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REP:9
Title:ABI Compatibility
Author:Josh Faust
Status:Active
Type:Informational
Content-Type:text/x-rst
Created:08-Oct-2010
Post-History:13-Oct-2010, 22-Oct-2010

Abstract

This REP describes the requirements for Application Binary Interface (ABI) compatibility for ROS stack releases. To keep this reasonable, only even-cycle patch releases for stacks whose versions are >1.0 are required to be ABI compatible. For information on what defines a patch release, see the Stack Version Policy [1] page.

Motivation

Without ABI compatibility between patch versions, any released software building on top of that stack needs to be re-released for every patch version. This means, for example, any time the ROS stack releases a patch version, every other stack needs to be released as well. With ABI compatibility for even-cycle patches this would no longer be true. While this is a win for our deb-building process, it also means that external people can release binary versions of software against, say, cturtle, and be guaranteed that it will work with all future cturtle versions, as long as it is only depending on stable stacks.

ABI Compatibility Requirements

ABI compatibility applies only to compiled/linked languages such as C or C++.

ABI compatibility is required only for even-cycle [1] patch releases. For example, ROS 1.2.N must be fully ABI compatible with ROS 1.2.0. This requirement is true for every stack whose version is greater than 1.0.

ABI compatibility is only required for public facing API. If a piece of an API is marked as internal and is only used in definition files (i.e. not included in any public-facing header files), its ABI may change, because it is not part of the public ABI.

Note that this requirement is "best effort". If a fix for a serious bug requires ABI changes, it will be evaluated on a case-by-case basis.

This REP goes into effect for the Diamondback distribution release.

Definition

The Application Binary Interface [2] describes the binary interface to a library, and should not be confused with the API. A library is binary compatible if a program linked to a previous version of the library continues to work with a newer version without being recompiled.

Much of the information in this section is based on:

  • KDE's binary compatibility page [3]
  • ABI compatibility checker's page [4]

What's allowed

Note that this may not be an exhaustive list. Anything with a large caveat has not been included.

You can...

  • Add new non-virtual functions
  • Add a new enum to a class
  • Append new enumerations to an existing enum
  • Remove private non-virtual functions if they are not called by any inline functions (and never have been)
  • Remove private static members if they are not used by any inline functions (and never have been)
  • Add new static data members
  • Add new classes/structs
  • Add or remove friend declarations to classes

What's not allowed

Note that this may not be an exhaustive list. There may be things not on this list that cause an ABI breakage.

You cannot...

  • For an existing class or struct
    • Remove the class
    • Change a class hierarchy in any way (add, remove or reorder base classes)
    • Change class template arguments in any way, including adding an argument with a default value
    • Add new non-static data members, even if they are protected or private
    • Remove existing data members
    • Change the order of declaration of non-static data members, even if they are protected or private
    • Change the type of a data member
  • For an existing function of any type
    • Remove it
    • inline it (this includes moving a member function's body to the class definition, even without the inline keyword)
    • Add an overload, if that function did not already have at least one overload
    • Change its signature in any way. This includes:
      • Adding a parameter, even if it has a default value
      • Removing a parameter, even if it has a default value
      • Changing the type of any of the parameters, including const/volatile qualifiers
      • Changing the return type in any way
      • Changing the const/volatile qualifiers of a member function
      • Changing the access rights to functions or data members, for example from public to private. If you need to make a private function protected or public, add a new function that calls the private one.
    • If inline, make certain changes to its implementation
  • For virtual member functions
    • Add new virtual functions
    • Change the order of virtual functions in the class declaration
    • Override an existing virtual function if that function is not in the top-level base class that has virtual functions
    • Override an existing virtual function if it has a covariant return type [5]
    • Remove a virtual function, even if it is a reimplementation of a virtual function from the base class
  • For static non-private members or global variables
    • Remove it
    • Change its type
    • Change its const/volatile qualifiers
  • Change #defined constants
  • For enumerations
    • Change values of members
    • Remove members
    • Add members anywhere but the end of the list
    • Rename members
  • Change function calling conventions [6]

Checking Compatibility

ABI compatibility problems will often but not always cause crashes with client code. They may also cause data corruption, or nothing at all. Because of this they can be difficult to catch if you're not careful.

The ABI compatibility checker [4] is an easy way to check most ABI compatibility issues.

Future-proofing

It is possible in many cases to future-proof classes to allow changes to internal structure without breaking ABI compatibility. Techniques like pimpl (also known as d-pointer) [7] and opaque pointers [8] are recommended for these cases. The performance implications of these techniques should be weighed for each specific use case.

References

[1](1, 2) Stack versioning policy (http://www.ros.org/wiki/StackVersionPolicy)
[2]Application binary interface, Wikipedia, (http://en.wikipedia.org/wiki/Application_binary_interface)
[3]KDE's binary compatibility page, (http://techbase.kde.org/Policies/Binary_Compatibility_Issues_With_C++)
[4](1, 2) ABI compatibility checker, (http://ispras.linux-foundation.org/index.php/ABI_compliance_checker)
[5]Covariant Return Types, (http://www.tolchz.net/?p=33)
[6]Calling convention, Wikipedia, (http://en.wikipedia.org/wiki/Calling_convention)
[7]Compilation Firewalls, Herb Sutter, (http://www.gotw.ca/gotw/024.htm)
[8]Opaque pointer, Wikipedia, (http://en.wikipedia.org/wiki/Opaque_pointer)