Defect Report #155

Submission Date: 16 Oct 95
Submittor: BSI
Source: Clive D.W. Feather
Question
Submitted to BSI by Clive D.W. Feather [email protected].
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 003: Zero-sized allocations
The use of the word "unique" in subclause 7.10.3 is ambiguous, and the handling of zero size allocations is incomplete.
Part 1
Subclause 7.10.3 reads
If the size of the space requested is zero, the behavior is implementation-defined; the value returned shall be either a null pointer or a unique pointer.
Does the term "unique" mean "different every time," or does it mean "there is a single pointer returned by all calls with size zero" (as might be presumed from the ordinary dictionary definition of "unique") ?
In other words, if malloc(0) does not return a null pointer, is the following expression:
malloc(0) == malloc(0)
always zero, always non-zero, or implementation-defined ?
Part 2
If unique means "there is a single pointer," what is the result of attempting to free that pointer? How does the wording of 7.10.3 apply:
The value of a pointer that refers to freed space is indeterminate.
Possibly nothing happens, because the pointer does not really point to a block of memory. In that case, is the following code strictly conforming?
#include <stdlib.h> /* ..... */ void *p = malloc(0); if (p != NULL) { free (p); /* Line A */ free (p); /* Line B */ }
What is the behavior if each of lines A and B are reached?
Part 3
If "unique" means "different every time," then each such call still consumes address space, even though no storage actually needs to be allocated, and therefore the call can fail due to exhaustion of memory. Thus malloc(0) can return a null pointer, while the C Standard seems to suggest that an implementation can return either null pointers or unique pointers, but not both. This is a defect in the existing wording.
Suggested Technical Corrigendum:
If "unique" means "there is a single pointer," then change the penultimate sentence of 7.10.3 from:
If the size of the space requested is zero, the behavior is implementation-defined; the value returned shall be either a null pointer or a unique pointer.
to:
If the size of the space requested is zero, the behavior is implementation-defined; the value returned shall be either a null pointer or a unique pointer. The values returned by two zero-length allocations shall compare equal. Freeing the value returned by a zero-length allocation shall have no effect. If that value is used as an operand of the unary * operator, or of a + or - operator except one whose other operand has integral type and value zero, the behavior is undefined.
If "unique" means "different every time," then change it to:
If the size of the space requested is zero, the behavior is implementation-defined; either a null pointer is always returned, or the behavior is as if the size were some unspecified non-zero value. In the latter case, if the returned pointer is not a null pointer and is used as an operand of the unary * operator, or of a + or - operator except one whose other operand has integral type and value zero, the behavior is undefined.
[See also Defect Report #158.]
Technical Corrigendum
In subclause 7.10.3, change the next to last sentence to read:
If the size of the space requested is zero, the behavior is implementation defined: either a null pointer is returned, or the behavior is as if the size were some unspecified nonzero value, except that the returned pointer shall not be used to access an object.
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