# Version numbers and ABIs

## The different types of version

Upstream version

- The version that Linus or a stable series maintainer uses for a release. Currently Linus will use the version format: 4.`x`[-rc`y`]. Stable series maintainers use the version format: 4.`x`.`y`.

Package version

- The version used in a Debian package. Following Debian policy, it should follow the format `upstreamversion`-`debianrevision`. However, for an upstream release candidate, the string '-rc' must be replaced with '~rc' so that it will be recognised as an earlier version than the following release.

Kernel version

- This is the version that appears in kernel messages, filenames, package names and the output of 'uname -r'. In official kernel packages it follows the format `upstreamversion`[-`abiname`][-`featureset`]-`flavour`. It is not changed for every new package version. The `abiname` is changed as explained below.

Many programs parse the kernel version string reported by the `uname` system call or command and expect to find at least 3 version components separated by dots. For compatibility, the official kernel packages currently add '.0' to the upstream version.

## The kernel ABI

An ABI (Application Binary Interface) is an interface between two software components, considered at the level of register allocation and memory layout. The ABI between the kernel and user-space is generally maintained carefully, and is not a concern here. However, the ABI between the kernel and its modules is not. In order to support out-of-tree modules, the kernel version should be changed when the ABI between the kernel and modules changes.

### The ABI name

In official kernel packages, we change the `abiname` part of the kernel version to mark ABI changes that aren't due to a new upstream version. This part comes from the `abiname` setting in `debian/config/defines`. We use either a number or 'trunk' (for experimental), but for a custom package it should be some other string.

### Maintaining and updating the ABI

In order to avoid the need for users to rebuild out-of-tree modules frequently, we try to avoid changing the kernel ABI during updates to a Debian stable or oldstable release. Most importantly, we avoid making such changes without changing the ABI name, except where it appears that out-of-tree modules do not depend on that part of the ABI.

Bug fixes or configuration changes to the kernel may alter the ABI. If an exported function is conditional on CONFIG_`FOO`, or it uses a type whose definition depends on CONFIG_`FOO`, then turning CONFIG_`FOO` on or off changes the ABI of that function, and thus of the kernel as a whole. Enabling or changing the configuration of a single driver usually doesn't change the ABI, because most drivers don't export anything.

The kernel build process generates a 'symbol version' for each exported function or variable. This is a hash of the definitions that it depends on, and should change whenever the function's ABI changes. The kernel module loader detects incompatible modules by comparing symbol versions. The whole set of symbol versions represents the kernel ABI.

We collect the symbol versions for previously uploaded packages under the directory `debian/abi` and then compare the new kernel with those. If the ABI name is unchanged but the ABI itself is changed - except for additions, or changes that we have marked as acceptable - then the build is aborted.

If the kernel ABI has changed you must then change the ABI name in `debian/config/defines`. Then run the command

    $ fakeroot debian/rules debian/control-real

to regenerate the package definitions for this ABI name.