MPI_PACK_EXTERNAL(3)				     Open MPI				      MPI_PACK_EXTERNAL(3)

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
	  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  <#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 <#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>/MPI_Init_thread  <#mpi-init-thread>,
       after  MPI_Finalize  <#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	<#
       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>/MPI_Comm_spawn_multiple	<#
       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 <#mpi-abort> was called on that communicator. If called
	 on a window or file, acts as if MPI_Abort <#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	 <#mpi-comm-create-errhandler>	   then	    MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	<#mpi-file-create-errhandler>	  then	   MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

       Note that MPI does not guarantee that an MPI program can continue past an error.

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

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

       See also:

	  • MPI_Pack_external_size <#mpi-pack-external-size>

	  • MPI_Send <#mpi-send>

	  • MPI_Unpack_external <#mpi-unpack-external>

	  • sscanf(3C)

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				      MPI_PACK_EXTERNAL(3)
