SYNTAX

   C Syntax
          #include <mpi.h>

          int MPI_Buffer_detach(void *buf, int *size)

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

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

   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

OUTPUT PARAMETERS

       • buf : Initial buffer address (choice).

       • size : Buffer size, in bytes (integer).

       • ierror : Fortran only: Error status (integer).

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

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.

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

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       2003-2025, The Open MPI Community

                                                  Feb 17, 2025                              MPI_BUFFER_DETACH(3)