fluidsynth/doc/usage/synth_context.txt

/*!

\page synth-context Understanding the "synthesis context"

When reading through the functions exposed via our API, you will often read the
note: "May or may not be called from synthesis context."

The reason for this is that some functions are intentionally not thread-safe.
Or they require to be called from this context to behave correctly.

FluidSynth's rendering engine is implemented by using the "Dispatcher Thread
Pattern". This means that a certain thread @c A which calls one of FluidSynth's
rendering functions, namely

- fluid_synth_process()
- fluid_synth_nwrite_float()
- fluid_synth_write_float()
- fluid_synth_write_s16()

automatically becomes the "synthesis thread". The terms "synthesis context" and
"synthesis thread" are equivalent. A few locations in our API provide hooks
that allow you to interfere this "synthesis context". At those locations you
can register your own custom functions that will always be called by thread
@c A. For this use-case, the following functions are of interest:

- new_fluid_audio_driver2()
- fluid_player_set_playback_callback()
- fluid_sequencer_register_client()

*/