Statically linking using pkg-config and JHBuild

tl;dr: Export PKG_CONFIG="pkg-config --static" or add it to your .jhbuildrc file.

Statically linking libraries which use libtool (specifically, the LT_INIT autoconf macro) is easy: you pass --enable-static --disable-shared to configure, and out pop static .a libraries instead of dynamic .so libraries.

However, things get more complex once you start to consider dependencies of your library. For example, if I’m statically linking libfoo against libbar, and libbar uses libwidgit internally (i.e. consumes the libwidgit symbols but does not expose any of them as part of libbar’s public interfaces), libfoo needs to know about libwidgit and needs to list it in the libfoo linker command. This is because UNIX archives (statically linked .a files) do not list their dependencies (whereas dynamically linked .so files do (on sensible UNIX platforms, including Linux)). Effectively the transitive closure of link dependencies needs to be resolved at link time, rather than at run time.

How can we cope with this when configuring libfoo? By using pkg-config --static instead of normal pkg-config. This can be achieved by setting the PKG_CONFIG environment variable to pkg-config --static or by adding the following lines to .jhbuildrc (which also set the libtool options mentioned above):

autogenargs = '--disable-shared --enable-static'
os.environ['PKG_CONFIG'] = 'pkg-config --static'

pkg-config’s --static option changes the program’s output to include the Libs.private and (recursively the) Requires.private variables. For example:

$ pkg-config --libs gobject-2.0
-L/opt/gnome3/build/lib64 -lgobject-2.0 -lglib-2.0
$ pkg-config --static --libs gobject-2.0
-pthread -L/opt/gnome3/build/lib64 -lgobject-2.0 -lffi -lglib-2.0

Here, libffi and pthread are both private dependencies of GObject: they’re used internally in GObject, but don’t appear in its external interface. When statically linking against GObject, though, their symbols must be resolved and hence they must appear in the linker command.

For more about the differences between pkg-config’s Libs and Libs.private variables, see the excellent guide to pkg-config. For more information about static linking with pkg-config, see the Autotools Mythbuster.

(There are probably errors or omissions in this post, since I’m nowhere near an expert on linking. Please comment with any corrections! Also, this is not meant to suggest that statically linking libraries is a good idea in general — there are only a few situations where it’s sensible. But for those situations, hopefully this post makes things a little easier.)