Ubuntu Manpage: MPI_Unpack - Unpacks a datatype into contiguous memory.

Provided by: openmpi-doc_4.1.6-7ubuntu2_all

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.