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.| | |
or 0xABAB or 0xABABABAB | LocalAlloc(). |
| LocalAlloc( LMEM_FIXED, ... ). It is memory that has been allocated but not yet written to. |
| HeapAlloc() or LocalAlloc(). |
or or | /GZ is automatically initialized the uninitialized variable with this value. |
or or | DEBUG_NEW was defined. Memory with this tag signifies memory that has been allocated (by malloc() or new) but never written to the application. |
or or | 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. |
or or | 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
http://en.wikipedia.org/wiki/Magic_number_(programming)
Magic debug values are specific values written to memory during allocation or deallocation, so that it will later be possible to tell whether or not they have become corrupted, and to make it obvious when values taken from uninitialized memory are being used.
Since it is unlikely that any given 32-bit integer would take this specific value, the appearance of such a number in a debugger or memory dump can indicate common errors such as buffer overflows, or uninitialized variables.
Memory is usually viewed in hexadecimal, so common values used are often repeated digits or hexspeak.
Famous and common examples include:
0x5F3759DF: Used in Quake 3 for an optimized inverse square root function. [8] 0x..FACADE: Used by a number of RTOSes 0xA5A5A5A5: Used in embedded development because the alternating bit pattern (10100101) creates an easily recognized pattern on oscilloscopes and logic analyzers. 0xABABABAB: Used by Microsoft's HeapAlloc() to mark "no man's land" guard bytes after allocated heap memory 0xABADBABE: Used by Apple as the "Boot Zero Block" magic number 0xABADCAFE: A startup to this value to initialize all free memory to catch errant pointers 0xBAADF00D: Used by Microsoft's LocalAlloc(LMEM_FIXED) to mark uninitialised allocated heap memory 0xBADBADBADBAD: Burroughs large systems "uninitialized" memory (48-bit words) 0xBADCAB1E: Error Code returned to the Microsoft eVC debugger when connection is severed to the debuggee 0xBADC0FFEE0DDF00D: Used on IBM RS/6000 64-bit systems to indicate uninitialized CPU registers 0xBADDCAFE: On Sun Microsystems' Solaris, marks uninitialised kernel memory (KMEM_UNINITIALIZED_PATTERN) 0xBEEFCACE: Used by Microsoft .NET as a magic number in resource files 0xC0DEDBAD: A memory leak tracking tool which it will change the MMU tables so that all references to address zero 0xCAFEBABE: Used by both Mach-O ("Fat binary" in both 68k and PowerPC) to identify object files and the Java programming language to identify .class files 0xCAFEFEED: Used by Sun Microsystems' Solaris debugging kernel to mark kmemfree() memory 0xCEFAEDFE: Seen in Intel Mach-O binaries on Apple Computer's Mac OS X platform (see 0xFEEDFACEbelow)0xCCCCCCCC: Used by Microsoft's C++ debugging runtime library to mark uninitialised stack memory 0xCDCDCDCD: Used by Microsoft's C++ debugging runtime library to mark uninitialised heap memory 0xDDDDDDDD: Used by MicroQuill's SmartHeap and Microsoft's C++ debugging heap to mark freed heap memory 0xDEADBABE: Used at the start of Silicon Graphics' IRIX arena files 0xDEADBEEF: Famously used on IBM systems such as the RS/6000, also used in the original Mac OS operating systems, OPENSTEP Enterprise, and the Commodore Amiga. On Sun Microsystems' Solaris, marks freed kernel memory (KMEM_FREE_PATTERN) 0xDEADDEAD: A Microsoft Windows STOP Error code used when the user manually initiates the crash. 0xDEADF00D: All the newly allocated memory which is not explicitly cleared when it is munged 0xEBEBEBEB: From MicroQuill's SmartHeap 0xFADEDEAD: Comes at the end to identify every OSA script 0xFDFDFDFD: Used by Microsoft's C++ debugging heap to mark "no man's land" guard bytes before and after allocated heap memory 0xFEEDFACE: Seen in PowerPC Mach-O binaries on Apple Computer's Mac OS X platform. On Sun Microsystems' Solaris, marks the red zone (KMEM_REDZONE_PATTERN) 0xFEEEFEEE: Used by Microsoft's HeapFree() to mark freed heap memory 0xFEE1DEAD: Used by Linux reboot() syscall
Note that most of these are each 32 bits long — the word size of most modern computers.
The prevalence of these values in Microsoft technology is no coincidence; they are discussed in detail in Steve Maguire's book Writing Solid Code from Microsoft Press. He gives a variety of criteria for these values, such as:
- They should not be useful; that is, most algorithms that operate on them should be expected to do something unusual. Numbers like zero don't fit this criterion.
- They should be easily recognized by the programmer as invalid values in the debugger.
- On machines that don't have byte alignment, they should be odd numbers, so that dereferencing them as addresses causes an exception.
- They should cause an exception, or perhaps even a debugger break, if executed as code.
Since they were often used to mark areas of memory that were essentially empty, some of these terms came to be used in phrases meaning "gone, aborted, flushed from memory"; e.g. "Your program is DEADBEEF".
Pietr Brandehörst's ZUG programming language initialized memory to either 0x0000, 0xDEAD or 0xFFFF in development environment and to 0x0000 in the live environment, on the basis that uninitialised variables should be encouraged to misbehave under development to trap them, but encouraged to behave in a live environment to reduce errorssince April 2008">[citation needed].
