Memory Debug Codes

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本文详细介绍了Microsoft在内存管理中使用的特殊值，包括Microsoft Visual C++运行时库中的调试代码、编译器初始化值以及Windows NT中的内存代码。这些特殊的值用于帮助开发者检测内存错误和跟踪内存使用情况。

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Microsoft's memory management functions often initialize memory with special values. The following article describes frequent used variants.

Microsoft Visual C++ Runtime library

C runtime library provides it own debug codes:

0xCD, 0xCDCDCDCD - New objects. New objects are filled with 0xCD when they are allocated.
0xFD, 0xFDFDFDFD - No-man's land memory. Extra bytes that belong to the internal block allocated, but not the block you requested. They are placed before and after requested blocks and used for data bound checking.
0xDD, 0xDDDDDDDD - Freed blocks. The freed blocks kept unused in the debug heap's linked list when the _CRTDBG_DELAY_FREE_MEM_DF flag is set are currently filled with 0xDD. Although in some cases you won't see magic 0xDDDDDDDD value, as it will be overwritten by another debug function (e.g. 0xFEEEFEEE for HeapFree).

These constants are defined in DbgHeap.c file as

static unsigned char _bNoMansLandFill = 0xFD; /* fill no-man's land with this */
static unsigned char _bDeadLandFill = 0xDD; /* fill free objects with this */
static unsigned char _bCleanLandFill = 0xCD; /* fill new objects with this */

Compiler initialisations

0xCC, 0xCCCCCCCC - The /GX Microsoft Visual C++ compiler option initialises all local variables not explicitly initialised by the program. It fills all memory used by these variables with 0xCC, 0xCCCCCCCC.

Windows NT memory codes

0xABABABAB - Memory following a block allocated by LocalAlloc().
0xBAADF00D - "Bad Food". This is memory allocated via LocalAlloc( LMEM_FIXED, ... ). It is memory that has been allocated but not yet written to.
0xFEEEFEEE - OS fill heap memory, which was marked for usage, but wasn't allocated by HeapAlloc() or LocalAlloc(). Or that memory just has been freed by HeapFree().