Does the abstract Fortran interface of a C callback function require bind(C) attribute?

Question

Consider the following Fortran code, where the C-interoperable subroutine runFoo4C(...) bind(C, name="runFoo") in module Foo_mod takes a C-callback function pointer getLogFuncFromC() as an argument,

module CallbackInterface_mod

    abstract interface
        function getLogFunc4C_proc(ndim,Point) result(logFunc) ! bind(C)
            use, intrinsic :: iso_c_binding, only : c_int32_t, c_double, c_int
            integer(c_int32_t), intent(in)  :: ndim
            real(c_double), intent(in)      :: Point(ndim)
            real(c_double)                  :: logFunc
        end function getLogFunc4C_proc
    end interface

end module CallbackInterface_mod

!***********************************************************************************************************************************
!***********************************************************************************************************************************

module Foo_mod

    interface
    module subroutine runFoo4C(ndim, getLogFuncFromC, inputString, inputStringLen) bind(C, name="runFoo")
        use, intrinsic :: iso_c_binding, only: c_int32_t, c_char, c_funptr, c_f_procpointer, c_size_t
        use CallbackInterface_mod, only: getLogFunc4C_proc
        implicit none
        integer(c_int32_t) , intent(in)                         :: ndim
        character(len=1, kind=c_char), dimension(*), intent(in) :: inputString
        integer(c_size_t) , intent(in)                          :: inputStringLen
        type(c_funptr), intent(in), value                       :: getLogFuncFromC
    end subroutine runFoo4C
    end interface

contains

    subroutine runFoo(ndim, getLogFunc, string)
        !use CallbackInterface_mod, only: getLogFunc_proc
        use CallbackInterface_mod, only: getLogFunc4C_proc
        use, intrinsic :: iso_fortran_env, only: RK => real64
        implicit none
        integer :: ndim
        procedure(getLogFunc4C_proc)    :: getLogFunc
        character(*), intent(in)        :: string
        real(RK)                        :: Point(ndim)
        character(:), allocatable       :: mystring
        Point = [1._RK,1._RK]
        write(*,*) "Hi again, this is a call from inside runFoo!"
        write(*,*) "getLogFunc(2,[1,1]) = ", getLogFunc(ndim,Point)
        write(*,*) "string = ", string
    end subroutine

end module Foo_mod

!***********************************************************************************************************************************
!***********************************************************************************************************************************

submodule (Foo_mod) Foo_smod

contains

    module subroutine runFoo4C(ndim, getLogFuncFromC, InputString, inputStringLen) bind(C, name="runFoo")

        use, intrinsic :: iso_c_binding, only: c_double, c_int32_t, c_char, c_funptr, c_f_procpointer, c_size_t
        use CallbackInterface_mod, only: getLogFunc4C_proc
        implicit none
        integer(c_int32_t) , intent(in)                         :: ndim
        character(len=1, kind=c_char), dimension(*), intent(in) :: InputString
        integer(c_size_t) , intent(in)                          :: inputStringLen
        type(c_funptr), intent(in), value                       :: getLogFuncFromC
        procedure(getLogFunc4C_proc), pointer                   :: getLogFunc
        real(c_double)                                          :: Point(ndim)
        character(:), allocatable                               :: inputString4tran
        integer                                                 :: i

        write(*,*) "InputString: ", InputString(1:inputStringLen)
        allocate( character(len=inputStringLen) :: inputString4tran )
        do i=1,inputStringLen
            inputString4tran(i:i) = InputString(i)
        end do
        write(*,*) "inputString4tran: ", inputString4tran

        ! associate the input C procedure pointer to a Fortran procedure pointer
        call c_f_procpointer(cptr=getLogFuncFromC, fptr=getLogFunc)
        Point = [1._c_double, 1._c_double]
        write(*,*) "Here we go: "
        write(*,*) "getLogFunc(ndim=2, [1._c_double, 1._c_double]): ", getLogFunc( ndim, Point )

        call runFoo(ndim, getLogFunc, inputString4tran)

    end subroutine runFoo4C

end submodule Foo_smod

The abstract Fortran interface of this callback function, is given by getLogFunc4C_proc() in module CallbackInterface_mod in the above code. Now the question:

Does this abstract interface require a bind(c) attribute to comply with the fortran standard? My own naive guess is that it does not need the bind(c) as it is not going to be called with the global identifier of the function in the interface, but the abstract interface simply determines the interface of the C callback function, a pointer to which is passed to Fortran to be called later from inside Fortran.

Indeed, commenting out this bind(c) attribute in the abstract interface does not lead to any compilation or runtime error using ifort (18.0.2 Windows compiler).

If it is not needed, then how about variable declarations in this abstract interface? Do they need to be declared by C-conforming kinds from the iso_c_binding intrinsic module?

Answer 1

The presence (or absence) of BIND(C) in the abstract interface changes the characteristics of the procedure pointer, but does so in a way that this program doesn't reveal. Because you do the call to getLogFunc through a pointer you converted from a C_FUNPTR, you prevent the compiler from noticing the mismatch should BIND(C) be omitted in the abstract interface. For example, should the procedure have a character(*) argument, lots of bad things would happen for a mismatch.

BIND(C), by itself, is fine in an abstract interface, as long as you don't also say NAME=. Since it changes how the procedure is called, you must specify it if the called procedure is interoperable.

Regarding "If it is not needed, then how about variable declarations in this abstract interface? Do they need to be declared by C-conforming kinds from the iso_c_binding intrinsic module?", you make the common error of conflating definitions in intrinsic module ISO_C_BINDING with being interoperable. The kind constants in that module are just numbers, there's nothing magical about them. You are required to have the actual and dummy arguments match in type, kind and rank (with some exceptions.)