MPI_Unpack(3) Open MPI MPI_Unpack(3)
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, 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 lo-
cation 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 loca-
tions occupied by the message that was unpacked. comm is the communica-
tor 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 re-
ceived. 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 op-
eration 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 in-
dividual 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 us-
ing 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 re-
sult 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++ 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
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
4.1.2 Nov 24, 2021 MPI_Unpack(3)
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