17.2.29. MPI_Bsend

MPI_Bsend - Basic send with user-specified buffering.

17.2.29.1. Syntax

17.2.29.1.1. C Syntax

#include <mpi.h>

int MPI_Bsend(const void *buf, int count, MPI_Datatype datatype,
   int dest, int tag, MPI_Comm comm)

17.2.29.1.2. Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'

MPI_BSEND(BUF, COUNT,DATATYPE, DEST, TAG, COMM, IERROR)
    <type>  BUF(*)
    INTEGER COUNT, DATATYPE, DEST, TAG, COMM, IERROR

17.2.29.1.3. Fortran 2008 Syntax

USE mpi_f08
MPI_Bsend(buf, count, datatype, dest, tag, comm, ierror)
    TYPE(*), DIMENSION(..), INTENT(IN) :: buf
    INTEGER, INTENT(IN) :: count, dest, tag
    TYPE(MPI_Datatype), INTENT(IN) :: datatype
    TYPE(MPI_Comm), INTENT(IN) :: comm
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

17.2.29.2. Input Parameters

  • buf : Initial address of send buffer (choice).

  • count : Number of entries in send buffer (nonnegative integer).

  • datatype : Datatype of each send buffer element (handle).

  • dest : Rank of destination (integer).

  • tag : Message tag (integer).

  • comm : Communicator (handle).

17.2.29.3. Output Parameters

  • IERROR : Fortran only: Error status (integer).

17.2.29.4. Description

MPI_Bsend performs a buffered-mode, blocking send.

17.2.29.5. Notes

This send is provided as a convenience function; it allows the user to send messages without worrying about where they are buffered (because the user must have provided buffer space with MPI_Buffer_attach).

In deciding how much buffer space to allocate, remember that the buffer space is not available for reuse by subsequent MPI_Bsends unless you are certain that the message has been received (not just that it should have been received). For example, this code does not allocate enough buffer space:

MPI_Buffer_attach( b, n*sizeof(double) + MPI_BSEND_OVERHEAD );
for (i=0; i<m; i++) {
    MPI_Bsend( buf, n, MPI_DOUBLE, ... );
}

because only enough buffer space is provided for a single send, and the loop may start a second MPI_Bsend before the first is done making use of the buffer.

In C, you can force the messages to be delivered by MPI_Buffer_detach( &b, &n ); MPI_Buffer_attach( b, n ); (The MPI_Buffer_detach will not complete until all buffered messages are delivered.)

17.2.29.6. Errors

Almost all MPI routines return an error value; C routines as the value; C routines as the value of the function and Fortran routines in the last argument.

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.