[CMake] Result of FIND_LIBRARY changing its value

[Eduard Bloch]

> > What can I do to solve that? Adding -L/usr/local/lib to the compiler
> > flags is a workaround, but is there any better way?
> 
> IIRC, for some Unix systems /usr/local/lib takes precedence over /usr/lib
> without having to specify -L/usr/local/lib. You can check your libraries
> with the ldd command (or whatever equivalent there is on AIX) to see
> exactly
> which libraries they are linked with.  It's quite possible cmake knows the
> AIX conventions and it does not specify the -L/usr/local/lib flag because
> it
> is actually not needed for that platform. Normally, though, cmake does
> specify the appropriate -L option for libraries that have been found in
> non-standard places.
> 
> If it turns out the -L/usr/local/lib flag is necessary on your version of
> AIX, and cmake does not supply it, then that is a bug in the platform
> support for cmake which I suggest you report with AIX version, ldd
> results,
> etc. via the CMake bug-tracking system.

It depends on what neccessary means. I played a bit with IBMs native compiler xlc and gcc and they have different behaviour. xlc's looks first in /usr/include, AIX ld looks first in /usr/lib, GCC looks first in /usr/local/include. And ld seems to use some voodoo to find the correct library which this source has been compiled for.

In cmake's default linker flags for various situations, it uses an order similar to xlc's, so it is not a real bug. However, I need to make sure that /usr/local/lib is used when /usr/local/lib/libiconv.a has been found. OTOH it may be any other path, eg. /opt/freeware/..., so the user should have an easy way to configure it. It was really easy with autodevtools, I have set LDFLAGS environment once and forgot about the issue.

With cmake, there are three variables for linker flags in the cache. Assuming that the user wants to set linker flags and does set those vars explicitely, it is still not enough. Because FIND_LIBRARY has another, independent lookup algorithm. Introducing another alien, some LIB variable (which I have never seen before, which is not documented well and which does not work AFAICS). And then there are the Cmake modules using another list of "required libraries". I have not looked at the details but it looks like they are processed trough cmake's library option-list building algorithms and so the final command call is not influenced by user's ld flags.

So what is the best way to say: I want -L/usr/local/bin be prepended to the command on whatever you do with gcc, and also be respected by ALL tests, etc.?

For my current purpose (linking with a library but only if it is there), I consider throwing the find_library test away and use self-made snippet with CheckCSourceCompiles module and the user ldflags passed trough CMAKE_REQUIRED_FLAGS.
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