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Weird problem with structure alignment

Started by jj2007, February 17, 2014, 08:09:52 PM

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jj2007

The problem is the 4 in bINPUT_RECORD STRUCT 4:

include \masm32\include\masm32rt.inc
aINPUT_RECORD STRUCT
  EventType             WORD ?
  two_byte_alignment    WORD ?
  UNION
    KeyEvent                KEY_EVENT_RECORD            <>
    MouseEvent              MOUSE_EVENT_RECORD          <>
    WindowBufferSizeEvent   WINDOW_BUFFER_SIZE_RECORD   <>
    MenuEvent               MENU_EVENT_RECORD           <>
    FocusEvent              FOCUS_EVENT_RECORD          <>
  ENDS
aINPUT_RECORD ENDS

bINPUT_RECORD STRUCT 4
  EventType             WORD ?
  UNION
    KeyEvent                KEY_EVENT_RECORD            <>
    MouseEvent              MOUSE_EVENT_RECORD          <>
    WindowBufferSizeEvent   WINDOW_BUFFER_SIZE_RECORD   <>
    MenuEvent               MENU_EVENT_RECORD           <>
    FocusEvent              FOCUS_EVENT_RECORD          <>
  ENDS
bINPUT_RECORD ENDS

.data
ira aINPUT_RECORD <>
irb bINPUT_RECORD <>

.code
start:
lea eax, ira.KeyEvent
sub eax, offset ira
print str$(eax), 9, "bytes", 13, 10
lea eax, irb.KeyEvent
sub eax, offset irb
print str$(eax), 9, "bytes", 13, 10
exit
end start


The good news is that JWasm is entirely happy with this, in line with the MASM Programmer's Guide.

The bad news is that Masm 6.14, 6.15, 8, 9 and 10 complain about internal errors.

Windows.inc avoids the problem by inserting a padding word, but that is just a workaround, not a proper solution. It seems it's a known bug, #4 in Japheth's list - how should it be treated?

hutch--

JJ,

I don't see the problem of setting internal alignment with padding, its orthodox technique and works fine.


INPUT_RECORD STRUCT
  EventType             WORD ?
  two_byte_alignment    WORD ?
  UNION
    KeyEvent                KEY_EVENT_RECORD            <>
    MouseEvent              MOUSE_EVENT_RECORD          <>
    WindowBufferSizeEvent   WINDOW_BUFFER_SIZE_RECORD   <>
    MenuEvent               MENU_EVENT_RECORD           <>
    FocusEvent              FOCUS_EVENT_RECORD          <>
  ENDS
INPUT_RECORD ENDS

sinsi

Workaround?

e_type UNION
    KeyEvent                KEY_EVENT_RECORD            <>
    MouseEvent              MOUSE_EVENT_RECORD          <>
    WindowBufferSizeEvent   WINDOW_BUFFER_SIZE_RECORD   <>
    MenuEvent               MENU_EVENT_RECORD           <>
    FocusEvent              FOCUS_EVENT_RECORD          <>
e_type ENDS

bINPUT_RECORD STRUCT 4
  EventType         WORD ?
  etype             e_type <>
bINPUT_RECORD ENDS


lea eax, irb.etype.FocusEvent

4
4

jj2007

Hutch, Sinsi,

Thanks. I had hoped there would be an "official and automatic" solution but it seems a manual intervention is needed, i.e. padding.

Gunther

Jochen,

Quote from: jj2007 on February 17, 2014, 10:04:29 PM
I had hoped there would be an "official and automatic" solution but it seems a manual intervention is needed, i.e. padding.

that doesn't mean that it would be an inferior solution.

Gunther
You have to know the facts before you can distort them.

sinsi

Windbg output

(1644.4d4): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
eax=00000000 ebx=0048f480 ecx=01232054 edx=00000204 esi=00000000 edi=0048f48c
eip=011999e7 esp=0048f458 ebp=0048f470 iopl=0         nv up ei pl zr na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010246
ml!mergeStructInst+0x3da:
011999e7 8a00            mov     al,byte ptr [eax]          ds:002b:00000000=??


ML output

MASM : fatal error A1016: Internal error

  Version 11.00.50727.1

  ExceptionCode            = C0000005
  ExceptionFlags           = 00000000
  ExceptionAddress         = 011999E7 (01170000) "F:\asm\32\ml.exe"
  NumberParameters         = 00000002
  ExceptionInformation[ 0] = 00000000
  ExceptionInformation[ 1] = 00000000

CONTEXT:
  Eax    = 00000000  Esp    = 0048F458
  Ebx    = 0048F480  Ebp    = 0048F470
  Ecx    = 01232054  Esi    = 00000000
  Edx    = 00000204  Edi    = 0048F48C
  Eip    = 011999E7  EFlags = 00010246
  SegCs  = 00000023  SegDs  = 0000002B
  SegSs  = 0000002B  SegEs  = 0000002B
  SegFs  = 00000053  SegGs  = 0000002B
  Dr0    = 00000000  Dr3    = 00000000
  Dr1    = 00000000  Dr6    = 00000000
  Dr2    = 00000000  Dr7    = 00000000


Using "aINPUT_RECORD STRUCT 4" works, possibly since the union is already aligned.
In the same way, "bINPUT_RECORD STRUCT 2" works (but obviously gives a wrong answer, 2 not 4).