RFR(xs): 8168542: os::realloc should return a valid pointer for input size=0

[David Holmes]

Hi Thomas,

On 24/10/2016 11:12 PM, Thomas Stüfe wrote:
> Dear all,
>
> please check this tiny bug fix.
>
> Bug report:
> https://bugs.openjdk.java.net/browse/JDK-8168542
>
> Webrev:
> http://cr.openjdk.java.net/~stuefe/webrevs/8168542-os_realloc_size_0/webrev.00/webrev/
>
> In short, this fixes a corner case for os::realloc() which currently
> returns NULL if input size is zero.
>
> But as we have coding which interprets a return value of NULL as OOM (See
> ReallocateHeap()), this is not a good solution. It is also inconsistent
> with how os::malloc() deals with the same situation and potentially with
> the way the native C-Runtime deals with it (currently, in a debug build we
> will return NULL in case of size=0 whereas in the release build we just
> call the native ::realloc() and return whatever it returns.)

Sorry but I do not like this. A native realloc with a size of zero and a 
non-NULL ptr acts like free(ptr). Our realloc does not do that. A native 
malloc that receives a size of zero "returns either NULL, or a unique 
pointer value that can later be successfully passed to free()". Our 
os::malloc returns 1 - and I see nothing that indicates that can 
successfully be passed to os::free.

So while the current handling of size==0 is a bit inconsistent and 
unclear, it is even less clear that returning 1 is a reasonable thing to do.

To me passing a size of zero (unless expecting it to act like a free!) 
is a bug that should be handled in the caller.

I welcome opinions from others on this.

David

PS. I will be traveling soon and unable to respond to emails until 
Wednesday afternoon at the earliest.

> Thank you,
>
> Thomas
>