# MPI_Bsend

Basic send with user-provided buffering

## Synopsis

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

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

## Input Parameters

buf
  initial address of send buffer (choice)
count
  number of elements in send buffer (non-negative integer)
datatype
  datatype of each send buffer element (handle)
dest
  rank of destination (integer)
tag
  message tag (integer)
comm
  communicator (handle)

## Notes

This send is provided as a convenience function; it allows the user to send messages without worring 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_Bsend`s 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.)

## 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 `malloc` or 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. `ierr` is 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 `INTEGER` in 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 guarantee 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_BUFFER
  Invalid buffer pointer. Usually a null buffer where one is not valid.

MPI_ERR_COMM
  Invalid communicator. A common error is to use a null communicator in a call (not even allowed in `MPI_Comm_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_TAG
  Invalid tag argument. Tags must be non-negative; tags in a receive (`MPI_Recv`, `MPI_Irecv`, `MPI_Sendrecv`, etc.) may also be `MPI_ANY_TAG`. The largest tag value is available through the the attribute `MPI_TAG_UB`.

MPI_ERR_TYPE
  Invalid datatype argument. Additionally, this error can occur if an uncommitted MPI_Datatype (see `MPI_Type_commit`) is used in a communication call.

MPI_ERR_OTHER
  Other error; use `MPI_Error_string` to get more information about this error code.

## See Also

MPI_Buffer_attach, MPI_Ibsend, MPI_Bsend_init