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17.

API and ABI

18.

At a high level, an API – <em>Application Programming Interface</em> – is the boundary between two components when developing against them. It is closely related to an ABI – <em>Application Binary Interface</em> – which is the boundary at runtime. It defines the possible ways in which other components can interact with a component. More concretely, this normally means the C headers of a library form its API, and compiled library symbols its ABI. The difference between an API and ABI is given by compilation of the code: there are certain things in a C header, such as <code>#define</code>s, which can cause a library’s API to change without changing its ABI. But these differences are mostly academic, and for all practical purposes, API and ABI can be treated interchangeably.

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Examples of API-incompatible changes to a C function would be to add a new parameter, change the function’s return type, or remove a parameter.

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However, many other parts of a project can form an API. If a daemon exposes itself on D-Bus, the interfaces exported there form an API. Similarly, if a C API is exposed in higher level languages by use of GIR, the GIR file forms another API — if it changes, any higher level code using it must also change.

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Other examples of more unusual APIs are configuration file locations and formats, and GSettings schemas. Any changes to these could require code using your library to change.

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Stability

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API stability refers to some level of guarantee from a project that its API will only change in defined ways in the future, or will not change at all. Generally, an API is considered ‘stable’ if it commits to backwards-compatibility (defined below); but APIs could also commit to being unstable or even forwards-compatible. The purpose of API stability guarantees is to allow people to use your project from their own code without worrying about constantly updating their code to keep up with API changes. Typical API stability guarantees mean that code which is compiled against one version of a library will run without problems against all future versions of that library with the same major version number — or similarly that code which runs against a daemon will continue to run against all future versions of that daemon with the same major version number.

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It is possible to apply different levels of API stability to components within a project. For example, the core functions in a library could be stable, and hence their API left unchanged in future; while the newer, less core functions could be left unstable and allowed to change wildly until the right design is found, at which point they could be marked as stable.

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Several types of stability commonly considered:

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Unstable