# OpenMP kernel (1D FDTD)

The **OpenMP kernel** distributes independent cell updates among CPU threads while preserving phase order. Here is its parallel region, with array pointers and dimensions prepared before entry:

```c
#pragma omp parallel \
    default(none) \
    firstprivate(Hy, Ez, Ch1, Ch2, Ce1, Ce2, width, nsteps) \
    shared(sim)
{
    for (size_t n = 0; n < nsteps; n++) {

        // Update Hy
        #pragma omp for schedule(static)
        for (size_t k = 0; k < width - 1; k++) {
            Hy[k] = Ch1[k] * Hy[k] 
                + Ch2[k] * (Ez[k + 1] - Ez[k]);
        }

        // Update Ez
        #pragma omp for schedule(static)
        for (size_t k = 1; k < width; k++) {
            Ez[k] = Ce1[k] * Ez[k]
                + Ce2[k] * (Hy[k] - Hy[k - 1]);
        }

        // Inject Ez source
        #pragma omp single
        {
            Fdtd_EzSourcesApply(sim->fsim_sources, sim);
            Fdtd_EzStateNextStep(sim->fsim_state);
        }
    }
}
```

Each magnetic iteration writes a different `Hy[k]`. The electric loop reads the completed magnetic phase, including values updated by other threads. The implicit barriers make those reads safe.

The `single` block applies each source and increments the shared counter once. Select this path with `--set-accel=openmp` or `Fdtd_EzSimSetAccel()`. The library does not automatically switch to serial execution for small grids; benchmark the work per thread and synchronization costs.

Source: [fdtd_ez_kernels.c](https://github.com/yanevskiv/fdtd-openmp/blob/main/src/fdtd/Ez/fdtd_ez_kernels.c).