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