# Name

`MPI_Bsend` - Basic send with user-specified buffering.

# Syntax

## C Syntax

```c
#include <mpi.h>
int MPI_Bsend(const void *buf, int count, MPI_Datatype datatype,
   int dest, int tag, MPI_Comm comm)
```

## Fortran Syntax

```fortran
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
```

## Fortran 2008 Syntax

```fortran
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
```

# 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).

# Output Parameters

* `IERROR` : Fortran only: Error status (integer).

# Description

`MPI_Bsend` performs a buffered-mode, blocking send.

# 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_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:
```c
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.)

# 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.