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A helper for library maintainers to use symbol versioning

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smap

A helper for library maintainers to use symbol versioning

Why use symbol versioning?

The main reason is to be able to keep the library [ABI] stable.

If a library is intended to be used for a long time, it will need updates for eventual bug fixes and/or improvement. This can lead to changes in the [API] and, in the worst case, changes to the [ABI].

Using symbol versioning, it is possible to make compatible changes and keep the applications working without recompiling. If incompatible changes were made (breaking the [ABI]), symbol versioning allows both incompatible versions to live in the same system without conflict. And even more uncommon situations, like an application to be linked to different (incompatible) versions of the same library.

For more information, I strongly recommend reading:

  • [HOW_TO] How to write shared libraries, by Ulrich Drepper

How to add symbol versioning to my library?

Adding version information to the symbols is easy. Keeping the [ABI] stable, unfortunately, is not. This project intends to help in the first part.

To add version information to symbols of a library, one can use version scripts (in Linux). Version scripts are files used by linkers to map symbols to a given version. It contains the symbols exported by the library grouped by the releases where they were introduced. For example:

LIB_EXAMPLE_1_0_0
  {
    global:
      symbol;
      another_symbol;
    local:
      *;
  };

In this example, the release LIB_EXAMPLE_1_0_0 introduces the symbols symbol and another_symbol. The * wildcard in local catches all other symbols, meaning only symbol and another_symbol are globally exported as part of the library [API].

If a compatible change is made, it would introduce a new release, like:

LIB_EXAMPLE_1_1_0
{
    global:
        new_symbol;
} LIB_EXAMPLE_1_0_0;

LIB_EXAMPLE_1_0_0
{
    global:
        symbol;
        another_symbol;
    local:
        *;
};

The new release LIB_EXAMPLE_1_1_0 introduces the symbol new_symbol. The * wildcard should be only in one version, usually in the oldest version. The } LIB_EXAMPLE_1_0_0; part in the end of the new release means the new release depends on the old release.

Suppose a new incompatible version LIB_EXAMPLE_2_0_0 released after LIB_EXAMPLE_1_1_0. Its map would look like:

LIB_EXAMPLE_2_0_0
{
    global:
        a_newer_symbol;
        another_symbol;
        new_symbol;
    local:
        *;
};

The symbol symbol was removed (and that is why it was incompatible). And a new symbol was introduced, a_newer_symbol.

Note that all global symbols in all releases were merged in a unique new release.

Installation:

At the command line:

pip install symver-smap

Documentation:

Check in Read the docs

0.1.0 (2018-05-09)

  • First release on PyPI.

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