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
- 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).
- 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 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 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 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_external, where the
first call provides
position=0, and each successive 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 successive related calls to MPI_Unpack_external, 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 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)