yes of course file handles are thread-specific, you wouldn’t want random bits of code trying 0, 1, 2 … to see what it could get. Actually they are process-specific, but the process running your dispatch queue threads is not you and even if it was you the threads were probably created before you created the filehandles so it wouldn’t have inherited them.

the easiest way is to use a normal thread, start it, select on it, finish it again.

if you want to be properly dispatch_* about it, create a READ (or WRITE) dispatch source with the file descriptor and queue. Then add the handler block and resume it. To do the same thing the select() timeout does you can install a dispatch_after to cancel it again. It’s annoyingly more code but that’s how libdispatch supports filehandles natively. You could easily wrap that in a class and just use when required.