.TH MPI_Bsend 3 "6/24/2006" "LAM/MPI 7.1.4" "LAM/MPI"
.SH NAME
MPI_Bsend \-  Basic send with user-specified buffering 
.SH SYNOPSIS
.nf
#include <mpi.h>
int MPI_Bsend(void *buf, int count, MPI_Datatype dtype, 
              int dest, int tag, MPI_Comm comm)
.fi
.SH INPUT PARAMETERS
.PD 0
.TP
.B buf 
- initial address of send buffer (choice) 
.PD 1
.PD 0
.TP
.B count 
- number of elements in send buffer (nonnegative integer) 
.PD 1
.PD 0
.TP
.B dtype 
- datatype of each send buffer element (handle) 
.PD 1
.PD 0
.TP
.B dest 
- rank of destination (integer) 
.PD 1
.PD 0
.TP
.B tag 
- message tag (integer) 
.PD 1
.PD 0
.TP
.B comm 
- communicator (handle) 
.PD 1

.SH 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 
.I must
have provided buffer space with 
.I MPI_Buffer_attach
).

In deciding how much buffer space to allocate, remember that the
buffer space is not available for reuse by subsequent 
.I 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

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

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

In C, you can force the messages to be delivered by
.nf
MPI_Buffer_detach(&b, &n);
MPI_Buffer_attach(b, n);
.fi

(The 
.I MPI_Buffer_detach
will not complete until all buffered messages
are delivered.)

It is generally a bad idea to use the 
.I MPI_Bsend
function, as it
guarantees that the entire message will suffer the overhead of an
additional memory copy.  For large messages, or when shared memory
message transports are being used, this overhead can be quite
expensive.

.SH NOTES FOR FORTRAN

All MPI routines in Fortran (except for 
.I MPI_WTIME
and 
.I MPI_WTICK
)
have an additional argument 
.I ierr
at the end of the argument list.
.I 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 
.I call
statement.

All MPI objects (e.g., 
.I MPI_Datatype
, 
.I MPI_Comm
) are of type
.I INTEGER
in Fortran.

.SH ERRORS

If an error occurs in an MPI function, the current MPI error handler
is called to handle it.  By default, this error handler aborts the
MPI job.  The error handler may be changed with 
.I MPI_Errhandler_set
;
the predefined error handler 
.I MPI_ERRORS_RETURN
may be used to cause
error values to be returned (in C and Fortran; this error handler is
less useful in with the C++ MPI bindings.  The predefined error
handler 
.I MPI::ERRORS_THROW_EXCEPTIONS
should be used in C++ if the
error value needs to be recovered).  Note that MPI does 
.I not
guarantee that an MPI program can continue past an error.

All MPI routines (except 
.I MPI_Wtime
and 
.I MPI_Wtick
) return an error
value; C routines as the value of the function and Fortran routines
in the last argument.  The C++ bindings for MPI do not return error
values; instead, error values are communicated by throwing exceptions
of type 
.I MPI::Exception
(but not by default).  Exceptions are only
thrown if the error value is not 
.I MPI::SUCCESS
\&.


Note that if the 
.I MPI::ERRORS_RETURN
handler is set in C++, while
MPI functions will return upon an error, there will be no way to
recover what the actual error value was.
.PD 0
.TP
.B MPI_SUCCESS 
- No error; MPI routine completed successfully.
.PD 1
.PD 0
.TP
.B MPI_ERR_COMM 
- Invalid communicator.  A common error is to use a
null communicator in a call (not even allowed in 
.I MPI_Comm_rank
).
.PD 1
.PD 0
.TP
.B MPI_ERR_COUNT 
- Invalid count argument.  Count arguments must be
non-negative; a count of zero is often valid.
.PD 1
.PD 0
.TP
.B MPI_ERR_TYPE 
- Invalid datatype argument.  May be an uncommitted
MPI_Datatype (see 
.I MPI_Type_commit
).
.PD 1
.PD 0
.TP
.B 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 (
.I MPI_Recv
, 
.I MPI_Irecv
, 
.I MPI_Sendrecv
, etc.) may also be
.I MPI_ANY_SOURCE
\&.

.PD 1
.PD 0
.TP
.B MPI_ERR_TAG 
- Invalid tag argument.  Tags must be non-negative;
tags in a receive (
.I MPI_Recv
, 
.I MPI_Irecv
, 
.I MPI_Sendrecv
, etc.)
may also be 
.I MPI_ANY_TAG
\&.
The largest tag value is available
through the the attribute 
.I MPI_TAG_UB
\&.

.PD 1

.SH SEE ALSO
MPI_Buffer_attach, MPI_Buffer_detach, MPI_Ibsend, MPI_Bsend_init
.br

.SH MORE INFORMATION

For more information, please see the official MPI Forum web site,
which contains the text of both the MPI-1 and MPI-2 standards.  These
documents contain detailed information about each MPI function (most
of which is not duplicated in these man pages).

.I http://www.mpi-forum.org/


.SH ACKNOWLEDGEMENTS

The LAM Team would like the thank the MPICH Team for the handy program
to generate man pages ("doctext" from
.I ftp://ftp.mcs.anl.gov/pub/sowing/sowing.tar.gz
), the initial
formatting, and some initial text for most of the MPI-1 man pages.
.SH LOCATION
bsend.c