qmake configured correctly for multiarch library directories?

[Stephen M. Webb]

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On 05/17/2012 04:09 PM, Josh Stratton wrote:
>>> Like I said, the majority of my libraries are still in /usr/lib and now qmake doesn't link against them by
>>> default. Is this a bug?  Is this just an accepted consequence of multiarch?  Or is there something I need to
>>> configure that wasn't handled properly in my upgrade?
>> 
>> Well, qmake only generates makefiles.  GCC has /usr/lib hardcoded in its library resolver search path and is
>> always used implicitly, so that should not be a problem.  If qmake is not using its linker symbol ($(GXX) or
>> $(LD)) to generate library search paths if it needs them explicitly, it is certainly a major bug in qmake and
>> should have been noticed long ago.  The multi-arch paths are in addition to the default paths, not instead of.
> 
> I'm not very familiar with how that works.  I know when I run cmake it will add the -L/usr/lib and
> -L/usr/lib/i386-linux-gnu to the makefiles, so is it just being redundant?  On another machine I tried a quick test
> of adding a file to the LIBS path and it worked without -L/usr/lib being set explicitly.

To see where GCC looks for libraries by default, run 'gcc -print-search-dirs | grep libraries'.  In the absence of
special options, using the -L switch will augment the library search path.  Adding the standard paths to the command
line is redundant.

>> How do you know the /usr/lib libraries are not being linked against?  What is the symptom, other than not
>> appearing explicitly on the command line?
> 
> The actual problem I experienced after upgrading was the missing library assimp, which was still sitting in my
> /usr/lib directory. Since the library file was still there and didn't see /usr/lib explicitly included, I figured
> that was the problem.  I'm having similar issues with including aqsis, so it seemed dubious that a handful of
> packages were broken and more likely that it was some issue with qmake specific to my machine.

It's possible the library in /usr/lib is the wrong architecture, which would cause GCC to ignore it. You can use the
'file' utility to see whether a file in /usr/lib was built for i386 (ELF 32-bit LSB shared object, Intel 80386) or
x86_64 (ELF 64-bit LSB shared object, x86-64).  You can also force -L or -l flags explicitly when running 'make' after
qmake by setting the LDFLAGS environment variable, if you want to experiment.

- -- 
Stephen M. Webb  <stephen at ubuntu.com>
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