DeinoMPI

The Great and Terrible implementation of MPI-2

MPI_Type_get_extent

Get the lower bound and extent for a Datatype

int MPI_Type_get_extent(
  MPI_Datatype datatype,
  MPI_Aint *lb,
  MPI_Aint *extent
);

Parameters

datatype: [in] datatype to get information on (handle)
lb: [out] lower bound of datatype (integer)
extent: [out] extent of datatype (integer)

Remarks

Returns the lower bound and the extent of datatype (as defined by the MPI-1 standard, Section 3.12.2).

MPI allows one to change the extent of a datatype, using lower bound and upper bound markers ( MPI_LB and MPI_UB). This is useful, as it allows to control the stride of successive datatypes that are replicated by datatype constructors, or are replicated by the count argument in a send or recieve call. However, the current mechanism for achieving it is painful; also it is restrictive. MPI_LB and MPI_UB are "sticky": once present in a datatype, they cannot be overridden (e.g., the upper bound can be moved up, by adding a new MPI_UB marker, but cannot be moved down below an existing MPI_UB marker). A new type constructor is provided to facilitate these changes. The use of MPI_LB and MPI_UB is deprecated.

Thread and Interrupt Safety

This routine is both thread- and interrupt-safe. This means that this routine may safely be used by multiple threads and from within a signal handler.

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

Example Code

The following sample code illustrates MPI_Type_get_extent.

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

int main(int argc, char **argv)
{
    int mpi_err, errs = 0, size;
    MPI_Aint lb, ub, extent;
    MPI_Datatype type;
    struct { float a; int b; } foo;

    MPI_Init(&argc, &argv);

    type = MPI_INT;
    MPI_Type_size(type, &size);
    if (size != sizeof(int)) {
        fprintf(stderr, "MPI_Type_size of MPI_INT incorrect size (%d); should be %d.\n", size, (int) sizeof(int));fflush(stderr);
        errs++;
    }

    MPI_Type_get_extent(type, &lb, &extent);
    if (extent != sizeof(int)) {
        fprintf(stderr, "MPI_Type_get_extent of MPI_INT returned incorrect extent (%d); should be %d.\n", (int) extent, (int) sizeof(int));fflush(stderr);
        errs++;
    }
    if (lb != 0) {
        fprintf(stderr, "MPI_Type_get_extent of MPI_INT returned incorrect lb (%d); should be 0.\n", (int) lb);fflush(stderr);
        errs++;
    }

    MPI_Type_ub(type, &ub);
    if (ub != extent - lb) {
        fprintf(stderr, "MPI_Type_ub of MPI_INT returned incorrect ub (%d); should be %d.\n", (int) ub, (int) (extent - lb));fflush(stderr);
        errs++;
    }

    type = MPI_FLOAT_INT;
    MPI_Type_size(type, &size);
    if (size != sizeof(float) + sizeof(int)) {
        fprintf(stderr, "MPI_Type_size of MPI_FLOAT_INT returned incorrect size (%d); should be %d.\n", size, (int) (sizeof(float) + sizeof(int)));fflush(stderr);
        errs++;
    }

    MPI_Type_get_extent(type, &lb, &extent);
    if (extent != sizeof(foo)) {
        fprintf(stderr, "MPI_Type_get_extent of MPI_FLOAT_INT returned incorrect extent (%d); should be %d.\n", (int) extent, (int) sizeof(foo));fflush(stderr);
        errs++;
    }
    if (lb != 0) {
        fprintf(stderr, "MPI_Type_get_extent of MPI_FLOAT_INT returned incorrect lb (%d); should be 0.\n", (int) lb);fflush(stderr);
        errs++;
    }

    MPI_Type_ub(type, &ub);
    if (ub != extent - lb) {
        fprintf(stderr, "MPI_Type_ub of MPI_FLOAT_INT returned incorrect ub (%d); should be %d.\n", (int) ub, (int) (extent - lb));fflush(stderr);
        errs++;
    }

    MPI_Finalize();
    return errs;
}

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