Issue 0064: What is a null pointer constant?

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

Authors: Clive Feather, WG14
Date: 1993-12-03
Submitted against: C90
Status: Closed
Converted from: dr.htm, dr_064.html

Item 1 - Null pointer constants

Consider the following translation unit:

char *f1 (int i, int *pi)
        {
        *pi = i;
        return 0;
        }

 char *f2 (int i, int *pi)
        {
        return (*pi = i, 0);
        }

In f1, the 0 is a null pointer constant (subclause 6.2.2.3). Since return acts as if by assignment (subclause 6.6.6.4) the function is strictly conforming.

In f2, the 0 is a null pointer constant. However, a constant expression cannot contain a comma operator (subclause 6.4), and so the expression being returned is not a null pointer constant per se. Which of the following is the case?

1. The property of being a null pointer constant percolates upwards through an expression, and the function f2 is strictly conforming.
2. The property of being a null pointer constant does not percolate upwards, and the expression being notionally assigned in the return statement, though of value zero, is not a null pointer constant but only of type int, thus violating a constraint (subclause 6.3.16.1).

Comment from WG14 on 1997-09-23:

Response

Function f2 is not strictly conforming, because it violates a constraint for simple assignment (which applies to converting the type of the return expression), because the return expression is not a null pointer constant.