CMake doesn't pick up INTERFACE_INCLUDE_DIRECTORIES of linked library

Question

I have this in my cmake-file

get_target_property(moggle_interface_includes moggle INTERFACE_INCLUDE_DIRECTORIES)
message("Moggle interface includes: ${moggle_interface_includes}")

target_link_libraries(motor
    moggle
)

get_target_property(motor_includes motor INCLUDE_DIRECTORIES)
message("MOTOR includes ${motor_includes}")

Which outputs this

Moggle interface includes: "/home/nick/code/onegame/motor/moggle/include"
Motor includes:" "

How can this be? When moggle is linked, it should also, according to this

CMake will also propagate "usage requirements" from linked library targets. Usage 
requirements affect compilation of sources in the <target>. They are specified by 
properties defined on linked targets. During generation of the build system, CMake 
integrates usage requirement property values with the corresponding build properties for 
<target>:

     INTERFACE_COMPILE_DEFINITONS: Appends to COMPILE_DEFINITONS
     INTERFACE_INCLUDE_DIRECTORIES: Appends to INCLUDE_DIRECTORIES

... pick up the INTERFACE_INCLUDE_DIRECTORIES and add them to motor's, so what am I doing wrong?

• CMake verison: cmake version 2.8.12.2
• OS: Arch Linux

Answer 1

CMake does some processing at 'configure time' and some processing at 'generate time'.

The message() is executed at configure time, but linked libraries are only evaluated later at generate time. Because your include directories depend on the linked libraries, the include directories are not fully resolved until generate time.

The file(GENERATE) command evaluates generator expression content at generate time and writes it to a file, so something like this will write the final include directories to includes.txt:

file(GENERATE 
    OUTPUT "includes.txt" 
    CONTENT "$<TARGET_PROPERTY:motor,INCLUDE_DIRECTORIES>\n"
)

If your purpose is debugging, then try setting CMAKE_VERBOSE_MAKEFILE to 1 to see the compiler command lines or try setting

set(CMAKE_DEBUG_TARGET_PROPERTIES INCLUDE_DIRECTORIES)

and it will show you a backtrace for where each of the include directories on each target comes from.

http://www.cmake.org/cmake/help/v3.0/manual/cmake-generator-expressions.7.html

http://www.cmake.org/cmake/help/v3.0/manual/cmake-buildsystem.7.html

http://www.cmake.org/cmake/help/v3.0/variable/CMAKE_DEBUG_TARGET_PROPERTIES.html

http://www.cmake.org/cmake/help/git-master/command/file.html