Ubuntu Manpage: MPI_Pack_external - Writes data to a portable format

NAME

       MPI_Pack_external - Writes data to a portable format

SYNTAX

C Syntax

       #include <mpi.h>
       int MPI_Pack_external(const char *datarep, const 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_BYTE, 1, 0,
                     MPI_COMM_WORLD);
            } else {       /* RECEIVER CODE */
                 MPI_Recv(buf, 1, MPI_BYTE, 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_BYTE. 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_BYTE.)

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.