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``
72HDLC_MAGIC            0x239e           n_hdlc                   ``drivers/char/n_hdlc.c``
73APM_BIOS_MAGIC        0x4101           apm_user                 ``arch/x86/kernel/apm_32.c``
74FASYNC_MAGIC          0x4601           fasync_struct            ``include/linux/fs.h``
75SLIP_MAGIC            0x5302           slip                     ``drivers/net/slip.h``
76TTY_MAGIC             0x5401           tty_struct               ``include/linux/tty.h``
77MGSL_MAGIC            0x5401           mgsl_info                ``drivers/char/synclink.c``
78TTY_DRIVER_MAGIC      0x5402           tty_driver               ``include/linux/tty_driver.h``
79MGSLPC_MAGIC          0x5402           mgslpc_info              ``drivers/char/pcmcia/synclink_cs.c``
80FULL_DUPLEX_MAGIC     0x6969                                    ``drivers/net/ethernet/dec/tulip/de2104x.c``
81CG_MAGIC              0x00090255       ufs_cylinder_group       ``include/linux/ufs_fs.h``
82RIEBL_MAGIC           0x09051990                                ``drivers/net/atarilance.c``
83NBD_REQUEST_MAGIC     0x12560953       nbd_request              ``include/linux/nbd.h``
84BAYCOM_MAGIC          0x19730510       baycom_state             ``drivers/net/baycom_epp.c``
85SAVEKMSG_MAGIC2       0x4B4D5347       savekmsg                 ``arch/*/amiga/config.c``
86COW_MAGIC             0x4f4f4f4d       cow_header_v1            ``arch/um/drivers/ubd_user.c``
87SAVEKMSG_MAGIC1       0x53415645       savekmsg                 ``arch/*/amiga/config.c``
88EEPROM_MAGIC_VALUE    0x5ab478d2       lanai_dev                ``drivers/atm/lanai.c``
89HDLCDRV_MAGIC         0x5ac6e778       hdlcdrv_state            ``include/linux/hdlcdrv.h``
90KV_MAGIC              0x5f4b565f       kernel_vars_s            ``arch/mips/include/asm/sn/klkernvars.h``
91FW_HEADER_MAGIC       0x65726F66       fw_header                ``drivers/atm/fore200e.h``
92LO_MAGIC              0x68797548       nbd_device               ``include/linux/nbd.h``
93NBD_REPLY_MAGIC       0x96744668       nbd_reply                ``include/linux/nbd.h``
94ENI155_MAGIC          0xa54b872d       midway_eprom	        ``drivers/atm/eni.h``
95CODA_MAGIC            0xC0DAC0DA       coda_file_info           ``fs/coda/coda_fs_i.h``
96YAM_MAGIC             0xF10A7654       yam_port                 ``drivers/net/hamradio/yam.c``
97CCB_MAGIC             0xf2691ad2       ccb                      ``drivers/scsi/ncr53c8xx.c``
98QUEUE_MAGIC_FREE      0xf7e1c9a3       queue_entry              ``drivers/scsi/arm/queue.c``
99QUEUE_MAGIC_USED      0xf7e1cc33       queue_entry              ``drivers/scsi/arm/queue.c``
100NMI_MAGIC             0x48414d4d455201 nmi_s                    ``arch/mips/include/asm/sn/nmi.h``
101===================== ================ ======================== ==========================================
102
103Note that there are also defined special per-driver magic numbers in sound
104memory management. See ``include/sound/sndmagic.h`` for complete list of them. Many
105OSS sound drivers have their magic numbers constructed from the soundcard PCI
106ID - these are not listed here as well.
107