README
1This directory attempts to document the ABI between the Linux kernel and
2userspace, and the relative stability of these interfaces. Due to the
3everchanging nature of Linux, and the differing maturity levels, these
4interfaces should be used by userspace programs in different ways.
5
6We have four different levels of ABI stability, as shown by the four
7different subdirectories in this location. Interfaces may change levels
8of stability according to the rules described below.
9
10The 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38 why it is obsolete and when it can be expected to be removed.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
44Every file in these directories will contain the following information:
45
46What: Short description of the interface
47Date: Date created
48KernelVersion: Kernel version this feature first showed up in.
49Contact: Primary contact for this interface (may be a mailing list)
50Description: Long description of the interface and how to use it.
51Users: 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
61How things move between levels:
62
63Interfaces in stable may move to obsolete, as long as the proper
64notification is given.
65
66Interfaces may be removed from obsolete and the kernel as long as the
67documented amount of time has gone by.
68
69Interfaces in the testing state can move to the stable state when the
70developers feel they are finished. They cannot be removed from the
71kernel tree without going through the obsolete state first.
72
73It's up to the developer to place their interfaces in the category they
74wish for it to start out in.
75
76
77Notable bits of non-ABI, which should not under any circumstances be considered
78stable:
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/process/stable-api-nonsense.rst.
88
