To: Deborah Donovan
From: David R Tribble on Wed, Feb 11, 1998 12:47 AM
Subject: Comments on ISO/IEC CD 9899 (C9X)
Message-Id: <2.2.32.19980211030046.00f4e1fc@central.beasys.com>
Mime-Version: 1.0
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Date: Tue, 10 Feb 1998 21:00:46 -0600
To: ddonovan@itic.nw.dc.us
From: David R Tribble <david.tribble@central.beasys.com>
Subject: Comments on ISO/IEC CD 9899 (C9X)

Public Comment Number(s) PC-____ to PC-____

ISO/IEC CD 9899 (SC22N2620) Public Comment
===========================================

Date:               1998-02-10
Author:             David R. Tribble
Author Affiliation: Self
Postal Address:     6004 Cave River Dr.
                    Plano, TX 75093-6951
                    USA
E-mail Address:     dtribble@technologist.com
                    david.tribble@central.beasys.com
                    dtribble@flash.net
Telephone Number:   +1 972 738 6125, 16:00-00:00 UTC
                    +1 972 964 1729, 01:00-05:00 UTC
Fax Number:         +1 972 738 6111
Number of individual comments: 1

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Comment 1. 

Category: Normative change to existing feature retaining the original
          intent

Committee Draft subsection: 7.1.6.

Title: NULL macro type

Detailed description:

 Section 7.1.6 [#3] states that the standard macro 'NULL' expands to an
 "implementation-defined null pointer constant".

 I feel that the standard ought to restrict the use of the 'NULL' macro
 to pointer expressions only, i.e., make it illegal to use 'NULL' as an
 integer expression.  As defined currently, 'NULL' can be defined as
 '0' or '0L' as well as '((void*)0)'.  The former two are integer
 constants, allowing for the possibility of 'NULL' being used as an
 integer constant on some implementations.  Disallowing 'NULL' as an
 integer constant would make this dubious practice illegal.

Proposal:

 I propose the following change to [7.1.6 #3]:

    [#3]  The macros are

        NULL

    which expands to an implementation-defined null pointer constant of
    pointer type; and ...

 (The rest of the sentence is unchanged.)

Discussion:

 [1]
 The use of 'NULL' as an integer constant is an inappropriate
 programming practice.  Some examples are:

    int     i =  NULL;         // Should be 0
    char    ch = NULL;         // Should be '\0' or NUL
    memset(p, NULL, sz);       // Should be '\0' or 0
    if (ch > NULL) ...         // Should be '\0' or 0

 The 'NULL' macro ought to represent a pointer value, not simply a zero
 value.

 [2]
 Many implementations could define 'NULL' thus:

    #define NULL  ((void*)0)

 which meets the requirements above.  A few implementations use a
 representation other than "all bits zero" to represent a null pointer,
 so they could define 'NULL' as something like this:

    #define NULL  ((void*)0x80000000)    // ala IBM CICS

 The definition above does not restrict the type of 'NULL' to anything
 other than a pointer type; it is not required to be 'void*' per se,
 but could be whatever type the compiler vendor deems appropriate.  For
 example:

    #define NULL  __null           // Vendor A

    #define NULL  ((__notype*)0)   // Vendor B

 In these cases, '__null' and '__notype' are implementation-specific
 reserved words with special meanings.  (It is assumed that in both
 cases the type of 'NULL' is "implementation-specific pointer type".)

------------------------------------------------------------------------