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1This directory attempts to document the ABI between the Linux kernel and2userspace, and the relative stability of these interfaces. Due to the3everchanging nature of Linux, and the differing maturity levels, these4interfaces should be used by userspace programs in different ways.5 6We have four different levels of ABI stability, as shown by the four7different subdirectories in this location. Interfaces may change levels8of 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 has14 defined to be stable. Userspace programs are free to use these15 interfaces with no restrictions, and backward compatibility for16 them will be guaranteed for at least 2 years. Most interfaces17 (like syscalls) are expected to never change and always be18 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 the24 current interface will not break by doing this, unless grave25 errors or security problems are found in them. Userspace26 programs can start to rely on these interfaces, but they must be27 aware of changes that can occur before these interfaces move to28 be marked stable. Programs that use these interfaces are29 strongly encouraged to add their name to the description of30 these interfaces, so that the kernel developers can easily31 notify them if any changes occur (see the description of the32 layout of the files below for details on how to do this.)33 34 obsolete/35 This directory documents interfaces that are still remaining in36 the kernel, but are marked to be removed at some later point in37 time. The description of the interface will document the reason38 why it is obsolete and when it can be expected to be removed.39 40 removed/41 This directory contains a list of the old interfaces that have42 been removed from the kernel.43 44Every file in these directories will contain the following information:45 46What: Short description of the interface47Date: Date created48KernelVersion: 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 when52 it changes. This is very important for interfaces in53 the "testing" stage, so that kernel developers can work54 with userspace developers to ensure that things do not55 break in ways that are unacceptable. It is also56 important to get feedback for these interfaces to make57 sure they are working in a proper way and do not need to58 be changed further.59 60 61Note:62 The fields should be use a simple notation, compatible with ReST markup.63 Also, the file **should not** have a top-level index, like::64 65 ===66 foo67 ===68 69How things move between levels:70 71Interfaces in stable may move to obsolete, as long as the proper72notification is given.73 74Interfaces may be removed from obsolete and the kernel as long as the75documented amount of time has gone by.76 77Interfaces in the testing state can move to the stable state when the78developers feel they are finished. They cannot be removed from the79kernel tree without going through the obsolete state first.80 81It's up to the developer to place their interfaces in the category they82wish for it to start out in.83 84 85Notable bits of non-ABI, which should not under any circumstances be considered86stable:87 88- Kconfig. Userspace should not rely on the presence or absence of any89 particular Kconfig symbol, in /proc/config.gz, in the copy of .config90 commonly installed to /boot, or in any invocation of the kernel build91 process.92 93- Kernel-internal symbols. Do not rely on the presence, absence, location, or94 type of any kernel symbol, either in System.map files or the kernel binary95 itself. See Documentation/process/stable-api-nonsense.rst.96