> would be simpler? In usage, it would cost checking a flag for each
> callout:
> 
> 	if (c->flags & CALLOUT_THREAD) {
> 		/* schedule thread */
> 	} else {
> 		/* call function */
> 	}
> 
> .. probably not too expensive?

it's what i specifically wanted to avoid by this code,
because it adds "some" overheads to common code for rarely used functionality.
but if people feel it better (it seems so), i can do it.

YAMAMOTO Takashi