Question about "virtual"

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ghostreaper

Joined: 07 Apr 2011
Posts: 2

ghostreaper 23 Aug 2011, 07:59

I　don't really know when and how to use "virtual", in other words, what situation I have to use it.
And the snippet below:

Code:

struc EXCEPTION_POINTERS {
    .ExceptionRecord dd ?  ; pointer to EXCEPTION_RECORD
    .ContextRecord   dd ?  ; pointer to CONTEXT
}

virtual at 0
    EXCEPTION_POINTERS EXCEPTION_POINTERS
end virtual

what does the "virtual" do exactly?
I'm really confused about using it.

23 Aug 2011, 07:59

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 23 Aug 2011, 08:45

Used this way with "struc", it defines symbols EXCEPTION_POINTERS.ExceptionRecord = 0 and EXCEPTION_POINTERS.ContextRecord = 4. Without this, you couldn't do something like "mov eax, [ebx + EXCEPTION_POINTERS.ContextRecord]", because "EXCEPTION_POINTERS.ContextRecord" symbol wouldn't be defined. "struc" doesn't define it, it is only a special kind of macro. FASM is very lowlevel assembler, and uses this kind of "trickery" quite often, in order to be very powerful by having only handful of simple features, which can be combined to build expected features.

"virtual" itself defines all labels inside it, but not the data. You don't need to really understand this, as long as you only use it for structures.

23 Aug 2011, 08:45

AsmGuru62

Joined: 28 Jan 2004
Posts: 1708
Location: Toronto, Canada

AsmGuru62 23 Aug 2011, 13:23

I use virtual to generate code for structures in IDE I am developing:

Code:

virtual at 0
RECT:
  .Left   INT32 ?
     .Top    INT32 ?
     .Right  INT32 ?
     .Bottom INT32 ?
     .size   =$
end virtual

In this case, I can access the members of a structure:

Code:

macro RECT_WIDTH r32_width, r32_pRect
{
       mov     r32_width, [r32_pRect + RECT.Right]
 sub     r32_width, [r32_pRect + RECT.Left]
}

Or allocate memory for the structure using .size member:

Code:

invoke       HeapAlloc, [hAllocator], HEAP_NO_SERIALIZE, RECT.size

23 Aug 2011, 13:23

ghostreaper

Joined: 07 Apr 2011
Posts: 2

ghostreaper 07 Sep 2011, 07:51

thank you all of you very much.I got it!

07 Sep 2011, 07:51

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