cmake - How to set include_directories from a CMakeLists.txt file?

Question

I seem to be having trouble setting the include path (-I) using the include_directories() command in CMake. My project directory is as follows:

Root
| - CMakeLists.txt
| - libs
| - | - CMakeLists.txt
| - | - inc
| - | - | - // lib specific includes
| - | - src
| - | - | - // lib specific sources
| - proj1
| - | - CMakeLists.txt
| - | - inc
| - | - | - // proj1 specific includes
| - | - src
| - | - | - // proj1 specific sources

The root CMakeLists.txt file looks like so:

project(ROOT)
add_subdirectory(libs)
add_subdirectory(proj1)

The CMakeLists.txt file under libs:

project(lib)
add_library(lib STATIC ${lib_hdrs} ${lib_srcs}) // for conciseness, omitted set() 

And lastly, the CMakeLists.txt file under proj1:

project(proj1)

include_directories("${ROOT_SOURCE_DIR}/lib/inc") # <- problem line?

add_executable(proj1 ${proj1_srcs})
target_link_libraries(proj1 lib)

The goal is to create the library from the source and header files in libs, then link against the executable generated under proj1. Proj1 has some files that #include stuff in libs include, so I need to add the directories to be used with -I. Based on the documentation, that's what include_directories() is supposed to do. However despite explicitly setting that and following it with a debug message(${INCLUDE_DIRECTORIES}), the INCLUDE_DIRECTORIES variable is an empty string, and no directories are specified for the include path, so my compilation of proj1 fails.

I've also attempted removing the quotes around ${ROOT_SOURCE_DIR}/inc to see if that helped but no luck.

Answer 1

include_directories() populates a directory property called INCLUDE_DIRECTORIES:

http://www.cmake.org/cmake/help/v2.8.12/cmake.html#prop_dir:INCLUDE_DIRECTORIES

Note that CMake 2.8.11 learned the target_include_directories command, which populates the INCLUDE_DIRECTORIES target property. http://www.cmake.org/cmake/help/v2.8.12/cmake.html#command:target_include_directories

Note also that you can encode the fact that 'to compile against the headers of the lib target, the include directory lib/inc is needed' into the lib target itself by using target_include_directories with the PUBLIC keyword.

add_library(lib STATIC ${lib_hdrs} ${lib_srcs}) # Why do you list the headers?
target_include_directories(lib PUBLIC "${ROOT_SOURCE_DIR}/lib/inc")

Note also I am assuming you don't install the lib library for others to use. In that case you would need to specify different header directories for the build location and for the installed location.

target_include_directories(lib
  PUBLIC
    # Headers used from source/build location:
    "$<BUILD_INTERFACE:${ROOT_SOURCE_DIR}/lib/inc>"
    # Headers used from installed location:
    "$<INSTALL_INTERFACE:include>"     
)

Anyway, that's only important if you are installing lib for others to use.

After the target_include_directories(lib ...) above you don't need the other include_directories() call. The lib target 'tells' proj1 the include directories it needs to use.

See also target_compile_defintions() and target_compile_options().