Synchronizing openFrameworks threads

[bitrex]

I have two Thread subclasses in the openFrameworks API that receive data
from a third thread via a lock-free FIFO, e.g:

<https://openframeworks.cc/documentation/utils/ofThreadChannel/>

process it a bit and then transmit it out to their own independent
hardware serial ports.

I am looking for a suggestion on what C++1x technique or library I could
use to allow synchronizing the output data transmission routine timing
to a thread-safe master reference clock, but allow the data input to the
threads to remain asynchronous.

[bitrex]

I guess since the threads are already getting their xmit data via a FIFO
of that type they could also receive timing clock data the same way...