Linux magic numbers¶
This file is a registry of magic numbers which are in use. When youadd a magic number to a structure, you should also add it to thisfile, since it is best if the magic numbers used by various structuresare unique.
It is avery good idea to protect kernel data structures with magicnumbers. This allows you to check at run time whether (a) a structurehas been clobbered, or (b) you’ve passed the wrong structure to aroutine. This last is especially useful --- particularly when you arepassing pointers to structures via a void * pointer. The tty code,for example, does this frequently to pass driver-specific and linediscipline-specific structures back and forth.
The way to use magic numbers is to declare them at the beginning ofthe structure, like so:
struct tty_ldisc { int magic; ...};Please follow this discipline when you are adding future enhancementsto the kernel! It has saved me countless hours of debugging,especially in the screwy cases where an array has been overrun andstructures following the array have been overwritten. Using thisdiscipline, these cases get detected quickly and safely.
Changelog:
Theodore Ts'o 31 Mar 94The magic table is current to Linux 2.1.55. Michael Chastain <mailto:mec@shout.net> 22 Sep 1997Now it should be up to date with Linux 2.1.112. Becausewe are in feature freeze time it is very unlikely thatsomething will change before 2.2.x. The entries aresorted by number field. Krzysztof G. Baranowski <mailto: kgb@knm.org.pl> 29 Jul 1998Updated the magic table to Linux 2.5.45. Right over the feature freeze,but it is possible that some new magic numbers will sneak into thekernel before 2.6.x yet. Petr Baudis <pasky@ucw.cz> 03 Nov 2002Updated the magic table to Linux 2.5.74. Fabian Frederick <ffrederick@users.sourceforge.net> 09 Jul 2003
Magic Name | Number | Structure | File |
|---|---|---|---|
PG_MAGIC | ‘P’ | pg_{read,write}_hdr |
|
APM_BIOS_MAGIC | 0x4101 | apm_user |
|
FASYNC_MAGIC | 0x4601 | fasync_struct |
|
SLIP_MAGIC | 0x5302 | slip |
|
BAYCOM_MAGIC | 19730510 | baycom_state |
|
HDLCDRV_MAGIC | 0x5ac6e778 | hdlcdrv_state |
|
KV_MAGIC | 0x5f4b565f | kernel_vars_s |
|
CODA_MAGIC | 0xC0DAC0DA | coda_file_info |
|
YAM_MAGIC | 0xF10A7654 | yam_port |
|
CCB_MAGIC | 0xf2691ad2 | ccb |
|
QUEUE_MAGIC_FREE | 0xf7e1c9a3 | queue_entry |
|
QUEUE_MAGIC_USED | 0xf7e1cc33 | queue_entry |
|
NMI_MAGIC | 0x48414d4d455201 | nmi_s |
|