halting problem :: pkg-config and paths

:: ~4 min read

This is something of a frequently asked question, as it comes up every once in a while. The pkg-config documentation is fairly terse, and even pkgconf hasn’t improved on that.

The problem

Let’s assume you maintain a project that has a dependency using pkg-config.

Let’s also assume that the project you are depending on loads some files from a system path, and your project plans to install some files under that path.

The questions are:

  • how can the project you are depending on provide an appropriate way for you to discover where that path is
  • how can the project you maintain use that information

The answer to both questions is: by using variables in the pkg-config file. Sadly, there’s still some confusion as to how those variables work, so this is my attempt at clarifying the issue.

Defining variables in pkg-config files

The typical preamble stanza of a pkg-config file is something like this:


Each variable can reference other variables; for instance, in the example above, all the other directories are relative to the prefix variable.

Those variables that can be extracted via pkg-config itself:

$ pkg-config --variable=includedir project-a

As you can see, the --variable command line argument will automatically expand the ${prefix} token with the content of the prefix variable.

Of course, you can define any and all variables inside your own pkg-config file; for instance, this is the definition of the giomoduledir variable inside the gio-2.0 pkg-config file:




This way, the giomoduledir variable will be expanded to /usr/lib/gio/modules when asking for it.

If you are defining a path inside your project’s pkg-config file, always make sure you’re using a relative path!

We’re going to see why this is important in the next section.

Using variables from pkg-config files

Now, this is where things get complicated.

As I said above, pkg-config will expand the variables using the definitions coming from the pkg-config file; so, in the example above, getting the giomoduledir will use the prefix provided by the gio-2.0 pkg-config file, which is the prefix into which GIO was installed. This is all well and good if you just want to know where GIO installed its own modules, in the same way you want to know where its headers are installed, or where the library is located.

What happens, though, if your own project needs to install GIO modules in a shared location? More importantly, what happens if you’re building your project in a separate prefix?

If you’re thinking: “I should install it into the same location as specified by the GIO pkg-config file”, think again. What happens if you are building against the system’s GIO library? The prefix into which it has been installed is only going to be accessible by the administrator user; or it could be on a read-only volume, managed by libostree, so sudo won’t save you.

Since you’re using a separate prefix, you really want to install the files provided by your project under the prefix used to configure your project. That does require knowing all the possible paths used by your dependencies, hard coding them into your own project, and ensuring that they never change.

This is clearly not great, and it places additional burdens on your role as a maintainer.

The correct solution is to tell pkg-config to expand variables using your own values:

$ pkg-config \
> --define-variable=prefix=/your/prefix \
> --variable=giomoduledir
> gio-2.0

This lets you rely on the paths as defined by your dependencies, and does not attempt to install files in locations you don’t have access to.

Build systems

How does this work, in practice, when building your own software?

If you’re using Meson, you can use the get_pkgconfig_variable() method of the dependency object, making sure to replace variables:

gio_dep = dependency('gio-2.0')
giomoduledir = gio_dep.get_pkgconfig_variable(
  define_variable: [ 'libdir', get_option('libdir') ],

This is the equivalent of the --define-variable/--variable command line arguments.

If you are using Autotools, sadly, the PKG_CHECK_VAR m4 macro won’t be able to help you, because it does not allow you to expand variables. This means you’ll have to deal with it in the old fashioned way:

giomoduledir=`$PKG_CONFIG --define-variable=libdir=$libdir --variable=giomoduledir gio-2.0`

Which is annoying, and yet another reason why you should move off from Autotools and to Meson. 😃


All of this, of course, works only if paths are expressed as locations relative to other variables. If that does not happen, you’re going to have a bad time. You’ll still get the variable as requested, but you won’t be able to make it relative to your prefix.

If you maintain a project with paths expressed as variables in your pkg-config file, check them now, and make them relative to existing variables, like prefix, libdir, or datadir.

If you’re using Meson to generate your pkg-config file, make sure that the paths are relative to other variables, and file bugs if they aren’t.


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