assembly and c

tokernizer · 04-15-2006, 02:35 PM

I am trying to modify the value of a member in a c structure by calling a function written in assembly. Here are the codes:
c code:

Code:

#include <stdio.h>
#include <stdlib.h>
typedef struct integer *intpointer;
typedef struct integer{
	int a;
}integer;

extern int zero(intpointer);
int main(){
	intpointer a = (intpointer)malloc(sizeof(integer));
	int b;
	a->a = 33;
	printf("before: %d\n", a->a);
	b = zero(a);
	printf("after: %d\n", a->a);
	printf("should be: %d\n", b);
	return 0;
}

assembly code:

Code:

global zero
zero:
	push ebp
	mov ebp, esp
	mov dword [ebp + 12], 0
	mov eax, [ebp + 12]
	pop ebp
	ret

I was expecting to see the output as:

Code:

before: 33
after: 0
should be: 0

but it came out as:

Code:

before: 33
after: 33
should be: 0

what am i doing wrong ?

95se · 04-15-2006, 03:44 PM

A few things.

If I'm not mistaken, the first pass arg is at esp+4, since your pushing ebp, it will be ebp+8 (or, esp+8). So you want the value at ebp+8, not ebp+12.

Second, the value at ebp+8 (12 you have) will be an address. The indirect addressing mode your using ([ebp+8]) will only get you the address, since what your getting is the value at the address that ebp holds, plus 8, not the address, of the address, there. All your doing is overwriting some value on that stack that the other program will never see. You should have something like this,

Code:

push ebp
push ebx
mov ebp, esp

;; It's +12 in this case because I pushed
;; ebx AS WELL as ebp =]
mov ebx, dword [ebp+12]
mov dword [ebx], 0
mov eax, dword [ebx]

mov esp, ebp
pop ebx
pop ebp
ret

You'll also notice, I restore my stack afterwards, something you don't. You should move the value of ebp back into esp. This just ensures that everything will be left untouched, after the function runs.

tokernizer · 04-15-2006, 03:57 PM

I got it now, thank you.

95se · 04-15-2006, 04:01 PM

It's nice to see your using NASM. I am not a big fan of the AT&T style of gas

llmmix · 04-15-2006, 09:19 PM

tokernizer:

How to compile your example? I like to know.

gcc -c a.c b.s -o c ?

i love gas anyway it standard..

95se · 04-15-2006, 09:27 PM

Well, he's using nasm, so it would be like,

Code:

nasm -f elf asm_code.asm
gcc -c c_code.c
gcc asm_code.o c_code.o -o executable

cupubboy · 04-16-2006, 02:51 AM

Quote:

Originally Posted by 95se

It's nice to see your using NASM. I am not a big fan of the AT&T style of gas

Ditto .. nasm is nice

addy86 · 04-16-2006, 08:19 AM

Starting with binutils 2.10, gas understands the Intel syntax; it is triggered with the .intel_syntax directive.

llmmix · 04-16-2006, 11:40 AM

Quote:

Originally Posted by 95se

Well, he's using nasm, so it would be like,

Code:

nasm -f elf asm_code.asm
gcc -c c_code.c
gcc asm_code.o c_code.o -o executable

Thanks!
By the way, next time i wouldn't post "Thanks" reply,
not because i am tired, but it just too formal and space wasted.

without post, i always Thanks to everybody here in my hearts.