[CMake] absolute library path, sorry for the question

[Mario Emmenlauer]

Dear Bo Zhou,

Thank you for your reply!! Your explanation is very helpful for writing
my own CMakeLists.txt. But in my case, I use the standard CMakeLists.txt
that comes with the libraries. For example libtiff, turbojpeg, thrift,
and many others...

As an example, I build libtiff in the following way:
        cmake ../$TIFFVERSION \
            -Wno-dev \
            -G"Unix Makefiles" \
            -DCMAKE_PREFIX_PATH="$THIRDPARTYTARGETDIR" \
            -DCMAKE_INSTALL_PREFIX="$THIRDPARTYTARGETDIR" \
            -DCMAKE_BUILD_TYPE="Release" \
            -DBUILD_SHARED_LIBS="ON" \
            -Djpeg="ON" \
            -DJPEG_INCLUDE_DIR="$THIRDPARTYTARGETDIR/include" \
            -DJPEG_LIBRARY="$THIRDPARTYTARGETDIR/lib/libjpeg.so" && \
        make VERBOSE="1" && make install

In the build log, I can see that cmake detects the jpeg library to be
'$THIRDPARTYTARGETDIR/lib/libjpeg.so'. However, in the later build, the
Makefile links with '-ljpeg' instead of '$THIRDPARTYTARGETDIR/lib/libjpeg.so'.
And this in turn will make 'ld' use '/usr/lib/x86_64-linux-gnu/libjpeg.so'
instead of my thirdparty libjpeg.

So at some point between FindJPEG.cmake and the final Makefile, cmake
changed the JPEG_LIBRARIES variable from absolute to relative. And I can
not understand why or when this happens. The documentation also does not
help me, because it explains that this should happen 'if the full path is
not know or if the full path is one of the system library dirs' (see
https://cmake.org/pipermail/cmake/2015-September/061462.html). In my case,
I override to use jpeg with a known, existing path that is not a system
library.

All the best,

   Mario




On 13.12.2017 11:12, Bo Zhou wrote:
> Hi
> 
> CMAKE_SKIP_RPATH usually is used for the shared module which might want to have the customized distributed path such as within the application. According
> personal experience on POSIX systems (Linux, UNIX, OSX), always enable CMAKE_SKIP_RPATH for the all local shared dependencies, and to the final distribution,
> (if needed) put shared libraries into the lib folder of distribution.
> 
> According your description, I'm not sure how did you pick up your library into the application from CMake, usually we have to make sure the CMake always chooses
> the libraries from 3rd-party directory not system or another places. In our system, it has a variable THIRDPARTY_DIR for command FIND_PATH() to locate the
> folder which contains the all built libraries, then use HINTS and NAMES in the command FIND_PATH() and FIND_LIBRARY() to locate the correct paths for the
> pre-built libraries, so the linking should be okay.
> 
> If everything has no error, but just the application always picks up the system's library not the 3rd-party libraries we prepared, one solution would add
> another loader script for the application, from the loader script it appends the local lib folder to LD_LIBRARY_PATH, then launch the actual application
> executable file. A lot of commercial application on Linux solve the similar issue by this way.
> 
> Wish my reply is helpful.
> 
> Thank you very much.
> 
> On Wed, Dec 13, 2017 at 6:06 PM, Mario Emmenlauer <mario at emmenlauer.de <mailto:mario at emmenlauer.de>> wrote:
> 
> 
>     Dear cmake users,
> 
>     I have found good documentation on the cmake wiki about RPATH and
>     its different variables, and also on the mailing lists. But somehow
>     it still does not add up for me. Can you please help?
> 
>     I use cmake 3.9.5 on different platforms, CentOS 6 and Ubuntu 14.04
>     amongst others. I build many standard libraries (like tiff and jpeg)
>     into my own thirdparty directory, and set CMAKE_PREFIX_PATH to find
>     them there. On newer Linux and Windows, cmake uses always the absolute
>     path to those libraries, and everything works fine. But on older Linux
>     like Ubuntu 14.04 and CentOS 6 it does not. cmake will first find the
>     library in the correct absolute thirdparty directory, and I can confirm
>     this by printing the 'XXX_LIBRARIES' variable in the find script. But
>     later the Makefile will link with '-lxxx' instead of the full path.
>     This makes 'ld' use the system library instead!
> 
>     I completely fail to understand at what point cmake changes XXX_LIBRARIES
>     from an absolute path to '-lxxx', for example for libjpeg.
> 
>     I have experimented with CMAKE_SKIP_BUILD_RPATH, but it does not help.
>     The only workaround I could find is to add the thirdparty directory to
>     LDFLAGS. Then 'ld' will also find the correct library.
> 
> 
>     Is this a bug, or an expected behavior? What parameters should I add
>     when I invoke cmake so that it will always use libraries with their
>     full path?
> 
> 
>     Cheers,
> 
>         Mario Emmenlauer
> 
> 
> 
>     PS: since I build thirdparty libraries, I prefer not to change their
>     CMakeLists.txt but would rather prefer to set better cmake command line
>     parameters.
> 
> 
>     References:
>     https://cmake.org/Wiki/CMake_RPATH_handling <https://cmake.org/Wiki/CMake_RPATH_handling>
>     https://cmake.org/pipermail/cmake/2015-September/061462.html <https://cmake.org/pipermail/cmake/2015-September/061462.html>
> 
> 
> 


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