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(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.