Introduction¶

This part of the documentation inside Documentation/ABI directoryattempts to document the ABI between the Linux kernel anduserspace, and the relative stability of these interfaces. Due to theeverchanging nature of Linux, and the differing maturity levels, theseinterfaces should be used by userspace programs in different ways.

We have four different levels of ABI stability, as shown by the fourdifferent subdirectories in this location. Interfaces may change levelsof stability according to the rules described below.

The different levels of stability are:

stable/

This directory documents the interfaces that the developer hasdefined to be stable. Userspace programs are free to use theseinterfaces with no restrictions, and backward compatibility forthem will be guaranteed for at least 2 years. Most interfaces(like syscalls) are expected to never change and always beavailable.

testing/

This directory documents interfaces that are felt to be stable,as the main development of this interface has been completed.The interface can be changed to add new features, but thecurrent interface will not break by doing this, unless graveerrors or security problems are found in them. Userspaceprograms can start to rely on these interfaces, but they must beaware of changes that can occur before these interfaces move tobe marked stable. Programs that use these interfaces arestrongly encouraged to add their name to the description ofthese interfaces, so that the kernel developers can easilynotify them if any changes occur (see the description of thelayout of the files below for details on how to do this.)

obsolete/

This directory documents interfaces that are still remaining inthe kernel, but are marked to be removed at some later point intime. The description of the interface will document the reasonwhy it is obsolete and when it can be expected to be removed.

removed/

This directory contains a list of the old interfaces that havebeen removed from the kernel.

Every file in these directories will contain the following information:

What:

Short description of the interface

Date:

Date created

KernelVersion:

(Optional) Kernel version this feature first showed up in.Note: git history often provides more accurate versioninfo, so this field may be omitted.

Contact:

Primary contact for this interface (may be a mailing list)

Description:

Long description of the interface and how to use it.

Users:

All users of this interface who wish to be notified whenit changes. This is very important for interfaces inthe “testing” stage, so that kernel developers can workwith userspace developers to ensure that things do notbreak in ways that are unacceptable. It is alsoimportant to get feedback for these interfaces to makesure they are working in a proper way and do not need tobe changed further.

Note:

The fields should be use a simple notation, compatible with ReST markup.Also, the fileshould not have a top-level index, like:

===foo===

How things move between levels:

Interfaces in stable may move to obsolete, as long as the propernotification is given.

Interfaces may be removed from obsolete and the kernel as long as thedocumented amount of time has gone by.

Interfaces in the testing state can move to the stable state when thedevelopers feel they are finished. They cannot be removed from thekernel tree without going through the obsolete state first.

It’s up to the developer to place their interfaces in the category theywish for it to start out in.

Notable bits of non-ABI, which should not under any circumstances be consideredstable:

• Kconfig. Userspace should not rely on the presence or absence of anyparticular Kconfig symbol, in /proc/config.gz, in the copy of .configcommonly installed to /boot, or in any invocation of the kernel buildprocess.

• Kernel-internal symbols. Do not rely on the presence, absence, location, ortype of any kernel symbol, either in System.map files or the kernel binaryitself. SeeThe Linux Kernel Driver Interface.

ABI symbols¶

ABI files¶