Calling C++ from Fortran with OpenMPI - c++

Question

I am having a compile-time issue which I have reduced to the following test case. I wish to call a C++ routine from fortran and have the C++ routine be MPI aware.
Consider the following sample code,
Fortran main:
! -- main.f90
program main
implicit none
external return_three
integer return_three
write(*,*) return_three()
end program main
C++ subroutine:
// -- subs.cpp
#include <mpi.h>
extern "C"
{
int return_three_();
}
int return_three_()
{
return 3;
}
Note that, for the problem to reproduce, I only need to include mpi.h.
Compiling with GCC 5.3 and OpenMPI 1.10.1 (I checked GCC 4.8 and PGI 15.10 too) gives the following problem during linking:
% mpic++ -c subs.cpp
% mpifort -c main.f90
% mpifort -o main subs.o main.o -lstdc++ -lgcc_s
subs.o: In function `MPI::Intracomm::Intracomm()':
subs.cpp:(.text._ZN3MPI9IntracommC2Ev[_ZN3MPI9IntracommC5Ev]+0x14): undefined reference to `MPI::Comm::Comm()'
subs.o: In function `MPI::Intracomm::Intracomm(ompi_communicator_t*)':
subs.cpp:(.text._ZN3MPI9IntracommC2EP19ompi_communicator_t[_ZN3MPI9IntracommC5EP19ompi_communicator_t]+0x19): undefined reference to `MPI::Comm::Comm()'
subs.o: In function `MPI::Op::Init(void (*)(void const*, void*, int, MPI::Datatype const&), bool)':
subs.cpp:(.text._ZN3MPI2Op4InitEPFvPKvPviRKNS_8DatatypeEEb[_ZN3MPI2Op4InitEPFvPKvPviRKNS_8DatatypeEEb]+0x24): undefined reference to `ompi_mpi_cxx_op_intercept'
subs.o:(.rodata._ZTVN3MPI3WinE[_ZTVN3MPI3WinE]+0x48): undefined reference to `MPI::Win::Free()'
subs.o:(.rodata._ZTVN3MPI8DatatypeE[_ZTVN3MPI8DatatypeE]+0x78): undefined reference to `MPI::Datatype::Free()'
collect2: error: ld returned 1 exit status
It seems to me like mpifort is missing some C++-related libraries. It's my understanding that mpifort should be used to compile a fortran main program, though. The problem doesn't occur with Intel 16.0 compiled against OpenMPI 1.10.1.
My questions are:
What's going on here? Why is Intel able to handle this sample code and PGI/GCC is not?
Is there a portable way to include C++ subroutines with MPI in a fortran code?
(if possible) Is there an easy way to fix my current problem? I'm trying to compile a package on my machine, so it'd be best if I could just add -lmagicfix or something.

Answer 1

I was able to compile your code with GCC 5.3.0 and openMPI 1.10.2 by adding -lmpi_cxx in the final step:
% mpic++ -c subs.cpp
% mpifort -c main.f90
% mpifort -o main main.o subs.o -lstdc++ -lmpi_cxx
The reason is that the openMPI wrapper compilers mpifort and mpic++ link to different MPI libraries. You can check this with the -showme:libs option:
% mpifort -showme:libs
mpi_usempif08 mpi_usempi_ignore_tkr mpi_mpifh mpi
% mpic++ -showme:libs
mpi_cxx mpi
So in order to use the C++ MPI library, you have to tell mpifort explicitly to link to it.