need help passing and modifying c++ array from assembly library

[soas]

Im trying to write a dll in assembly language that I can pass a pointer to an array from my c++ program to, to be modified.. I thought I had it figured out, but it crashes if the elements are more than 2, so I know Im doing it wrong

Here is what I have for my program so far...

in the c++ part I have

extern "C" void _cdecl Num(BYTE []);

int main()
{
  BYTE c[2];
  c[0] = 0;
  c[1] = 0;
  c[2] = 0;
  Num(c);
  printf("%d, %d, %d\n", c[0], c[1], c[2]);
  system("PAUSE");	
  return 0;
}

and on in the library part I have

Num proc public
	push ebp
	push eax
	push ebx
	mov ebp, esp

	mov eax, 10
	mov [ebx], eax
	mov eax, 12
	mov [ebx+1], eax
	mov eax, 22
	mov [ebx+2], eax
	
	pop ebx
	pop eax
	pop ebp
	ret
Num endp

When I run this progam it works fine, as long as c doesnt have more than c[2] elements.. I'd like it to be BYTE c[128], and to be able to modify those elements..

I dont know alot about using arrays in assembly, I've never tried passing them between languages especialy..

Im using masm32 just in case that makes a difference, if anyone knows what Im doing wrong could please give me some hints or point me in a direction where I could find more info about this, I'd really appreciate it

 

[soas]

when I was pasting my assembly program in I skipped over a line, I only realized it after reading through it for the 3rd time... its supposed to say mov ebx, [ebp] below mov ebp, esp .. I hope that doesnt make things more confusing

 

[AmkG]

You're not putting bytes. You're putting DWORDS. EAX is a DWORD. To use a byte, use al/ah,bl/bh,cl/ch,dl/dh "Information has a tendency to be free. Which means someone will always tell you something you don't want to know."

 

[AmkG]

Also, since you're putting constants, why bother going through the registers?

mov byte ptr [ebx],10

NOTE: for many assemblers, a 'byte ptr' or '* ptr' is often recommended when using register indices without any offsets:

mov word ptr [ebx],ax

"Information has a tendency to be free. Which means someone will always tell you something you don't want to know."

 

[soas]

okay, well I dont intend it to be constants, Im just starting out like that to test it

And I made the necessary changes to change the registers to byte registers instead of DWORDS but it still crashes if the array I try to pass has more than 3 elements (c[2])..

any ideas about that part of it?

 

[AmkG]

At the time you put:
mov ebp,esp

the stack was:
| pointer to passed array
| Return address (4 bytes)
| pushed ebp (4 bytes)
| pushed eax (4 bytes)
| pushed ebx (4 bytes) <-esp points here

so since you put mov ebp, esp, ebp now points to the pushed ebx. To get the pointer to array, you need
mov ebx,[ebp+16]
since the processor pushed 4 bytes (assuming 32-bit near or 16-bit far call), and you pushed a total of 12 bytes before putting esp into ebp.

Have fun! &quot;Information has a tendency to be free. Which means someone will always tell you something you don't want to know.&quot;