# Restricted pointers (1D FDTD)

**Restricted pointers** tell a C compiler how memory is accessed through a pointer during its scope. FDTD OpenMP declares local pointers to its field and coefficient arrays with `restrict` in both simulation kernels:

```c
Fdtd_FloatType * restrict Hy = sim->fsim_fields->ff_hy;
Fdtd_FloatType * restrict Ez = sim->fsim_fields->ff_ez;
```

When an object is modified through a restricted pointer, accesses to that object must follow the corresponding restriction. The independently allocated field arrays satisfy the intended separation: updating `Hy[k]` does not also change an overlapping `Ez` element.

This information lets the compiler reason about dependencies without conservatively assuming that the field pointers overlap. It can help with instruction scheduling and automatic vectorization of the spatial loops. The kernels do not contain an explicit `omp simd` directive.

`restrict` does not allocate memory or perform a runtime overlap check. It is a promise that the implementation must honor, separate from the OpenMP rules for [[fdtd-openmp-data-sharing|data sharing]] and synchronization between threads.