# MPI_Buffer_detach

Removes an existing buffer (for use in MPI_Bsend etc)

## Synopsis

    #include "mpi.h"
    int MPI_Buffer_detach(
            void *bufferptr,
            int *size )

## Output Parameters

| buffer | 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.

## 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.

Location:buffree.c