Defect Report #158

Submission Date: 16 Oct 95
Submittor: BSI
Source: Clive D.W. Feather
Question
Submitted to BSI by Clive D.W. Feather clive@sco.com.
In this Defect Report, identifiers lexically identical to those declared in standard headers refer to the identifiers declared in those standard headers, whether or not the header is explicitly mentioned.
This Defect Report has been prepared with considerable help from Mark Brader, Jutta Degener, Ronald Guilmette, and a person whose employment conditions require anonymity. However, except where stated, opinions expressed or implied should not be assumed to be those of any person other than myself.
Defect Report UK 006: Null pointer conversions
The C Standard does not define semantics for the explicit conversion of null pointer constants and for the implicit conversion of null pointers.
Subclause 6.2.2.3 reads in part:
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. Such a pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.
Two null pointers, converted through possibly different sequences of casts to pointer types, shall compare equal.
Given the definitions:
void * p = 0; int * i = 0;
does the standard guarantee that the expression p == i always evaluates to 1? The last quoted sentence only covers casts, and not the implicit conversions of that comparison. Conversely, do the expressions (int *)0 or 1 ? 0 : (int *)0 yield null pointers of type (int *)? The quoted text does not cover the case of a null pointer constant being converted other than by assignment or in a test for equality, yet expressions such as these are widely used.
Suggested Technical Corrigendum:
In subclause 6.2.2.3, change:

Two null pointers, converted through possibly different sequences of casts to pointer types, shall compare equal.

to:

Conversion of a null pointer to another pointer type yields a null pointer of that type. Any two null pointers shall compare equal.

Alternatively, a common term could be introduced to more conveniently describe the various forms of pointer that cannot be dereferenced. In this case, replace the last two paragraphs of subclause 6.2.2.3 with:

For each pointer type, there exist values which can participate in assignment and equality operations, but which cause undefined behavior if dereferenced. These are referred to as undereferenceable. An undereferenceable pointer compares unequal to any other value of the same pointer type. For each pointer type, one particular undereferenceable pointer value is called the null pointer. [Footnote: Since there is only one such value, all null pointers of the same type compare equal.]

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 with an object of pointer type, or cast to pointer type, then it is converted to the null pointer of that type. Conversion of a null pointer to another pointer type produces the null pointer of that type.

If the answer to Defect Report #155 is that unique means different each time, then replace the last two sentences of subclause 7.10.3 with:

If the size of the space requested is zero, an undereferenceable pointer is returned. It is implementation-defined whether this is always a null pointer or whether the implementation attempts to produce a value distinct from any other undereferenceable pointer. Any pointer value returned by an allocation can be passed to the free function; if the value is not a null pointer, it becomes indeterminate. [Footnote: A subsequent allocation may return a pointer value with the same bit pattern, but a strictly conforming program can't detect this.] The value of a pointer that refers to any part of a freed object is also indeterminate.

Response

Does the Standard guarantee that p == i? Yes.

Does (int *) 0 yield a null pointer of type int *? Yes.

Does 1 ? 0 : (int *) 0 yield a null pointer of type int *? Yes.

The intent of the part of Subclause 6.2.2.3 that you quote is to state that a null pointer results when a null pointer constant is converted to a pointer. By singling out assignment and comparison, the Standard is misleadingly specific. This should be clarified in the Standard.

Future Change

In Subclause 6.2.2.3
Change

"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. Such a pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.

Two null pointers, converted through possibly different sequences of casts to pointer types, shall compare equal."

"The result of converting a null pointer constant to a pointer type is called a null pointer. A null pointer is guaranteed to compare unequal to a pointer to any object or function.
Conversion of a null pointer to another pointer type yields a null pointer of that type. Any two null pointers shall compare equal."

In Subclause 6.3.9
Add a new paragraph after the first paragraph in Semantics

"If a null pointer constant is compared for equality to a pointer, the constant is converted to the type of the pointer."

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