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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)