10. Introducing GNU Libtool

Libtool takes care of all the peculiarities of creating, linking and loading shared and static libraries across a great number of platforms, providing a uniform command line interface to the developer. By using Libtool to manage your project libraries, you only need to concern yourself with Libtool's interface: when someone else builds your project on a platform with a different library architecture, Libtool invokes that platform's compiler and linker with the correct environment and command line switches. It will install libraries and library using binaries according to the conventions of the host platform, and follows that platform's rules for library versioning and library interdependencies.

Libtool empowers you to treat a library as an implementation of a well defined interface of your choosing. This Libtool library may be manifest as a collection of compiler objects, a static ar archive, or a position independent runtime loadable object. By definition, native libraries are fully supported by Libtool since they are an implementation detail of the Libtool library abstraction. It's just that until Libtool achieves complete world domination, you might need to bear in mind what is going on behind the command line interface when you first add Libtool support to your project.

The sheer number of uses of the word `library' in this book could be easily very confusing. In this chapter and throughout the rest of the book, I will refer to various kinds of libraries as follows:

`native'

Low level libraries, that is, libraries provided by the host architecture.

`Libtool library'

The kind of library built by Libtool. This encompasses both the shared and static native components of the implementation of the named library.

`pseudo-library'

The high level `.la' file produced by Libtool. The `pseudo-library' is not a library in its own right, but is treated as if it were from outside the Libtool interface.

Futhermore, in the context of Libtool, there is another subtle (but important) distinction to be drawn:

`static library'

A Libtool library which has no shared archive component.

`static archive'

The static component of a Libtool library.

Many developers use Libtool as a black box which requires adding a few macros to `configure.in' and tweaking a project's `Makefile.am'. The next chapter addresses that school of thought in more detail. In this chapter I will talk a little about the inner workings of Libtool, and show you how it can be used directly from your shell prompt -- how to build various kinds of library, and how those libraries can be used by an application. Before you can do any of this, you need to create a libtool script that is tailored to the platform you are using it from.