The Great and Terrible implementation of MPI-2

function index


Packs a datatype into contiguous memory
int MPI_Pack(
  void *inbuf,
  int incount,
  MPI_Datatype datatype,
  void *outbuf,
  int outcount,
  int *position,
  MPI_Comm comm


[in] input buffer
[in] input count
[in] datatype
[in] output buffer
[in] output count
[in] position
[in] communicator


Packs the message in the send buffer specified by inbuf, incount, datatype into the buffer space specified by outbuf and outcount. The input buffer can be any communication buffer allowed in MPI_SEND. The output buffer is a contiguous storage area containing outsize bytes, starting at the address outbuf (length is counted in bytes, not elements, as if it were a communication buffer for a message of type MPI_PACKED).

The input value of position is the first location in the output buffer to be used for packing. position is incremented by the size of the packed message, and the output value of position is the first location in the output buffer following the locations occupied by the packed message. The comm argument is the communicator that will be subsequently used for sending the packed message.

Notes for Fortran

All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK) have an additional argument ierr at the end of the argument list. ierr is an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the call statement.

All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran.


All MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with MPI_Comm_set_errhandler (for communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for RMA windows). The MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

No error; MPI routine completed successfully.

Example Code

The following sample code illustrates MPI_Pack.

#include "mpi.h"
#include <stdio.h>

int main(int argc, char *argv[])
    int rank, size;
    int i;
    char c[100];
    char buffer[110];
    int position = 0;
    MPI_Status status;

    MPI_Init(&argc, &argv);
    MPI_Comm_size(MPI_COMM_WORLD, &size);
    if (size < 2)
        printf("Please run with 2 processes.\n");fflush(stdout);
        return 1;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    if (rank == 0)
        for (i=0; i<100; i++)
            c[i] = i;
        i = 123;
        MPI_Pack(&i, 1, MPI_INT, buffer, 110, &position, MPI_COMM_WORLD);
        MPI_Pack(c, 100, MPI_CHAR, buffer, 110, &position, MPI_COMM_WORLD);
        MPI_Send(buffer, position, MPI_PACKED, 1, 100, MPI_COMM_WORLD);

    if (rank == 1)
        MPI_Recv(buffer, 110, MPI_PACKED, 0, 100, MPI_COMM_WORLD, &status);
        MPI_Unpack(buffer, 110, &position, &i, 1, MPI_INT, MPI_COMM_WORLD);
        MPI_Unpack(buffer, 110, &position, c, 100, MPI_CHAR, MPI_COMM_WORLD);
        printf("i=%d\nc[0] = %d\n...\nc[99] = %d\n", i, (int)c[0], (int)c[99]);fflush(stdout);

    return 0;