CMake Fortran Module Directory to be used with add_library - module

Question

I have a CMake 3.5.2 project that creates a library: libtest.a, which is then linked to by an executable.
The source code is Fortran, and the libtest.a produces a module file: "main.mod"
The executable also needs to include this main.mod file, so to make main.mod accessible, when building the project I set the variable, CMAKE_Fortran_MODULE_DIRECTORY to a known location, and add it to the relevant include paths.
This works great when building the entire project, main.mod is built in a known location, and it is there for whatever code needs it. My usage, however, makes it necessary to only build libtest.a by itself, and leave the executable to be built by a downstream user sometimes.
The issue I am having is that when I go into the libtest source and treat it as its own CMake project, the library will build and install, but the main.mod file is always left in the BINARY_DIR and is not built in the CMAKE_Fortran_MODULE_DIRECTORY, dispite setting in the the CMakeList.txt within libtest.
Is the Fortran_MODULE_DIRECTORY only honored when add_executable() is being called? And just ignored for the library builds alone? Or am I missing something.
Thanks for the help.
EDIT: This will reproduce my issue.
test_mod.f90:
module main
implicit none
real, parameter :: pi=3.2
end module main
tt.f90:
program test
use main
implicit none
real :: a, area
a =10
area = a * 100
end program test
CMakeList.txt:
CMAKE_minimum_required( VERSION 3.5 )
enable_language( Fortran )
project( tt )
file( GLOB test_SOURCES ${CMAKE_CURRENT_SOURCE_DIR}/*.f90 )
add_library( tt STATIC ${test_SOURCES} )
set( CMAKE_Fortran_MODULE_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/Mod )
install( TARGETS ${PROJECT_NAME} DESTINATION ${CMAKE_CURRENT_SOURCE_DIR}/Lib/ )
If I build and install the above code, I will get a libtt.a library installed in the Lib directory, however my main.mod will remain in my build directory and is not build in a Mod folder.

Answer 1

Here I assume that the "user" uses cmake to build the project while having access to the source of your project.
The steps to a working build.
There is a CMakeLists.txt file for libtest that specifies CMAKE_Fortran_MODULE_DIRECTORY. This should be enough for main.mod to appear there.
There is a CMakeLists.txt file for buiding the "client" program. This file should include the libtest project with add_subdirectory.
Add target_link_libraries(NAME_OF_PROGRAM NAME_OF_LIBRARY). This only takes care of the linking of libraries and is not sufficient (for solution B below anyway) for the module to be known to the client program.
Now, make your own adventure:
Solution A: in the libtest CMakeLists.txt, place the module where "all modules will go", for instance set(CMAKE_Fortran_MODULE_DIRECTORY ${CMAKE_BINARY_DIR}/modules) (you need to do this also for the "client" CMakeLists.txt). The variable ${CMAKE_BINARY_DIR} is set by the "client" cmake invocation and will be the same for all included cmake projects. This directory will be listed in the build commands for Fortran programs.
Solution B: in the libtest CMakeLists.txt, place the module of this library in a dedicated directory. You can achieve this, for instance, with set(CMAKE_Fortran_MODULE_DIRECTORY ${PROJECT_BINARY_DIR}/modules). You need then to manually specify this location with include_directories(PATH_THAT_DEPENDS_ON_THE_NAME_OF_THE_SUBPROJECT) in the client CMakeLists.txt.
If you wish the library to be installable, you need to specify paths for installing the library and the module file. With Fortran, you should think of this with the target OS, compiler and architecture in mind.
Links to the CMake documentation:
PROJECT_BINARY_DIR
CMAKE_Fortran_MODULE_DIRECTORY
CMAKE_BINARY_DIR
Following the addition of your sample code, the following modification should do it:
CMAKE_minimum_required( VERSION 3.5 )
enable_language( Fortran )
project( tt )
set( CMAKE_Fortran_MODULE_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/Mod )
file( GLOB test_SOURCES ${CMAKE_CURRENT_SOURCE_DIR}/*.f90 )
add_library( tt STATIC ${test_SOURCES} )
install( TARGETS ${PROJECT_NAME} DESTINATION ${CMAKE_CURRENT_SOURCE_DIR}/Lib/ )
install(DIRECTORY ${CMAKE_Fortran_MODULE_DIRECTORY} DESTINATION ${CMAKE_CURRENT_SOURCE_DIR})
Make sure that set( CMAKE_Fortran_MODULE_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/Mod ) occurs before any add_library line.
Add install(DIRECTORY ${CMAKE_Fortran_MODULE_DIRECTORY} DESTINATION ${CMAKE_CURRENT_SOURCE_DIR}) to actually install the .mod file. Module files (as header files in C) have to installed in addition to the library file.
The setup you created is a bit unusual in that you locate everything within the source build whereas "usual" install locations are made relative to CMAKE_INSTALL_PREFIX