# MPI_Buffer_detach

Removes an existing buffer (for use in MPI_Bsend etc)

## Synopsis

    #ifdef ENABLE_QMPI
    #ifndef MPICH_MPI_FROM_PMPI
    int QMPI_Buffer_detach(QMPI_Context context, int tool_id, void *buffer_addr,
                           int *size) MPICH_API_PUBLIC

## Output Parameters

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

## Notes

The reason that `MPI_Buffer_detach` returns the address and size of the buffer being detached is to allow nested libraries to replace and restore the buffer. For example, consider

        int size, mysize, idummy;
        void *ptr, *myptr, *dummy;
        MPI_Buffer_detach(&ptr, &size);
        MPI_Buffer_attach(myptr, mysize);
        ...
        ... library code ...
        ...
        MPI_Buffer_detach(&dummy, &idummy);
        MPI_Buffer_attach(ptr, size);

This is much like the action of the Unix signal routine and has the same strengths (it is simple) and weaknesses (it only works for nested usages).

Note that for this approach to work, MPI_Buffer_detach must return MPI_SUCCESS even when there is no buffer to detach. In that case, it returns a size of zero. The MPI 1.1 standard for `MPI_BUFFER_DETACH` contains the text

       The statements made in this section describe the behavior of MPI for
       buffered-mode sends. When no buffer is currently associated, MPI behaves
       as if a zero-sized buffer is associated with the process.

This could be read as applying only to the various Bsend routines. This implementation takes the position that this applies to `MPI_BUFFER_DETACH` as well.

## 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_attach`.

## 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.

The Fortran binding for this routine is different. Because Fortran does not have pointers, it is impossible to provide a way to use the output of this routine to exchange buffers. In this case, only the size field is set.

## Notes for C

Even though the `bufferptr` argument is declared as `void *`, it is really the address of a void pointer. See the rationale in the standard for more details.

## 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_ARG
  Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., `MPI_ERR_RANK`).

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

## See Also

MPI_Buffer_attach