[CMake] add_subdirectory and link_directories

[Michael Hertling]

On 01/10/2011 06:24 PM, Andrea Galeazzi wrote:
> I've got a main project which relies on several sub-library projects, so 
> the main CMakeLists.txt
> add_subdirectory("W:/Omega/Kernel"  "${CMAKE_CURRENT_BINARY_DIR}/myLib1")
> ...........
> and then I also specify the directory where all libraries have been 
> built (by setting ARCHIVE_OUTPUT_DIRECTORY for each library)
> link_directories("${ROOT_LIB}/{CMAKE_SYSTEM_NAME}/${CMAKE_BUILD_TYPE}")
> Anyway I noticed that the build process works fine also without 
> declaring the row above.
> My question: is specifying  link_directories useless when I use 
> add_subdirectory in order to build a library? If yes, is that true 
> regardless the target (I only tested it in VS2010 and MinGW)?

Since CMake prefers to specify libraries by path instead of using -l/-L
or the like, there's usually no need for the LINK_DIRECTORIES() command,
IMO. For the project's targets, CMake tracks their paths by itself and
takes OUTPUT_DIRECTORY properties and explicit binary directories into
account; the find modules and configuration files of external packages
also use full paths for their libraries, either immediately or via the
IMPORTED_LOCATION properties of imported targets, so you should rarely
have to use the LINK_DIRECTORIES() command, at least for new CMakified
projects. Moreover, it might involve the danger of luring the linker
into the wrong directories, see [1], and makes it harder to choose a
static library instead of a shared one, see [2]. Thus, I'd recommend
against using LINK_DIRECTORIES() unless there's a good reason for it.

Regards,

Michael

[1] http://www.mail-archive.com/cmake@cmake.org/msg33519.html
[2] http://www.cmake.org/Wiki/CMake_2.6_Notes#Link_Line_Generation