README 3.9 KB

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 why it is obsolete and when it can be expected to be removed.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