assembly ,structure references to members! thanks

topheraholic · 07-09-2011, 03:44 AM

Code:

WNDCLASS struct
  style         DWORD
  lpfnWndProc   DWORD
...
WNDCLASS ends

.data?
stWndClass WNDCLASS <>
...
mov esi,offset stWndClass
mov eax,[esi+WNDCLASS.lpfnWndProc]

mov eax,[esi+stWndClass.lpfnWndProc]

if the address of stWndClass is 403000h, WHY it would be compile as
mov eax,[esi+403004] and the first one would be
mov eax,[esi+4] ? I know how to get the values 403004h and 4,but do not know why.thanks very much

axet · 07-09-2011, 04:33 AM

because you are reffering to second element and length of first is 4 bytes

change to:

stWndClass.style

topheraholic · 07-09-2011, 08:15 AM

i know 4, it is not the point! you did not get what i am saying! my point it why it is 304000h while the other is 4? why they are not both 4!?

johnsfine · 07-09-2011, 08:27 AM

Quote:

Originally Posted by topheraholic

i know 4, it is not the point! you did not get what i am saying! my point it why it is 304000h while the other is 4? why they are not both 4!?

WNDCLASS.lpfnWndProc is the offset of lpfnWndProc within the definition of the struct, so it is 4, which is the offset of that member relative to the start of the struct.

stWndClass.lpfnWndProc is the address of that member of that specific instance of the struct within memory.

topheraholic · 07-09-2011, 08:41 AM

So why WNDCLASS.lpfnWndProc is not the address of WNDCLASS plus the offset of lpfnWndProc ?

and why stWndClass.lpfnWndProc is not the offset of lpfnWndProc within the definition of the struc,which it should be 4?

why stWndClass.lpfnWndProc is the address of that member in memory which WNDCLASS.lpfnWndProc is not?

i mean is it because stWNdClass is a sturcture variable? so it has a address in memory? but what does WNDCLASS? it is a structure,not a variable?so it is no address in memory? i am so comfused! thanks a lot!

johnsfine · 07-09-2011, 11:10 AM

Quote:

Originally Posted by topheraholic

So why WNDCLASS.lpfnWndProc is not the address of WNDCLASS plus the offset of lpfnWndProc ?

You came near to answering your question yourself (see below).

WNDCLASS is not an object in memory. It has no address.

Quote:

is it because stWNdClass is a sturcture variable? so it has a address in memory?

Yes.

Quote:

WNDCLASS? it is a structure,not a variable?so it is no address in memory?

Right. A structure definition is not an object in memory and it has no address.