# Data sharing (1D FDTD)

**Data sharing** specifies which variables are shared among OpenMP threads and which have a separate instance per thread. FDTD OpenMP makes these choices explicit on its parallel region:

```c
#pragma omp parallel \
    default(none) \
    firstprivate(Hy, Ez, Ch1, Ch2, Ce1, Ce2, width, nsteps) \
    shared(sim)
```

`firstprivate` gives each thread initialized copies of the array pointers, grid width, and step count. Copying a pointer copies its address, not the array it points to. All threads therefore still access the same fields and coefficients.

The `sim` pointer is shared so the source-processing code can access the common simulation object. The time-loop counter declared inside the region is private to each thread, and the work-sharing loop indices are private too.

`default(none)` requires explicit classification of applicable variables instead of relying on implicit sharing. It helps expose omissions, while correctness still depends on distinct write destinations and the [[fdtd-openmp-barriers|barriers]] between phases.