Fortran Explicit and implicit interfaces Internal/module subprograms and explicit interfaces

Example

A subprogram (which defines a procedure), can be either a subroutine or a function; it is said to be an internal subprogram if it is called or invoked from the same program or subprogram that contains it, as follows

program my_program

  ! declarations
  ! executable statements,
  ! among which an invocation to
  ! internal procedure(s),
  call my_sub(arg1,arg2,...)
  fx = my_fun(xx1,xx2,...)

contains
  
  subroutine my_sub(a1,a2,...)
    ! declarations
    ! executable statements
  end subroutine my_sub

  function my_fun(x1,x2,...) result(f)
    ! declarations
    ! executable statements
  end function my_fun

end program my_program

In this case the compiler will know all about any internal procedure, since it treats the program unit as a whole. In particular, it will "see" the procedure's interface, that is

• whether it is a function or subroutine,
• which are the names and properties of the arguments a1, a2, x1, x2, ...,
• which are the properties of the result f (in the case of a function).

Being the interface known, the compiler can check whether the actual arguments (arg1, arg2, xx1, xx2, fx, ...) passed to the procedure match with the dummy arguments (a1, a2, x1, x2, f, ...).

In this case we say that the interface is explicit.

A subprogram is said to be module subprogram when it is invoked by a statement in the containing module itself,

module my_mod

  ! declarations

contains
  
  subroutine my_mod_sub(b1,b2,...)
    ! declarations
    ! executable statements
    r = my_mod_fun(b1,b2,...)
  end subroutine my_sub

  function my_mod_fun(y1,y2,...) result(g)
    ! declarations
    ! executable statements
  end function my_fun

end module my_mod

or by a statement in another program unit that uses that module,

program my_prog

  use my_mod

  call my_mod_sub(...)

end program my_prog

As in the preceding situation, the compiler will know everything about the subprogram and, therefore, we say that the interface is explicit.