# Name

`MPI_Buffer_attach` - Attaches a user-defined buffer for sending.

# Syntax

## C Syntax

```c
#include <mpi.h>

int MPI_Buffer_attach(void *buf, int size)
```

## Fortran Syntax

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

MPI_BUFFER_ATTACH(BUF, SIZE, IERROR)
	<type>	BUF(*)
	INTEGER	SIZE, IERROR
```

## Fortran 2008 Syntax

```fortran
USE mpi_f08

MPI_Buffer_attach(buffer, size, ierror)
	TYPE(*), DIMENSION(..), ASYNCHRONOUS :: buffer
	INTEGER, INTENT(IN) :: size
	INTEGER, OPTIONAL, INTENT(OUT) :: ierror
```

# Input Parameters

* `buf` : Initial buffer address (choice).
* `size` : Buffer size, in bytes (integer).

# Output Parameter

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

# Description

Provides to MPI a buffer in the user's memory to be used for buffering
outgoing messages. The buffer is used only by messages sent in buffered
mode. Only one buffer can be attached to a process at a time.

# Notes

The size given should be the sum of the sizes of all outstanding Bsends
that you intend to have, plus `MPI_BSEND_OVERHEAD` bytes for each Bsend
that you do. For the purposes of calculating size, you should use
`MPI_Pack_size`. In other words, in the code

```c
MPI_Buffer_attach( buf, size );
MPI_Bsend( ..., count=20, datatype=type1, ... );
//...
MPI_Bsend( ..., count=40, datatype=type2, ... );
```
the value of size in the `MPI_Buffer_attach` call should be greater than the value computed by

```c
MPI_Pack_size( 20, type1, comm, &s1 );
MPI_Pack_size( 40, type2, comm, &s2 );
size = s1 + s2 + 2 * MPI_BSEND_OVERHEAD;
```

`MPI_BSEND_OVERHEAD` gives the maximum amount of buffer space that may be
used by the  Bsend  routines.  This value is in mpi.h for C and mpif.h
for Fortran.

# Errors

Almost all MPI routines return an error 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.


# See Also

[MPI_Buffer_detach(3)](MPI_Buffer_detach.html)