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  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.
  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.
 10 The different levels of stability are:
 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.
 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.)
 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 38         why it is obsolete and when it can be expected to be removed.
 40   removed/
 41         This directory contains a list of the old interfaces that have
 42         been removed from the kernel.
 44 Every file in these directories will contain the following information:
 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.
 61 How things move between levels:
 63 Interfaces in stable may move to obsolete, as long as the proper
 64 notification is given.
 66 Interfaces may be removed from obsolete and the kernel as long as the
 67 documented amount of time has gone by.
 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.
 73 It's up to the developer to place their interfaces in the category they
 74 wish for it to start out in.
 77 Notable bits of non-ABI, which should not under any circumstances be considered
 78 stable:
 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.
 85 - Kernel-internal symbols.  Do not rely on the presence, absence, location, or
 86   type of any kernel symbol, either in files or the kernel binary
 87   itself.  See Documentation/process/stable-api-nonsense.rst.

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