I created a project demonstrating the problem I'm having: https://github.com/TheComet93/cmake-sucks

I have a library which links against python and boost_python. The library compiles fine, but if I try to link against my library from an executable, I get these linking errors:

$ make
[ 50%] Built target library
Linking CXX executable app
../../cmake-dep/lib/libboost_python.so: undefined reference to `PyBytes_Size'
../../cmake-dep/lib/libboost_python.so: undefined reference to `PyUnicode_FromString'
../../cmake-dep/lib/libboost_python.so: undefined reference to `PyUnicode_FromFormat'
../../cmake-dep/lib/libboost_python.so: undefined reference to `PyBytes_AsString'
../../cmake-dep/lib/libboost_python.so: undefined reference to `PyUnicode_InternFromString'
../../cmake-dep/lib/libboost_python.so: undefined reference to `PyUnicode_FromEncodedObject'
../../cmake-dep/lib/libboost_python.so: undefined reference to `PyUnicode_AsWideChar'
../../cmake-dep/lib/libboost_python.so: undefined reference to `PyModule_Create2'
../../cmake-dep/lib/libboost_python.so: undefined reference to `PyUnicode_FromStringAndSize'
../../cmake-dep/lib/libboost_python.so: undefined reference to `PyUnicode_AsUTF8String'
../../cmake-dep/lib/libboost_python.so: undefined reference to `PyUnicode_AsUTF8'
collect2: error: ld returned 1 exit status
app/CMakeFiles/app.dir/build.make:91: recipe for target 'app/app' failed
make[2]: *** [app/app] Error 1
CMakeFiles/Makefile2:160: recipe for target 'app/CMakeFiles/app.dir/all' failed
make[1]: *** [app/CMakeFiles/app.dir/all] Error 2
Makefile:75: recipe for target 'all' failed
make: *** [all] Error 2

Enabling verbose output reveals the core of the issue:

/usr/bin/c++       CMakeFiles/app.dir/main.cpp.o  -o app  -L/home/thecomet/documents/programming/c++/cmake-sucks/cmake-dep/lib -rdynamic ../library/liblibrary.a -lpython2.7 ../../cmake-dep/lib/libboost_python.so -Wl,-rpath,/home/thecomet/documents/programming/c++/cmake-sucks/cmake-dep/lib

liblibrary.a is being linked before python2.7 and boost_python, which shouldn't be the case.

I'm stuck at how I'm supposed to fix this.

If you look at the CMakeLists.txt files, you'll see I'm basically just doing:

add_library(library ${files})
target_link_library(library python boost_python)

and:

add_executable(app ${files})
target_link_library(app library)

This is already enough to generate these linker errors.

It's hilarious, I tried to compile and I got:

make[2]: *** No rule to make target `/usr/lib/x82.74-linux-gnu/libboost_python.so', needed by `app/app'.  Stop.

CMake actually thought the "6_6" part in "x86_64" was some kind of version number and replaced it by 2.7. Epic fail. Anyway, have you tried linking boost_python before python? It is my understanding that the normal linker order is left-to-right, i.e. dependencies come last, unless you are using a nonstandard linker like on Windows or some special option such as the "scan twice" option of GNU ld to resolve circular dependencies.

CMake actually thought the "6_6" part in "x86_64" was some kind of version number and replaced it by 2.7. Epic fail.

        # hack to force boost to use python2.7
        string (REGEX REPLACE "[0-9].[0-9]" "2.7" Boost_PYTHON_LIBRARY ${Boost_PYTHON_LIBRARY})

Anyway, have you tried linking boost_python before python? It is my understanding that the normal linker order is left-to-right, i.e. dependencies come last, unless you are using a nonstandard linker like on Windows or some special option such as the "scan twice" option of GNU ld to resolve circular dependencies.