3.4.3.1. A skeleton of the user’s main program

The following is a skeleton of the user’s main program (or calling program) for the integration of an ODE IVP using the ERKStep module. Most of the steps are independent of the NVECTOR implementation used. For the steps that are not, refer to §9 for the specific name of the function to be called or macro to be referenced.

Initialize parallel or multi-threaded environment, if appropriate.

For example, call MPI_Init to initialize MPI if used, or set num_threads, the number of threads to use within the threaded vector functions, if used.
Create the SUNDIALS simulation context object.

Call SUNContext_Create() to allocate the SUNContext object.
Set problem dimensions, etc.

This generally includes the problem size, N, and may include the local vector length Nlocal.

Note

The variables N and Nlocal should be of type sunindextype.
Set vector of initial values

To set the vector y0 of initial values, use the appropriate functions defined by the particular NVECTOR implementation.

For native SUNDIALS vector implementations (except the CUDA and RAJA based ones), use a call of the form
```
y0 = N_VMake_***(..., ydata);
```
if the sunrealtype array ydata containing the initial values of \(y\) already exists. Otherwise, create a new vector by making a call of the form
```
y0 = N_VNew_***(...);
```
and then set its elements by accessing the underlying data where it is located with a call of the form
```
ydata = N_VGetArrayPointer_***(y0);
```
For details on each of SUNDIALS’ provided vector implementations, see the corresponding sections in §9 for details.
Create ERKStep object

Call arkode_mem = ERKStepCreate(...) to create the ERKStep memory block. ERKStepCreate() returns a void* pointer to this memory structure. See §3.4.3.2.1 for details.
Specify integration tolerances

Call ERKStepSStolerances() or ERKStepSVtolerances() to specify either a scalar relative tolerance and scalar absolute tolerance, or a scalar relative tolerance and a vector of absolute tolerances, respectively. Alternatively, call ERKStepWFtolerances() to specify a function which sets directly the weights used in evaluating WRMS vector norms. See §3.4.3.2.2 for details.
Set optional inputs

Call ERKStepSet* functions to change any optional inputs that control the behavior of ERKStep from their default values. See §3.4.3.2.5 for details.
Specify rootfinding problem

Optionally, call ERKStepRootInit() to initialize a rootfinding problem to be solved during the integration of the ODE system. See §3.4.3.2.3 for general details, and §3.4.3.2.5 for relevant optional input calls.
Advance solution in time

For each point at which output is desired, call
```
ier = ERKStepEvolve(arkode_mem, tout, yout, &tret, itask);
```
Here, itask specifies the return mode. The vector yout (which can be the same as the vector y0 above) will contain \(y(t_\text{out})\). See §3.4.3.2.4 for details.
Get optional outputs

Call ERKStepGet* functions to obtain optional output. See §3.4.3.2.7 for details.
Deallocate memory for solution vector
Upon completion of the integration, deallocate memory for the vector y (or yout) by calling the NVECTOR destructor function:
N_VDestroy(y);
Free solver memory

Call ERKStepFree() to free the memory allocated for the ERKStep module.
Finalize MPI, if used

Call MPI_Finalize to terminate MPI.