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

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

       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.