HOWTO: Convert Between Signed and Unsigned Numbers

ID: Q189323

The information in this article applies to:

Microsoft Visual Basic for Applications, version 5.0
Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, version 5.0
Microsoft Visual Basic Standard, Professional, and Enterprise Editions, 32-bit only, for Windows, version 4.0
Microsoft Access versions 7.0, 97

SUMMARY

Visual Basic for Applications only supports signed 2- and 4-byte Integers, while other languages, such as C, support both signed and unsigned Integers. This article provides conversion functions between signed and unsigned 4-byte Integers and between signed and unsigned 2-byte Integers.

MORE INFORMATION

In VBA, the range of Integer values is from -32768 to +32767, and for Long values from -2147483648 to 2147483647. When making API calls or calling a DLL written in C, you may be requested to pass in or receive unsigned values in the range of 0 to 65535 or 0 to 4294967296. The conversion functions provided below convert an unsigned Integer to a Long and also from an unsigned Long to a Double for purposes of input and display or other calculations.

The four functions are:

   UnsignedToLong
   LongToUnsigned
   UnsignedToInteger
   IntegerToUnsigned

UnsignedToLong

The function takes a Double containing a value in the range of an unsigned Long and returns a Long that you can pass to an API that requires an unsigned Long.

LongToUnsigned

The function takes an unsigned Long from an API and converts it to a Double for display or arithmetic purposes.

UnsignedToInteger

The function takes a Long containing a value in the range of an unsigned Integer and returns an Integer that you can pass to an API that requires an unsigned Integer.

IntegerToUnsigned

The function takes an unsigned Integer from and API and converts it to a Long for display or arithmetic purposes.

Typical Usage

   Declare Function MyAPI Lib "xxx" (Value As Long) As Long
   Dim uResult As Long
   uResult = MyAPI(UnsignedToLong(3300000000))
   Debug.Print "Return Code: " & LongToUnsigned(uResult)

In the above example, the MyAPI API accepts an unsigned Long as a parameter and returns an unsigned Long as a result code. Because VBA only understands signed Longs, the DECLARE statement uses signed Longs. The UnsignedToLong function converts a number outside the range of signed Long (but within the range of an unsigned Long) into a signed Long for purposes of calling the API. The LongToUnsigned function performs the opposite conversion.

Without these functions, the input argument would have to have been specified as a signed Long, in this case -994967296, and the return value would have to be displayed as a signed value, possibly also as a negative number and non-intuitive.

Step by Step Example

1. Create a new VBA project.

2. Add the following code to a Module:

      Option Explicit

      Private Const OFFSET_4 = 4294967296#
      Private Const MAXINT_4 = 2147483647
      Private Const OFFSET_2 = 65536
      Private Const MAXINT_2 = 32767

      Function UnsignedToLong(Value As Double) As Long
        If Value < 0 Or Value >= OFFSET_4 Then Error 6 ' Overflow
        If Value <= MAXINT_4 Then
          UnsignedToLong = Value
        Else
          UnsignedToLong = Value - OFFSET_4
        End If
      End Function

      Function LongToUnsigned(Value As Long) As Double
        If Value < 0 Then
          LongToUnsigned = Value + OFFSET_4
        Else
          LongToUnsigned = Value
        End If
      End Function

      Function UnsignedToInteger(Value As Long) As Integer
        If Value < 0 Or Value >= OFFSET_2 Then Error 6 ' Overflow
        If Value <= MAXINT_2 Then
          UnsignedToInteger = Value
        Else
          UnsignedToInteger = Value - OFFSET_2
        End If
      End Function

      Function IntegerToUnsigned(Value As Integer) As Long
        If Value < 0 Then
          IntegerToUnsigned = Value + OFFSET_2
        Else
          IntegerToUnsigned = Value
        End If
      End Function

3. Visual Basic only: Run the project and pause it.

4. Type the following in the Immediate/Debug Window:

      ?UnsignedToLong(3300000000)
      ?LongToUnsigned(-55)
      ?UnsignedToInteger(45000)
      ?IntegerToUnsigned(-3000)

5. You will get the following results:

REFERENCES

For additional information, please see the following article in the Microsoft Knowledge Base:

   ARTICLE-ID: Q112673
   TITLE     : How To Pass & Return Unsigned Integers to DLLs from VB

Version           : WINDOWS:4.0,5.0,7.0,97
Platform          : WINDOWS
Issue type        : kbhowto

Last Reviewed: July 16, 1998