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Fortran wrapper

MPTRAC has been equiped with a wrapper to access the C functions from Fortran. The project started as a coding sprint in the natESM project LAGOOn.

To integrate multi language programming with Fortran and C in a single program the ISO_C_BINDINGS module is since Fortran 2003 the standard approach that facilitates interoperability between the two languages. In Fortran, the compiler is informed with the BIND(C) attribute that a symbol shall be interoperable with C.

Compilation and testing

To compile the Fortran wrapper, navigate to the src directory and run the following command:

$ make wrapper

To test the compiled wrapper, use the following command in the same directory:

$ make wrapper_test

Application

trac_fortran.f90

This program calculates trajectories by calling MPTRAC's time step routine. It is the Fortran counterpart to trac.c. The program is called with a directory list, a control file name, and an atmospheric input file name:

# calling trac_fortran
$ ./trac_fortran <dirlist> <ctl> <atm_in> [KEY VALUE ...]

The required arguments are:

  • dirlist: A file containing directories to be processed. Each directory has to have an own control parameter file and a starting point file.

  • ctl: In the control parameter file the configuration parameters can be set.

  • atm_in: The starting point file contains a list of starting points for the trajectory calculation.

Example from the repository:

# minimal required input to run trac_fortran
$ ./trac_fortran data/dirlist trac.ctl atm_split.tab

In addition it is possible to append control parameters, giving the flag name first, followed by the parameter, all separated by a space. Example:

#
$ ./trac_fortran data/dirlist trac.ctl atm_split.tab ATM_BASENAME atm_diff GRID_BASENAME grid_diff

Interface

mptrac_fortran.f90

To ensure the interoperability between Fortran and C an interface is needed. This interface includes some general dimension variables, the structures atm_t, cache_t, clim_photo_t, clim_t, clim_ts_t, clim_zm_t, ctl_t, dd_t, met_t and the functions mptrac_alloc, mptrac_free, mptrac_get_met, mptrac_init, mptrac_read_atm, mptrac_read_clim, mptrac_read_ctl, mptrac_read_met, mptrac_run_timestep, mptrac_write_atm, mptrac_write_met, mptrac_write_output, mptrac_update_device, and mptrac_update_host. The functions have the prefix mptrac to indicate that they are interfaces to the original MPTRAC functions.

Checking order and array sizes

It is crucial that the order and array sizes in the Fortran interface match those in the original C structure. The structures in MPTRAC consist of extensive variable lists (up to O100 for the control parameter list) and data types that may also depend on self-defined structures (struct of struct). A shell script find-vars.sh (in tests/wrapper_test/) can be used to check differences in the structures and variable dimensions. This script is automatically executed when the wrapper test is performed.

Example

An example of using the Fortran wrapper for trajectory calculations can be found in tests/wrapper_test/run.sh.

This file executes the trajectory calculation first with the original C code trac.c. The output is stored in the directory data/c/. Afterwards, the Fortran code trac_fortran.f90 is called. This output is stored in the directory data/fortran/. Both directories are compared to make sure that the Fortran version produces the same results as the C version.