How to pass subroutine names as arguments in Fortran?

Question

What is the syntax for passing subroutine names as arguments? Schematically:

  .
  .
call action ( mySubX ( argA, argB ) )
  .
  .

subroutine action ( whichSub ( argA, argB ) )
  ...
call subroutine whichSub ( argA, argB )
  ...
end subroutine action

The goal is to have call subroutine whichSub ( argA, argB ) act as call subroutine mySubX ( argA, argB ). My preference is to avoid avoid passing a switch parameter and then use SELECT CASE.

Answer 1

It is

call action(mySubX)

provided action looks as

subroutine action(sub)
  !either - not recommmended, it is old FORTRAN77 style
  external sub
  !or - recommended
  interface
    subroutine sub(aA, aB)
      integer,intent(...) :: aA, aB
    end subroutine
  end interface
  ! NOT BOTH!!

  call sub(argA, argB)

provided action knows what to put there as argA, argB to represent aA, aB.

Otherwise, if you want to pass also the arguments

call action(mySubX, argA, argB)

subroutine action(sub, argA, argB)
  !either - not recommmended, it is old FORTRAN77 style
  external sub
  !or - recommended
  interface
    subroutine sub(aA, aB)
      integer,intent(...) :: aA, aB
    end subroutine
  end interface

  integer, intent(...) :: argA, argB

  call sub(argA, argB)

I don't think it is good to use function pointers here, they are good when you have to change the value of the pointer (the subroutine it points to) sometimes. Normal procedure arguments worked in FORTRAN77 and continue to work even now.

---

So as requested in the comment, if you are in a module and procedure with the right interface is accessible from the module (perhaps in the same module), you can use the procedure statement to get rod of the interface block:

module subs_mod
contains
  subroutine example_sub(aA, aB)
    integer,intent(...) :: aA, aB
    !the real example code
  end subroutine
end module

module action_mod
contains

  subroutine action(sub)
    use subs_mod
    procedure(example_sub) :: sub

    call sub(argA, argB)
  end subroutine
end module

but more likely, instead of a real subroutine you will create an abstract interface which you would reference with the procedure statement so in the end everything will be similar as before:

module action_mod

  abstract interface
    subroutine sub_interface(aA, aB)
      integer,intent(...) :: aA, aB
    end subroutine
  end interface

contains

  subroutine action(sub)
    procedure(sub_interface) :: sub

    call sub(argA, argB)
  end subroutine
end module