Manpages - MPI_Get_elements.3

MPI_Get_elements, MPI_Get_elements_x - Returns the number of basic elements in a data type.

  #include <mpi.h>
  int MPI_Get_elements(const MPI_Status *status, MPI_Datatype datatype,
  	int *count)
  int MPI_Get_elements_x(const MPI_Status *status, MPI_Datatype datatype,
  	MPI_Count *count)

  USE MPI
  ! or the older form: INCLUDE 'mpif.h'
  MPI_GET_ELEMENTS(STATUS, DATATYPE, COUNT, IERROR)
  	INTEGER	STATUS(MPI_STATUS_SIZE), DATATYPE, COUNT, IERROR
  MPI_GET_ELEMENTS_X(STATUS, DATATYPE, COUNT, IERROR)
  	INTEGER	STATUS(MPI_STATUS_SIZE), DATATYPE
          INTEGER(KIND=MPI_COUNT_KIND) COUNT
          INTEGER IERROR

  USE mpi_f08
  MPI_Get_elements(status, datatype, count, ierror)
  	TYPE(MPI_Status), INTENT(IN) :: status
  	TYPE(MPI_Datatype), INTENT(IN) :: datatype
  	INTEGER, INTENT(OUT) :: count
  	INTEGER, OPTIONAL, INTENT(OUT) :: ierror
  MPI_Get_elements_x(status, datatype, count, ierror)
  	TYPE(MPI_Status), INTENT(IN) :: status
  	TYPE(MPI_Datatype), INTENT(IN) :: datatype
  	INTEGER(KIND = MPI_COUNT_KIND), INTENT(OUT) :: count
  	INTEGER, OPTIONAL, INTENT(OUT) :: ierror

  #include <mpi.h>
  int Status::Get_elements(const Datatype& datatype) const

status

Return status of receive operation (status).

datatype

Datatype used by receive operation (handle).

count Number of received basic elements (integer).

IERROR

Fortran only: Error status (integer).

MPI_Get_elements and MPI_Get_elements_x behave different from MPI_Get_count, which returns the number of “top-level entries” received, i.e., the number of “copies” of type datatype. MPI_Get_count may return any integer value k, where 0 =< k =< count. If MPI_Get_count returns k, then the number of basic elements received (and the value returned by MPI_Get_elements and MPI_Get_elements_x) is n * k, where n is the number of basic elements in the type map of datatype. If the number of basic elements received is not a multiple of n, that is, if the receive operation has not received an integral number of datatype “copies,” then MPI_Get_count returns the value MPI_UNDEFINED. For both functions, if the count parameter cannot express the value to be returned (e.g., if the parameter is too small to hold the output value), it is set to MPI_UNDEFINED.

Example: Usage of MPI_Get_count and MPI_Get_element:

    ...
    CALL MPI_TYPE_CONTIGUOUS(2, MPI_REAL, Type2, ierr)
    CALL MPI_TYPE_COMMIT(Type2, ierr)
    ...
    CALL MPI_COMM_RANK(comm, rank, ierr)
    IF(rank.EQ.0) THEN
          CALL MPI_SEND(a, 2, MPI_REAL, 1, 0, comm, ierr)
          CALL MPI_SEND(a, 3, MPI_REAL, 1, 0, comm, ierr)
    ELSE
          CALL MPI_RECV(a, 2, Type2, 0, 0, comm, stat, ierr)
          CALL MPI_GET_COUNT(stat, Type2, i, ierr)     ! returns i=1
          CALL MPI_GET_ELEMENTS(stat, Type2, i, ierr)  ! returns i=2
          CALL MPI_RECV(a, 2, Type2, 0, 0, comm, stat, ierr)
          CALL MPI_GET_COUNT(stat, Type2, i, ierr) ! returns i=MPI_UNDEFINED
          CALL MPI_GET_ELEMENTS(stat, Type2, i, ierr)  ! returns i=3
    END IF

The function MPI_Get_elements can also be used after a probe to find the number of elements in the probed message. Note that the two functions MPI_Get_count and MPI_Get_elements return the same values when they are used with primitive data types.

Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument. C++ functions do not return errors. If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism will be used to throw an MPI::Exception object.

Before the error value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.

The MPI standard prescribes portable Fortran syntax for the COUNT argument of MPI_Get_elements_x only for Fortran 90. FORTRAN 77 users may use the non-portable syntax

       INTEGER*MPI_COUNT_KIND COUNT

where MPI_COUNT_KIND is a constant defined in mpif.h and gives the length of the declared integer in bytes.

MPI_Get_count