Providing include directory outside source folder for static library users - cmake

Question

I am developing a simple static C library for learning purposes using cmake.
Some projects like GLFW provide an include folder on the root, so library users can copy it and use it as an include directory.
In my library, I want to have an include folder on the root, so when I use the library on other projects, I can just copy this folder and set it as an include directory.
Here is a simplified folder structure of my library:
include
+--mylib.h
src
+--myheader.h
+--mysource.c
+--CMakeLists.txt
CmakeLists.txt
The src folder has my headers and implementation files, and a CMakeLists.txt for building a static library out of mysource.c.
The CMakeLists on the root folder just sets the project and adds src as a subdirectory.
I want the mylib.h file to have a #include <myheader.h>.
Here's a detour to talk about how I want to use it when it's done.
The idea is that when using the lib on another project, I can have something like this:
deps
+--include
+--mylib.h
src
+--main.c
And in the main.c file, include mylib.h and use what's defined on myheader.h
Here the detour ends, and I'm talking about my actual lib project again.
How can I achieve this using cmake? As far as I know, the mylib.h file needs to know it's including files from the src diretory, but I see no way of setting that, as for exemple in GLFW this directory does not have a CMakeLists.txt.

Answer 1

I am gonna quess that this is a design issue since it would make sense to you if you would have installed the library to a system before you tried to use it. That is, not using add_subdirectory() but find_library() at usage.
First, if you are using a external library, but not installing it, you would include all files in you deps-folder. All files then include source-files and so on and will be compiled besides you main.c. This is done with add_subdirectory(deps/MyLib) and later also included in you main-project.
Example:
add_subdirectory(deps/MyLib EXCLUDE_FROM_ALL)
target_link_libraries(${PROJECT_NAME} PRIVATE MyLib)
target_include_directories(${PROJECT_NAME} PRIVATE MyLib)
If you do not want to compile it all the time, then you must instruct cmake where it can find headers and library-files. Preferred way is to use find_library() which does some magic for you. Since you do not mention any installation i will assume that it does not exist and your only option is then to use add_subdirectory().
"I can just copy this folder and set it as an include directory."
CMake wants to handle these things for you so you should never copy headers around. You should either use add_subdirectory() to include a project/headers or make use of the find_library() which make sure you find where the headers are in the system.
I suggest that you push yourself to learn howto install a library into a system and how to utilize it later, but only by using find_library(). Then the library will be global for all projects and also not duplicated.
Adding some kind of psudo-code in hope it all makes more sense. Although it is around add_subdirectory() since the code for installing is quite large unfortunately.
CMakeLists.txt for main.c
cmake_minimum_required(VERSION 3.8)
project(MyLibTest)
add_executable(${PROJECT_NAME}
src/main.c
)
add_subdirectory(external/MyLib EXCLUDE_FROM_ALL)
target_link_libraries(${PROJECT_NAME} PRIVATE MyLib)
target_include_directories(${PROJECT_NAME} PRIVATE MyLib)
CMakeLists.txt for library
cmake_minimum_required(VERSION 3.8)
project(MyLib)
add_library(${PROJECT_NAME} STATIC
src/MyLib.c
)
target_include_directories(${PROJECT_NAME}
PUBLIC
$<INSTALL_INTERFACE:include>
$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
)
The structure for the project would then be:
/
external/MyLib
external/MyLib/src
MyLib.c
external/MyLib/include
MyLib.h
src
main.c
CMakeLists.txt