Q249962: INFO: Differences in Padding UDTs Between VB 5.0 and VB 6.0

Article: Q249962
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:5.0,6.0
Operating System(s): 
Keyword(s): kbinterop kbVBp kbVBp500 kbVBp600 kbGrpDSVB kbDSupport
Last Modified: 11-JAN-2001

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The information in this article applies to:

- Microsoft Visual Basic Learning Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Professional Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0 
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SUMMARY
=======

The amount of memory allocated for a UDT in Visual Basic can be different in
Visual Basic 6.0 and Visual Basic 5.0 depending on the data types used in the
UDT and their order.

MORE INFORMATION
================

Both Visual Basic 5.0 and Visual Basic 6.0 use an alignment factor of 4. This
means that every element of a UDT must start on a byte which is a multiple of
the size of the element itself or a multiple of the alignment factor, whichever
is less. For example, an element of type Integer must start on a byte address
that is a multiple of 2, while an element of type Double must start on a byte
address that is a multiple of 4.

In addition, some compilers add padding bytes at the end of the UDT to make the
size of the UDT a multiple of the alignment factor or a multiple of the largest
data size used in the structure, whichever is less. For example, a UDT in which
the largest data type is an Integer should have the total size of the UDT a
multiple of 2 while an UDT in which the largest data type is a Double should
have a size multiple of 4.

The difference between Visual Basic 5.0 and Visual Basic 6.0 is that the Visual
Basic 5.0 compiler only adds the required padding if you use an array of UDTs
while the Visual Basic 6.0 compiler (and the C compiler) always do it regardless
of whether or not you are using an array of structures. The Visual Basic 6.0
approach is consistent with the C approach.

Programmers who use only Visual Basic, or who don't pass UDTs to functions
written in other languages don't have to take this into consideration. However,
programmers passing UDTs to functions written in other languages should
understand this difference and evaluate how this can affect their applications.

The following sample project demonstrates this difference:

1. Create a new Standard EXE project. Form1 is created by default

2. In the Project menu, select Add Module. Module1 will be created.

3. In Module1, add the following code:

  Option Explicit
  Type Type1
  	m as Long
  	i as Integer
  End Type

  Type Type2
  	m as Long
  	t1 as Type1
  	i as Integer
  End Type

  Type Type3
  	t1(0) as Type1
  End Type

4. On Form1, add a CommandButton.

5. On the Command1_Click event, add the following code:

  Dim t1 as Type1
  Dim t2 as Type2
  Dim t3 as Type3

  MsgBox "Size of Type1 is: " & LenB(t1) 
  MsgBox "Size of Type2 is: " & LenB(t2) 
  MsgBox "Size of Type3 is: " & LenB(t3) 

Run the project to see the size of each structure. You should see the following
results:

+-------------------+
| Type  | VB5 | VB6 | 
+-------------------+
| Type1 | 6   | 8   | 
+-------------------+
| Type2 | 12  | 16  | 
+-------------------+
| Type3 | 8   | 8   | 
+-------------------+

Potential problems:

The problem arises in Visual Basic 5.0 when you pass a structure of Type2 to a
function written in C because the C compiler assumes Type1 as being of size 8
while Visual Basic 5.0 assumes size 6. The problem in this case is with the 3rd
member of the structure (element i). In Visual Basic this member starts at the
11th byte of the structure while the C compiler assumes it to start at the 13th
byte.

Workaround:

One possible workaround is to add filler elements at the end of the structure to
make the size in Visual Basic match the size in C. In the preceeding case you
could define the UDTs as follows:

  Type Type1
  	m as Long
  	i as Integer
  	filler as Integer ' added to match UDT's size
  End Type

   Type Type2
  	m as Long
  	t1 as Type1
  	i as Integer
  	filler as Integer ' added to match UDT's size
  End Type

Additional query words: UDType

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Keywords          : kbinterop kbVBp kbVBp500 kbVBp600 kbGrpDSVB kbDSupport 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVBA500 kbVBA600 kbVB500 kbVB600
Version           : WINDOWS:5.0,6.0
Issue type        : kbinfo

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