MPI_UNPACK_EXTERNAL(3)				     Open MPI				    MPI_UNPACK_EXTERNAL(3)

MPI_Unpack_external — Reads data from a portable format

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Unpack_external(const char datarep[], const void *inbuf,
	       MPI_Aint insize, MPI_Aint *position,
	       void *outbuf, int outcount,
	       MPI_Datatype datatype)

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'
	  MPI_UNPACK_EXTERNAL(DATAREP, INBUF, INSIZE, POSITION,
	       OUTBUF, OUTCOUNT, DATATYPE, IERROR)

	       INTEGER	       OUTCOUNT, DATATYPE, IERROR
	       INTEGER(KIND=MPI_ADDRESS_KIND) INSIZE, POSITION
	       CHARACTER*(*)   DATAREP
	       <type>	       INBUF(*), OUTBUF(*)

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

INPUT PARAMETERS
       • datarep: Data Representation (string).

       • inbuf: Input buffer start (choice).

       • insize: Size of input buffer, in bytes (integer).

       • outcount: Number of items to be unpacked (integer).

       • datatype: Datatype of each output data item (handle).

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_Unpack_external unpacks data from 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  a  contiguous  storage area pointed to by inbuf containing insize bytes. The output
       buffer can be any communication buffer allowed  in  MPI_Recv  <#mpi-recv>,  and	is  specified  by  outbuf,
       outcount, and datatype.

       The input value of position is the first position in inbuf to be read for unpacking (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 inbuf following the message that was
       unpacked. This way it may be used as input to a subsequent call to MPI_Unpack_external.

NOTES
       Note the difference between MPI_Recv <#mpi-recv> and  MPI_Unpack_external:  In  MPI_Recv	 <#mpi-recv>,  the
       count  argument	specifies  the	maximum	 number of items that can be received. In MPI_Unpack_external, the
       outcount argument specifies the actual number of items that are to be  unpacked.	 With  a  regular  receive
       operation,  the	incoming  message  size	 determines  the  number of components that will be received. With
       MPI_Unpack_external, it is up to the user to specify how many components to unpack, since the user may wish
       to unpack the received message multiple times into various buffers.

       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 <#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>/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 <#mpi-pack-external>

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

	  • MPI_Recv <#mpi-recv>

	  • sscanf(3C)

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				    MPI_UNPACK_EXTERNAL(3)
