1This part of the documentation inside Documentation/ABI directory 2attempts to document the ABI between the Linux kernel and 3userspace, and the relative stability of these interfaces. Due to the 4everchanging nature of Linux, and the differing maturity levels, these 5interfaces should be used by userspace programs in different ways. 6 7We have four different levels of ABI stability, as shown by the four 8different subdirectories in this location. Interfaces may change levels 9of stability according to the rules described below. 10 11The different levels of stability are: 12 13 stable/ 14 This directory documents the interfaces that the developer has 15 defined to be stable. Userspace programs are free to use these 16 interfaces with no restrictions, and backward compatibility for 17 them will be guaranteed for at least 2 years. Most interfaces 18 (like syscalls) are expected to never change and always be 19 available. 20 21 testing/ 22 This directory documents interfaces that are felt to be stable, 23 as the main development of this interface has been completed. 24 The interface can be changed to add new features, but the 25 current interface will not break by doing this, unless grave 26 errors or security problems are found in them. Userspace 27 programs can start to rely on these interfaces, but they must be 28 aware of changes that can occur before these interfaces move to 29 be marked stable. Programs that use these interfaces are 30 strongly encouraged to add their name to the description of 31 these interfaces, so that the kernel developers can easily 32 notify them if any changes occur (see the description of the 33 layout of the files below for details on how to do this.) 34 35 obsolete/ 36 This directory documents interfaces that are still remaining in 37 the kernel, but are marked to be removed at some later point in 38 time. The description of the interface will document the reason 39 why it is obsolete and when it can be expected to be removed. 40 41 removed/ 42 This directory contains a list of the old interfaces that have 43 been removed from the kernel. 44 45Every file in these directories will contain the following information: 46 47What: Short description of the interface 48Date: Date created 49KernelVersion: (Optional) Kernel version this feature first showed up in. 50 Note: git history often provides more accurate version 51 info, so this field may be omitted. 52Contact: Primary contact for this interface (may be a mailing list) 53Description: Long description of the interface and how to use it. 54Users: All users of this interface who wish to be notified when 55 it changes. This is very important for interfaces in 56 the "testing" stage, so that kernel developers can work 57 with userspace developers to ensure that things do not 58 break in ways that are unacceptable. It is also 59 important to get feedback for these interfaces to make 60 sure they are working in a proper way and do not need to 61 be changed further. 62 63 64Note: 65 The fields should be use a simple notation, compatible with ReST markup. 66 Also, the file **should not** have a top-level index, like:: 67 68 === 69 foo 70 === 71 72How things move between levels: 73 74Interfaces in stable may move to obsolete, as long as the proper 75notification is given. 76 77Interfaces may be removed from obsolete and the kernel as long as the 78documented amount of time has gone by. 79 80Interfaces in the testing state can move to the stable state when the 81developers feel they are finished. They cannot be removed from the 82kernel tree without going through the obsolete state first. 83 84It's up to the developer to place their interfaces in the category they 85wish for it to start out in. 86 87 88Notable bits of non-ABI, which should not under any circumstances be considered 89stable: 90 91- Kconfig. Userspace should not rely on the presence or absence of any 92 particular Kconfig symbol, in /proc/config.gz, in the copy of .config 93 commonly installed to /boot, or in any invocation of the kernel build 94 process. 95 96- Kernel-internal symbols. Do not rely on the presence, absence, location, or 97 type of any kernel symbol, either in System.map files or the kernel binary 98 itself. See Documentation/process/stable-api-nonsense.rst. 99