Comment (by Peter Diener):

With the things you have tried already, I’m afraid that the only path forward I see, is to use c_loc from iso_c_binding. Maybe it’s time for cactus to require a modern enough fortran compiler that iso_c_binding is provided. However, using this might not be without problems, as the argument has to have the pointer or target attribute.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2403/compile-failure-with-gfortran-10-when