NULL be defined as 4-4?This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.
Authors: Robert Paul Corbett, WG14
Date: 1992-12-10
Reference document: X3J11/92-004
Submitted against: C90
Status: Fixed
Fixed in: C90 TC1
Converted from: dr.htm, dr_043.html
Defining NULL
Subclause 7.1.6 defines NULL to be a macro “which expands to an
implementation-defined null pointer constant.” Subclause 6.2.2.3 defines a null
pointer constant to be “an integral constant expression with the value 0, or
such an expression cast to type void *.” The expression 4-4 is an integral
constant expression with the value 0. Therefore, Standard C appears to permit
#define NULL 4 - 4 as one of the ways NULL can be defined in the standard
headers. By allowing such a definition, Standard C forces programmers to
parenthesize NULL in several contexts if they wish to ensure portability. For
example, when NULL is cast to a pointer type, NULL must be parenthesized in
the cast expression.
At least one book about Standard C suggests defining NULL as
#define NULL (void *) 0 That definition leads to a subtler version of the
problem described above. Consider the expression NULL[p], where p is an
array of pointers. The expression expands to (void *)0[p] which is equivalent
to (void *)(p[0]). I doubt many users would expect such a result.
Have I correctly understood Standard C's requirements regarding NULL? If not,
what are those requirements?
Comment from WG14 on 1997-09-23:
Add to subclause 7.1.2, page 96 (before Forward references):
Any definition of an object-like macro described in this clause shall expand to code that is fully protected by parentheses where necessary, so that it groups in an arbitrary expression as if it were a single identifier.