Public Comment Number PC-UK0266

ISO/IEC CD2 9899 (SC22N2794) Public Comment

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

Date: 1998-09-22
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

Category: Normative change to existing feature retaining the original intent
Committee Draft subsection: 6.2.6.2, 6.4.4.1 etc.

Title: Problems with multiple zero representations etc.

Detailed description:

PC-UK0122 and other public comments raised some serious problems with
multiple representations of zero, but the FCD has addressed only some of
them.  The remaining comments in PC-UK0122 have been separated out, and
this is one of them.

Return and initial values of zero and unspecified non-zero are defined
throughout the standard, but the standard now defines two forms of zero for
signed magnitude and ones' complement, which causes serious ambiguity and
conflicts.  Signed integer values are commonly used to hold flag bits,
and this will stop working if the constant zero can have all of its bits
set (as is possible for ones' complement arithmetic.)

While no function in the C standard has an int value that may be either
zero or used for flag bits, this is a common convention, and existing
practice is that such functions return the value of all bits zero for
the value of zero.

The following should be added to 6.2.6.2:

    When other sections of this standard requires a function call to
    return zero, a constant to have value zero, or an expression to
    evaluate to zero, the implementation shall use the value consisting
    of all bits zero.