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Over time, Unix systems added support for shared libraries.
Conventional libraries, or static libraries, are linked into a program image. This means that each program which uses a static library includes some or all of the library in the program binary on disk.
Shared libraries, on the other hand, are a separate file. A program which uses a shared library does not include a copy of the library; it only includes the name of the library. Many programs can use a single shared library.
Using a shared library reduces disk space requirements. Since the system can generally share a single executable instance of the shared library among many programs, it also reduces swap space requirements at run time. Another advantage is that it is possible to fix a bug by updating the single shared library file on disk, without requiring all the programs which use the library to be rebuilt.
The first Unix shared library implementation was in System V release 3 from AT&T. The idea was rapidly adopted by other Unix vendors, appearing in SunOS, HP-UX, AIX, and Digital Unix among others. Unfortunately, each implementation differed in the creation and use of shared libraries and in the specific features which were supported.
Naturally, packages distributed as source code which included libraries wanted to be able to build their own shared libraries. Several different implementations were written in the Autoconf/Automake framework.
In 1996, Gordon Matzigkeit began work on a package known as Libtool. Libtool is a collection of shell scripts which handle the differences between shared library generation and use on different systems. It is closely tied to Automake, although it is possible to use it independently.
Over time, Libtool has been enhanced to support more Unix variants and to provide an interface for standardizing shared library features.
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