Ubuntu Manpage: MPI_Unpack_external - Reads data from a portable format

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.