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. Memory is usually viewed in hexadecimal, so memorable repeating or hexspeak values are common. Numerically odd values may be preferred so that processors without byte addressing will fault when attempting to use them as pointers (which must fall at even addresses). Values should be chosen that are away from likely addresses (the program code, static data, heap data, or the stack). Similarly, they may be chosen so that they are not valid codes in the instruction set for the given architecture.
Since it is very unlikely, although possible, that a 32-bit integer would take this specific value, the appearance of such a number in a debugger or memory dump most likely indicates an error such as a buffer overflow or an uninitialized variable.
Famous and common examples include:
| Code | Description |
|---|---|
..FACADE |
"Facade", Used by a number of RTOSes |
1BADB002 |
"Bad booze", Multiboot header magic number |
A5A5A5A5 |
Used in embedded development because the alternating bit pattern (1010 0101) creates an easily recognized pattern on oscilloscopes and logic analyzers. |
A5 |
Used in FreeBSD's PHK malloc(3) for debugging when /etc/malloc.conf is symlinked to "-J" to initialize all newly allocated memory as this value is not a NULL pointer or ASCII NUL character. |
ABABABAB |
Used by Microsoft's HeapAlloc to mark "no man's land" guard bytes after allocated heap memory |
ABADBABE |
"A bad babe", Used by Apple as the "Boot Zero Block" magic number |
ABADCAFE |
"A bad cafe", Used to initialize all unallocated memory (Mungwall, AmigaOS). |
0DEFACED |
"Defaced", Required by Microsoft's Hyper-V hypervisor to be used by Linux guests as their "guest signature", after changing from original 0xB16B00B5 |
BAADF00D |
"Bad food", Used by Microsoft's LocalAlloc(LMEM_FIXED) to mark uninitialized allocated heap memory |
BADBADBADBAD |
"Bad bad bad bad", Burroughs large systems "uninitialized" memory (48-bit words) |
BADC0FFEE0DDF00D |
"Bad coffee odd food", Used on IBM RS/6000 64-bit systems to indicate uninitialized CPU registers |
BADDCAFE |
"Bad cafe", On Sun Microsystems' Solaris, marks uninitialised kernel memory (KMEM_UNINITIALIZED_PATTERN) |
BBADBEEF |
"Bad beef", Used in WebKit |
BEEFCACE |
"Beef cake", Used by Microsoft .NET as a magic number in resource files |
CAFED00D |
"Cafe dude", Used by Java for their pack200 compression |
CAFEFEED |
"Cafe feed", Used by Sun Microsystems' Solaris debugging kernel to mark kmemfree memory |
CCCCCCCC |
Used by Microsoft's C++ debugging runtime library and many DOS environments to mark uninitialized stack memory. CC resembles the opcode of the INT 3 debug breakpoint interrupt on x86 processors. |
CDCDCDCD |
Used by Microsoft's C++ debugging runtime library to mark uninitialized heap memory |
D15EA5E |
"Disease", Used as a flag to indicate regular boot on the Nintendo GameCube and Wii consoles |
DDDDDDDD |
Used by MicroQuill's SmartHeap and Microsoft's C++ debugging heap to mark freed heap memory |
DEADBABE |
"Dead babe", Used at the start of Silicon Graphics' IRIX arena files |
DEADBEEF |
"Dead beef", 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) |
DEADC0DE |
"Dead code", Used as a marker in OpenWRT firmware to signify the beginning of the to-be created jffs2 file system at the end of the static firmware |
DEADF00D |
"Dead food", Used by Mungwall on the Commodore Amiga to mark allocated but uninitialized memory |
DEFEC8ED |
"Defecated", Used for OpenSolaris core dumps |
EBEBEBEB |
From MicroQuill's SmartHeap |
FADEDEAD |
"Fade dead", Comes at the end to identify every AppleScript script |
FDFDFDFD |
Used by Microsoft's C++ debugging heap to mark "no man's land" guard bytes before and after allocated heap memory |
FEE1DEAD |
"Feel dead", Used by Linux reboot syscall |
FEEDFACE |
"Feed face", Seen in PowerPC Mach-O binaries on Apple Inc.'s Mac OS X platform. On Sun Microsystems' Solaris, marks the red zone (KMEM_REDZONE_PATTERN) |
FEEEFEEE |
"Fee fee", Used by Microsoft's HeapFree to mark freed heap memory |
Note that most of these are each 32 bits long — the dword size of 32-bit architecture 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".
Read more about this topic: Deadbeef
Famous quotes containing the words magic and/or values:
“I revere the memory of Mr. F. as an estimable man and most indulgent husband, only necessary to mention Asparagus and it appeared or to hint at any little delicate thing to drink and it came like magic in a pint bottle; it was not ecstasy but it was comfort.”
—Charles Dickens (1812–1870)
“What we often take to be family values—the work ethic, honesty, clean living, marital fidelity, and individual responsibility—are in fact social, religious, or cultural values. To be sure, these values are transmitted by parents to their children and are familial in that sense. They do not, however, originate within the family. It is the value of close relationships with other family members, and the importance of these bonds relative to other needs.”
—David Elkind (20th century)