11-167
To: J3
From: Nick Maclaren
Subject: Interop TR: more on constraints
Date: 2011 June 07
Reference: N1854


Upon looking it over, I have come back to the current approach.
I have also spotted a couple of other nasties that I had missed before.
I believe that this covers Reinhold's and my points, and minimises the
changes.  I have also included minor fixes to CFI_establish, CFI_section
and CFI_select_part which look like historical artifacts.

A point raised by Reinhold and Bill is that we should define rules that
apply to C interfaces as well as ones between C and Fortran.  I don't
see any practical way of doing that, without the structure of a BIND(C)
interface to tie things to.

There is a significant overlap with 5.2.7, but I think that it better
to keep them separate, to avoid some extremely messy wording.


{{{ The following deletions in 15:32-35, 17:12-15 and 18:9-12 are moved
to 5.2.6 in an expanded form. }}}

[15:32-35] Delete "It shall not point to a C descriptor that describes
an object that is described by a C descriptor pointed to by a formal
parameter that corresponds to a Fortran dummy argument. It shall not
point to a C descriptor that describes an allocated allocatable object."

[16:19] Change "to the C descriptor established" to "to become a valid C
descriptor".

[17:12-15] Delete "It shall not point to a C descriptor that describes
an object that is described by a C descriptor pointed to by a formal
parameter that corresponds to a Fortran dummy argument. It shall not
point to a C descriptor that describes an allocated allocatable object."

[17:29] Change "to the C descriptor established" to "to become a valid C
descriptor".

[18:9-12] Delete "It shall not point to a C descriptor that describes
an object that is described by a C descriptor pointed to by a formal
parameter that corresponds to a Fortran dummy argument. It shall not
point to a C descriptor that describes an allocated allocatable object."

[18:30-31] Change "to the C descriptor established" to "to become a valid C
descriptor".


{{{ In the following, I have referred to a dummy argument when it might
be more natural to refer to an actual argument.  The reason for that is
that I am referring to the interface, which Fortran requires to be the
same as any procedure definition. }}}

[19:33-35] Delete "A C descriptor that is pointed to by a formal
parameter that corresponds to a Fortran dummy argument with the
INTENT(IN) attribute shall not be updated."

[19:36] After "a C descriptor", add "that is pointed to by a formal
parameter or actual argument that corresponds to a dummy argument in a
BIND(C) interface".

[19:41+] Add the new paragraphs:

"The following restrictions apply to a C descriptor that is pointed to
by a formal parameter or actual argument that corresponds to a dummy
argument in a BIND(C) interface:

    * It shall not be modified if either the Fortran dummy argument has
the INTENT(IN) attribute or the descriptor is for an assumed-shape
array.

    * No modification shall have the effect of changing the rank, type
or attribute members.  The elem_len member shall not be changed unless
the descriptor is for a character type.

    * If it has the ALLOCATABLE attribute, it shall be modified only by
calling the functions CFI_allocate and CFI_deallocate."

    * If it has the POINTER attribute, it shall be modified only by
calling the functions CFI_allocate, CFI_deallocate, CFI_section,
CFI_select_part and CFI_setpointer.

    NOTE
    In this context, modification refers to any change to the location
    or contents of the descriptor, including establishment and update.
    The intent of these restrictions is that C descriptors remain intact
    at all times they are accessible to an active Fortran procedure, so
    that the Fortran code is not required to copy them.  C programmers
    should note that doing things with descriptors that are not possible
    in Fortran may well confuse compilers and cause misbehaviour; the
    restrictions here exclude only the more obvious issues."