[Mpi3-bwcompat] MPI_Count mixing with int...

[Fab Tillier]

Rajeev Thakur wrote on Thu, 8 Jul 2010 at 18:24:16

> You can define a filetype that is many Gbytes and read only a small part
> of it. The filetype is specified in MPI_File_set_view and is not passed
> in the read/write calls. The datatype passed in the read/write calls
> refers to the buffer passed to that call.

Perfect, thanks for the example!  Does this imply that MPI_Type_get_extent, which returns an MPI_Aint, might overflow the extent parameter on a 32-bit platform on which a user defines a many Gbyte filetype?  How should we address this (there are probably other APIs affected similarly)?

-Fab 

>> -----Original Message-----
>> From: Fab Tillier
>> Sent: Thursday, July 08, 2010 3:32 PM
>> 
>> Rajeev Thakur wrote on Thu, 8 Jul 2010 at 12:52:58
>> 
>>>> As an aside, can count ever be larger than size_t?
>>> 
>>> Probably not in the communication or I/O functions since count refers
>>> to the number of items in the memory buffer, but it could be for the
>>> datatype constructor functions since datatypes can
>> represent file layout.
>> 
>> How would you use a datatype that is larger than your address
>> space?  You can't read or write it, you can't send or receive
>> it, you can't pack/unpack it...
>> 
>> Am I missing something and a datatype like this would be useful?
>> 
>> -Fab