FIX: GP Fault When For Loop Used with Unsigned Char Index

The information in this article applies to:

• The C/C++ Compiler (CL.EXE) included with: Microsoft Visual C++, 32-bit Edition, versions 2.0 and 2.1 on the following platform: x86 only

SYMPTOMS

If you access a structure member when using an unsigned char index in a For loop, a general protection (GP) fault occurs.

CAUSE

When the compiler is optimizing for speed, only the low byte of the register where the index is stored is zeroed. Later on, the full extended register is used in calculating the offset of the structure member. Because the rest of the register is never zeroed, the offset used is invalid.

RESOLUTION

Use either one of the following workarounds:

• Change the type of the For loop index to any type other than unsigned char.

  -or-

• Do not use speed optimizations.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug was corrected in Microsoft Visual C++, 32-bit Edition, version 4.0.

MORE INFORMATION

The sample code below can be used to demonstrate the problem.

Due to the complexity of the optimization algorithm, some structures may work while others fail. However until this bug is fixed, it should be considered unsafe to use unsigned char indexing in a For loop with speed optimization.

Sample Code to Reproduce Problem

/* Compile options needed: any speed optimizations
*/

   char    filler[37];
   int     x;

void main()

   unsigned char i = 1;

   for(i=0; i<2; i++)
   {
      xyz[i].x = -1;
   }