# MPI_Buffer_attach

Attaches a user-provided buffer for sending

## Synopsis

    int MPI_Buffer_attach(void *buffer, int size)

## Input Parameters

buffer
  initial buffer address (choice)
size
  buffer size, in bytes (integer)

## Notes

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

         MPI_Buffer_attach(buffer, 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

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

The `MPI_BSEND_OVERHEAD` gives the maximum amount of space that may be used in the buffer for use by the BSEND routines in using the buffer. This value is in `mpi.h` (for C) and `mpif.h` (for Fortran).

## Thread and Interrupt Safety

The user is responsible for ensuring that multiple threads do not try to update the same MPI object from different threads. This routine should not be used from within a signal handler.

The MPI standard defined a thread-safe interface but this does not mean that all routines may be called without any thread locks. For example, two threads must not attempt to change the contents of the same `MPI_Info` object concurrently. The user is responsible in this case for using some mechanism, such as thread locks, to ensure that only one thread at a time makes use of this routine. Because the buffer for buffered sends (e.g., `MPI_Bsend`) is shared by all threads in a process, the user is responsible for ensuring that only one thread at a time calls this routine or `MPI_Buffer_detach`.

## 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 guarentee 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_INTERN
  An internal error has been detected. This is fatal. Please send a bug report to `mpi-bugs@mcs.anl.gov`.

## See Also

MPI_Buffer_detach, MPI_Bsend