MPI_Buffer_detach

Removes an existing buffer (for use in MPI_Bsend etc)

Synopsis

int MPI_Buffer_detach(void *buffer, 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.

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.

See Also

MPI_Buffer_attach

Location:buffree.c