1.. _magicnumbers:
2
3Linux magic numbers
4===================
5
6This file is a registry of magic numbers which are in use.  When you
7add a magic number to a structure, you should also add it to this
8file, since it is best if the magic numbers used by various structures
9are unique.
10
11It is a **very** good idea to protect kernel data structures with magic
12numbers.  This allows you to check at run time whether (a) a structure
13has been clobbered, or (b) you've passed the wrong structure to a
14routine.  This last is especially useful --- particularly when you are
15passing pointers to structures via a void * pointer.  The tty code,
16for example, does this frequently to pass driver-specific and line
17discipline-specific structures back and forth.
18
19The way to use magic numbers is to declare them at the beginning of
20the structure, like so::
21
22	struct tty_ldisc {
23		int	magic;
24		...
25	};
26
27Please follow this discipline when you are adding future enhancements
28to the kernel!  It has saved me countless hours of debugging,
29especially in the screwy cases where an array has been overrun and
30structures following the array have been overwritten.  Using this
31discipline, these cases get detected quickly and safely.
32
33Changelog::
34
35					Theodore Ts'o
36					31 Mar 94
37
38  The magic table is current to Linux 2.1.55.
39
40					Michael Chastain
41					<mailto:mec@shout.net>
42					22 Sep 1997
43
44  Now it should be up to date with Linux 2.1.112. Because
45  we are in feature freeze time it is very unlikely that
46  something will change before 2.2.x. The entries are
47  sorted by number field.
48
49					Krzysztof G. Baranowski
50					<mailto: kgb@knm.org.pl>
51					29 Jul 1998
52
53  Updated the magic table to Linux 2.5.45. Right over the feature freeze,
54  but it is possible that some new magic numbers will sneak into the
55  kernel before 2.6.x yet.
56
57					Petr Baudis
58					<pasky@ucw.cz>
59					03 Nov 2002
60
61  Updated the magic table to Linux 2.5.74.
62
63					Fabian Frederick
64					<ffrederick@users.sourceforge.net>
65					09 Jul 2003
66
67
68===================== ================ ======================== ==========================================
69Magic Name            Number           Structure                File
70===================== ================ ======================== ==========================================
71PG_MAGIC              'P'              pg_{read,write}_hdr      ``include/linux/pg.h``
72APM_BIOS_MAGIC        0x4101           apm_user                 ``arch/x86/kernel/apm_32.c``
73FASYNC_MAGIC          0x4601           fasync_struct            ``include/linux/fs.h``
74SLIP_MAGIC            0x5302           slip                     ``drivers/net/slip.h``
75BAYCOM_MAGIC          0x19730510       baycom_state             ``drivers/net/baycom_epp.c``
76HDLCDRV_MAGIC         0x5ac6e778       hdlcdrv_state            ``include/linux/hdlcdrv.h``
77KV_MAGIC              0x5f4b565f       kernel_vars_s            ``arch/mips/include/asm/sn/klkernvars.h``
78CODA_MAGIC            0xC0DAC0DA       coda_file_info           ``fs/coda/coda_fs_i.h``
79YAM_MAGIC             0xF10A7654       yam_port                 ``drivers/net/hamradio/yam.c``
80CCB_MAGIC             0xf2691ad2       ccb                      ``drivers/scsi/ncr53c8xx.c``
81QUEUE_MAGIC_FREE      0xf7e1c9a3       queue_entry              ``drivers/scsi/arm/queue.c``
82QUEUE_MAGIC_USED      0xf7e1cc33       queue_entry              ``drivers/scsi/arm/queue.c``
83NMI_MAGIC             0x48414d4d455201 nmi_s                    ``arch/mips/include/asm/sn/nmi.h``
84===================== ================ ======================== ==========================================
85