cmake with -DBUILD_matlab=ON fails

[wookayin]

Hello, I suspect that this might be a bug introduced after the commit 3d30510 (PR #2059).

The environment with which I've tested is: Mac OS X 10.10. I haven't tested with Ubuntu, since I don't have any Linux machine with matlab installed as of now.

$ cd build
$ cmake .. -DBUILD_matlab=ON && make -j4

However, building matcaffe fails with the message ld: library not found for -lpython2.

[100%] Building Matlab interface: ${CAFFE_ROOT}/matlab/caffe/caffe.mexmaci64
Building with 'Xcode Clang++'.
ld: library not found for -lpython2
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Let's see the command line argument passed (absolute paths were replaced by ${CAFFE_ROOT}).
There are python-related link flags, -lpython2 and -lboost_python-mt, which were added from ${PYTHON_LIBRARIES}

/Applications/MATLAB_R2014b.app/bin/mex -output ${CAFFE_ROOT}/matlab/caffe/caffe.mexmaci64 ${CAFFE_ROOT}/matlab/caffe/matcaffe.cpp -DCPU_ONLY -DWITH_PYTHON_LAYER -DGTEST_USE_OWN_TR1_TUPLE -I${CAFFE_ROOT}/src -I/usr/local/include -I${CAFFE_ROOT}/build/include -I/usr/local/include/opencv -I/System/Library/Frameworks/Accelerate.framework/Versions/Current/Frameworks/vecLib.framework/Versions/Current/Headers -I/System/Library/Frameworks/Python.framework/Headers -I/usr/local/lib/python2.7/site-packages/numpy/core/include -I${CAFFE_ROOT}/include -I${CAFFE_ROOT}/build -L${CAFFE_ROOT}/build/lib -L/usr/local/lib -L/usr/local/lib -L/usr/local/lib -L/usr/local/lib -L/usr/lib -lcaffe -lboost_system-mt -lboost_thread-mt -lglog -lgflags -lhdf5_hl -lhdf5 -llmdb -lleveldb -lsnappy -lopencv_core -lopencv_highgui -lopencv_imgproc -lcblas -lpython2  -lboost_python-mt -lprotobuf -v

The reason why ld fails might be that the ordering of link flags (-L and -l) has changed in the internal invocation, and thus it was not able to link the python2 dynamic module.

... (omitted) ...
/usr/bin/xcrun -sdk macosx10.9 (..omitted...)
-lcaffe  -lboost_system-mt  -lboost_thread-mt  -lglog  -lgflags -lhdf5_hl  -lhdf5  -llmdb  -lleveldb  -lsnappy  -lopencv_core  -lopencv_highgui  -lopencv_imgproc  -lcblas  -lpython2  -lboost_python-mt  -lprotobuf   -L${CAFFE_ROOT}/build/lib  -L/usr/local/lib  -L/usr/local/lib  -L/usr/local/lib  -L/usr/local/lib  -L/usr/lib  -L"/Applications/MATLAB_R2014b.app/bin/maci64" -lmx -lmex -lmat -o "${CAFFE_ROOT}matlab/caffe/caffe.mexmaci64

This build step with cmake, succeeds prior to 3d30510.

As I am doubtful these flags (e.g. -lpython) are necessary to build matlab targets, these flags could be removed when building matlab bindings so that the build could be successful out-of-box. Currently, I tried a workaround setting BUILD_python_layer to OFF, but after inspection to find better solution, I think I could submit a PR to fix this. Thanks.

[eshelman]

I am experiencing the same issue (but on Scientific Linux 6.6 with the g++ compiler). From what I can tell, the issue is that the compilation line is using -lpython2 instead of the actual Python version, such as -lpython2.7

The flag -lpython2 tells the linker to look for a library file named libpython2.so. As far as I can tell, no version of Python has shipped such a file for ages.

It's possible to work around this by manually changing that flag to -lpython2.7. You can also create a symlink from libpython2.so to libpython2.7.so (although that's a nasty hack).

Because python doesn't need to be linked here, the cleanest would be to remove the -lpython2 from the Matlab build line. I'm submitting a pull request with the change.

[ronghanghu]

See https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/caffe-users/1sVAqSDLvpk/r9LMUXp7EgAJ

[adinriv]

Just for completeness, this answer (http://stackoverflow.com/a/35286218/424986) solves the problem of missing lbypthon2 library with Matlab by updating the CMake rules.