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.
   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.
  10The 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  38        why it is obsolete and when it can be expected to be removed.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.
  44Every file in these directories will contain the following information:
  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.
  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::
  65        ===
  66        foo
  67        ===
  69How things move between levels:
  71Interfaces in stable may move to obsolete, as long as the proper
  72notification is given.
  74Interfaces may be removed from obsolete and the kernel as long as the
  75documented amount of time has gone by.
  77Interfaces in the testing state can move to the stable state when the
  78developers feel they are finished.  They cannot be removed from the
  79kernel tree without going through the obsolete state first.
  81It's up to the developer to place their interfaces in the category they
  82wish for it to start out in.
  85Notable bits of non-ABI, which should not under any circumstances be considered
  88- Kconfig.  Userspace should not rely on the presence or absence of any
  89  particular Kconfig symbol, in /proc/config.gz, in the copy of .config
  90  commonly installed to /boot, or in any invocation of the kernel build
  91  process.
  93- Kernel-internal symbols.  Do not rely on the presence, absence, location, or
  94  type of any kernel symbol, either in files or the kernel binary
  95  itself.  See Documentation/process/stable-api-nonsense.rst.