DeinoMPI

The Great and Terrible implementation of MPI-2

MPI_File_read

Read using individual file pointer

int MPI_File_read(
  MPI_File mpi_fh,
  void *buf,
  int count,
  MPI_Datatype datatype,
  MPI_Status *status
);

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)
status: [out] status object (Status)

Remarks

MPI_FILE_READ reads a file using the individual file pointer.

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 this routine.

Example Code

The following sample code illustrates MPI_File_read.

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

/* Test reading and writing zero bytes (set status correctly) */

int main( int argc, char *argv[] )
{
    int errs = 0;
    int size, rank, i, *buf, count;
    MPI_File fh;
    MPI_Comm comm;
    MPI_Status status;

    MPI_Init( &argc, &argv );

    comm = MPI_COMM_WORLD;
    MPI_File_open( comm, "test.ord", MPI_MODE_RDWR | MPI_MODE_CREATE | MPI_MODE_DELETE_ON_CLOSE, MPI_INFO_NULL, &fh );
    MPI_Comm_size( comm, &size );
    MPI_Comm_rank( comm, &rank );
    buf = (int *)malloc( size * sizeof(int) );
    buf[0] = rank;

    /* Write to file */
    MPI_File_write_ordered( fh, buf, 1, MPI_INT, &status );
    MPI_Get_count( &status, MPI_INT, &count );
    if (count != 1) {
        errs++;
        fprintf( stderr, "Wrong count (%d) on write-ordered\n", count );fflush(stderr);
    }

    /* Set the individual pointer to 0, since we want to use a read_all */
    MPI_File_seek( fh, 0, MPI_SEEK_SET );

    /* Read nothing (check status) */
    memset( &status, 0xff, sizeof(MPI_Status) );
    MPI_File_read( fh, buf, 0, MPI_INT, &status );
    MPI_Get_count( &status, MPI_INT, &count );
    if (count != 0) {
        errs++;
        fprintf( stderr, "Count not zero (%d) on read\n", count );fflush(stderr);
    }

    /* Write nothing (check status) */
    memset( &status, 0xff, sizeof(MPI_Status) );
    MPI_File_write( fh, buf, 0, MPI_INT, &status );
    if (count != 0) {
        errs++;
        fprintf( stderr, "Count not zero (%d) on write\n", count );fflush(stderr);
    }

    /* Read shared nothing (check status) */
    MPI_File_seek_shared( fh, 0, MPI_SEEK_SET );
    /* Read nothing (check status) */
    memset( &status, 0xff, sizeof(MPI_Status) );
    MPI_File_read_shared( fh, buf, 0, MPI_INT, &status );
    MPI_Get_count( &status, MPI_INT, &count );
    if (count != 0) {
        errs++;
        fprintf( stderr, "Count not zero (%d) on read shared\n", count );fflush(stderr);
    }

    /* Write nothing (check status) */
    memset( &status, 0xff, sizeof(MPI_Status) );
    MPI_File_write_shared( fh, buf, 0, MPI_INT, &status );
    if (count != 0) {
        errs++;
        fprintf( stderr, "Count not zero (%d) on write\n", count );fflush(stderr);
    }

    MPI_Barrier( comm );

    MPI_File_seek_shared( fh, 0, MPI_SEEK_SET );
    for (i=0; i<size; i++) buf[i] = -1;
    MPI_File_read_ordered( fh, buf, 1, MPI_INT, &status );
    if (buf[0] != rank) {
        errs++;
        fprintf( stderr, "%d: buf = %d\n", rank, buf[0] );fflush(stderr);
    }

    free( buf );
    MPI_File_close( &fh );

    MPI_Finalize();
    return errs;
}

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