Ubuntu Manpage: MPI_Unpack_external - Reads data from a portable format

NAME

       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

       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(*)

C++ Syntax

       #include <mpi.h>
       void MPI::Datatype::Unpack_external(const char* datarep,
            const void* inbuf, MPI::Aint insize,
            MPI_Aint& position, void *outbuf,
            int outcount) const

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  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.