[petsc-dev] PetscMalloc for size zero

[Matthew Knepley]

On Thu, Jan 30, 2014 at 12:50 PM, Matthew Knepley <knepley at gmail.com> wrote:

> On Thu, Jan 30, 2014 at 12:46 PM, Brad Aagaard <baagaard at usgs.gov> wrote:
>
>> On 01/30/2014 10:34 AM, Jed Brown wrote:
>>
>>> Brad Aagaard <baagaard at usgs.gov> writes:
>>>
>>>  Matt and Jed,
>>>>
>>>> I see that Jed pushed some changes (jed/malloc-zero) for PetscMalloc to
>>>> deal with memory alignment and a zero size. It looks like the pointer
>>>> will NOT be NULL for a size of 0. Is this true?
>>>>
>>>
>>> Yes, just like malloc(), it can be either a unique pointer or NULL.  You
>>> need the size anyway to know how many elements are in the array.
>>>
>>
>> I thought it was a nice feature that PETSc improved on malloc() and
>> free() by returning NULL for zero sized allocation (although this wasn't
>> true for --with-debugging=0 due to memory alignment) and set pointers to
>> NULL after freeing.
>>
>> What is the rationale for not returning NULL for mallocs of size zero
>> other than conforming to C malloc behavior?
>
>
> There is no such thing as "conforming to C malloc", since that WOULD allow
> returning NULL in this case.
>

Actually, we are violating the malloc standard that Jed sent out. It says
that for 0 size, malloc must return

  NULL or a unique pointer

However, if we have

  PetscMalloc2(0,&r1,1,&r2)

then r1 == r2 with the optimized implementation, and the pointers are not
unique.

   Matt


> I think we are doing users a disservice by not enforcing this.
>
>    Matt
>
>
>>
>> Brad
>>
>>
>>
>
>
> --
> What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> -- Norbert Wiener
>



-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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