Trouble with Pointers

[CSIRAC]

Hi,

I'm trying to translate the following code to MASM:

   
   if ((message == WM_NCCALCSIZE) && (wParam == TRUE)) // Handle the non-client size message.
   {
       // Calculate new NCCALCSIZE_PARAMS based on custom NCA inset.
       NCCALCSIZE_PARAMS *pncsp = reinterpret_cast<NCCALCSIZE_PARAMS*>(lParam); //lParam contains a pointer to a NCCALCSIZE_PARAMS structure

       pncsp->rgrc[0].left   = pncsp->rgrc[0].left   + 0;
       pncsp->rgrc[0].top    = pncsp->rgrc[0].top    + 0;
       pncsp->rgrc[0].right  = pncsp->rgrc[0].right  - 0;
       pncsp->rgrc[0].bottom = pncsp->rgrc[0].bottom - 0;

       lRet = 0;
       // No need to pass message on to the DefWindowProc.
       fCallDWP = false;

I tried to access the fields of the NCCALCSIZE_PARAMS structure by doing this:

MOV   [lParam].NCCALCSIZE_PARAMS.rgrc.left, [lParam].NCCALCSIZE_PARAMS.rgrc.left
; ect

Why won't this work? (I'm completely new to assembly, so please bear with me if I've done something dumb)

[dedndave]

you cannot move directly from memory to memory, unless you use a MOVS instruction
MOVS requires the ESI and EDI registers be set to the source and destination addresses
if it is a dword value (32-bits) or smaller, you can MOV it into one of the general registers, then to the destination from there
on a 64-bit machine with a 64-bit OS, you can MOV 64 bits
http://www.arl.wustl.edu/~lockwood/class/cs306/books/artofasm/Chapter_6/CH06-1.html

[jj2007]

MOV   [lParam].NCCALCSIZE_PARAMS.rgrc.left, [lParam].NCCALCSIZE_PARAMS.rgrc.left

Why won't this work?

As Dave rightly said, direct mem to mem is not possible. You need to get a pointer first, and then use either eax as an intermediate, or the stack. Here two examples assuming that you want to increase .left by 123:

Code Select Expand

mov edx, lParam
mov eax, [edx.NCCALCSIZE_PARAMS.rgrc.left]
add eax, 123
mov [edx.NCCALCSIZE_PARAMS.rgrc.left], eax


Code Select Expand

mov edx, lParam
push [edx.NCCALCSIZE_PARAMS.rgrc.left]
add dword ptr [esp], 123
pop [edx.NCCALCSIZE_PARAMS.rgrc.left]


If you use include \masm32\include\masm32rt.inc, you have access to the Masm32 macros. One of them is m2m or "mem to mem", and it does the push/pop thing in one line:

Code Select Expand

m2m [edx.NCCALCSIZE_PARAMS.rgrc.left], [edx.NCCALCSIZE_PARAMS.rgrc.top]

[CSIRAC]

Cheers dedndave and jj2007,

My bad for making such a simple error (and forgeting to add to [lParam].NCCALCSIZE_PARAMS.rgrc.left) :'(
I spent ages looking at it (I assumed I'd done something dumb with the lParam pointer). Oh well... I suppose
I'm glad that I don't have to learn another esoteric rule with pointers  :wink