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Based on kernel version 3.16. Page generated on 2014-08-06 21:35 EST.

1	This directory attempts to document the ABI between the Linux kernel and
2	userspace, and the relative stability of these interfaces.  Due to the
3	everchanging nature of Linux, and the differing maturity levels, these
4	interfaces should be used by userspace programs in different ways.
5	
6	We have four different levels of ABI stability, as shown by the four
7	different subdirectories in this location.  Interfaces may change levels
8	of stability according to the rules described below.
9	
10	The different levels of stability are:
11	
12	  stable/
13		This directory documents the interfaces that the developer has
14		defined to be stable.  Userspace programs are free to use these
15		interfaces with no restrictions, and backward compatibility for
16		them will be guaranteed for at least 2 years.  Most interfaces
17		(like syscalls) are expected to never change and always be
18		available.
19	
20	  testing/
21		This directory documents interfaces that are felt to be stable,
22		as the main development of this interface has been completed.
23		The interface can be changed to add new features, but the
24		current interface will not break by doing this, unless grave
25		errors or security problems are found in them.  Userspace
26		programs can start to rely on these interfaces, but they must be
27		aware of changes that can occur before these interfaces move to
28		be marked stable.  Programs that use these interfaces are
29		strongly encouraged to add their name to the description of
30		these interfaces, so that the kernel developers can easily
31		notify them if any changes occur (see the description of the
32		layout of the files below for details on how to do this.)
33	
34	  obsolete/
35	  	This directory documents interfaces that are still remaining in
36		the kernel, but are marked to be removed at some later point in
37		time.  The description of the interface will document the reason
38		why it is obsolete and when it can be expected to be removed.
39	
40	  removed/
41		This directory contains a list of the old interfaces that have
42		been removed from the kernel.
43	
44	Every file in these directories will contain the following information:
45	
46	What:		Short description of the interface
47	Date:		Date created
48	KernelVersion:	Kernel version this feature first showed up in.
49	Contact:	Primary contact for this interface (may be a mailing list)
50	Description:	Long description of the interface and how to use it.
51	Users:		All users of this interface who wish to be notified when
52			it changes.  This is very important for interfaces in
53			the "testing" stage, so that kernel developers can work
54			with userspace developers to ensure that things do not
55			break in ways that are unacceptable.  It is also
56			important to get feedback for these interfaces to make
57			sure they are working in a proper way and do not need to
58			be changed further.
59	
60	
61	How things move between levels:
62	
63	Interfaces in stable may move to obsolete, as long as the proper
64	notification is given.
65	
66	Interfaces may be removed from obsolete and the kernel as long as the
67	documented amount of time has gone by.
68	
69	Interfaces in the testing state can move to the stable state when the
70	developers feel they are finished.  They cannot be removed from the
71	kernel tree without going through the obsolete state first.
72	
73	It's up to the developer to place their interfaces in the category they
74	wish for it to start out in.
75	
76	
77	Notable bits of non-ABI, which should not under any circumstances be considered
78	stable:
79	
80	- Kconfig.  Userspace should not rely on the presence or absence of any
81	  particular Kconfig symbol, in /proc/config.gz, in the copy of .config
82	  commonly installed to /boot, or in any invocation of the kernel build
83	  process.
84	
85	- Kernel-internal symbols.  Do not rely on the presence, absence, location, or
86	  type of any kernel symbol, either in System.map files or the kernel binary
87	  itself.  See Documentation/stable_api_nonsense.txt.
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