DeinoMPI

The Great and Terrible implementation of MPI-2

MPI_Get_count

Gets the number of "top level" elements

int MPI_Get_count(
  MPI_Status *status,
  MPI_Datatype datatype,
  int *count
);

Parameters

status: [in] return status of receive operation (Status)
datatype: [in] datatype of each receive buffer element (handle)
count: [out] number of received elements (integer)

Remarks

Returns the number of entries received. (Again, we count entries, each of type datatype, not bytes.) The datatype argument should match the argument provided by the receive call that set the status variable.

If the size of the datatype is zero, this routine will return a count of zero. If the amount of data in status is not an exact multiple of the size of datatype (so that count would not be integral), a count of MPI_UNDEFINED is returned instead.

[] Rationale.

Some message passing libraries use INOUT count, tag and source arguments, thus using them both to specify the selection criteria for incoming messages and return the actual envelope values of the received message. The use of a separate status argument prevents errors that are often attached with INOUT argument (e.g., using the MPI_ANY_TAG constant as the tag in a receive). Some libraries use calls that refer implicitly to the "last message received." This is not thread safe.

The datatype argument is passed to MPI_GET_COUNT so as to improve performance. A message might be received without counting the number of elements it contains, and the count value is often not needed. Also, this allows the same function to be used after a call to MPI_PROBE. ( End of rationale.)

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

Example Code

The following sample code illustrates MPI_Get_count.

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

/* Receive partial datatypes and check that MPI_Getelements gives the correct version */

int main( int argc, char *argv[] )
{
    int errs = 0;
    MPI_Datatype outtype, oldtypes[2];
    MPI_Aint offsets[2];
    int blklens[2];
    MPI_Comm comm;
    int size, rank, src, dest, tag;

    MPI_Init( &argc, &argv );

    comm = MPI_COMM_WORLD;
    MPI_Comm_rank( comm, &rank );
    MPI_Comm_size( comm, &size );
    if (size < 2) {
        printf( "This test requires at least 2 processes\n" );fflush(stdout);
        MPI_Abort( MPI_COMM_WORLD, 1 );
    }

    src = 0;
    dest = 1;

    if (rank == src) {
        int buf[128], position, cnt;
        /* sender */
        /* Create a datatype and send it (multiple of sizeof(int)) */
        /* Create a send struct type */
        oldtypes[0] = MPI_INT;
        oldtypes[1] = MPI_CHAR;
        blklens[0] = 1;
        blklens[1] = 4*sizeof(int);
        offsets[0] = 0;
        offsets[1] = sizeof(int);
        MPI_Type_struct( 2, blklens, offsets, oldtypes, &outtype );
        MPI_Type_commit( &outtype );

        buf[0] = 4*sizeof(int);
        MPI_Send( buf, 1, outtype, dest, 0, comm );
        MPI_Type_free( &outtype );

        /* Create a datatype and send it (not a multiple of sizeof(int)) */
        /* Create a send struct type */
        oldtypes[0] = MPI_INT;
        oldtypes[1] = MPI_CHAR;
        blklens[0] = 1;
        blklens[1] = 4*sizeof(int)+1;
        offsets[0] = 0;
        offsets[1] = sizeof(int);
        MPI_Type_struct( 2, blklens, offsets, oldtypes, &outtype );
        MPI_Type_commit( &outtype );

        buf[0] = 4*sizeof(int) + 1;
        MPI_Send( buf, 1, outtype, dest, 1, comm );
        MPI_Type_free( &outtype );

        /* Pack data and send as packed */
        position = 0;
        cnt = 7;
        MPI_Pack( &cnt, 1, MPI_INT, buf, 128*sizeof(int), &position, comm );
        MPI_Pack( "message", 7, MPI_CHAR, buf, 128*sizeof(int), &position, comm );
        MPI_Send( buf, position, MPI_PACKED, dest, 2, comm );
    }
    else if (rank == dest) {
        MPI_Status status;
        int buf[128], i, elms, count;
        /* Receiver */
        /* Create a receive struct type */
        oldtypes[0] = MPI_INT;
        oldtypes[1] = MPI_CHAR;
        blklens[0] = 1;
        blklens[1] = 256;
        offsets[0] = 0;
        offsets[1] = sizeof(int);
        MPI_Type_struct( 2, blklens, offsets, oldtypes, &outtype );
        MPI_Type_commit( &outtype );

        for (i=0; i<3; i++) {
            tag = i;
            MPI_Recv( buf, 1, outtype, src, tag, comm, &status );
            MPI_Get_elements( &status, outtype, &elms );
            if (elms != buf[0] + 1) {
                errs++;
                printf( "For test %d, Get elements gave %d but should be %d\n", i, elms, buf[0] + 1 );fflush(stdout);
            }
            MPI_Get_count( &status, outtype, &count );
            if (count != MPI_UNDEFINED) {
                errs++;
                printf( "For partial send, Get_count did not return MPI_UNDEFINED\n" );fflush(stdout);
            }
        }
        MPI_Type_free( &outtype );
    }

    MPI_Finalize();
    return errs;
}

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