Function background_started 

pub fn background_started() -> bool

Has the process-global background executor been built yet?

C’s twin is epicsAtomicGetIntT(&cbState) != cbInit — the guard callbackSetQueueSize and callbackParallelThreads refuse on (callback.c:107, :162). Sizing knobs are read once, when the pool is constructed, so a write after this returns true changes nothing and has to be reported rather than silently accepted.