NASM Forum > Programming with NASM

Structure Size member

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munair:
Some NASM examples I found put 'size:' as a final member of a structure and use it to obtain the structure's size. Is this correct?

The following code is compiler generated and NASM has no trouble compiling it:

--- Code: ---struc _D27
  _D27_fname:          resd 1
  _D27_sname:          resd 1
  _D27_age:            resd 1
  size:
endstruc

SECTION .text

global  _start
global  _end

_start:
  mov     eax, [_I0 + _D27_age]

_end:
  mov     ebx, 0
  mov     eax, 1
  int     80h

SECTION .data

_I0: istruc _D27
  at _D27_fname,          dd 0
  at _D27_sname,          dd 0
  at _D27_age,            dd 0
iend

--- End code ---

If the 'size:' member is correct, can it also be prepended, or is the name 'size' default for NASM to be recognized?

Frank Kotler:
I THINK that Nasm defaults to "_D27_len" for the size of the structure. Check me on that, though.

Best,
Frank

munair:
That would be _D27_size, which doesn't require the 'size' member (_D27_len does not compile).

munair:
Hmm, it looks like I figured it out. The following code gives me the size:

--- Code: ---struc _D27
  _D27_fname:          resd 1
  _D27_sname:          resd 1
  _D27_age:            resd 1
endstruc

SECTION .text

global  _start
global  _end

_start:

  mov     eax, _D27_size
  ; printing eax outputs 12

_end:
  mov     ebx, 0
  mov     eax, 1
  int     80h
--- End code ---

BTW, what happens here:

--- Code: ---mov     eax, [_D27_size]
--- End code ---

That's what I tried first but it causes a segmentation fault when trying to print eax to the standard output device.

debs3759:
That would treat _D27_size as a pointer and would load eax with whatever is at the memory address it points to.

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