MPI_Unpack_external(3)

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 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)