1 .. _magicnumbers:
2
3 Linux magic numbers
4 ===================
5
6 This file is a registry of magic numbers which are in use. When you
7 add a magic number to a structure, you should also add it to this
8 file, since it is best if the magic numbers used by various structures
9 are unique.
10
11 It is a **very** good idea to protect kernel data structures with magic
12 numbers. This allows you to check at run time whether (a) a structure
13 has been clobbered, or (b) you've passed the wrong structure to a
14 routine. This last is especially useful --- particularly when you are
15 passing pointers to structures via a void * pointer. The tty code,
16 for example, does this frequently to pass driver-specific and line
17 discipline-specific structures back and forth.
18
19 The way to use magic numbers is to declare them at the beginning of
20 the structure, like so::
21
22 struct tty_ldisc {
23 int magic;
24 ...
25 };
26
27 Please follow this discipline when you are adding future enhancements
28 to the kernel! It has saved me countless hours of debugging,
29 especially in the screwy cases where an array has been overrun and
30 structures following the array have been overwritten. Using this
31 discipline, these cases get detected quickly and safely.
32
33 Changelog::
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 ===================== ================ ======================== ==========================================
69 Magic Name Number Structure File
70 ===================== ================ ======================== ==========================================
71 PG_MAGIC 'P' pg_{read,write}_hdr ``include/linux/pg.h``
72 APM_BIOS_MAGIC 0x4101 apm_user ``arch/x86/kernel/apm_32.c``
73 FASYNC_MAGIC 0x4601 fasync_struct ``include/linux/fs.h``
74 SLIP_MAGIC 0x5302 slip ``drivers/net/slip.h``
75 BAYCOM_MAGIC 0x19730510 baycom_state ``drivers/net/baycom_epp.c``
76 HDLCDRV_MAGIC 0x5ac6e778 hdlcdrv_state ``include/linux/hdlcdrv.h``
77 KV_MAGIC 0x5f4b565f kernel_vars_s ``arch/mips/include/asm/sn/klkernvars.h``
78 CODA_MAGIC 0xC0DAC0DA coda_file_info ``fs/coda/coda_fs_i.h``
79 YAM_MAGIC 0xF10A7654 yam_port ``drivers/net/hamradio/yam.c``
80 CCB_MAGIC 0xf2691ad2 ccb ``drivers/scsi/ncr53c8xx.c``
81 QUEUE_MAGIC_FREE 0xf7e1c9a3 queue_entry ``drivers/scsi/arm/queue.c``
82 QUEUE_MAGIC_USED 0xf7e1cc33 queue_entry ``drivers/scsi/arm/queue.c``
83 NMI_MAGIC 0x48414d4d455201 nmi_s ``arch/mips/include/asm/sn/nmi.h``
84 ===================== ================ ======================== ==========================================
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