PRB: LoadBitmap GPFaults on Compressed Bitmaps in Windows 3.1

• Microsoft Windows Software Development Kit (SDK) for Windows, version 3.1

SYMPTOMS

Calling LoadBitmap from an application causes a General Protection Fault (GPF) when attempting to load a compressed bitmap.

CAUSE

There is a problem with the way the Windows 3.x standard VGA driver handles compressed bitmaps. When a bitmap is saved as compressed, non-VGA drivers tend to leave the biClrUsed member of the BITMAPINFOHEADER at zero. This behavior causes the VGA driver to GPF as it tries to load the bitmap because the VGA driver attempts to decode the color table as though it were RLE encoded data. This can easily lead to running off the end of the data.

RESOLUTION

One work around would be to use uncompressed bitmaps. However, if compression is absolutely necessary, another way would be to save the bitmap compressed using the VGA driver so the biClrUsed member of the BITMAPINFOHEADER structure is properly set to the number of colors in the color table. Every other driver that then attempts to load the bitmap should be able to do so successfully.

MORE INFORMATION

Applications typically save images in compressed format in order to save space. This behavior can be accomplished in Microsoft Visual C++ version 1.52 by loading the bitmap in App Studio, selecting the Resource.Properties menu item and checking the "Save Compressed" checkbox.

STATUS

This behavior is by design.