CMake add_subdirectory with find_package? - c

Question

I have the following structure of projects/source:
MyProject
|-MyProject
| |-src
| | |-project.c
| |
| |-CMakeLists.txt
|
|-MyLibrary
|-include
| |-hello.h
|
|-src
| |-hello.c
|
|-CMakeLists.txt
MyProject/src/project.c includes MyLibrary/include/hello.h, which in turn includes libusb.h from the system (found using CMake).
And I am adding the libusb dependency in MyLibrary/CMakeLists.txt using the following code:
# Find libusb
find_package(LibUsb REQUIRED)
# Include libusb
include_directories(${LIBUSB_INCLUDE_DIR})
# Add dependencies
target_link_libraries(owi535 ${LIBUSB_LIBRARY})
However when compiling MyProject (which includes MyLibrary using add_subdirectory and target_link_libraries), I get an error stating libusb.h cannot be found.
I can compile MyLibrary on it's own, however compiling MyProject requires libusb.h to be in it's include path, which it is not.
Is there a way to make it so that by adding MyLibrary as a dependency, MyProject pulls libusb.h through it? This would mean that I don't need to repeat the find_package code for every project that includes MyLibrary.
The other issue I am having is similar; I am getting an error stating that there are Undefined symbols when calling libusb functions in MyLibrary when I compile MyProject, however when compiling MyLibrary on it's own, there are no errors.
Undefined symbols for architecture x86_64:
"_libusb_close", referenced from:
_my_close_method in libMyLibrary.a(hello.c.o)
"_libusb_exit", referenced from:
_my_exit_method in libMyLibrary.a(hello.c.o)
"_libusb_init", referenced from:
_my_init_method in libMyLibrary.a(hello.c.o)
"_libusb_open_device_with_vid_pid", referenced from:
_my_open_method in libMyLibrary.a(hello.c.o)
ld: symbol(s) not found for architecture x86_64

Answer 1

From the cmake documentation:
PUBLIC and INTERFACE items will populate the INTERFACE_INCLUDE_DIRECTORIES property of <target>.
Targets may populate this property [INTERFACE_INCLUDE_DIRECTORIES] to publish the include directories required to compile against the headers for the target. Consuming targets can add entries to their own INCLUDE_DIRECTORIES property such as $<TARGET_PROPERTY:foo,INTERFACE_INCLUDE_DIRECTORIES> to use the include directories specified in the interface of foo.
So, you need to use this in MyLibrary/CMakeLists.txt:
target_include_directories(MyLibrary PUBLIC ${LIBUSB_INCLUDE_DIR})