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Subject: Re: [OMPI users] Mixing the FORTRAN and C APIs.
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2011-05-07 17:01:52

In addition to what Tim Prince said:

1. you can mix C and Fortran MPI function calls in a single program

2. as such, there's no reason for your C++ app to call mpi_init__() (for example)

3. you should be able to call *all* MPI C functions as you normally would

4. the only things that you should need MPI_<foo>_f2c / MPI_<foo>_c2f for are when you exchange MPI handles between your app and PARPACK. For example, if you have to pass an MPI datatype down to PARPACK, you'd actually need to pass MPI_Type_f2c(MPI_INTEGER), or MPI_Type_f2c(MPI_DOUBLE_PRECISION).

5. yes, it might be a pain to figure out how to link your C++ app with a fortran library, but I think you need to figure that out. You *may* actually end up linking your final executable with the Fortran compiler/linker, not the C++ compiler/linker. But you may need to specify some extra support libraries regardless of which linker you end up using -- you might need to google around and see if other people have tried linking C++ and Fortran together with your compiler suite.

Make sense?

On May 6, 2011, at 1:50 PM, Tim Prince wrote:

> On 5/6/2011 10:22 AM, Tim Hutt wrote:
>> On 6 May 2011 16:45, Tim Hutt<tdhutt_at_[hidden]> wrote:
>>> On 6 May 2011 16:27, Tim Prince<tcprince_at_[hidden]> wrote:
>>>> If you want to use the MPI Fortran library, don't convert your Fortran to C.
>>>> It's difficult to understand why you would consider f2c a "simplest way,"
>>>> but at least it should allow you to use ordinary C MPI function calls.
>>> Sorry, maybe I wasn't clear. Just to clarify, all of *my* code is
>>> written in C++ (because I don't actually know Fortran), but I want to
>>> use some function from PARPACK which is written in Fortran.
>> Hmm I converted my C++ code to use the C OpenMPI interface instead,
>> and now I get link errors (undefined references). I remembered I've
>> been linking with -lmpi -lmpi_f77, so maybe I need to also link with
>> -lmpi_cxx or -lmpi++ ... what exactly do each of these libraries
>> contain?
>> Also I have run into the problem that the communicators are of type
>> "MPI_Comm" in C, and "integer" in Fortran... I am using MPI_COMM_WORLD
>> in each case so I assume that will end up referring to the same
>> thing... but maybe you really can't mix Fortran and C. Expert opinion
>> would be very very welcome!
> If you use your OpenMPI mpicc wrapper to compile and link, the MPI libraries should be taken care of.
> Style usage in an f2c translation is debatable, but you have an #include "f2c.h" or "g2c.h" which translates the Fortran data types to legacy C equivalent. By legacy I mean that in the f2c era, the inclusion of C data types in Fortran via USE iso_c_binding had not been envisioned.
> One would think that you would use the MPI header data types on both the Fortran and the C side, even though you are using legacy interfaces.
> Slip-ups in MPI data types often lead to run-time errors. If you have an error-checking MPI library such as the Intel MPI one, you get a little better explanation at the failure point.
> --
> Tim Prince
> _______________________________________________
> users mailing list
> users_at_[hidden]

Jeff Squyres
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