In many applications, a Fortran submodule is defined in a different file than the Fortran module that uses it. Meta-build systems such as CMake and Meson must consider that each Fortran compiler treats Fortran submodule interface file naming distinctively. Fortran module interface files are like generated header files. Fortran module files are automatically generated and generally not human-readable.

A brief taxonomy of file naming conventions for Fortran submodules follows. Assume a two-file program:

file basic.f90

module demo
real, parameter :: pi = 4.*atan(1.)
real :: tau

interface
  module subroutine hello(pi,tau)
    real, intent(in) :: pi
    real, intent(out) :: tau
  end subroutine hello
end interface
contains
end module demo

program sm
use demo
call hello(pi, tau)
print *,'pi=',pi, 'tau=', tau
end program

file basic_sub.f90

submodule (demo) hi
contains
module procedure hello
  tau = 2*pi
end procedure hello
end submodule hi

The order of the commands for each compiler is significant. The module must be built before the submodule, to generate the necessary module interface files BEFORE compiling the submodule. Each compiler creates corresponding basic.o and basic_sub.o object files.

1. gfortran -c basic.90 creates files: basic.o demo.mod demo.smod.
2. gfortran -c basic_sub.f90 -o basic_sub.o creates files: basic_sub.o demo@hi.smod.
3. gfortran basic.o basic_sub.o -o basic creates executable basic

Here is a table of the module interface files generated by each compiler. “module” are the files generated by step #1, building the file containing the Fortran module. “submodule” are the files generated by step #2, building the containing the Fortran submodule.

Related:

Fortran submodule reference

Fortran module file format