# 17.2.32. MPI_Buffer_detach

MPI_Buffer_detach — Removes an existing buffer (for use in in MPI_Bsend, etc.)

## 17.2.32.1. SYNTAX

### 17.2.32.1.1. C Syntax

    #include <mpi.h>

    int MPI_Buffer_detach(void *buf, int *size)

### 17.2.32.1.2. Fortran Syntax

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

    MPI_BUFFER_DETACH(BUF, SIZE, IERROR)
        <type>  BUF(*)
        INTEGER SIZE, IERROR

### 17.2.32.1.3. Fortran 2008 Syntax

    USE mpi_f08

    MPI_Buffer_detach(buffer_addr, size, ierror)
        USE, INTRINSIC :: ISO_C_BINDING, ONLY
        TYPE(C_PTR), INTENT(OUT) :: buffer_addr
        INTEGER, INTENT(OUT) :: size
        INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 17.2.32.2. OUTPUT PARAMETERS

- `buf` : Initial buffer address (choice).

- `size` : Buffer size, in bytes (integer).

- `ierror` : Fortran only: Error status (integer).

## 17.2.32.3. DESCRIPTION

Detach the buffer currently associated with MPI. The call returns the address and the size of the detached buffer. This operation will block until all messages currently in the buffer have been transmitted. Upon return of this function, the user may reuse or deallocate the space taken by the buffer.

Example: Calls to attach and detach buffers.

    #define BUFFSIZE 10000

    int size char *buff;
    MPI_Buffer_attach( malloc(BUFFSIZE), BUFFSIZE);

    // a buffer of 10000 bytes can now be used by MPI_Bsend

    MPI_Buffer_detach( &buff, &size); // Buffer size reduced to zero
    MPI_Buffer_attach( buff, size); // Buffer of 10000 bytes available again

## 17.2.32.4. 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’s simple) and weak‐nesses (it only works for nested usages).

For 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.

For C: Even though the buf argument is declared as void, it is really the address of a void pointer. See Rationale, below, for more details.

Even though the C functions MPI_Buffer_attach and MPI_Buffer_detach both have a first argument of type void*, these arguments are used differently: A pointer to the buffer is passed to MPI_Buffer_attach; the address of the pointer is passed to MPI_Buffer_detach, so that this call can return the pointer value.

## 17.2.32.5. ERRORS

Almost all MPI routines return an error value; C routines as the return result of the function and Fortran routines in the last argument.

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. If no communication object is associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all other MPI functions.

Open MPI includes three predefined error handlers that can be used:

- `MPI_ERRORS_ARE_FATAL` Causes the program to abort all connected MPI processes.

- `MPI_ERRORS_ABORT` An error handler that can be invoked on a communicator, window, file, or session. When called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a window or file, acts as if MPI_Abort was called on a communicator containing the group of processes in the corresponding window or file. If called on a session, aborts only the local process.

- `MPI_ERRORS_RETURN` Returns an error code to the application.

MPI applications can also implement their own error handlers by calling:

- MPI_Comm_create_errhandler then MPI_Comm_set_errhandler

- MPI_File_create_errhandler then MPI_File_set_errhandler

- MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init

- MPI_Win_create_errhandler then MPI_Win_set_errhandler

Note that MPI does not guarantee that an MPI program can continue past an error.

See the MPI man page for a full list of MPI error codes.

See the Error Handling section of the MPI-3.1 standard for more information.

See also

- MPI_Buffer_attach