Re: [Qemu-devel] issue: linking 64bit glib when building for cpu=i386

[Daniel P. Berrange]

On Fri, Aug 22, 2014 at 09:28:05AM +0100, Peter Maydell wrote:
> On 22 August 2014 09:20, Daniel P. Berrange <address@hidden> wrote:
> > Distros will install pkg-config .pc files for non-native architectures
> > in a different location normally. The supported / recommended way to
> > tell pkg-config to look in these alternative dirs is to set the env
> > variable  PKG_CONFIG_LIBDIR. This replaces the built-in default search
> > directory that looks for native.
> >
> > So on a Fedora / RHELL system, to make pkg-config use 32-bit libs you
> > want to set
> >
> >    PKG_CONFIG_LIBDIR=/usr/lib/pkgconfig
> >
> > which replaces the default location of /usr/lib64/pkgconfig. This is
> > the same thing you'd need to do to build QEMU for say, mingw32 where
> > you must set something like
> >
> >    PKG_CONFIG_LIBDIR=/usr/i686-w64-mingw32/sys-root/mingw/lib/pkgconfig/
> 
> Yes, but this should be done by the i686-w64-mingw32-pkg-config
> wrapper IMHO. (That's how I have my mingw setup configured,
> anyway.)

Sure if your distro wants to rebuild the pkg-config binary for each
arch that is a valid approach too, but it is by no means required
in order to get non-native builds working.

> > Note, i say PKG_CONFIG_LIBDIR here, *not* PKG_CONFIG_PATH. The latter
> > variable adds the default search path - you want to stop it looking in
> > the default search path completely because it is the wrong arch, so
> > must use PKG_CONFIG_LIBDIR
> 
> Interestingly, Debian's cross-compile pkg-config wrapper
> (aarch64-linux-gnu-pkg-config etc) sets PKG_CONFIG_PATH,
> not PKG_CONFIG_LIBDIR. Maybe that's a bug, but it works...

Yes, that would be a bug. Consider if you have mistakenly only installed
the 64-bit dev package for libusb and not the 32-bit package. Now the
'libusb-1.0.pc' file will only be present in the directory
/usr/lib64/pkgconfig but not in /usr/lib/pkgconfig.

If you used PKG_CONFIG_PATH to point to the 32-bit directory, then
it won't find the 32-bit libusb-1.0, so will fallback to looking in
the 64-bit directory and repo the 64-bit version which is definitely
not what you want. That'll lead to pain & suffering with wierd build
and/or link errors. If you used PKG_CONFIG_LIBDIR then you'll get an
immediate error from pkg-config telling you the 32-bit libusb-1.0
package was not installed.

Regards,
Daniel
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