Cmake cannot find library using "link_directories"

Question

I Ubuntu, I am learning about cmake and make, and just trying a simple example. I have two directories: src and build. In src, I have two files: main.cpp, and CMakeLists.txt, which has (only) the following text:

add_executable(test main.cpp) link_directories(/usr/lib/x86_64-linux-gnu) target_link_libraries(test protobuf) 

In /usr/lib/x86_64-linux-gnu, there is a shared library called libprotobuf.so, which I want to link against. My main.cpp uses functions in this library, by including the releveant header file, #include <google/protobuf/message.h>.

Now, in my build directory, I run cmake ../src, and then make. However, I then get linker errors telling me that there are undefined references to some of the functions in the protobuf library. If I do a search through all the files and subdirectories in build, there is not mention of anything related to protobuf.

However, if I remove the link_directories line in my CMakeLists.txt file, and instead write the full path to the library when specifying the executable, i.e. target_link_libraries(test /usr/lib/x86_64-linux-gnu/libprotobuf.so), it compiles and links fine.

Why is link_directories not allowing cmake to find this library?

Answer 1

Do not use link_directories like this in CMake.

This is a common beginner's mistake, as many other build environments work like this, but in CMake it's just asking for trouble. Even the manpage specifically advises against it:

Note that this command [link_directories] is rarely necessary. Library locations returned by find_package() and find_library() are absolute paths. Pass these absolute library file paths directly to the target_link_libraries() command. CMake will ensure the linker finds them.

So instead, always pass absolute paths to target_link_libraries and use find_library to resolve the link directory:

find_library(PROTOBUF_LIBRARY protobuf HINTS /usr/lib/x86_64-linux-gnu) target_link_libraries(test PUBLIC ${PROTOBUF_LIBRARY}) 

This has the huge benefit that you will probably get a diagnostic at CMake configure time if the expected library cannot be found, instead of a random linker error at compile time. Also, this allows the user to specify a library location via the GUI if the target machine has a non-standard directory layout.

So if it doesn't work right away, be sure to check the result of the find_library call and consult the manpage to track down why it doesn't find your library as intended.