struct sigevent:
This element specifies how the calling process is notified once the operation terminates. If the `sigev_notify' element is `SIGEV_NONE', no notification is sent. If it is
`SIGEV_SIGNAL', the signal determined by `sigev_signo' is sent. Otherwise, `sigev_notify' must be `SIGEV_THREAD'. In this case, a thread is created which starts executing the function pointed to by `sigev_notify_function'.