Libtool

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Libtool

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6.2 Libtool's versioning system

Libtool has its own formal versioning system. It is not as flexible as some, but it is definitely the simplest of the more powerful versioning systems.

Think of a library as exporting several sets of interfaces, arbitrarily represented by integers. When a program is linked against a library, it may use any subset of those interfaces.

Libtool's description of the interfaces that a program uses is simple: it encodes the least and the greatest interface numbers in the resulting binary (first-interface, last-interface).

The dynamic linker is guaranteed that if a library supports every interface number between first-interface and last-interface, then the program can be relinked against that library.

Note that this can cause problems because libtool's compatibility requirements are actually stricter than is necessary.

Say `libhello' supports interfaces 5, 16, 17, 18, and 19, and that libtool is used to link `test' against `libhello'.

Libtool encodes the numbers 5 and 19 in `test', and the dynamic linker will only link `test' against libraries that support every interface between 5 and 19. So, the dynamic linker refuses to link `test' against `libhello'!

In order to eliminate this problem, libtool only allows libraries to declare consecutive interface numbers. So, `libhello' can declare at most that it supports interfaces 16 through 19. Then, the dynamic linker will link `test' against `libhello'.

So, libtool library versions are described by three integers:

current: The most recent interface number that this library implements.
revision: The implementation number of the current interface.
age: The difference between the newest and oldest interfaces that this library implements. In other words, the library implements all the interface numbers in the range from number current - age to current.

If two libraries have identical current and age numbers, then the dynamic linker chooses the library with the greater revision number.