(j3.2006) MPI usage problems in Fortran

[Craig Rasmussen]

On Mar 19, 2008, at 12:20 PM, Bill Long wrote:

>>
>>> 1. VALUE and ASYNCHRONOUS lead to a compile error (is this correct?)
>>>
>> Yes it is correct, but don't ask me why.
>>
>
> Right, it is a constraint violation.  And it does make sense.  The  
> point
> of giving a dummy variable the asynchronous attribute is to make sure
> that the physical memory referenced through it is the same memory  
> as is
> associated with the corresponding actual argument.  That's because an
> async I/O operation initiated in the subprogram might not finish  
> before
> control is transferred back to the caller, and the I/O libraries  
> need a
> stable target in memory for the data transfers throughout the I/O
> operation.  The VALUE attribute has essentially the opposite  
> effect. It,
> at least conceptually, implies that the dummy is associated with a
> memory location local to the subprogram that gets its initial value  
> from
> the caller, but from that point on is disassociated from the
> corresponding actual argument. In particular, any changes made to the
> value in the local memory location are thrown away on return.

But in this case the variable type was type(C_PTR).  I was passing by  
value to get to a C routine (I believe it doesn't work otherwise, but  
I should check again).  I don't really care about the memory address  
of the C_PTR itself, the asynchronous aspect of the operation refers  
to the memory referenced by the C_PTR.  So I certainly see your  
point, it's just that I don't think it should apply in this instance.

Regards,
Craig

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