Interoperating with C

use iso_c_binding provides kinds that match C types: c_int, c_double, c_char, c_ptr, c_null_char. Use them for every argument that crosses the boundary.

bind(c, name='abs') gives a procedure C linkage. In an interface block it declares a C function you can call; on your own procedure it exports a C-callable symbol.

Fortran passes by reference; C usually passes by value. Mark scalar arguments value to match a C signature like int abs(int).

C strings are NUL-terminated arrays of character(kind=c_char); append c_null_char when passing a Fortran string to C.

Derived types declared bind(c) match C structs field for field, and c_loc/c_f_pointer convert between C pointers and Fortran pointers.

program cinterop
  use iso_c_binding, only: c_int, c_size_t, c_char, c_null_char
  implicit none
  interface
    function c_abs(x) bind(c, name='abs') result(r)
      import :: c_int
      integer(c_int), value :: x
      integer(c_int) :: r
    end function c_abs
    function c_strlen(s) bind(c, name='strlen') result(n)
      import :: c_char, c_size_t
      character(kind=c_char), intent(in) :: s(*)
      integer(c_size_t) :: n
    end function c_strlen
  end interface
  print '(i0)', c_abs(-7_c_int)
  print '(i0)', c_strlen('fortran' // c_null_char)
end program cinterop