A small, portable Fortran dependency generator (modules, submodules, and CPP includes)

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Hi all,

I wanted to share a small tool I’ve been using privately for some time and see whether it might be useful to others. It’s a lightweight Fortran dependency extractor focused specifically on modules and submodules. The tool emits Make-compatible dependency rules:

  • use statements: .mod dependencies,
  • module / submodule definitions: .mod / .smod targets,
  • CPP #include directives (including recursive includes via preprocessing).

For example, for:

!file.f90
module a
  use b
  ![...]
end

submodule(a) c
  use d
  ![...]
end

submodule(e) f
  use a
  ![...]
end

running FortranDep -l file.f90 produces:

file.o:  file.f90  \
  b.mod  \
  d.mod  \
  e.smod
a.mod:  file.f90  \
  b.mod
a@c.smod:  file.f90  \
  d.mod
e@f.smod:  file.f90  \
  e.smod

The motivation is simple: Fortran compilers already compute module dependencies internally, but this information is rarely exposed in a form that traditional build systems can consume. This tool exists to expose that information in a usable way (similarly to gcc -MMD in the C/C++ world). It is fully portable and can be used out of the box.

Repository:

Context

In recent years, tools like fpm have made it much easier to build and manage Fortran projects, especially for new codebases. However, many projects—particularly in HPC and scientific computing—still rely on existing Make-based workflows, where introducing a full build system is not always practical.

At the same time, there are a number of dependency generators and scripts floating around (often Python-based or compiler-specific), but in my experience they tend to:

  • only partially handle module dependencies,
  • ignore submodules or CPP #include s, or
  • be tightly coupled to a specific compiler

This tool is an attempt to fill that gap with something small, portable, and predictable.

A few notes on scope and intent:

  • It works well with gfortran, which is what I primarily use.
  • It does not aim to support every compiler dialect or extension.
  • It does not replace fpm or any existing build system.
  • It’s designed for people who already manage builds with Make and want reliable module dependency tracking.

At the moment it’s a small, self-contained utility I’ve been using successfully on medium-sized codebases.

Thanks for reading.

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