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List:       cmake
Subject:    Re: [CMake] Library search path ordering
From:       "William A. Hoffman" <billlist () nycap ! rr ! com>
Date:       2005-08-25 13:23:32
Message-ID: 6.2.3.4.2.20050825091812.0916fb38 () pop ! nycap ! rr ! com
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The reason it is needed is that often times libraries are specified with full
paths to cmake.   And not as -L and -l, but rather /full/path/libA.a \
/full/path/libB.a. However, to work with linkers cmake splits those into -L/full/path \
and -lA -lB, the order of the -l's must be preserved.  However, the -L may need to be \
reordered so that the correct library is found.   Say you have something like this:

/A/liba.a
/A/libb.a
/B/libb.a

Then you tell cmake to link /A/liba.a and /B/libb.a, in the old cmake
you got:  -L/A -L/B -la -lb, and the /A/libb.a was used instead of the
specified /B/libb.a.  With the new cmake, you will get -L/B -L/A -la -lb,
and you will get the libraries specified.   In most cmake projects libraries
are specified as full paths because FIND_LIBRARY is used.    If it is a library
that is being built by cmake, then you just have to use the library name.

-Bill


At 07:34 AM 8/25/2005, David Flitney wrote:
> Bill,
> 
> Thanks for that. If I understand correctly, what you're implying here is that I'll \
> need to use: 
> TARGET_LINK_LIBRARIES(Project ${Project_BINARY_DATA}/lib/libmiscmaths,a)
> 
> to force my preferred ordering? I hadn't realised this was possible/necessary. \
> Previously with CMake 2.0.6 - and also with 2.2 on non MacOSX platforms - I got the \
> correct ordering without this (just lucky?). 
> Okay, that appears to work. It does leave me a little confused as to why there is a \
> need for a configuration time reordering. Surely the developer could do this work \
> through the order in the CMakeLists.txt file? As appears to be the case with the \
> INCLUDE_DIRECTORIES directives. 
> Thanks again,
> 
> Dave
> 
> On 24 Aug 2005, at 22:17, William A. Hoffman wrote:
> 
> > To try and use -L and to make sure that full paths to libraries specified are
> > used, cmake reorders paths using the following algorithm:
> > 
> > 
> > /** \class cmOrderLinkDirectories
> > * \brief Compute the best -L path order
> > *
> > * This class computes the best order for -L paths.
> > * It tries to make sure full path specified libraries are
> > * used.  For example if you have /usr/mylib/libfoo.a on as
> > * a link library for a target, and you also have /usr/lib/libbar.a
> > * and you also have /usr/lib/libfoo.a, then you would
> > * want -L/usr/mylib -L/usr/lib to make sure the correct libfoo.a is
> > * found by the linker.  The algorithm is as follows:
> > * - foreach library create a vector of directories it exists in.
> > * - foreach directory create a vector of directories that must come
> > *   after it, put this in a map<dir, vector<dir>> mapping from a directory
> > *   to the vector of directories that it must be before.
> > * - put all directories into a vector
> > * - sort the vector with a compare function CanBeBefore
> > *   CanBeBefore returns true if a directory is OK to be before
> > *   another directory.  This is determined by looking at the
> > *   map<dir vector<dir>> and seeing if d1 is in the vector for d2.
> > 
> > 
> > Can you be more specific about all the full path libraries you are using, and
> > are there any warnings from cmake about not being able to come up with a correct \
> > order? 
> > -Bill
> > 
> > 
> > At 01:07 PM 8/24/2005, David Flitney wrote:
> > > OS: MacOSX 10.3.9 (Panther)
> > > CMake Version: 2.2-beta
> > > 
> > > I'm having a problem with -L directives. They are being reordered (optimised \
> > > :-) ?) such that I can't control which gets searched first, e.g., say I have a \
> > > locally built library: 
> > > ${Project_BINARY_DIR}/lib/libmiscmaths.a
> > > 
> > > but also there is an older version in /usr/local/maths/lib then I can't ensure \
> > > that my locally built one is linked against. 
> > > I've tried:
> > > 
> > > LINK_DIRECTORIES(${Project_BINARY_DIR}/lib /usr/local/maths/lib)
> > > TARGET_LINK_LIBRARY(Project miscmaths)
> > > 
> > > but this ordering isn't being obeyed.
> > > 
> > > -- 
> > > Cheers, Dave
> > > 
> > > Dave Flitney, IT Manager
> > > Oxford Centre for Functional MRI of the Brain
> > > E:flitney@fmrib.ox.ac.uk W:+44-1865-222713 F:+44-1865-222717
> > > URL: http://www.fmrib.ox.ac.uk/~flitney
> > > 
> > > _______________________________________________
> > > CMake mailing list
> > > CMake@cmake.org
> > > http://www.cmake.org/mailman/listinfo/cmake
> > 
> -- 
> Cheers, Dave
> 
> Dave Flitney, IT Manager
> Oxford Centre for Functional MRI of the Brain
> E:flitney@fmrib.ox.ac.uk W:+44-1865-222713 F:+44-1865-222717
> URL: http://www.fmrib.ox.ac.uk/~flitney


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