Not clear where pkg-config is finding information or how .pc files are named

Question

I followed a tutorial on GTK which used this command to generate the build flags:

$ pkg-config --cflags --libs gtk+-3.0

This outputs coherent flags. From research, I have found that pkg-config searches for .pc files in /usr/lib/pkginfo, usr/share/pkgconfig, in the /local equivalents, and in the folder indicated by the PKG_CONFIG_PATH variable.

None of the folders contains the GTK file and the environment variable is not set. Where is pkg-config getting the flags?

Answer 1

It’s finding them in /usr/lib/x86_64-linux-gnu/pkgconfig/gtk+-3.0.pc (assuming you’re on amd64). On Debian, pkg-config also searches the multi-arch directory for the target, i.e. /usr/lib/$(dpkg-architecture -q DEB_TARGET_MULTIARCH)/pkgconfig.

Answer 2

There are a couple of nice tricks given in the man page for getting this information from pkg-config itself.

PKG-CONFIG DERIVED VARIABLES

       pc_path
              The default search path used by pkg-config  when  searching  for
              .pc files. This can be used in a query for the pkg-config module
              itself itself:
                $ pkg-config --variable pc_path pkg-config

       pcfiledir
              The installed location of the .pc file.  This  can  be  used  to
              query  the location of the .pc file for a particular module, but
              it can also be used to make .pc files relocatable.

So for example to see the actual search path for your system / architecture:

$ pkg-config --variable pc_path pkg-config | tr ':' '\n'
/usr/local/lib/x86_64-linux-gnu/pkgconfig
/usr/local/lib/pkgconfig
/usr/local/share/pkgconfig
/usr/lib/x86_64-linux-gnu/pkgconfig
/usr/lib/pkgconfig
/usr/share/pkgconfig

and to get the location where a particular package's .pc file is found

$ pkg-config --variable pcfiledir gtk+-3.0
/usr/lib/x86_64-linux-gnu/pkgconfig