DeinoMPI

The Great and Terrible implementation of MPI-2

MPI_File_iread

Nonblocking read using individual file pointer

int MPI_File_iread(
  MPI_File mpi_fh,
  void *buf,
  int count,
  MPI_Datatype datatype,
  MPI_Request *request
);

Parameters

mpi_fh: [in] file handle (handle)
buf: [out] initial address of buffer (choice)
count: [in] number of elements in buffer (nonnegative integer)
datatype: [in] datatype of each buffer element (handle)
request: [out] request object (handle)

Remarks

MPI_FILE_IREAD is a nonblocking version of the MPI_FILE_READ interface.

MPI maintains one individual file pointer per process per file handle. The current value of this pointer implicitly specifies the offset in the data access routines described in this section. These routines only use and update the individual file pointers maintained by MPI. The shared file pointer is not used nor updated.

The individual file pointer routines have the same semantics as the data access with explicit offset routines with the following modification:

the offset is defined to be the current value of the MPI-maintained individual file pointer.

After an individual file pointer operation is initiated, the individual file pointer is updated to point to the next etype after the last one that will be accessed. The file pointer is updated relative to the current view of the file.

If MPI_MODE_SEQUENTIAL mode was specified when the file was opened, it is erroneous to call the routine.

Example Code

The following sample code illustrates MPI_File_iread.

#include "mpi.h"
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#define SIZE (65536)
/* Uses asynchronous I/O. Each process writes to separate files and
reads them back. The file name is taken as a command-line argument,
and the process rank is appended to it.*/

int main(int argc, char **argv)
{
    int *buf, i, rank, nints, len;
    char *filename, *tmp;
    int errs=0;
    MPI_File fh;
    MPI_Status status;
    MPI_Request request;

    MPI_Init(&argc,&argv);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    /* process 0 takes the file name as a command-line argument and broadcasts it to other processes */
    if (rank == 0)
    {
        i = 1;
        while ((i < argc) && strcmp("-fname", *argv)) {
            i++;
            argv++;
        }
        if (i >= argc) {
            /* Use a default filename of testfile */
            len = 8;
            filename = (char *)malloc(len + 10);
            strcpy( filename, "testfile" );
        }
        else {
            argv++;
            len = (int)strlen(*argv);
            filename = (char *) malloc(len+10);
            strcpy(filename, *argv);
        }
        MPI_Bcast(&len, 1, MPI_INT, 0, MPI_COMM_WORLD);
        MPI_Bcast(filename, len+10, MPI_CHAR, 0, MPI_COMM_WORLD);
    }
    else {
        MPI_Bcast(&len, 1, MPI_INT, 0, MPI_COMM_WORLD);
        filename = (char *) malloc(len+10);
        MPI_Bcast(filename, len+10, MPI_CHAR, 0, MPI_COMM_WORLD);
    }

    buf = (int *) malloc(SIZE);
    nints = SIZE/sizeof(int);
    for (i=0; i<nints; i++) buf[i] = rank*100000 + i;

    /* each process opens a separate file called filename.'myrank' */
    tmp = (char *) malloc(len+10);
    strcpy(tmp, filename);
    sprintf(filename, "%s.%d", tmp, rank);
    MPI_File_open(MPI_COMM_SELF, filename, MPI_MODE_CREATE | MPI_MODE_RDWR, MPI_INFO_NULL, &fh);
    MPI_File_set_view(fh, 0, MPI_INT, MPI_INT, "native", MPI_INFO_NULL);
    MPI_File_iwrite(fh, buf, nints, MPI_INT, &request);
    MPI_Wait( &request, &status );
    MPI_File_close(&fh);

    /* reopen the file and read the data back */
    for (i=0; i<nints; i++) buf[i] = 0;
    MPI_File_open(MPI_COMM_SELF, filename, MPI_MODE_CREATE | MPI_MODE_RDWR, MPI_INFO_NULL, &fh);
    MPI_File_set_view(fh, 0, MPI_INT, MPI_INT, "native", MPI_INFO_NULL);
    MPI_File_iread(fh, buf, nints, MPI_INT, &request);
    MPI_Wait( &request, &status );
    MPI_File_close(&fh);

    /* check if the data read is correct */
    for (i=0; i<nints; i++) {
        if (buf[i] != (rank*100000 + i)) {
            errs++;
            fprintf(stderr, "Process %d: error, read %d, should be %d\n", rank, buf[i], rank*100000+i);fflush(stderr);
        }
    }
    free(buf);
    free(filename);
    free(tmp);
    MPI_Finalize();
    return errs;
}

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