[Mpi-22] Ticket #46: Addconst Keyword to the C bindings- implementation

[Jeff Squyres]

Do you end up casting away the const when invoking the Windows network  
interfaces?

On Linux, I think the following interfaces would require casting the  
const away:

- shared memory (even using Linux vmsplice())
- MX
- TCP
- OpenFabrics (both verbs and udapl)

These interfaces do not require casting away const:

- SCTP (I *think* -- I'm not an SCTP expert, but a quick look in  
sctp.h showed a "const" argument for the sctp_sendmsg() function)

I can't check these offhand:

- Portals
- Elan

On Mar 12, 2009, at 6:06 PM, Erez Haba wrote:

> I urge you, please read the ticket proposal again. It discusses the  
> merit of this proposal.
>
> As for IB; yes you'd copy small messages (which const is fine with)  
> and some implement RDMA read or write for large messages; you might  
> need to cast the const away if your IB interface does not expose  
> const SGE's.
>
> As for the compiler; no you are not violating the contact;  
> (otherwise we wouldn't have casts); as long as you guarantee that  
> you are not changing the buffer. It's a way for the programmer to  
> use old, non const correct, interfaces.
>
> .Erez
>
> -----Original Message-----
> From: mpi-22-bounces_at_[hidden] [mailto:mpi-22-bounces_at_[hidden] 
> ] On Behalf Of Underwood, Keith D
> Sent: Thursday, March 12, 2009 2:03 PM
> To: MPI 2.2
> Subject: Re: [Mpi-22] Ticket #46: Add const Keyword to the C  
> bindings- implementation
>
> I didn't think that typical MPI implementations over IB copied the  
> data for large messages (as opposed to the bounce buffers for small  
> messages).
>
> If you are going to have to cast away the const, what is the point  
> of putting it in?  In fact, if you have to cast away the const,  
> aren't you violating your contract with the compiler?  Should this  
> change be rolled out from the bottom up?  i.e. common low-level APIs  
> first and then MPI?
>
> Keith
>
> >-----Original Message-----
> >From: mpi-22-bounces_at_[hidden] [mailto:mpi-22-bounces_at_lists.mpi-
> >forum.org] On Behalf Of Erez Haba
> >Sent: Thursday, March 12, 2009 2:10 PM
> >To: MPI 2.2
> >Subject: Re: [Mpi-22] Ticket #46: Add const Keyword to the C  
> bindings-
> >implementation
> >
> >Possibly you'll have to cast.
> >There are few interfaces that take const for send buffer (like  
> sockets).
> >In other cases you copy the buffer (like with IB) which keeps the  
> const
> >property.
> >
> >Thanks,
> >.Erez
> >
> >-----Original Message-----
> >From: mpi-22-bounces_at_[hidden] [mailto:mpi-22-bounces_at_lists.mpi-
> >forum.org] On Behalf Of Underwood, Keith D
> >Sent: Thursday, March 12, 2009 12:05 PM
> >To: MPI 2.2
> >Subject: Re: [Mpi-22] Ticket #46: Add const Keyword to the C  
> bindings-
> >implementation
> >
> >It would seem like const would eventually have to be cast away for  
> most
> >lower level interfaces.  Portals, Elan, IB Verbs, and as best I can  
> tell MX
> >don't have const on their send buffers...
> >
> >Keith
> >
> >>-----Original Message-----
> >>From: mpi-22-bounces_at_[hidden] [mailto:mpi-22-bounces_at_lists.mpi-
> >>forum.org] On Behalf Of Erez Haba
> >>Sent: Thursday, March 12, 2009 12:44 PM
> >>To: MPI 2.2
> >>Subject: Re: [Mpi-22] Ticket #46: Add const Keyword to the C  
> bindings-
> >>implementation
> >>
> >>No, it's not being cast away (although it is a quick way for you to
> >>implement it).
> >>In MPICH2 implementation it trickles down to the lower functions.  
> There
> >are
> >>very few places where the const is being cast away because ticket  
> #46 does
> >>not to break backward compatibility.
> >>For example, when the user function is called (const was not added  
> to the
> >>MPI_User_function) the implementation must cast away const from  
> the send
> >>buffer before calling the user function.
> >>
> >>Thanks,
> >>.Erez
> >>
> >>-----Original Message-----
> >>From: mpi-22-bounces_at_[hidden] [mailto:mpi-22-bounces_at_lists.mpi-
> >>forum.org] On Behalf Of Jeff Squyres
> >>Sent: Thursday, March 12, 2009 11:34 AM
> >>To: MPI 2.2
> >>Subject: Re: [Mpi-22] Ticket #46: Add const Keyword to the C  
> bindings-
> >>implementation
> >>
> >>More importantly, how is const handled?  Is it just cast away inside
> >>the MPI library?
> >>
> >>
> >>On Mar 12, 2009, at 2:34 PM, Underwood, Keith D wrote:
> >>
> >>> Since the main purpose of an implementation for something like
> >>> adding const is to see how invasive the change is, would it be
> >>> possible to get a diff against the trees that these were created  
> from?
> >>>
> >>> Thanks,
> >>> Keith
> >>>
> >>> From: mpi-22-bounces_at_[hidden] [mailto:mpi-22-
> >>bounces_at_[hidden]
> >>> ] On Behalf Of Erez Haba
> >>> Sent: Thursday, March 12, 2009 12:24 PM
> >>> To: MPI 2.2
> >>> Subject: [Mpi-22] Ticket #46: Add const Keyword to the C  
> bindings -
> >>> implementation
> >>>
> >>>
> >>> Implementation for ticket #46 is now available from ANL (thanks  
> Pavan)
> >>>
> >>>
> >>> tarballs
> >>>
> >>http://www.mcs.anl.gov/research/projects/mpich2/downloads/tarballs/nightly
> >/
> >>const
> >>>
> >>>
> >>> thanks,
> >>> .Erez
> >>>
> >>> _______________________________________________
> >>> mpi-22 mailing list
> >>> mpi-22_at_[hidden]
> >>> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi-22
> >>
> >>
> >>--
> >>Jeff Squyres
> >>Cisco Systems
> >>
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> >>
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-- 
Jeff Squyres
Cisco Systems