The Great and Terrible implementation of MPI-2

function index


Returns the upper bound on the amount of space needed to pack a message
int MPI_Pack_size(
  int incount,
  MPI_Datatype datatype,
  MPI_Comm comm,
  int *size


[in] count argument to packing call (integer)
[in] datatype argument to packing call (handle)
[in] communicator argument to packing call (handle)
[out] upper bound on size of packed message, in bytes (integer)


The MPI standard document describes this in terms of MPI_Pack, but it applies to both MPI_Pack and MPI_Unpack. That is, the value size is the maximum that is needed by either MPI_Pack or MPI_Unpack.


The call returns an upper bound, rather than an exact bound, since the exact amount of space needed to pack the message may depend on the context (e.g., first message packed in a packing unit may take more space).

Thread and Interrupt Safety

This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as malloc or other non-MPICH runtime routines that are themselves not interrupt-safe.

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.
Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank).
Invalid datatype argument. May be an uncommitted MPI_Datatype (see MPI_Type_commit).
Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK).

Example Code

The following sample code illustrates MPI_Pack_size.

#include "mpi.h"
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char *argv[])
int a[100][100], b[100][100];
    MPI_Datatype row, xpose;
    MPI_Aint sizeofint;
int err, errs = 0;
    int bufsize, position = 0;
    void *buffer;
    int i, j;

/* Initialize a to some known values. */
    for(i = 0; i < 100; i++) {
        for(j = 0; j < 100; j++) {
            a[i][j] = i*1000+j;
            b[i][j] = -1;
    MPI_Init(&argc, &argv);
    MPI_Type_extent(MPI_INT, &sizeofint);
    /* Create datatypes. */
    MPI_Type_vector(100, 1, 100, MPI_INT, &row);
    MPI_Type_hvector(100, 1, sizeofint, row, &xpose);
    /* Pack it. */
    MPI_Pack_size(1, xpose, MPI_COMM_WORLD, &bufsize);
    buffer = (
char *) malloc((unsigned) bufsize);
    /* To improve reporting of problems about operations, we change the error handler to errors return */
    err = MPI_Pack(a, 1, xpose, buffer, bufsize, &position, MPI_COMM_WORLD);
/* Unpack the buffer into b. */
    position = 0;
    err = MPI_Unpack(buffer, bufsize, &position, b, 100*100, MPI_INT, MPI_COMM_WORLD);
    for (i = 0; i < 100; i++) {
        for (j = 0; j < 100; j++) {
            if(b[i][j] != a[j][i]) {
                fprintf(stderr, "b[%d][%d] = %d, should be %d\n", i, j, b[i][j], a[j][i]);fflush(stderr);
    return 0;