MPI_TYPE_CREATE_STRUCT(3)

MPI_Type_create_struct — Creates a structured data type.

SYNTAX

  C Syntax

    #include <mpi.h>
    int MPI_Type_create_struct(int count, int array_of_blocklengths[],
         const MPI_Aint array_of_displacements[], const MPI_Datatype array_of_types[],
         MPI_Datatype *newtype)

  Fortran Syntax

    USE MPI
    ! or the older form: INCLUDE 'mpif.h'
    MPI_TYPE_CREATE_STRUCT(COUNT, ARRAY_OF_BLOCKLENGTHS,
                 ARRAY_OF_DISPLACEMENTS, ARRAY_OF_TYPES, NEWTYPE, IERROR)
         INTEGER COUNT, ARRAY_OF_BLOCKLENGTHS(*), ARRAY_OF_TYPES(*),
         INTEGER NEWTYPE, IERROR
         INTEGER(KIND=MPI_ADDRESS_KIND) ARRAY_OF_DISPLACEMENTS(*)

  Fortran 2008 Syntax

    USE mpi_f08
    MPI_Type_create_struct(count, array_of_blocklengths,
                 array_of_displacements, array_of_types, newtype, ierror)
         INTEGER, INTENT(IN) :: count, array_of_blocklengths(count)
         INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(IN) ::
         array_of_displacements(count)
         TYPE(MPI_Datatype), INTENT(IN) :: array_of_types(count)
         TYPE(MPI_Datatype), INTENT(OUT) :: newtype
         INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS

•   count: Number of blocks (integer) — also number of entries in arrays array_of_types, array_of_displacements, and array_of_blocklengths.

•   array_of_blocklengths: Number of elements in each block (array of integers).

•   array_of_displacements: Byte displacement of each block (array of integers).

•   array_of_types: Type of elements in each block (array of handles to data-type objects).

OUTPUT PARAMETERS

•   newtype: New data type (handle).

•   ierror: Fortran only: Error status (integer).

DESCRIPTION

MPI_Type_create_struct creates a structured data type. This routine replaces MPI_Type_struct, which is now deprecated.

MPI_Type_create_struct is the most general type constructor. It further generalizes MPI_Type_create_hindexed in that it allows each block to consist of replications of different datatypes.

Example 1: Let type1 have type map

    {(double, 0), (char, 8)}

with extent 16. Let B = (2, 1, 3), D = (0, 16, 26), and T = (MPI_FLOAT, type1, MPI_CHAR). Then a call to MPI_Type_create_struct(3, B, D, T, newtype) returns a datatype with type map

    {
     (float, 0), (float,4),             // 2 float
     (double, 16), (char, 24),          // 1 type1
     (char, 26), (char, 27), (char, 28) // 3 char
    }

That is, two copies of MPI_FLOAT starting at 0, followed by one copy of type1 starting at 16, followed by three copies of MPI_CHAR, starting at 26.

Example 2:

An example of a struct with only some components part of the type

    struct MyStruct {
        double x[2], y;
        char a;
        int n;
    };
    // create a new type where we only send x, y and n
    int B[] = {
        2, // 2 double's
        1, // 1 double
        1, // 1 int
        1  // alignment padding
    };
    MPI_Aint D[] = {
        offsetof(struct MyStruct, x),
        offsetof(struct MyStruct, y),
        offsetof(struct MyStruct, n),
        sizeof(struct MyStruct)
    };
    MPI_Datatype T[] = {
        MPI_DOUBLE,
        MPI_DOUBLE,
        MPI_INT,
        MPI_UB
    };
    MPI_Datatype mpi_dt_mystruct;
    MPI_Type_create_struct(4, B, D, T, &mpi_dt_mystruct);
    MPI_Type_commit(&mpi_dt_mystruct);
    // We can now send a struct (omitting a)
    struct MyStruct values[3];
    if ( rank == 0 ) {
        // ... initialize values
        MPI_Send(values, 3, mpi_dt_mystruct, 1, 0, MPI_COMM_WORLD);
    } else if ( rank == 1 ) {
        MPI_Recv(values, 3, mpi_dt_mystruct, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
    }

For more information, see section 5.1.2 of the MPI-4.0 Standard.

NOTES

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

The MPI-1 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);

defining the extent of a datatype as including an epsilon, which 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, a clarification to the standard made epsilon zero. Thus, 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. See the above example.

ERRORS

Almost all MPI routines return an error value; C routines as the return result of the function and Fortran routines in the last argument.

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. If no communication object is associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all other MPI functions.

Open MPI includes three predefined error handlers that can be used:

•   MPI_ERRORS_ARE_FATAL Causes the program to abort all connected MPI processes.

•   MPI_ERRORS_ABORT An error handler that can be invoked on a communicator, window, file, or session. When called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a window or file, acts as if MPI_Abort was called on a communicator containing the group of processes in the corresponding window or file. If called on a session, aborts only the local process.

•   MPI_ERRORS_RETURN Returns an error code to the application.

    MPI applications can also implement their own error handlers by calling:

•   MPI_Comm_create_errhandler then MPI_Comm_set_errhandler

•   MPI_File_create_errhandler then MPI_File_set_errhandler

•   MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init

•   MPI_Win_create_errhandler then MPI_Win_set_errhandler

    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 the Error Handling section of the MPI-3.1 standard for more information.

    SEE ALSO:

•   MPI_Type_struct

•   MPI_Type_create_hindexed

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