Using cmake for coupling C++ and FORTRAN (mpif90) with iso_c_binding

Question

I am trying to write the CMakeLists.txt for coupling a simple FORTRAN program which calls a C++ function using iso_c_binding. When I use gfortran as the FORTRAN compiler, the code works perfectly, but when I switch to openMPI (mpif90) instead of gfortran, I get the error:

CMakeFiles/test.exe.dir/main.f90.o: In function `MAIN__':
main.f90:(.text+0x2d): undefined reference to `mpi_init_'
main.f90:(.text+0x47): undefined reference to `mpi_comm_size_'
main.f90:(.text+0x61): undefined reference to `mpi_comm_rank_'
main.f90:(.text+0x1c3): undefined reference to `mpi_finalize_'
collect2: ld returned 1 exit status
make[2]: *** [test.exe] Error 1
make[1]: *** [CMakeFiles/test.exe.dir/all] Error 2
make: *** [all] Error 2

I am attaching the gfortran version (working). In order to switch to the mpif90, I just uncomment the commented lines in the main.f90:

program main
   use iso_c_binding
   implicit none
!   include 'mpif.h'

   interface
      function func (n,x) bind (C, name="func")
         import
         integer(c_int):: func
         integer(c_int):: n
         real(c_double), dimension(1:n), intent(in):: x
      end function func
   end interface

   integer(c_int):: n
   real(c_double), dimension(:), allocatable:: x
   integer(c_int):: result
!   integer, parameter:: master = 0
!   integer:: numtasks, taskid, ierr, dest
!   integer  status(mpi_status_size)

!   call mpi_init(ierr)
!   call mpi_comm_size(mpi_comm_world, numtasks, ierr)
!   call mpi_comm_rank(mpi_comm_world, taskid, ierr)
!   if (taskid .eq. master) then
      n = 3
      allocate(x(1:n))
      x = (/1., 2., 3./)
      result = func(n,x)
      deallocate(x)
!   end if
!   call mpi_finalize(ierr)
end program main

func.c:

#include <iostream>
using namespace std;
#ifdef __cplusplus
  extern"C" {
#endif
int func(int& n, double x[]) {
   std::cout << x[0] << "  " << x[1] << "  " << x[2] << std::endl;
   return 0;
}
#ifdef __cplusplus
  }
#endif

CMakeLists.txt:

PROJECT(test)
cmake_minimum_required(VERSION 2.6)
enable_language(C Fortran)
# -------------------------
# Setting the compilers
# -------------------------
#set (CMAKE_Fortran_COMPILER /usr/local/openmpi/bin/mpif90)
set (CMAKE_Fortran_COMPILER /usr/bin/gfortran)
set (CMAKE_C_COMPILER /usr/bin/g++)
# -------------------------
# Setting the flags
# -------------------------
set_source_files_properties(main.f90 func.cpp PROPERTIES COMPILE_FLAGS "-c -lstdc++")
# -------------------------
# Making the executable
# -------------------------
ADD_EXECUTABLE(test.exe main.f90 func.cpp)

I would appreciate any help on this issue.

Answer 1

The problem is that you are not linking to MPI anywhere. Besides, setting the CMAKE_Fortran_compiler by hand is the wrong way to do it as it is not portable at all. Here is a simplified version of what I have in one of my projects. Note that I am using the FindMPI module that comes with recent versions of cmake. I do not know if cmake version 2.6 has that module but you can easily find it online.

cmake_minimum_required (VERSION 2.8)
project(DYNAMO Fortran C)
find_package(MPI REQUIRED)
set(DYNAMO_LINK_LIBRARIES ${MPI_Fortran_LIBRARIES})
set(DYNAMO_SRCS main.f90)
add_executable(dynamo ${DYNAMO_SRCS})
set_target_properties(dynamo PROPERTIES COMPILE_FLAGS "${MPI_Fortran_COMPILE_FLAGS}")
set_target_properties(dynamo PROPERTIES LINK_FLAGS "${MPI_Fortran_LINK_FLAGS}")
target_link_libraries(dynamo ${DYNAMO_LINK_LIBRARIES})
install(TARGETS dynamo RUNTIME DESTINATION bin)