How CMake supports Fortran modules

[ivan-pi]

I just came across the following talk,

Keynote: CMake: One Tool To Build Them All - Bill Hoffman [ CppNow 2021 ]

which goes into a few details of how CMake builds the dependency graph for Fortran modules. Amusingly, Kitware maintained their own fork of ninja for a few years, until Google realized the dynamic dependency generation feature CMake devs had built for Fortran, would also be useful for supporting C++20 modules. The CMake module dependency generator evolved from makedepf90. The talk also explains how CMake preprocesses the files first (cc @arteevraina, @milancurcic).

A few more links to C++ proposal documents were provided during the talk:

• How CMake supports Fortran Modules
• Format for describing dependencies of source files

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For anyone interested in the topic of generating dependency graphs, there were some decent discussions at Discourse:

• Module dependencies
• How do you generate file dependence when writing Makefile?

If you search for "Makefile" at comp.lang.fortran you will find that the introduction of modules in Fortran 90 caused a lot of pain for Fortran users in the era 1990-2000 (and arguably continuing until this day). The common approach was to write their own tools (e.g. awk scripts or rudimentary Fortran parsers) to output module and include file dependencies.