Library depend on a header file - cmake

Question

Let's assume I have a project with a series of libraries. I also need to generate a header file that will be used by all of these projects. So I created a CMake file, like this:
project(main)
add_subdirectory(sub_1)
add_subdirectory(sub_2)
# ...
add_subdirectory(sub_n)
add_custom_command(
OUTPUT CustomHeader.h
COMMENT "Generating custom header for all the libraries"
COMMAND ...)
add_library(${PROJECT_NAME} STATIC ${OBJECT_LIST})
The problem is, that I don't know how to tell CMake to run my custom command (that generates this CustomHeader.h) before it would try to build the libraries in the subfolders.
I tried add_custom_target(TARGET MyPrebuild PRE_BUILD ...) but I'm running on Linux, and this option only works on Windows platform according to the documentation.
add_dependencies only work between targets, and not a target and a single file.
I could, in theory, add the header to be among the source files of the individual libraries (in the sub_1, .., sub_n folders) but it feels wrong, as the header is not required to be part of those libraries.
So I just have no idea how I can make a library depend on an include file, that is not part of it.
Any tips how I can overcome this problem?

Answer 1

For make header file (re)built before a library in subdirectory is compiled, you may create target, which builds the file, and make the library dependent from the target:
# *CMakeLists.txt*
# ...
add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/CustomHeader.h ...)
add_custom_target(generate_custom_header DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/CustomHeader.h)
# *sub/CMakeLists.txt*
# ...
add_library(libA ...)
add_dependencies(libA generate_custom_header)
Instead of using add_dependencies, you may create header-only library which "implements" you header and link with it:
# *CMakeLists.txt*
# ...
add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/CustomHeader.h ...)
add_custom_target(generate_custom_header DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/CustomHeader.h)
add_library(libCustom INTERFACE) # Header only library
add_dependencies(libCustom generate_custom_header) # which depends on generated header
# You may even assign include directories for the header-only library
target_include_directories(libCustom INTERFACE ${CMAKE_CURRENT_BINARY_DIR})
# *sub/CMakeLists.txt*
# ...
add_library(libA ...)
target_link_libraries(libA libCustom) # Common linking with a header-only library.
Note, that INTERFACE library is a "fake" - it is never created by itself. Instead, all "features" of INTERFACE library are just propagated to its users.

Answer 2

I would suggest to add another library target that will both keep track of the generated headers and will help to properly configure other libraries to know where to find them (i.e. target_include_directories).
cmake_minimum_required(VERSION 3.0)
project(testable)
set(CustomHeaderInPath ${CMAKE_CURRENT_SOURCE_DIR}/CustomHeader.example)
set(CustomHeaderPath ${CMAKE_CURRENT_BINARY_DIR}/CustomHeader.h)
add_custom_command(
OUTPUT ${CustomHeaderPath}
COMMAND cp ${CustomHeaderInPath} ${CustomHeaderPath}
COMMENT "Generated file"
DEPENDS ${CustomHeaderInPath})
add_library(CustomHeaderLibrary ${CustomHeaderPath})
target_include_directories(CustomHeaderLibrary PUBLIC ${CMAKE_CURRENT_BINARY_DIR})
set_target_properties(CustomHeaderLibrary PROPERTIES LINKER_LANGUAGE C)
add_library(LibA a.c)
target_link_libraries(LibA CustomHeaderLibrary)
add_library(LibB b.c)
target_link_libraries(LibB CustomHeaderLibrary)
add_executable(${PROJECT_NAME} main.c)
target_link_libraries(${PROJECT_NAME} PUBLIC LibA LibB)
Note that I had to explicitly set the LINKER_LANGUAGE of the new target as cmake won't be able to deduce it properly if no c or cpp files are added to the library.