# 18.2.83. MPI_Dims_create

MPI_Dims_create — Creates a division of processors in a Cartesian grid.

## 18.2.83.1. SYNTAX

### 18.2.83.1.1. C Syntax

    #include <mpi.h>

    int MPI_Dims_create(int nnodes, int ndims, int dims[])

### 18.2.83.1.2. Fortran Syntax

    USE MPI
    ! or the older form: INCLUDE 'mpif.h'
    MPI_DIMS_CREATE(NNODES, NDIMS, DIMS, IERROR)
         INTEGER NNODES, NDIMS, DIMS(*), IERROR

### 18.2.83.1.3. Fortran 2008 Syntax

    USE mpi_f08
    MPI_Dims_create(nnodes, ndims, dims, ierror)
         INTEGER, INTENT(IN) :: nnodes, ndims
         INTEGER, INTENT(INOUT) :: dims(ndims)
         INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 18.2.83.2. INPUT PARAMETERS

- `nnodes`: Number of nodes in a grid (integer).

- `ndims`: Number of Cartesian dimensions (integer).

## 18.2.83.3. IN/OUT PARAMETER

- `dims`: Integer array of size ndims specifying the number of nodes in each dimension.

## 18.2.83.4. OUTPUT PARAMETER

- `ierror`: Fortran only: Error status (integer).

## 18.2.83.5. DESCRIPTION

For Cartesian topologies, the function MPI_Dims_create helps the user select a balanced distribution of processes per coordinate direction, depending on the number of processes in the group to be balanced and optional constraints that can be specified by the user. One use is to partition all the processes (the size of MPI_COMM_WORLD’s group) into an n-dimensional topology.

The entries in the array dims are set to describe a Cartesian grid with ndims dimensions and a total of nnodes nodes. The dimensions are set to be as close to each other as possible, using an appropriate divisibility algorithm. The caller may further constrain the operation of this routine by specifying elements of array dims. If dims[i] is set to a positive number, the routine will not modify the number of nodes in dimension i; only those entries where dims[i] = 0 are modified by the call.

Negative input values of dims[i] are erroneous. An error will occur if nnodes is not a multiple of ((pi) over (i, dims[i] != 0)) dims[i].

For dims[i] set by the call, dims[i] will be ordered in nonincreasing order. Array dims is suitable for use as input to routine MPI_Cart_create. MPI_Dims_create is local.

Example:

    dims
    before                                       dims
    call         function call           on return
    -----------------------------------------------------
    (0,0)        MPI_Dims_create(6, 2, dims)     (3,2)
    (0,0)        MPI_Dims_create(7, 2, dims)     (7,1)
    (0,3,0)      MPI_Dims_create(6, 3, dims)     (2,3,1)
    (0,3,0)      MPI_Dims_create(7, 3, dims)     erroneous call
    ------------------------------------------------------

## 18.2.83.6. 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.