NAME

       MPI_Unpack - Unpacks a datatype into contiguous memory.

SYNTAX

C Syntax

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

Fortran Syntax

       USE MPI
       ! or the older form: INCLUDE 'mpif.h'
       MPI_UNPACK(INBUF, INSIZE, POSITION, OUTBUF, OUTCOUNT,
            DATATYPE, COMM, IERROR)
            <type>    INBUF(*), OUTBUF(*)
            INTEGER   INSIZE, POSITION, OUTCOUNT, DATATYPE,
                 COMM, IERROR

Fortran 2008 Syntax

       USE mpi_f08
       MPI_Unpack(inbuf, insize, position, outbuf, outcount, datatype, comm,
                 ierror)
            TYPE(*), DIMENSION(..), INTENT(IN) :: inbuf
            TYPE(*), DIMENSION(..) :: outbuf
            INTEGER, INTENT(IN) :: insize, outcount
            INTEGER, INTENT(INOUT) :: position
            TYPE(MPI_Datatype), INTENT(IN) :: datatype
            TYPE(MPI_Comm), INTENT(IN) :: comm
            INTEGER, OPTIONAL, INTENT(OUT) :: 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