fortran 90 passing user defined type member to a c fucntion

[burlen]

When calling a c function from a fortran 90 program with members of a 
user defined types for output arguments of the called subroutine the 
user defined types aren't being modified as they should be (according to 
my understanding). But if I pass in native types (eg integer), these are 
modified upon return as expected. So for the user defined types it's 
behaving like pass by value, rather than pass by reference. The c 
function has no interface defined, it's just linked in, so in my 
understanding all arguments should be treated like intent inout, and I 
think that means pass by reference. Am I correct in thinking that all 
arguments passed to the c function should be passed by reference?

An example of passing the user defined type members to a fortran 
subroutine showed that modifications of the arguments inside the 
subroutine are visible to the caller when the subroutine returns. Which 
is what I expected. There must be something about calling the c function 
that changes the behavior.

Am I overlooking something simple here? If not, can someone explain why 
the user defined type members are passed by value?
Thanks
Burlen


Here is an excerpt of the code:



module CartesianDecompModule
  use BoxModule
  
!============================================================================
  type CartesianDecomp
    integer   WorldRank          ! my rank in comm world
    integer   WorldSize          ! number of processes in the communicator
    !----------------------------
    integer   Comm               ! optimized cartesian communicator
    logical   Periodicity(3)     ! periodic BC flags
    logical   Reorder            ! if set optimize the communicator
    integer   NDims              ! dimensionality (1,2, or 3)
    integer   NSubDomains(3)     ! number of sub-domains
    integer   SubDomainCoords(3) ! index space coordinates into decomp
    type(Box) Domain             ! simulation domain
    type(Box) SubDomain          ! local portion of the domain

  end type CartesianDecomp

contains
  
!----------------------------------------------------------------------------
  subroutine CreateCommunicator(c)
    use mpi

    implicit none
    type(CartesianDecomp) :: c ! object to initialize
    integer :: iErr
    integer :: comm,nsubs(3),subs(3)

    ! ! This doesn't work. output arguments 
c%NSubDomains,c%Comm,c%SubDomainCoords, aren't modified ??
    ! call MPI_Dims_create(c%WorldSize,c%NDims,c%NSubDomains,iErr)
    ! call 
MPI_Cart_create(MPI_COMM_WORLD,c%NDims,c%NSubDomains,c%Periodicity,c%Reorder,c%Comm,iErr)
    ! call 
MPI_Cart_get(c%Comm,c%NDims,c%NSubDomains,c%Periodicity,c%SubDomainCoords,iErr)

    ! This works.
    subs(:)=0
    nsubs(:)=0
    call MPI_Dims_create(c%WorldSize,c%NDims,nsubs,iErr)
    call 
MPI_Cart_create(MPI_COMM_WORLD,c%NDims,nsubs,c%Periodicity,c%Reorder,comm,iErr)
    call MPI_Cart_get(comm,c%NDims,nsubs,c%Periodicity,subs,iErr)
    c%Comm=comm
    c%NSubDomains=nsubs
    c%SubDomainCoords=subs

  end subroutine

end module