# MPI_Type_indexed

Creates an indexed datatype

## Synopsis

    int MPI_Type_indexed(int count,
                        int blocklens[],
                        int indices[],
                        MPI_Datatype old_type,
                        MPI_Datatype *newtype)

## Input Parameters

count
  number of blocks -- also number of entries in indices and blocklens
blocklens
  number of elements in each block (array of nonnegative integers)
indices
  displacement of each block in multiples of old_type (array of integers)
old_type
  old datatype (handle)

## Output Parameter

newtype
  new datatype (handle)

## 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.

The indices are displacements, and are based on a zero origin. A common error is to do something like to following

        integer a(100)
        integer blens(10), indices(10)
        do i=1,10
             blens(i)   = 1
    10       indices(i) = 1 + (i-1)*10
        call MPI_TYPE_INDEXED(10,blens,indices,MPI_INTEGER,newtype,ierr)
        call MPI_TYPE_COMMIT(newtype,ierr)
        call MPI_SEND(a,1,newtype,...)

expecting this to send `a(1),a(11),...` because the indices have values `1,11,...`. Because these are displacements from the beginning of `a`, it actually sends `a(1+1),a(1+11),...`.

If you wish to consider the displacements as indices into a Fortran array, consider declaring the Fortran array with a zero origin

        integer a(0:99)

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

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

MPI_ERR_ARG
  Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., `MPI_ERR_RANK`).

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

Location:type_indexed.c