11-175r1
To: J3
From: Nick Maclaren / Toon Moene
Subject: Interop TR: CFI_section and strides
Date: 2011 June 28
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 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 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 );
"