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MPI_Pack_external(3) man page (version 1.4.5)

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MPI_Pack_external - Writes data to a portable format


C Syntax

#include <mpi.h>
int MPI_Pack_external(char *datarep, void *inbuf,
    int incount, MPI_Datatype datatype,
    void *outbuf, MPI_Aint outsize,
    MPI_Aint *position)

Fortran Syntax

INCLUDE ’mpif.h’
    <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

Data representation (string).
Input buffer start (choice).
Number of input data items (integer).
Datatype of each input data item (handle).
Output buffer size, in bytes (integer).

Input/Output Parameter

Current position in buffer, in bytes (integer).

Output Parameters

Output buffer start (choice).
Fortran only: Error status (integer).


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. 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_PACKED, 1, 0,
    } else {        /* RECEIVER CODE */
        MPI_Recv(buf, 1, MPI_PACKED, 0, 0, MPI_COMM_WORLD,
        MPI_Unpack_external("external32", buf, 1000,
            MPI_INT, &i, 1, &position);
        MPI_Unpack_external("external32", buf, 1000,
            MPI_DOUBLE, &msg, i, &position);


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_PACKED. 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_PACKED.)

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.


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.

See Also


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