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MPI_Unpack - Unpacks a datatype into contiguous memory.
int MPI_Unpack(const 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
- Input buffer start (choice).
- Size of input
buffer, in bytes (integer).
- Number of items to be unpacked (integer).
- Datatype of each output data item (handle).
- Communicator for
packed message (handle).
- Current position
in bytes (integer).
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.
- Output buffer start (choice).
- Fortran only: Error status (integer).
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.
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
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.
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.
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