Generic interface with subroutine and function

[ivanpribec]

For the setlocale case, I personally wouldn’t bother wrapping the routine going the full way:

@Pap

module locale
use, intrinsic :: iso_c_binding
implicit none
private

interface
  ! The original one, using C conventions
  function c_setlocale(category, locale) bind(c, name="setlocale")
     import :: c_ptr, c_int, c_char
     type(c_ptr) :: c_setlocale
     integer(kind=c_int), intent(in), value :: category
     character(kind=c_char), intent(in) :: locale(*)  ! A null-terminated string
  end function setlocale
end interface

public :: setlocale, c_setlocale

contains

subroutine setlocale(category, locale, result)
   integer(kind=c_int), intent(in) :: category
   character(kind=c_char,len=*), intent(in) :: locale
   character(kind=c_char,len=:), intent(out), allocatable, optional :: result

   type(c_ptr) :: ptr
   character(c_char), pointer :: f_ptr(:) => null()

   ptr = c_setlocale(category, f_c_string(locale)))
   if (present(result)) then
      if (c_associated(ptr)) then
         ! Allocate and copy output string
         associate(n => c_strlen(ptr))
            call c_f_pointer(ptr,f_ptr,[n])
            allocate(character(kind=c_char,len=n) :: result)
            do i = 1, n
               result(i:i) = f_ptr(i)
            end do
         end associate
      end if
    end if
        
end subroutine setlocale

end module locale

This way, both Fortran users not familiar with C conventions (null-terminated strings) and power-users can select the option they prefer:

integer(c_int), parameter :: LC_ALL = 1
character(len=:), allocatable :: locale_US
type(c_ptr) :: dptr  ! Dummy pointer

! --- Fortran users ---

call setlocale(LC_ALL,"en_US.UTF-8",locale_US) ! switch to US
call setlocale(LC_ALL,"el_GR.utf8")            ! switch to Greek

! Restore US locale
call setlocale(LC_ALL,locale_US)

! --- Advanced users familiar with C ---
dptr = c_setlocale(1,"en_US.UTF-8"//c_null_char)