(simple example) MPI parallel io writing garbage

问题

I am trying to make multiple processes write an integer buffer into a file at the same time using MPI parallel io. To achieve this goal I searched various websites:

MPI and Parallel IO
Google book search
Parallel IO with MPI

And I tried to learn their teachings. To test them i created the following simple code in c++:

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>

#define BUFSIZE 100

using namespace std;

int main(int argc, char *argv[])
{
    int myrank, buf[BUFSIZE], rcode;
    MPI_File thefile;

    MPI_Init(&argc, &argv);
    MPI_Comm_rank(MPI_COMM_WORLD, &myrank);

    for (int i=0; i<BUFSIZE; i++)
        buf[i] = myrank * BUFSIZE + i;

    rcode = MPI_File_open(MPI_COMM_WORLD, "testfile", MPI_MODE_CREATE | MPI_MODE_RDWR,
                                MPI_INFO_NULL, &thefile);

    if(rcode != MPI_SUCCESS){
        cerr << "Did not open file" << endl;
        return EXIT_FAILURE;
    }

    rcode = MPI_File_set_view(thefile, myrank * (MPI_Offset)BUFSIZE * sizeof(int), MPI_INT, MPI_INT,
                                        "native", MPI_INFO_NULL);
    if(rcode != MPI_SUCCESS){
        cerr << "Problem setting process view" << endl;
        return EXIT_FAILURE;
    }


    MPI_File_write(thefile, buf, BUFSIZE, MPI_INT, MPI_STATUS_IGNORE);
    if(rcode != MPI_SUCCESS){
        cerr << "Problem writting file" << endl;
        return EXIT_FAILURE;
    }

    MPI_File_close(&thefile);
    MPI_Finalize();

    return EXIT_SUCCESS;
}

However, when I try to read the file with Kate, I get random garbage: a bunch of squares, rectangles, !$"%&!!/ symbols and no integers at all.

What I am doing wrong?

回答1:

You seem to have a text/binary data misconception.

The ints written by MPI_File_write(..., MPI_INT,...) will be written as binary data, not ASCII or Unicode text. "A bunch of squares, rectangles, @#$@#! symbols" reads to me as if you are opening the file with a text editor and looking for ASCII numbers.

If you want formatted output, you have to format it yourself. If you want a text editor to read your file, you need to write text, not ints.

来源：https://stackoverflow.com/questions/15602521/simple-example-mpi-parallel-io-writing-garbage

标签

c++

mpi