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(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)




1.4.1                            Jan 14, 2010           MPI_Unpack_external(3)
