Defect Report #112
Submission Date: 03 Dec 93
Submittor: WG14
Source: Ron Guilmette
Question
ANSI/ISO C Defect report #rfg19:
Subject: Null pointer constants and relational comparisons.
a) Does the following code involve usage which requires
a diagnostic from a conforming implementation?
b) Does the following code involve usage which renders
the code itself not strictly conforming?
void test (void *vp)
{
(vp > (void*)0); /* ? */
}
Background:
Subclause 6.2.2.3:
An integral constant expression with the value 0, or such an
expression cast to type void *,
is called a null pointer constant.
If a null pointer constant is assigned to or compared for equality
to a pointer, the constant is converted to a pointer of that type.
This last paragraph of subclause 6.2.2.3 seems to suggest
that zero-valued integral constant expressions
which are cast to void *
(and then called null pointer constants) can only be used in
assignments and/or equality comparisons, but not in relational comparisons.
(It was probably the Committee's intent to permit such expression
to be used in all ways, and in all contexts where any other
kind of void *
non-lvalued expressions can be used, but the current wording of subclause
6.2.2.3 does not make that fact altogether apparent and unambiguous.)
Response
The code does not require a diagnostic but has undefined behavior,
so it renders the translation unit not strictly conforming.
Subclause 6.3.8 makes clear that this behavior is undefined.
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