You remind me. I now realize that it is not an matter of compiler, but an issue of C language. The printf() function in C doesn't print messages onto the standard ouput immediately, but instead stores them in a buffer. Only in some cases does the standard output work, defined in standard C:
1. The buffer is full or freshed compulsively (like use fflush()).
2. When a newline comes (\n).
3. Needs reading from buffer (such as scanf()).
Some compilers may deal with this function, and compiled by Mpich2 this code seems be well-off.
Thanks very much.
On Wed, Mar 30, 2011 at 3:39 PM, Pascal Deveze
<Pascal.Deveze@bull.net> wrote:
Maybe this could solve your problem: Just add \n in the string you want
to display:
printf("Please give N= \n");
Of course, this will return, but the string is displayed. This run by
me without the fflush().
On the other hand, do you really observe that the time of the scanf ()
and the time to enter "N" be insignificant ?
Pascal
Meilin Bai a écrit :
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@ias.edu> 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
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Meilin Bai
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Meilin Bai
School of Electronic Engineering and Computer Science, Peking University
Beijing 100871, China
E-Mail:
meilin.bai@gmail.com