VB6 array TYPE bug - Can anyone explain this please ???

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  12 years ago

  by MartinFletcher

  Martin Fletcher

  , United States

  Joined 12 years ago

  Run following code to see the problem as .BAS module.

  'Word' should be same as an Integer - i.e. 2 bytes, but it actually takes 3 bytes in the boot_fat16_bad Type structure!

  To fix it, uncomment the res type in both structures.

  Using the IDE Watch feature, it is easy to see an extra byte has been declared at the end of the 8 byte array.

  Why is this happening???

   

   

  
  Option Explicit

  
  Public Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (Destination As Any, Source As Any, ByVal Length As Long)

  Public Type Word
    lsb As Byte
    msb As Byte
  End Type

  'note: using Word type here as using integers seems to sometimes us 3 bytes instead of 2!!!

  Private Type boot_fat16
      jmp(2) As Byte          'Must be 0xEB, 0x3C, 0x90
      sys_id(7) As Byte       '3 "MSDOS5.0"
  '    res As Byte            'this fixes the bug
      Sect_Size As Word       '0bh *Sector size in bytes (512)
  End Type

  
  Private Type boot_fat16_bad
      jmp(2) As Byte          'Must be 0xEB, 0x3C, 0x90
      sys_id(7) As Byte       '3 "MSDOS5.0"
  '    res As Byte            'this fixed the bug
      Sect_Size As Integer    '0bh *Sector size in bytes (512)
  End Type

  
  Sub Main()
  Dim b As boot_fat16         'good structure
  Dim bb As boot_fat16_bad    'bad structure

  b.Sect_Size.lsb = &H3
  b.Sect_Size.msb = &H2
  bb.Sect_Size = &H203
  Dim SectorBuff1() As Byte
  Dim SectorBuff2() As Byte
  ReDim SectorBuff1(15)
  ReDim SectorBuff2(15)
  Call CopyMemory(SectorBuff1(0), b.jmp(0), Len(b))     'dest, source, no bytes
  Call CopyMemory(SectorBuff2(0), bb.jmp(0), Len(bb))   'dest, source, no bytes
  MsgBox SectorBuff2(12) & " should be same as " & SectorBuff1(12)

  
  End Sub

  
   

   

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