11-175r2
To: J3
From: Nick Maclaren / Toon Moene / Bill Long
Subject: Interop TR: CFI_section and strides
Date: 2011 June 29
Reference: N1854, 11-168, 11-175


For clarity, this doesn't repeat the discussion in 11-175 and proposes
only the second alternative.  This separates out the bounds and strides
and makes CFI_section essentially a C equivalent of Fortran section
syntax.

During the meeting, it was pointed out that the proposal in 11-168
means that the attribute argument is unnecessary, and serves only
to introduce a source of error.  It is therefore removed.


Edits to N1854
--------------

[17:7-8] Delete "CFI attribute t attribute," and replace "const
CFI_dim_t dim[]" by "const CFI_index_t lower_bounds[], const CFI_index_t
upper_bounds[], const CFI_index_t strides[]".

[17:18] Delete the specification of the attribute parameter.

[17:19-24] Replace the paragraph specifying the dim argument by:

"lower_bounds points to an array specifying the subscripts of the
element in the given array that is the first element,
in Fortran array element order (6.5.3.2 of ISO/IEC 1539-1:2010),
 of the array
section.  If it is NULL, the subscripts of the first element of source
are used; otherwise, the number of elements shall be source->rank.

upper_bounds points to an array specifying the subscripts of the element
in the given array that is the last element,
in Fortran array element order (6.5.3.2 of ISO/IEC 1539-1:2010),
 of the array section.  If it
is NULL, the subscripts of the last element of source are used;
otherwise, the number of elements shall be source->rank.

strides points to an array specifying the strides of the array section
in units of elements of argument source; if an element is 0, the section
subscript for the dimension is a subscript and the corresponding
elements of lower_bounds and upper_bounds shall be equal.  If it is
NULL, the strides are treated as being all 1; otherwise, the number of
elements shall be source->rank."

[17:26-28] Replace "The attribute argument determines whether the C
descriptor describes an assumed-shape array or pointer object. The value
of result->rank is source->rank minus the number of dim entries for
which the extent member is equal to -1." by "The value of result->rank
is source->rank minus the number of stride elements which have value 0."

[17:29] Replace "the C descriptor established" by "an established C
descriptor".

[17:34-39] and [18:1-2] Replace the example by:

"the following code fragment updates a C descriptor to describe the
array section A(3::5).

    CFI_index_t lower_bounds[] = {2}, strides[] = {5};
    CFI_CDESC_T(1) section;
    int ind;
    /* Establish the C descriptor section. */
    ind = CFI_section ( (CFI_cdesc_t *) &section, source,
        lower_bounds, NULL, strides );

If source already points to a C descriptor for the rank-two
assumed-shape array A declared in Fortran as

    real A(100,100)

the following code fragment updates a C descriptor to describe the
rank-one array section A(:,42).

    CFI_index_t lower_bounds[] = {source->dim[0].lower_bound,41},
        upper_bounds[] = {source->dim[0].upper_bound,41},
        strides[] = {1,0};
    CFI_CDESC_T(1) section;
    int ind;
    /* Establish the C descriptor section. */
    ind = CFI_section ( (CFI_cdesc_t *) &section, source,
        lower_bounds, upper_bounds, strides );
"