Issue 0263: all-zero bits representations

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: Clive D.W. Feather <clive@demon.net>, UK C Panel
Date: 2001-09-07
Submitted against: C99
Status: Fixed
Fixed in: C99 TC2
Converted from: summary-c99.htm, dr_263.htm

Problem

Consider the code:

   int v [10];
    memset (v, 0, sizeof v);

Most programmers would expect this code to set all the elements of v to zero. However, the code is actually undefined: it is possible for int to have a representation in which all-bits-zero is a trap representation (for example, if there is an odd-parity bit in the value).

Consider also:

   int *p;
    p = calloc (n_members, sizeof (int));

This problem applies to all integer types except for unsigned char. I believe that the idiom is well-enough known that it should be made a part of the Standard.

Suggested Technical Corrigendum

Append to 6.2.6.2#5:

For any integer type, the object representation where all the bits are zero shall be a representation of the value zero in that type.

Comment from WG14 on 2002-03-07:

Technical Corrigendum

Append to 6.2.6.2#5:

For any integer type, the object representation where all the bits are zero shall be a representation of the value zero in that type.