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       MPI_Pack_external - Writes data to a portable format


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’

            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


       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).


       position  Current position in buffer, in bytes (integer).


       outbuf    Output buffer start (choice).

       IERROR    Fortran only: Error status (integer).


       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

       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,
            } else {       /* RECEIVER CODE */
                 MPI_Recv(buf, 1, MPI_PACKED, 0, 0, MPI_COMM_WORLD,
                 MPI_Unpack_external("external32", buf, 1000,
                     MPI_INT, &i, 1, &position);
                 MPI_Unpack_external("external32", buf, 1000,
                     MPI_DOUBLE, &msg, i, &position);


       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

       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.


       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.