11-167r1
To: J3
From: Nick Maclaren / Bill Long
Subject: Interop TR: more on constraints
Date: 2011 June 30
Reference: N1854, 11-167, 11-168r2, 11-175r2


Clean up of restrictions on what can be done with C descriptors.


Edits to N1854:

[13:8+] Move para 2 of 5.2.6 [19:36-38] to here.

{{{Strictly, the following INTENT constraints do not belong in the TR,
as they are more like a fix to the existing interoperability.  C's
rules on 'read-only' data and when data may be references are wildly
different from Fortran's, and I believe that spelling Fortran's rules
out is a good idea.  There is already such wording in 5.2.5.1, so we
could add: }}}

[13:8++] Add the paragraph following the one moved by the previous
edit:

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

    * It shall not be modified if the Fortran dummy argument has the
INTENT(IN) attribute.

    * It shall not be accessed before it is given a value if the Fortran
dummy argument has the INTENT(OUT) attribute."


[15:32-35] Replace:

"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."

by

"It shall not point to a C descriptor that is pointed to by a formal
parameter that corresponds to a Fortran actual argument."


[16:19] Change "to the C descriptor established" to "to an established
C descriptor".

[17:12-15] Replace: "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."

by

"If /result/ points to a C descriptor that is pointed to by a formal
parameter that corresponds to a Fortran actual argument, the attribute
member shall not have the value CFI_attribute_assumed."

[17:17] At the end of the source description, add "The corresponding
values of the elem_len and type members shall be the same in the C
descriptors pointed to by source and result."

[18:9-12] Replace: "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."

by

"If /result/ points to a C descriptor that is pointed to by a formal
parameter that corresponds to a Fortran actual argument, the attribute
member shall not have the value CFI_attribute_assumed."


[18:14] At the end of the source description, add "The corresponding
values of the rank member shall be the same in the C descriptors
pointed to by source and result."

[19:12] At the end of the source description, add "If source is not
NULL, the corresponding values of the elem_len, rank, and type,
members shall be in same in the C descriptors pointed to by source and
result."


[19:33-41] Replace the remaining contents of section 5.2.6 by:

A C descriptor shall not be initialized, updated or copied other than
by calling the functions specified here.

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
or unknown-size array.

    * Its base_addr member shall not be accessed before it is given a
value if the Fortran dummy argument is a pointer and has the
INTENT(OUT) attribute.

    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 misbehavior."