Hi,
The "callback" flag instructs the Pd scheduler to run within callbacks from the audio system; otherwise the audio system might still be opened in callback mode (in portaudio or jack) or not (in alsa or mmio) but if yes, the scheduler and the callback routine communicate via FIFOs... I chose whichever seemed to allow the lowest latency, which depends on the system.
Yes, it is good to have that choice. Is there a way to enable this worthy parameter from the command line or perhaps a way of setting it statically, maybe in s_audio_jack.c?
all the best, P