The system provides each process with three interval timers, defined in <
sys/time.h>. The
getitimer() call returns the current value for the timer specified in
which in the structure at
value. The
setitimer() call sets a timer to the specified
value (returning the previous value of the timer if
ovalue is non-nil).
A timer value is defined by the
itimerval structure:
struct itimerval {
struct timeval it_interval; /* timer interval */
struct timeval it_value; /* current value */
};
If
it_value is non-zero, it indicates the time to the next timer expiration. If
it_interval is non-zero, it specifies a value to be used in reloading
it_value when the timer expires. Setting
it_value to 0 disables a timer. Setting
it_interval to 0 causes a timer to be disabled after its next expiration (assuming
it_value is non-zero).
Time values smaller than the resolution of the system clock are rounded up to this resolution (typically 10 milliseconds).
The
ITIMER_REAL timer decrements in real time. A
SIGALRM signal is delivered when this timer expires.
The
ITIMER_VIRTUAL timer decrements in process virtual time. It runs only when the process is executing. A
SIGVTALRM signal is delivered when it expires.
The
ITIMER_PROF timer decrements both in process virtual time and when the system is running on behalf of the process. It is designed to be used by interpreters in statistically profiling the execution of interpreted programs. Each time the
ITIMER_PROF timer expires, the
SIGPROF signal is delivered. Because this signal may interrupt in-progress system calls, programs using this timer must be prepared to restart interrupted system calls.