You use io_service incorrectly, please see asio tutorial/examples.
while (ios1.poll() > 0) ; while (ios2.poll() > 0) ;
io_services are already running - in other threads - why do you poll() them here? In this example, poll() is there to execute the f() handlers and wait for them to finish.
You are right, not polling the io_services from the main thread makes the f() calls run in the other threads as intended. In that case is there a way to check from the main thread if the io_services finished executing the posted handlers (some kind of a peek())? I have a program where there are several parallel io_threads and I want to run two distinct library functions in each io_service first thing an io_service is started and last thing before an io_service is stopped. These functions are posted from the main thread. My problem is with the function that is to be executed last: how can I wait for that to be executed before stopping the io_service and joining the thread? Calling poll()/dispatch() from the main thread is not an option, it seems. Vil