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+.. _magicnumbers:
+
+Linux magic numbers
+===================
+
+This file is a registry of magic numbers which are in use. When you
+add a magic number to a structure, you should also add it to this
+file, since it is best if the magic numbers used by various structures
+are unique.
+
+It is a **very** good idea to protect kernel data structures with magic
+numbers. This allows you to check at run time whether (a) a structure
+has been clobbered, or (b) you've passed the wrong structure to a
+routine. This last is especially useful --- particularly when you are
+passing pointers to structures via a void * pointer. The tty code,
+for example, does this frequently to pass driver-specific and line
+discipline-specific structures back and forth.
+
+The way to use magic numbers is to declare them at the beginning of
+the structure, like so::
+
+ struct tty_ldisc {
+ int magic;
+ ...
+ };
+
+Please follow this discipline when you are adding future enhancements
+to the kernel! It has saved me countless hours of debugging,
+especially in the screwy cases where an array has been overrun and
+structures following the array have been overwritten. Using this
+discipline, these cases get detected quickly and safely.
+
+Changelog::
+
+ Theodore Ts'o
+ 31 Mar 94
+
+ The magic table is current to Linux 2.1.55.
+
+ Michael Chastain
+ <mailto:mec@shout.net>
+ 22 Sep 1997
+
+ Now it should be up to date with Linux 2.1.112. Because
+ we are in feature freeze time it is very unlikely that
+ something will change before 2.2.x. The entries are
+ sorted by number field.
+
+ Krzysztof G. Baranowski
+ <mailto: kgb@knm.org.pl>
+ 29 Jul 1998
+
+ Updated 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 the
+ kernel before 2.6.x yet.
+
+ Petr Baudis
+ <pasky@ucw.cz>
+ 03 Nov 2002
+
+ Updated 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 ``include/linux/pg.h``
+APM_BIOS_MAGIC 0x4101 apm_user ``arch/x86/kernel/apm_32.c``
+FASYNC_MAGIC 0x4601 fasync_struct ``include/linux/fs.h``
+SLIP_MAGIC 0x5302 slip ``drivers/net/slip.h``
+BAYCOM_MAGIC 0x19730510 baycom_state ``drivers/net/baycom_epp.c``
+HDLCDRV_MAGIC 0x5ac6e778 hdlcdrv_state ``include/linux/hdlcdrv.h``
+KV_MAGIC 0x5f4b565f kernel_vars_s ``arch/mips/include/asm/sn/klkernvars.h``
+CODA_MAGIC 0xC0DAC0DA coda_file_info ``fs/coda/coda_fs_i.h``
+YAM_MAGIC 0xF10A7654 yam_port ``drivers/net/hamradio/yam.c``
+CCB_MAGIC 0xf2691ad2 ccb ``drivers/scsi/ncr53c8xx.c``
+QUEUE_MAGIC_FREE 0xf7e1c9a3 queue_entry ``drivers/scsi/arm/queue.c``
+QUEUE_MAGIC_USED 0xf7e1cc33 queue_entry ``drivers/scsi/arm/queue.c``
+NMI_MAGIC 0x48414d4d455201 nmi_s ``arch/mips/include/asm/sn/nmi.h``
+===================== ================ ======================== ==========================================