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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, 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 and MPI_Unpack_external: In 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, 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_errhandler
• MPI_File_create_errhandler then MPI_File_set_errhandler
• MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init
• MPI_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
• MPI_Pack_external_size
• MPI_Recv
• sscanf(3C)
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Sep 06, 2025 MPI_UNPACK_EXTERNAL(3)