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Subject: Re: [OMPI users] printf and scanf problem of C code compiled with Open MPI
From: Prentice Bisbal (prentice_at_[hidden])
Date: 2011-03-29 14:38:10

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

Neither behavior is "wrong".