# Open vSwitch Library ABI Updates

This file describes the manner in which the Open vSwitch shared library manages different ABI and API revisions. This document aims to describe the background, goals, and concrete mechanisms used to export code-space functionality so that it may be shared between multiple applications.

## Definitions

Definitions for terms appearing in this document
Term | Definition
--- | ---
ABI | Abbreviation of Application Binary Interface
API | Abbreviation of Application Programming Interface
Application Binary Interface | The low-level runtime interface exposed by an object file.
Application Programming Interface | The source-code interface descriptions intended for use in multiple translation units when compiling.
Code library | A collection of function implementations and definitions intended to be exported and called through a well-defined interface.
Shared Library | A code library which is imported at run time.

## Overview

C and C++ applications often use ‘external’ functionality, such as printing specialized data types or parsing messages, which has been exported for common use. There are many possible ways for applications to call such external functionality, for instance by including an appropriate inline definition which the compiler can emit as code in each function it appears. One such way of exporting and importing such functionality is through the use of a library of code.

When a compiler builds object code from source files to produce object code, the results are binary data arranged with specific calling conventions, alignments, and order suitable for a run-time environment or linker. This result defines a specific ABI.

As library of code develops and its exported interfaces change over time, the resulting ABI may change as well. Therefore, care must be taken to ensure the changes made to libraries of code are effectively communicated to applications which use them. This includes informing the applications when incompatible changes are made.

The Open vSwitch project exports much of its functionality through multiple such libraries of code. These libraries are intended for multiple applications to import and use. As the Open vSwitch project continues to evolve and change, its exported code will evolve as well. To ensure that applications linking to these libraries are aware of these changes, Open vSwitch employs libtool version stamps.

## ABI Policy

Open vSwitch will export the ABI version at the time of release, such that the library name will be the major.minor version, and the rest of the release version information will be conveyed with a libtool interface version.

The intent is for Open vSwitch to maintain an ABI stability for each minor revision only (so that Open vSwitch release 2.5 carries a guarantee for all 2.5.ZZ micro-releases). This means that any porting effort to stable branches must take not to disrupt the existing ABI.

In the event that a bug must be fixed in a backwards-incompatible way, developers must bump the libtool ‘current’ version to inform the linker of the ABI breakage. This will signal that libraries exposed by the subsequent release will not maintain ABI stability with the previous version.

## Coding

At build time, if building shared libraries by passing the –enable-shared arguments to ./configure, version information is extracted from the `$PACKAGE_VERSION` automake variable and formatted into the appropriate arguments. These get exported for use in Makefiles as `$OVS_LTINFO`, and passed to each exported library along with other `LDFLAGS`.

Therefore, when adding a new library to the build system, these version flags should be included with the `$LDFLAGS` variable. Nothing else needs to be done.

Changing an exported function definition (from a file in, for instance lib/*.h) is only permitted from minor release to minor release. Likewise changes to library data structures should only occur from minor release to minor release.