# MPI_Type_struct

Creates a struct datatype

## Synopsis

    #include "mpi.h"
    int MPI_Type_struct(
            int count,
            int blocklens[],
            MPI_Aint indices[],
            MPI_Datatype old_types[],
            MPI_Datatype *newtype )

## Input Parameters

| count | number of blocks (integer) -- also number of entries in arrays array_of_types , array_of_displacements and array_of_blocklengths
| blocklens | number of elements in each block (array)
| indices | byte displacement of each block (array)
| old_types | type of elements in each block (array of handles to datatype objects)

## Output Parameter

newtype
  new datatype (handle)

## Notes

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The MPI standard originally made vague statements about padding and alignment; this was intended to allow the simple definition of structures that could be sent with a count greater than one.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

## 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.

## 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_Errhandler_set`; 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.

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.

Location:type_struct.c