MPI_Unpack(3)                      Open MPI                      MPI_Unpack(3)



NAME
       MPI_Unpack - Unpacks a datatype into contiguous memory.


SYNTAX
C Syntax
       #include <mpi.h>
       int MPI_Unpack(void *inbuf, int insize, int *position,
            void *outbuf, int outcount, MPI_Datatype datatype,
            MPI_Comm comm)


Fortran Syntax
       INCLUDE 'mpif.h'
       MPI_UNPACK(INBUF, INSIZE, POSITION, OUTBUF, OUTCOUNT,
            DATATYPE, COMM, IERROR)
            <type>    INBUF(*), OUTBUF(*)
            INTEGER   INSIZE, POSITION, OUTCOUNT, DATATYPE,
                 COMM, IERROR


C++ Syntax
       #include <mpi.h>
       void Datatype::Unpack(const void* inbuf, int insize,
            void *outbuf, int outcount, int& position,
            const Comm& comm) const


INPUT PARAMETERS
       inbuf     Input buffer start (choice).

       insize    Size of input buffer, in bytes (integer).

       outcount  Number of items to be unpacked (integer).

       datatype  Datatype of each output data item (handle).

       comm      Communicator for packed message (handle).


INPUT/OUTPUT PARAMETER
       position  Current position in bytes (integer).


OUTPUT PARAMETERS
       outbuf    Output buffer start (choice).

       IERROR    Fortran only: Error status (integer).


DESCRIPTION
       Unpacks a message into the receive buffer specified by outbuf,
       outcount, datatype from the buffer space specified by inbuf and insize.
       The output buffer can be any communication buffer allowed in MPI_Recv.
       The input buffer is a contiguous storage area containing insize bytes,
       starting at address inbuf. The input value of position is the first
       location in the input buffer occupied by the packed message. position
       is incremented by the size of the packed message, so that the output
       value of position is the first location in the input buffer after the
       locations occupied by the message that was unpacked. comm is the
       communicator used to receive the packed message.


NOTES
       Note the difference between MPI_Recv and MPI_Unpack: In MPI_Recv, the
       count argument specifies the maximum number of items that can be
       received. The actual number of items received is determined by the
       length of the incoming message. In MPI_Unpack, the count argument
       specifies the actual number of items that are to be unpacked; the
       "size" of the corresponding message is the increment in position. The
       reason for this change is that the "incoming message size" is not
       predetermined since the user decides how much to unpack; nor is it easy
       to determine the "message size" from the number of items to be
       unpacked.

       To understand the behavior of pack and unpack, it is convenient to
       think of the data part of a message as being the sequence obtained by
       concatenating the successive values sent in that message. The pack
       operation stores this sequence in the buffer space, as if sending the
       message to that buffer. The unpack operation retrieves this sequence
       from buffer space, as if receiving a message from that buffer. (It is
       helpful to think of internal Fortran files or sscanf in C for a similar
       function.)

       Several messages can be successively packed into one packing unit. This
       is effected by several successive related calls to MPI_Pack, where the
       first call provides position = 0, and each successive call inputs the
       value of position that was output by the previous call, and the same
       values for outbuf, outcount, and comm. This packing unit now contains
       the equivalent information that would have been stored in a message by
       one send call with a send buffer that is the "concatenation" of the
       individual send buffers.

       A packing unit can be sent using type MPI_Packed. Any point-to-point or
       collective communication function can be used to move the sequence of
       bytes that forms the packing unit from one process to another. This
       packing unit can now be received using any receive operation, with any
       datatype: The type-matching rules are relaxed for messages sent with
       type MPI_Packed.

       A message sent with any type (including MPI_Packed) can be received
       using the type MPI_Packed. Such a message can then be unpacked by calls
       to MPI_Unpack.

       A packing unit (or a message created by a regular, "typed" send) can be
       unpacked into several successive messages. This is effected by several
       successive related calls to MPI_Unpack, where the first call provides
       position = 0, and each successive call inputs the value of position
       that was output by the previous call, and the same values for inbuf,
       insize, and comm.

       The concatenation of two packing units is not necessarily a packing
       unit; nor is a substring of a packing unit necessarily a packing unit.
       Thus, one cannot concatenate two packing units and then unpack the
       result as one packing unit; nor can one unpack a substring of a packing
       unit as a separate packing unit. Each packing unit that was created by
       a related sequence of pack calls or by a regular send must be unpacked
       as a unit, by a sequence of related unpack calls.


ERRORS
       Almost all MPI routines return an error value; C routines as the value
       of the function and Fortran routines in the last argument. C++
       functions do not return errors. If the default error handler is set to
       MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism
       will be used to throw an MPI:Exception object.

       Before the error value is returned, the current MPI error handler is
       called. By default, this error handler aborts the MPI job, except for
       I/O function errors. The error handler may be changed with
       MPI_Comm_set_errhandler; 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.


SEE ALSO
       MPI_Pack
       MPI_Pack_size




1.4.1                            Jan 14, 2010                    MPI_Unpack(3)
