Linux – How to exclude some library paths listed in “ pkg-config –variable pc_path pkg-config”

compilinglibrarieslinuxmakepkg-config

I have Ubuntu 14.04 upgraded from 12.04 making dist-upgrades. I did many manual installations such as ffmpeg, libglib and so on, in the past. I have a nice custom distro now, it works well but I have problems while trying to compile applications. It stems from library conflicts between manually installed packages from source code and native distro libraries. A guy advised me to rename /usr/local it works but boot failed on next reboot.

When I look for directories added by pkg-config with 

  pkg-config --variable pc_path pkg-config

it lists 

 /usr/local/lib/i386-linux-gnu/pkgconfig:/usr/local/lib/pkgconfig:/usr/local/share/pkgconfig:/usr/lib/i386-linux-gnu/pkgconfig:/usr/lib/pkgconfig:/usr/share/pkgconfig

I don't want it to look for paths in /usr/local/lib...

How can I ban those paths not to let pkg-config look for?

Best Answer

Stuff in /usr/local usually supersedes stuff in /usr, so I'm a bit confused as to why you would install libraries there to have a "a nice custom distro", but then not want to compile against them. Those are the libraries the system will use actually use.

Anyway, man pkg-config claims the base search path:

is libdir/pkgconfig:datadir/pkgconfig where libdir is the libdir for pkg-config and datadir is the datadir for pkg-config when it was installed.

This implies they are compiled in. I notice it is different on ubuntu than fedora -- the former is long and inclusive, whereas the latter is short and exclusive; on fedora I have to set a $PKG_CONFIG_PATH to include /usr/local.

Since paths in $PKG_CONFIG_PATH are checked first, you could just set:

PKG_CONFIG_PATH=/usr/lib/i386-linux-gnu/pkgconfig:/usr/lib/pkgconfig:/usr/share/pkgconfig

The fact that these are at the end of the built-in paths won't matter; if the check makes it to there without finding anything, there's nothing to be found.

To demonstrate how this works, create a temporary directory /opt/bs/pkg and copy a .pc file from one of the directories in the default path into it -- e.g., alsa.pc. First check;

> pkg-config --libs alsa
-lasound

Now go into /opt/bs/pkg/alsa.pc and change -lasound (it's in the Libs: field) to -foobar. Set $PKG_CONFIG_PATH and try again:

> PKG_CONFIG_PATH=/opt/bs/pkg pkg-config --libs alsa
-foobar

Eureka, $PKG_CONFIG_PATH has overridden the built-in paths...you can delete /opt/bs/pkg, of course.

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