DeinoMPI

The Great and Terrible implementation of MPI-2

MPI_Get

Get data from a memory window on a remote process

int MPI_Get(
  void *origin_addr,
  int origin_count,
  MPI_Datatype origin_datatype, 
  int target_rank,
  MPI_Aint target_disp,
  int target_count,
  MPI_Datatype target_datatype,
  MPI_Win win
);

Parameters

origin_addr: [in] Address of the buffer in which to receive the data
origin_count: [in] number of entries in origin buffer (nonnegative integer)
origin_datatype: [in] datatype of each entry in origin buffer (handle)
target_rank: [in] rank of target (nonnegative integer)
target_disp: [in] displacement from window start to the beginning of the target buffer (nonnegative integer)
target_count: [in] number of entries in target buffer (nonnegative integer)
target_datatype: [in] datatype of each entry in target buffer (handle)
win: [in] window object used for communication (handle)

Remarks

Similar to MPI_PUT, except that the direction of data transfer is reversed. Data are copied from the target memory to the origin. The origin_datatype may not specify overlapping entries in the origin buffer. The target buffer must be contained within the target window, and the copied data must fit, without truncation, in the origin buffer.

Thread and Interrupt Safety

This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as mallocor other non-MPICH runtime routines that are themselves not interrupt-safe.

Notes for Fortran

All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK) have an additional argument ierr at the end of the argument list. ierris an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the call statement. All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGERin Fortran.

Errors

All MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with MPI_Comm_set_errhandler (for communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for RMA windows). The MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

MPI_SUCCESS: No error; MPI routine completed successfully.

MPI_ERR_ARG: Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK).

MPI_ERR_COUNT: Invalid count argument. Count arguments must be non-negative; a count of zero is often valid.

MPI_ERR_RANK: Invalid source or destination rank. Ranks must be between zero and the size of the communicator minus one; ranks in a receive (MPI_Recv, MPI_Irecv, MPI_Sendrecv, etc.) may also be MPI_ANY_SOURCE.

MPI_ERR_TYPE: Invalid datatype argument. May be an uncommitted MPI_Datatype (see MPI_Type_commit).

MPI_ERR_WIN: Invalid MPI window object

Example Code

The following sample code illustrates MPI_Get.

#include "mpi.h"
#include "stdio.h"

/* This does a transpose with a get operation, fence, and derived
    datatypes. Uses vector and hvector (Example 3.32 from MPI 1.1
    Standard). Run on 2 processes */

#define NROWS 100
#define NCOLS 100

int main(int argc, char *argv[])
{
    int rank, nprocs, A[NROWS][NCOLS], i, j;
    MPI_Win win;
    MPI_Datatype column, xpose;
    int errs = 0;

    MPI_Init(&argc,&argv);
    MPI_Comm_size(MPI_COMM_WORLD,&nprocs);
    MPI_Comm_rank(MPI_COMM_WORLD,&rank);
    if (nprocs != 2) {
        printf("Run this program with 2 processes\n");fflush(stdout);
        MPI_Abort(MPI_COMM_WORLD, 1);
    }

    if (rank == 0)
    {
        for (i=0; i<NROWS; i++)
            for (j=0; j<NCOLS; j++)
                A[i][j] = -1;

        /* create datatype for one column */
        MPI_Type_vector(NROWS, 1, NCOLS, MPI_INT, &column);
        /* create datatype for matrix in column-major order */
        MPI_Type_hvector(NCOLS, 1, sizeof(int), column, &xpose);
        MPI_Type_commit(&xpose);

        MPI_Win_create(NULL, 0, 1, MPI_INFO_NULL, MPI_COMM_WORLD, &win);

        MPI_Win_fence(0, win);

        MPI_Get(A, NROWS*NCOLS, MPI_INT, 1, 0, 1, xpose, win);

        MPI_Type_free(&column);
        MPI_Type_free(&xpose);

        MPI_Win_fence(0, win);

        for (j=0; j<NCOLS; j++)
        {
            for (i=0; i<NROWS; i++)
            {
                if (A[j][i] != i*NCOLS + j)
                {
                    if (errs < 50)
                    {
                        printf("Error: A[%d][%d]=%d should be %d\n", j, i, A[j][i], i*NCOLS + j);fflush(stdout);
                    }
                    errs++;
                }
            }
        }
        if (errs >= 50)
        {
            printf("Total number of errors: %d\n", errs);fflush(stdout);
        }
    }
    else
    { /* rank = 1 */
        for (i=0; i<NROWS; i++)
            for (j=0; j<NCOLS; j++)
                A[i][j] = i*NCOLS + j;
        MPI_Win_create(A, NROWS*NCOLS*sizeof(int), sizeof(int), MPI_INFO_NULL, MPI_COMM_WORLD, &win);
        MPI_Win_fence(0, win);
        MPI_Win_fence(0, win);
    }

    MPI_Win_free(&win);
    MPI_Finalize();
    return errs;
}

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