Defect Report #155
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 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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