Cmake not setting RPATH when adding link_library with -L

Question

When setting link libraries in the following manner

target_link_libraries (SOME_TARGET -L/somedir -lfoo)

cmake doesn't handle RPATHs. Is using '-L' and '-l' not best practice, or actually plain wrong? When creating my own Find*.cmake I usually use find_library() but the find script I got doesn't do this and resorts to the above form using '-L' and '-l'.

The documentation doesn't really explain how RPATHs are gathered, also the documentation isn't really clear how it handles "-l" and "-L" the only pointer you get is

"Item names starting with -, but not -l or -framework, are treated as linker flags"

Answer 1

Specifying toolchain-dependent flags like -l and -L is generally not recommended, as it breaks portability and might have different effects than you expect.

The correct way to set the linker path would be the link_directories command.

The idiomatic solution in CMake is to use find_library for locating the library and then pass the full path to the linker, so you do not need to worry about link directories at all.

Now, the RPATH is a different beast, as it also determines where dynamic libraries can be located at runtime. Usually, the default settings work reasonably fine here. If you ever find yourself in the unfortunate situation where it does not, there is a number of target properties and CMake variables influencing this:

There are a few properties used to specify RPATH rules. INSTALL_RPATH is a semicolon-separated list specifying the rpath to use in installed targets (for platforms that support it). INSTALL_RPATH_USE_LINK_PATH is a boolean that if set to true will append directories in the linker search path and outside the project to the INSTALL_RPATH. SKIP_BUILD_RPATH is a boolean specifying whether to skip automatic generation of an rpath allowing the target to run from the build tree. BUILD_WITH_INSTALL_RPATH is a boolean specifying whether to link the target in the build tree with the INSTALL_RPATH. This takes precedence over SKIP_BUILD_RPATH and avoids the need for relinking before installation. INSTALL_NAME_DIR is a string specifying the directory portion of the “install_name” field of shared libraries on Mac OSX to use in the installed targets. When the target is created the values of the variables CMAKE_INSTALL_RPATH, CMAKE_INSTALL_RPATH_USE_LINK_PATH, CMAKE_SKIP_BUILD_RPATH, CMAKE_BUILD_WITH_INSTALL_RPATH, and CMAKE_INSTALL_NAME_DIR are used to initialize these properties.

(From the set_target_properties docs)

Also, you might want to have a look at the CMake Wiki page for RPATH handling.

The whole RPATH business is unfortunately rather complex and a thorough explanation would require far more space than is appropriate for a StackOverflow answer, but I hope this is enough to get you started.

Answer 2

Basically, You're using target_link_libraries() wrong. According to documentation, You should provide target, libraries and maybe some CMake specific linkage flags.

For example something like that:

target_link_libraries(my_build_target somedir/foo.so)

If You're using Your own crafted Find*.cmake solutions, it's usualy being done like this:

find_library(foo)
//build main target somewhere here
//now link it:
target_link_libraries(my_build_target ${FOO_LIBRARIES})

NOTE: I assume Your crafted Find*.cmake files follows these guidelines and fills CMake variables like SOMELIB_LIBRARIES, and/or SOMELIB_INCLUDE_DIRS, etc.

NOTE2: for my personal opinion, target_link_directories() is pain in a butt and You should avoid using it if not really needed. It's difficult to maintain and uses paths relative to current source directory.