Public Comment Number PC-UK0170

ISO/IEC CD 9899 (SC22N2620) Public Comment

=========================================== 

Date: 1998-03-26
Author: N.M Maclaren
Author Affiliation: Self
Postal Address:
    University of Cambridge,
    Computer Laboratory,
    New Museums Site,
    Pembroke Street,
    Cambridge CB3 3QG,
    United Kingdom
E-mail Address: <nmm1@cam.ac.uk>
Telephone Number: +44 1223 334761
Fax Number:       +44 1223 334679
Number of individual comments: 1


Comment 1. 

Category: Request for information/clarification

Committee Draft subsection: 5.1.2.3

Title: Ordering of sequence points


Detailed description:

Paragraphs 2, 4 and 6 refer to the previous and next (or subsequent)
sequence points, but a expression parse tree does not have a canonical
ordering.  Statements do, but they are irrelevant in this context.  This
has caused considerable confusion with C89 in a fair number of obscure
but realistic constructions, as well in perverse ones like the
following:

    (a,++b,a)+(a,++b,a)

I suggest adding a paragraph saying something like:

    The ordering of sequence points within an expression is only
    partially specified by the syntax rules and operator precedences,
    and an implementation may reorder expressions in any way that is
    compatible with those.  A strictly conforming program shall not rely
    on any ordering of sequence points that is not required by the
    syntax rules and operator precedences.

I believe that this is normative in the sense that it makes it clear
that relying on any parse ordering is not strictly conforming, but that
is probably what was always meant.