Thanks folks! Forward progress is made...

Unfortunately, programs don't seem to write out their threadscope event logs until they terminate, and mine hangs until I kill it, so I can't get at the event log.

Tracing has taught me that before the hang-cause, my program splits its time in pthread_cond_wait in two different threads, and select in a third. After the hang, it no longer calls select and one of those pthread_cond_waits  in the other. In the version without -threaded that doesn't hang, it never does any pthread_cond_wait and never misses the select.

Now to go figure out what impossible condition it's waiting on, I guess.