SUBROUTINE TESTCPP ( II, JJ, KK, LL )
  INCLUDE 'COMON4.INS'

  integer*4, external :: CPPFUNCTION

  INTEGER a, b, c, test

  IF (.NOT. IFLAG) RETURN

  a = ICON(II)
  b = ICON(II + 1)
  test = CPPFUNCTION( a , b, c )
  WRITE ( ITERM, * ) 'C = ', c
  RETURN
  END

extern "C" {
      void _CPPFUNCTION(int a, int b, int *c);
}

void _CPPFUNCTION(int a, int b, int *c) {
      *c = a + b;
    }

cl /nologo /MD /c /W3 /O2 /FD /EHsc /errorReport:prompt /D"MSWINDOWS" /D"WIN32" ^
   /D"_WINDOWS" /D"NDEBUG" "cpp_code.cpp"

IFORT /nologo /Od /Oy- /assume:buffered_io /traceback /libs:dll /threads /c /Qip ^
      /extend_source:132 /noaltparam /fpscomp:logicals /warn:nodeclarations ^
      /warn:unused /warn:truncated_source /Qauto /Op /iface:cvf /define:DLLI ^
      /include:"C:Program Files (x86)PTIPSSE32PSSLIB" ^
      /object:"fort_code.OBJ" ^
      "fort_code.f"

lib /out:fort_cpp.lib fort_code.obj cpp_code.obj

ifort /nologo /assume:buffered_io /traceback /libs:dll /threads /c /Qip /extend_source:132 /noaltparam /fpscomp:logicals /Qprec /warn:declarations /warn:unused /warn:truncated_source /Qauto /fp:source /iface:cvf /define:DLLI /include:"C:Program Files (x86)PTIPSSE32PSSLIB" /object:"C:tempINIT_62028911hw2ap_conec.obj" /module:"C:tempINIT_620289" "11hw2ap_conec.f"


 ifort /nologo /assume:buffered_io /traceback /libs:dll /threads /c /Qip /extend_source:132 /noaltparam /fpscomp:logicals /Qprec /warn:declarations /warn:unused /warn:truncated_source /Qauto /fp:source /iface:cvf /define:DLLI /include:"C:Program Files (x86)PTIPSSE32PSSLIB" /object:"C:tempINIT_62028911hw2ap_conet.obj" /module:"C:tempINIT_620289" "11hw2ap_conet.f"


 link /INCREMENTAL:NO /NOLOGO /DLL /SUBSYSTEM:WINDOWS /MACHINE:X86 /ERRORREPORT:PROMPT @"C:tempINIT_620289linkfilestod9p1.txt" /OUT:"C:tempINIT_62028911hw2ap_dsusr.dll" /map:"C:tempINIT_62028911hw2ap_dsusr.map"

 fort_cpp.lib(fort_code.obj) : error LNK2019: unresolved external symbol _CPPFUNCTION@12 referenced in function _TESTCPP
 C:tempINIT_62028911hw2ap_dsusr.dll : fatal error LNK1120: 1 unresolved externals

 ERROR during link(1)... Aborted

SUBROUTINE CONEC
C
      INCLUDE 'COMON4.INS'
C
      CALL TESTCPP         ( 55791,     0,     0,     0)
C
      RETURN
      END
      SUBROUTINE CONET
C
      INCLUDE 'COMON4.INS'
C
      IF (.NOT. IFLAG) GO TO 9000
C
C  NETWORK MONITORING MODELS
C
C
 9000 CONTINUE
C
      RETURN
      END

program example

    use iso_c_binding
    implicit none

    interface
       integer(c_int) function add1(x) bind(C,name="add1")
         !DEC$ ATTRIBUTES DLLEXPORT :: add1
         use iso_c_binding
         integer(c_int), value :: x
       end function add1
    end interface

    call callingadd1()
  contains
    subroutine callingadd1()
      write(*,*) '1+1=', add1(1)
    end subroutine callingadd1
  end program example

extern "C" {
  int add1(int x);
}

int add1(int x) {
  return(x+1);
}

subroutine callingadd1() bind(C, name="callingadd1")
  !DEC$ ATTRIBUTES DLLEXPORT :: callingadd1
  use iso_c_binding
  implicit none
  interface
     integer(c_int) function add1(x) bind(C,name="add1")
       !DEC$ ATTRIBUTES DLLEXPORT :: add1
       use iso_c_binding
       integer(c_int), value :: x
     end function add1
  end interface
  write(*,*) '1+1=', add1(1)
end subroutine callingadd1