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Subject: Re: [OMPI devel] RFC: add support for large counts using derived datatypes
From: George Bosilca (bosilca_at_[hidden])
Date: 2013-07-16 17:08:32


On Jul 16, 2013, at 23:03 , Nathan Hjelm <hjelmn_at_[hidden]> wrote:

> On Tue, Jul 16, 2013 at 10:22:33PM +0200, George Bosilca wrote:
>> Nathan,
>>
>> I read your code and it's definitively looking good. I have however few minor issues with your patch.
>>
>> 1. MPI_Aint is unsigned as it must represent the difference between two memory arbitrary locations. In your MPI_Type_get_[true_]extent_x you go through size_t possibly reducing it's extent. I would suggest you used ssize_t instead.
>> 2. In several other locations size_t is used as a conversion base. In some of these location there is even a comment talking about ssize_t ?
>
> I looked at the code in question and there shouldn't be an issue. Where we want to return MPI_Aint it is never converted to a size_t. The size_t is to ensure that if we return an MPI_Count that the value is never larger than SSIZE_MAX or negative. Am I wrong in assuming MPI_Count can never be negative?

Based on the standard it is both a size and a displacement (including relative) in a file, so my understanding is that it can be negative.

  George.

> If so I can change the checks in MPI_Type_get_[true_]_extent_x to not loose this value.
>
> The other places that use size_t (MPI_Get_elements for example) are in places where I beleive the value will never legally be negative so it is safe to assume the returned value should be MPI_UNDEFINED in those cases. Is there a particular case I should look at?
>
> -Nathan
>
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