DeinoMPI

The Great and Terrible implementation of MPI-2

MPI_Type_struct

Creates a struct datatype

int MPI_Type_struct(
  int count,
  int blocklens[],
  MPI_Aint indices[],
  MPI_Datatype old_types[],
  MPI_Datatype *newtype
);

Parameters

count: [in] number of blocks (integer) -- also number of entries in arrays array_of_types , array_of_displacements and array_of_blocklengths
blocklens: [in] number of elements in each block (array)
indices: [in] byte displacement of each block (array)
old_types: [in] type of elements in each block (array of handles to datatype objects)
newtype: [out] new datatype (handle)

Deprecated Function

The MPI-2 standard deprecated a number of routines because MPI-2 provides better versions. This routine is one of those that was deprecated. The routine may continue to be used, but new code should use the replacement routine. The replacement for this routine is MPI_Type_create_struct

Remarks

If an upperbound is set explicitly by using the MPI datatype MPI_UB, the corresponding index must be positive.

The MPI standard originally made vague statements about padding and alignment; this was intended to allow the simple definition of structures that could be sent with a count greater than one. For example,

    struct { int a; char b; } foo;

may have sizeof(foo) > sizeof(int) + sizeof(char); for example, sizeof(foo) == 2*sizeof(int). The initial version of the MPI standard defined the extent of a datatype as including an epsilon that would have allowed an implementation to make the extent an MPI datatype for this structure equal to 2*sizeof(int). However, since different systems might define different paddings, there was much discussion by the MPI Forum about what was the correct value of epsilon, and one suggestion was to define epsilon as zero. This would have been the best thing to do in MPI 1.0, particularly since the MPI_UB type allows the user to easily set the end of the structure. Unfortunately, this change did not make it into the final document. Currently, this routine does not add any padding, since the amount of padding needed is determined by the compiler that the user is using to build their code, not the compiler used to construct the MPI library. A later version of MPICH may provide for some natural choices of padding (e.g., multiple of the size of the largest basic member), but users are advised to never depend on this, even with vendor MPI implementations. Instead, if you define a structure datatype and wish to send or receive multiple items, you should explicitly include an MPI_UB entry as the last member of the structure. For example, the following code can be used for the structure foo

    blen[0] = 1; indices[0] = 0; oldtypes[0] = MPI_INT;
    blen[1] = 1; indices[1] = &foo.b - &foo; oldtypes[1] = MPI_CHAR;
    blen[2] = 1; indices[2] = sizeof(foo); oldtypes[2] = MPI_UB;
    MPI_Type_struct( 3, blen, indices, oldtypes, &newtype );

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 mallocor 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. ierris 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 INTEGERin Fortran.

Errors

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.

MPI_SUCCESS: No error; MPI routine completed successfully.

MPI_ERR_TYPE: Invalid datatype argument. May be an uncommitted MPI_Datatype (see MPI_Type_commit).

MPI_ERR_COUNT: Invalid count argument. Count arguments must be non-negative; a count of zero is often valid.

MPI_ERR_INTERN: This error is returned when some part of the MPICH implementation is unable to acquire memory.

Example Code

The following sample code illustrates MPI_Type_struct.

#include "mpi.h"
#include <stdio.h>

/*
* This test checks to see if we can create a simple datatype
* made from many contiguous copies of a single struct. The
* struct is built with monotone decreasing displacements to
* avoid any struct->contig optimizations.
*/

int main( int argc, char **argv )
{
    int blocklens[8], psize, i, rank;
    MPI_Aint displs[8];
    MPI_Datatype oldtypes[8];
    MPI_Datatype ntype1, ntype2;

    MPI_Init( &argc, &argv );
    MPI_Comm_rank( MPI_COMM_WORLD, &rank );

    for (i=0; i<8; i++) {
        blocklens[i] = 1;
        displs[i] = (7-i) * sizeof(long);
        oldtypes[i] = MPI_LONG;
    }
    MPI_Type_struct( 8, blocklens, displs, oldtypes, &ntype1 );
    MPI_Type_contiguous( 65536, ntype1, &ntype2 );
    MPI_Type_commit( &ntype2 );

    MPI_Pack_size( 2, ntype2, MPI_COMM_WORLD, &psize );

    MPI_Type_free( &ntype2 );
    MPI_Type_free( &ntype1 );

    MPI_Finalize();
    return 0;
}

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