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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
          USE MPI
          ! or the older form: 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(*)

   Fortran 2008 Syntax
          USE mpi_f08
          MPI_Pack_external(datarep, inbuf, incount, datatype, outbuf, outsize,
                       position, ierror)
               CHARACTER(LEN=*), INTENT(IN) :: datarep
               TYPE(*), DIMENSION(..), INTENT(IN) :: inbuf
               TYPE(*), DIMENSION(..) :: outbuf
               INTEGER, INTENT(IN) :: incount
               TYPE(MPI_Datatype), INTENT(IN) :: datatype
               INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(IN) :: outsize
               INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(INOUT) :: position
               INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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 return result of the function and
       Fortran routines in the last argument.

       Before the error value is returned, the current MPI  error  handler  associated  with  the  communication
       object  (e.g.,  communicator, window, file) is called.  If no communication object is associated with the
       MPI call, then the call is considered attached to MPI_COMM_SELF and will call the  associated  MPI  error
       handler.   When   MPI_COMM_SELF   is   not  initialized  (i.e.,  before  MPI_Init/MPI_Init_thread,  after
       MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial  error  handler.
       The  initial  error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using
       the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info  key  to  MPI_Comm_spawn/‐
       MPI_Comm_spawn_multiple.   If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN
       error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is  called  for  all
       other MPI functions.

       Open MPI includes three predefined error handlers that can be used:

       • MPI_ERRORS_ARE_FATAL Causes the program to abort all connected MPI processes.

       • MPI_ERRORS_ABORT An error handler that can be invoked on a communicator, window, file, or session. When
         called  on  a  communicator,  it  acts  as if MPI_Abort was called on that communicator. If called on a
         window or file, acts as if MPI_Abort was called on a communicator containing the group of processes  in
         the corresponding window or file. If called on a session, aborts only the local process.

       • MPI_ERRORS_RETURN Returns an error code to the application.

       MPI applications can also implement their own error handlers by calling:

       • MPI_Comm_create_errhandler then MPI_Comm_set_errhandlerMPI_File_create_errhandler then MPI_File_set_errhandlerMPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_initMPI_Win_create_errhandler then MPI_Win_set_errhandler

       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 the Error Handling section of the MPI-3.1 standard for more information.

       SEE ALSO:MPI_Pack_external_sizeMPI_SendMPI_Unpack_externalsscanf(3C)

COPYRIGHT

       2003-2025, The Open MPI Community

                                                  Feb 17, 2025                              MPI_PACK_EXTERNAL(3)