MPI_Unpack_external(3OpenMPI) MPI_Unpack_external(3OpenMPI)
NAME
MPI_Unpack_external - Reads data from a portable format
SYNTAX
C Syntax
#include <mpi.h>
int MPI_Unpack_external(char *datarep, void *inbuf,
MPI_Aint insize, MPI_Aint *position,
void *outbuf, int outcount,
MPI_Datatype datatype)
Fortran Syntax
INCLUDE 'mpif.h'
MPI_UNPACK_EXTERNAL(DATAREP, INBUF, INSIZE, POSITION,
OUTBUF, OUTCOUNT, DATATYPE, IERROR)
INTEGER OUTCOUNT, DATATYPE, IERROR
INTEGER (KIND=MPI_ADDRESS_KIND) INSIZE, POSITION
CHARACTER*(*) DATAREP
<type> INBUF(*), OUTBUF(*)
C++ Syntax
#include <mpi.h>
void MPI::Datatype::Unpack_external(const char* datarep,
const void* inbuf, MPI::Aint insize,
MPI_Aint& position, void *outbuf,
int outcount) const
INPUT PARAMETERS
datarep Data Representation (string).
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).
INPUT/OUTPUT PARAMETER
position Current position in buffer, in bytes (integer).
OUTPUT PARAMETERS
outbuf Output buffer start (choice).
IERROR Fortran only: Error status (integer).
DESCRIPTION
MPI_Unpack_external unpacks data from the external32 format, a universal data representation defined
by the MPI Forum. This format is useful for exchanging data between MPI implementations, or when
writing data to a file.
The input buffer is a contiguous storage area pointed to by inbuf containing insize bytes. The output
buffer can be any communication buffer allowed in MPI_Recv, and is specified by outbuf, outcount, and
datatype.
The input value of position is the first position in inbuf to be read for unpacking (measured in
bytes, not elements, relative to the start of the buffer). When the function returns, position is
incremented by the size of the packed message, so that it points to the first location in inbuf fol-lowing following
lowing the message that was unpacked. This way it may be used as input to a subsequent call to
MPI_Unpack_external.
NOTES
Note the difference between MPI_Recv and MPI_Unpack_external: In MPI_Recv, the count argument speci-fies specifies
fies the maximum number of items that can be received. In MPI_Unpack_external, the outcount argument
specifies the actual number of items that are to be unpacked. With a regular receive operation, the
incoming message size determines the number of components that will be received. With
MPI_Unpack_external, it is up to the user to specify how many components to unpack, since the user
may wish to unpack the received message multiple times into various buffers.
To understand the behavior of pack and unpack, it is convenient to think of the data part of a mes-sage message
sage 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 suc-cessive successive
cessive related calls to MPI_Pack_external, where the first call provides position=0, and each suc-cessive successive
cessive call inputs the value of position that was output by the previous call, along with the same
values for outbuf and outcount. 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 packing unit can be unpacked into several successive messages. This is effected by several succes-sive successive
sive related calls to MPI_Unpack_external, where the first call provides position=0, and each succes-sive successive
sive call inputs the value of position that was output by the previous call, and the same values for
inbuf and insize.
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 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.
See the MPI man page for a full list of MPI error codes.
SEE ALSO
MPI_Pack_external
MPI_Pack_external_size
MPI_Recv
sscanf(3C)
Open MPI 1.2 September 2006 MPI_Unpack_external(3OpenMPI)
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