaking inside main is the first available address to stop inside the function when it is being executed.

Note that 0x080483cd is the first place a debugger can stop as it is modifying a variable (ie assigning zero to sum)

When you are breaking at 0x080483c4 this is before the setup assembler that C knows nothing about

answered Sep 26, 2012 at 2:23

Adrian Cornish

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