A source callback is a function called once per simulation step to apply an excitation. FDTD OpenMP separates the waveform calculation from the field-update kernel, allowing new sources without changing the FDTD loops.

A callback has this signature:

void Source(Fdtd_EzSim *sim, Fdtd_EzSourceData *data);

Register it with Fdtd_EzSimAddSource(sim, Source, parameters). The parameter structure provides six numeric fields and a void * pointer for custom data. Registration copies the structure by value; data referenced by its pointer must remain valid while the callback uses it. A newly created simulation has room for sixteen sources.

Inside the callback, Fdtd_EzSimGetTime() returns the current step counter. Fdtd_EzSimInjectEz() adds the calculated value to $E_z$ at the selected cell. The caller must choose a valid cell index.

Callbacks execute after the electric update and before the counter advances. In the OpenMP kernel, one thread executes the entire source list. Other threads wait before starting the next step, preserving the ordering between injection and field updates.