ACC: How to Pass a Single Byte of Data to Windows API Calls

• Microsoft Access versions 1.0, 1.1, 2.0

SUMMARY

Advanced: Requires expert coding, interoperability, and multiuser skills.

Microsoft Access versions 1.x and 2.0 do not have a single-byte data type. To pass a byte to an external function, such as a Windows application programming interface (API) function or a dynamic-link library (DLL) in Microsoft Access versions 1.x and 2.0, you should declare the byte as an integer.

If you need to pass a single byte of information in a data type, such as the RGBQUAD structure, you should pass the data structure as a string type.

NOTE: Microsoft Access for Windows 95 version 7.0 now has a Byte data type.

MORE INFORMATION

The Intel chip supports only a full word on the stack. You cannot put just 8 bits on the stack; you must use the full 16 bits. If you attempt to pass only 8 bits, you will end up passing 16 bits (your 8 bits of data plus an additional 8 bits that will be "padded"). This is a limitation of the processor, not of Microsoft Access.

To pass a single byte of data in a structure such as the RGBQUAD, use a fixed-length string of 1. For example, suppose you want to pass a single byte in the "C" RGBQUAD data structure, which looks like this:

   typedef struct tagRGBQUAD{
      BYTE   rgbBlue;
      BYTE   rgbGreen;
      BYTE   rgbRed;
      BYTE   rgbReserve;
   } RGBQUAD

   Type RGBQUAD
      rgbBlue As String * 1
      rgbGreen As String * 1
      rgbRed As String * 1
      rgbReserve As String * 1
   End Type

   Dim RGB As RGBQUAD

   RGB.rgbBlue = Chr$(10)
   RGB.rgbRed = Chr$(14)
   RGB.rgbGreen = Chr$(15)

   NOTE: In the above example, Blue=10, Red=14, and Green=15.

REFERENCES

For more information about this topic, search for "Type Statement" using the Microsoft Access Help menu.