[SciPy-user] Is it possible to pass Fortran derived data types to Python via C and SWIG?

[John Sal]

Thank you all for your help. I think that writing a set of subroutines
that creates derived data types from arguments passed to it and vice
versa is probably my best bet, but I may try Huard's solution.

On Dec 1, 9:58 am, "David Huard" <david.hu... at gmail.com> wrote:
> John,
>
> this is something I've wanted to look at. Here is what I had planned to do,
> so there is no guarantee that it will actually work...
>
> The ISO_C_BINDING module is part of the 2003 standard and allows
> interoperability between C and Fortran (it is included in the latest
> gfortran compiler). It allows interoperability of Fortran derived types with
> C structures (with certain restrictions). For example,
>
> use iso_c_binding
> type, bind(c) :: mytype
>   real(c_float) :: data
>   integer(c_int) :: n
> end type
>
> is interoperable with
>
> typedef struct {
>   float data;
>   int n;
>
> } mytype
>
> Now, I am just guessing, but if such a module was built into a shared
> library, maybe it could be accessed from python using ctypes STRUCTURES.
>
> Regards,
>
> David
>
> On Sun, Nov 30, 2008 at 4:38 PM, Berthold Höllmann <
>
> berth... at xn--hllmanns-n4a.de> wrote:
> > Matthieu Brucher <matthieu.brucher <at> gmail.com> writes:
>
> > > 2008/11/30 David Cournapeau <david <at> ar.media.kyoto-u.ac.jp>:
> > > > John Salvatier wrote:
> > > >> I have a Fortran 90 algorithm which uses a derived data type to return
> > > >> data, and I would like to make a python wrapper for this algorithm. I
> > > >> understand that f2py cannot wrap derived data types; is it possible to
> > > >> do so with a C interface for the Fortran algorithm and SWIG? I would
> > > >> have to pass the derived data type into a C struct and then to Python.
>
> > > > It is possible as long as you can pass the structure from fortran to C.
> > > > I don't know anything about Fortran derived data types, but if it is a
> > > > non trivial object (more than a set of fundamental types), I am afraid
> > > > it will be difficult. Does F90 supports POD data ? Otherwise, you will
> > > > need a scheme for marshalling your data from Fortran to C (to match
> > > > exactly how the structure would look like in C at the binary level).
>
> > > > David
>
> > > I've read an article (I don't remember where though, possibly CiSE)
> > > that stated that it's really not an easy task, as each Fortran
> > > compiler can do as it pleases it. So depending on the compiler and the
> > > Fortran standard, it can be possible, or not. So as there are no
> > > guaranties, you should write a function that transforms the Fortran
> > > structure in several pieces that are then passed to the C function.
>
> > > Matthieu
>
> > A feasible way to achieve this would be to write a Fortran wrapper
> > around your routine(x) that decomposes your derived data type to
> > standard types and exposes these in the interface. Than you can compose
> > the derived data type again in the wrapper and pass it to the original
> > routine. ::
>
> >   module geom
> >     type Point
> >        real :: x, y
> >     end type Point
> >     type Circle
> >        type (Point) :: Center
> >        real :: Radius
> >     end type Circle
> >   end module geom
> >   subroutine test(c)
> >     use geom
> >     type (Circle) :: c
> >     print*, c%Radius
> >     print*, c%Center%X
> >     print*, c%Center%Y
> >   end subroutine test
> >   subroutine w_test(x, y, r)
> >     use geom
> >     real :: x, y, z
> >     type (Circle) :: C
> >     c%Radius = r
> >     c%Center%X = x
> >     c%Center%Y = y
> >     call test(c)
> >   end subroutine w_test
>
> > Wrapping w_test should be trivial using f2py
>
> > Regards
> > Berthold
>
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>
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