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MPI_Unpack - Unpacks a datatype into contiguous memory.
int MPI_Unpack(void *inbuf, int insize, int *position,
void *outbuf, int outcount, MPI_Datatype datatype,
MPI_UNPACK(INBUF, INSIZE, POSITION, OUTBUF, OUTCOUNT,
DATATYPE, COMM, IERROR)
<type> INBUF(*), OUTBUF(*)
INTEGER INSIZE, POSITION, OUTCOUNT, DATATYPE,
void Datatype::Unpack(const void* inbuf, int insize,
void *outbuf, int outcount, int& position,
const Comm& comm) const
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).
position Current position in bytes (integer).
outbuf Output buffer start (choice).
IERROR Fortran only: Error status (integer).
Unpacks a message into the receive buffer specified by outbuf, out-
count, 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
received. The actual number of items received is determined by the
length of the incoming message. In MPI_Unpack, the count argument spec-
ifies 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
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
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
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.
Almost all MPI routines return an error value; C routines as the value
of the function and Fortran routines in the last argument. C++ func-
tions do not return errors. If the default error handler is set to
MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism
Open MPI 1.2 September 2006 MPI_Unpack(3OpenMPI)
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