Cmake cannot find library using "link_directories"

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?

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.

Make sure that your call to link_directories takes place before your call to the relevant add_executable.

I had mistakenly believed it only needed to be before the call to target_link_libraries, but that's not the case. After moving the call, the library is linked properly.

Make sure that the order will be link_directories, set PROJECT_LINK_LIBS, add_executable and then target_link_libraries.

Below is example to demonstarte it:

cmake_minimum_required(VERSION 2.8.9)
project (Logging)
include_directories(include)
file(GLOB LOGGINGSOURCES "libsrc/*.cpp")
file(GLOB SOURCES "src/*.cpp")
add_library(convertString SHARED ${LOGGINGSOURCES})
install(TARGETS convertString DESTINATION /root/Deepak/)
link_directories( /root/Deepak/ )
set(PROJECT_LINK_LIBS libconvertString.so)
add_executable(hello ${SOURCES})
target_link_libraries(hello ${PROJECT_LINK_LIBS} )