Only one signal that is sent as the result of a timer expiring can be sent to the timer's process at a time. So, if a signal of that timer is still pending, and the timer expires again, no signal is queued, and a timer overrun occurs. timer_getoverrun() returns the number of overruns that have happened due to signals being sent from a timer. It can be run after the timer expiration signal is finally delivered or accepted. The overrun count == the number of _extra_ timer expirations that happened after the most recent signal delivered/accepted was generated and before it was delivered/accepted. timer_getoverrun() can only return a value up to but not including {DELAYTIMER_MAX}. If the number of overruns > {DELAYTIMER_MAX}, timer_getoverrun() returns {DELAYTIMER_MAX}. timer_getoverrun() returns the overrun count on success. timer_getoverrun() returns -1 on failure if timerid is not a valid timer ID, timer_getoverrun() may sets errno == EINVAL