MPI_Pack_external(3)               Open MPI               MPI_Pack_external(3)



NAME
       MPI_Pack_external - Writes data to a portable format


SYNTAX
C Syntax
       #include <mpi.h>
       int MPI_Pack_external(char *datarep, void *inbuf,
            int incount, MPI_Datatype datatype,
            void *outbuf, MPI_Aint outsize,
            MPI_Aint *position)


Fortran Syntax
       INCLUDE 'mpif.h'
       MPI_PACK_EXTERNAL(DATAREP, INBUF, INCOUNT, DATATYPE,
            OUTBUF, OUTSIZE, POSITION, IERROR)

            INTEGER        INCOUNT, DATATYPE, IERROR
            INTEGER (KIND=MPI_ADDRESS_KIND) OUTSIZE, POSITION
            CHARACTER*(*)  DATAREP
            <type>         INBUF(*), OUTBUF(*)


C++ Syntax
       #include <mpi.h>
       void MPI::Datatype::Pack_external(const char* datarep,
            const void* inbuf, int incount,
            void* outbuf, MPI::Aint outsize,
            MPI::Aint& position) const


INPUT PARAMETERS
       datarep   Data representation (string).

       inbuf     Input buffer start (choice).

       incount   Number of input data items (integer).

       datatype  Datatype of each input data item (handle).

       outsize   Output buffer size, in bytes (integer).


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_Pack_external packs data into 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 specified by inbuf, incount and datatype, and may
       be any communication buffer allowed in MPI_Send. The output buffer
       outbuf must be a contiguous storage area containing outsize bytes.

       The input value of position is the first position in outbuf to be used
       for packing (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
       outbuf following the packed message. This way it may be used as input
       to a subsequent call to MPI_Pack_external.


       Example: An example using MPI_Pack_external:

            int position, i;
            double msg[5];
            char buf[1000];

            ...

            MPI_Comm_rank(MPI_COMM_WORLD, &myrank);
            if (myrank == 0) {  /* SENDER CODE */
                 position = 0;
                 i = 5; /* number of doubles in msg[] */
                 MPI_Pack_external("external32", &i, 1, MPI_INT,
                     buf, 1000, &position);
                 MPI_Pack_external("external32", &msg, i, MPI_DOUBLE,
                     buf, 1000, &position);
                 MPI_Send(buf, position, MPI_PACKED, 1, 0,
                     MPI_COMM_WORLD);
            } else {       /* RECEIVER CODE */
                 MPI_Recv(buf, 1, MPI_PACKED, 0, 0, MPI_COMM_WORLD,
                     MPI_STATUS_IGNORE);
                 MPI_Unpack_external("external32", buf, 1000,
                     MPI_INT, &i, 1, &position);
                 MPI_Unpack_external("external32", buf, 1000,
                     MPI_DOUBLE, &msg, i, &position);
            }


NOTES
       The datarep argument specifies the data format. The only valid value in
       the current version of MPI is "external32". The argument is provided
       for future extensibility.

       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_size
       MPI_Send
       MPI_Unpack_external
       sscanf(3C)




1.4.1                            Jan 14, 2010             MPI_Pack_external(3)
