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Date: Fri, 26 Mar 1999 07:47:53 -0600
To: Nicholas Nevin - Sun HPC High Performance Computing <nevin@XXXXXXXXXXXXXXXXX>
From: William Gropp <gropp@XXXXXXXXXXX>
Subject: Re: clarification on inter-language interoperability of
  attributes
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At 04:37 PM 3/25/99 -0500, Nicholas Nevin - Sun HPC High Performance
Computing wrote:
>
>Hi, All. I need some clarification on the behaviour of MPI_Comm_get_attr
>in the case of an architecture with 32 bit ints and 64 bit pointers.
>
>Section 4.12.7 of the MPI-2 standard states that MPI will store
>internally *address* sized attributes. It also states that an integer
>valued attribute when accessed from C via MPI_Comm_get_attr will return
>the address of a pointer to the integer valued attribute. 
>
>This seems to imply that MPI_Comm_get_attr will return a pointer to
>a 64 bit quantity. This however will break codes which do this.

No, it means that the attribute is stored as the address itself.  The thing
that the attribute points to may be an int, a long long, or a structure.
That's why an MPI_TAG_UB is returned as the address of an int.  We've been
doing this for a long time with MPICH on many architectures with 32 bit C
ints and 64 bit pointers (it is one of the reasons why Get_attr returns a
pointer to the integer containing the tag UB, rather than the value of the
UB itself).

Bill