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

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(*)

C++ Syntax

#include <mpi.h>
void MPI::Datatype::Pack_external(const char* datarep,
        const void* inbuf, int incount,
        void* outbuf, MPI::Aint outsize, 
        MPI::Aint& position) const

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

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

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.