realloc and malloc and zero-sized objects

[Larry Jones]

In article <1989Apr5.000608.16582 at sq.com>, msb at sq.com (Mark Brader) writes:
> [Much interesting text deleted - see original if interester]
> 
> Since 0 is not a possible size for an object, the invocation malloc(0)
> falls under the general rule [for out-of-range arguments to
> library functions] and is undefined behavior.  It would of
> course be permissible for an implementation to define its behavior in
> this circumstance; this would be an extension to the language.

This would be true except for the statement in 4.10.3 Memory
Management Functions:

"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."

Thus, even though there ain't no such thing as a zero sized
object, malloc and friends are allowed to return non-null
pointers when given a size of zero.

> #  Returns
> #  The realloc function returns either a null pointer or a pointer to the
> #  possibly moved allocated space.
> 
> Note the either-or.  Since a null pointer cannot point to allocated space,
> this is really an exclusive or.  Since an object cannot have zero size
> and also because of the last sentence in the Description section, it is
> clear that realloc is *not* allocating any space if it is called as
> realloc(p,0) where p is a valid non-null pointer.  Therefore realloc *must*
> return a null pointer.

But the statement above gives realloc the liberty to allocate
space if it wants to, so whether it returns a null pointer or not
is implementation defined as I said originally.

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