WFC Technical Note 006 - Funny Memory Values

$Revision: 1 $ Last Modified $Date: 3/26/00 11:01a $

Introduction

Microsoft has put quite a lot of memory leak detection helpers in Windows NT. They have not done a good job of advertising it. This document describes some of the things I've deciphered while debugging code.

Funny Memory Values

Many times while debugging programs, I will come across memory that is filled with "funny" values. After some playing around (i.e. hacking) with the Win32 API, I was able to figure out what they meant. Some of these values have been documented in places but never all together. The values for the tags presented here are hexadecimal because that's the way Developer's Studio presents them in the memory window.

Value	Meaning
`0xAB` or `0xABAB` or `0xABABABAB`	Memory following a block allocated by `LocalAlloc()`.
`0xBAADF00D`	Bad Food. Get it? This is memory allocated via `LocalAlloc( LMEM_FIXED, ... )`. It is memory that has been allocated but not yet written to.
`0xFEEE` `0xFEEEFEEE`	This seems to be memory that has been dedicated to a heap but not yet allocated by `HeapAlloc()` or `LocalAlloc()`.
`0xCC` or `0xCCCC` or `0xCCCCCCCC`	Microsoft Visual C++ compiled code with the `/GZ` is automatically initialized the uninitialized variable with this value.
`0xCD` or `0xCDCD` or `0xCDCDCDCD`	Microsoft Visual C++ compiled code with memory leak detection turned on. Usually, `DEBUG_NEW` was defined. Memory with this tag signifies memory that has been allocated (by `malloc()` or `new`) but never written to the application.
`0xDD` or `0xDDDD` or `0xDDDDDDDD`	Microsoft Visual C++ compiled code with memory leak detection turned on. Usually, `DEBUG_NEW` was defined. Memory with this tag signifies memory that has been freed (by `free()` or `delete`) by the application. It is how you can detect writing to memory that has already been freed. For example, if you look at an allocated memory structure (or C++ class) and most of the members contain this tag value, you are probably writing to a structure that has been freed.
`0xFD` or `0xFDFD` or `0xFDFDFDFD`	Microsoft Visual C++ compiled code with memory leak detection turned on. Usually, `DEBUG_NEW` was defined. Memory with this tag signifies memory that is in "no-mans-land." These are bytes just before and just after an allocated block. They are used to detect array-out-of-bounds errors. This is great for detecting off-by-one errors.

Samuel R. Blackburn