11-167r2
To: J3
From: Nick Maclaren / Bill Long
Subject: Interop TR: more on constraints
Date: 2011 July 1
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 if an object 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 if a C descriptor 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 will cause undefined
    behavior."

All TR: Throughout the TR, replace "NULL" with "a NULL pointer" except
in code examples and elsewhere NULL is needed.q