Project:Quality Assurance/Handling Libtool Archives

Libtool is a GNU project to build libraries in a portable system-independent way. It closely ties into the rest of the GNU Autotools system, but can be used outside GNU Autoconf and Automake as well. Libtool's goal is to produce two types of final products:


 * 1) static libraries, which usually have a   suffix on GNU/Linux
 * 2) dynamic libraries, which usually have a   suffix on GNU/Linux

Static libraries were the first kind of libraries on UNIX and UNIX-like systems. They are usually just archives of compiled object files. When linking to a static library, this is just as if you were to include more  files into the final product.

A significant disadvantage of static libraries is that they cannot, by the very nature of the archive they sit in, encode their required dependencies. GNU Libtool has devised a way of encoding all these dependencies. By writing all dependencies into a text file known as a Libtool Archive ( extension), this information is stored and can be used when linking statically to such a library.

On the other hand, modern dynamic libraries on GNU/Linux encode their dependencies in their ELF header, thus obviating the use of Libtool archives.

Historically, Gentoo has had multiple difficulties with Libtool archives in the past. See a nice documentation of how Libtool archives lead to massive unnecessary rebuilds: https://blog.flameeyes.eu/2011/08/library-soname-bumps-and-la-files-some-visual-clues It boils down to a number of interactions that make Libtool archives problematic on Gentoo:


 * 1) Libtool pulls in all direct and indirect dependencies into the   files it creates. This leads to massive overlinking, which is toxic to the Gentoo ecosystem, as it leads to a massive number of unnecessary rebuilds.
 * 2) Furthermore, the previous overlinking cannot be prevented by any LDFLAGS wizardry, as useful flags such as   become active only during invocation of ld.

Tackling .la files
The only way to avoid such problems is to delete the libtool archives. In 95% of cases, deleting the libtool archives can be done safely. In fact, there exists only one case in which .la should not be deleted: If  installs


 * 1) a static library that
 * 2) has private dependencies (i.e. depends on other libraries) and
 * 3) does not use pkg-config   files to document these dependencies

In all other cases,  can usually be safely deleted. See the flowchart by flameeyes documenting this in a more graphically intuitive way: