To: J3                                                     10-225
From: Bill Long
Subject: Interop TR: Requirement R9d
Date: 2010 October 07
Reference: 10-165r2, N1820, 10-007

Discussion:

Requirement R9d in N1820 is:

  INTENT(OUT) ALLOCATABLE dummy arguments are permitted in a BIND(C)
  routine.  It is the responsibility of the processor to deallocate
  the actual argument in any cross-language call.  It is not required
  for the processor to handle this in C-to-C calls.

There are three possible cases.

Case 1: If the caller is a Fortran program unit, then this text of
paragraph 6 of 6.7.3.2 'Deallocation of allocatable variables' in
10-007:

"When a procedure is invoked, any allocated allocatable object that is
an actual argument corresponding to an INTENT (OUT) allocatable dummy
argument is deallocated.."

clearly already requires the deallocation.

Case 2: If the caller is a C function and the callee is written in
Fortran, there is a clear expectation that the argument will be
deallocated. Stating this expectation is the intent of R9d.

Case 3: If the caller and callee are C functions, it is outside the
scope of the TR to require that the C compiler generates the
deallocation code.

The required deallocation is done by extra code inserted by the
compiler. The simplest implementation is to include the code in the
callee, since this involves only one instance of the code, rather than
generating separate code at each call site.  This implementation would
automatically cover Case 2.  Case 1 effectively requires that the
deallocation be done on the caller side for a BIND(C) callee, since
there is no way to know, in general, whether the callee is written in
Fortran or some other language. If the caller does the deallocation,
and the callee is a Fortran procedure, the deallocation coded there
will have no additional effect because the argument is not allocated.

Since the thrust of R9d is essentially a message to implementors, it
will be included in the TR as a Note.


Edits to 10-165r2:

[5:12+] After Note 3.1 in 3.3 'Argument association', add new Note:

"NOTE 3.2
 INTENT(OUT) ALLOCATABLE dummy arguments are permitted in a BIND(C)
 routine.  It is the responsibility of the processor to deallocate the
 actual argument if either the calling or called procedure is a
 Fortran program unit.  It is not required for the processor to handle
 this if neither the calling nor called procedure is a Fortran program
 unit."