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Subject: Re: [OMPI users] printf and scanf problem of C code compiled with Open MPI
From: Meilin Bai (meilin.bai_at_[hidden])
Date: 2011-03-29 22:27:28


So it means that MPI doesn't suit to interactive programming? Though we can
really use fflush(stdout) to get the right order, it takes too more time,
and it's said that using fflush() is not a good progrmming style in C.

On the other hand, in Fortran language, this situation won't exist. Maybe it
is because I/O implement is a built-in part of Fortran, while in C/C++ it is
realized only through function like scanf, printf, et al?

On Wed, Mar 30, 2011 at 2:38 AM, Prentice Bisbal <prentice_at_[hidden]> wrote:

> On 03/29/2011 01:29 PM, Meilin Bai wrote:
> > Dear open-mpi users:
> >
> > I come across a little problem when running a MPI C program compiled
> > with Open MPI 1.4.3. A part of codes as follows:
> >
> > MPI_Init(&argc, &argv);
> > MPI_Comm_size(MPI_COMM_WORLD, &numprocs);
> > MPI_Comm_rank(MPI_COMM_WORLD, &myid);
> > MPI_Get_processor_name(processor_name, &namelen);
> > if (myid == 0) {
> > printf("Please give N= ");
> > //fflush(stdout);
> > scanf("%d", &n);
> > startwtime = MPI_Wtime();
> > }
> >
> > If comment out the sentence of "fflush(stdout);", it doesn't print out
> > the message till I input an integer n. And if I add the fflush function
> > between them, it works as expected, though comsumming time obviously.
> >
> > However, when I compiled it with Mpich2-1.3.2p1, without fflush function
> > in the code, it works correctly.
> >
> > Can anyone know what the matter is.
> >
>
> The Open MPI Developers (Jeff, Ralph, etc) can confirm this:
>
> The MPI standard doesn't have a lot of strict requirements for I/O
> behavior like this, so implementations are allowed to buffer I/O if they
> want. There is nothing wrong with requiring fflush(stdout) in order to
> get the behavior you want. In fact, if you check some text books on MPI
> programming, I'm pretty sure they recommend using fflush to minimize
> this problem.
>
> MPICH behaves differently because its developers made different design
> choices.
>
> Neither behavior is "wrong".
>
> --
> Prentice
> _______________________________________________
> users mailing list
> users_at_[hidden]
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>

-- 
Meilin Bai