ValidateFreeSpaces(), GlobalCompact(), and Wild Pointers

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.0 and 3.1

SUMMARY

The ValidateFreeSpaces() function does not work as documented on page 4-454 of the "Microsoft Windows Software Development Kit Reference Volume 1" for version 3.0. On page 964 of the "Programmer's Reference, Volume 2: Functions" manual from the Windows 3.1 SDK, the ValidateFreeSpaces() function is correctly documented.

MORE INFORMATION

ValidateFreeSpaces() returns a void, not a LPSTR, as documented. Under the debugging kernel, ValidateFreeSpaces() will RIP if it finds overwritten free space; under the retail kernel, it does nothing.

To use ValidateFreeSpaces(), the WIN.INI file must include the following statements:

   [KERNEL]
   EnableHeapChecking=1
   EnableFreeChecking=1

To build a fairly solid detector of wild pointers, an application writer can use the following statements in the [KERNEL] section:

1. EnableHeapChecking=1 to cause the kernel to regularly check the global arena.

2. EnableFreeChecking=1 to let ValidateFreeSpaces() check global free space.

3. EnableSegmentChecksum=1 to cause the kernel to check for corrupted code segments.

Please note the following:

• CCh is used for the free-space fill byte because it will cause the debugger to breakpoint if a jump into free space is executed.
• If EnableSegmentChecksum=1 is used, setting breakpoints may cause spurious 0x0409 RIPs.
• Setting all of these checks will noticeably slow the system.
• If bad free spaces exist, check for fatal exit FFs and LLLLLLLs before the address actually appears. A few fatal exits may have to be ignored before the address appears.