MPI_Unpack_external(3) Open MPI MPI_Unpack_external(3)
NAME
MPI_Unpack_external - Reads data from a portable format
SYNTAX
C Syntax
#include <mpi.h>
int MPI_Unpack_external(const char datarep[], const void *inbuf,
MPI_Aint insize, MPI_Aint *position,
void *outbuf, int outcount,
MPI_Datatype datatype)
Fortran Syntax
USE MPI
! or the older form: 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(*)
Fortran 2008 Syntax
USE mpi_f08
MPI_Unpack_external(datarep, inbuf, insize, position, outbuf, outcount,
datatype, ierror)
CHARACTER(LEN=*), INTENT(IN) :: datarep
TYPE(*), DIMENSION(..), INTENT(IN) :: inbuf
TYPE(*), DIMENSION(..) :: outbuf
INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(IN) :: insize
INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(INOUT) :: position
INTEGER, INTENT(IN) :: outcount
TYPE(MPI_Datatype), INTENT(IN) :: datatype
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
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 univer-
sal 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 con-
taining 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 following 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 specifies the maximum number of items that
can be received. In MPI_Unpack_external, the outcount argument speci-
fies 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 buf-
fers.
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_external,
where the first call provides position=0, and each successive call in-
puts 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_BYTE. 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_BYTE.
A packing unit can be unpacked into several successive messages. This
is effected by several successive related calls to MPI_Unpack_external,
where the first call provides position=0, and each successive call in-
puts 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 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 must be unpacked as a unit by a se-
quence 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 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)
4.1.2 Nov 24, 2021 MPI_Unpack_external(3)
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