#include <sys/time.h> int getitimer(int which, struct itimerval *curr_value);
int setitimer(int which, const struct itimerval *new_value, struct itimerval *old_value);
Three types of timers---specified via the which argument---are provided, each of which counts against a different clock and generates a different signal on timer expiration:
In conjunction with ITIMER_VIRTUAL, this timer can be used to profile user and system CPU time consumed by the process.
A process has only one of each of the three types of timers.
Timer values are defined by the following structures:
struct itimerval {
struct timeval it_interval; /* Interval for periodic timer */
struct timeval it_value; /* Time until next expiration */
};
struct timeval {
time_t tv_sec; /* seconds */
suseconds_t tv_usec; /* microseconds */
};
The it_value substructure is populated with the amount of time remaining until the next expiration of the specified timer. This value changes as the timer counts down, and will be reset to it_interval when the timer expires. If both fields of it_value are zero, then this timer is currently disarmed (inactive).
The it_interval substructure is populated with the timer interval. If both fields of it_interval are zero, then this is a single-shot timer (i.e., it expires just once).
If either field in new_value.it_value is nonzero, then the timer is armed to initially expire at the specified time. If both fields in new_value.it_value are zero, then the timer is disarmed.
The new_value.it_interval field specifies the new interval for the timer; if both of its subfields are zero, the timer is single-shot.
A child created via fork(2) does not inherit its parent's interval timers. Interval timers are preserved across an execve(2).
POSIX.1 leaves the interaction between setitimer() and the three interfaces alarm(2), sleep(3), and usleep(3) unspecified.
The standards are silent on the meaning of the call:
setitimer(which, NULL, &old_value);
Many systems (Solaris, the BSDs, and perhaps others) treat this as equivalent to:
getitimer(which, &old_value);
In Linux, this is treated as being equivalent to a call in which the new_value fields are zero; that is, the timer is disabled. Don't use this Linux misfeature: it is nonportable and unnecessary.
On Linux kernels before 2.6.16, timer values are represented in jiffies. If a request is made set a timer with a value whose jiffies representation exceeds MAX_SEC_IN_JIFFIES (defined in include/linux/jiffies.h), then the timer is silently truncated to this ceiling value. On Linux/i386 (where, since Linux 2.6.13, the default jiffy is 0.004 seconds), this means that the ceiling value for a timer is approximately 99.42 days. Since Linux 2.6.16, the kernel uses a different internal representation for times, and this ceiling is removed.
On certain systems (including i386), Linux kernels before version 2.6.12 have a bug which will produce premature timer expirations of up to one jiffy under some circumstances. This bug is fixed in kernel 2.6.12.
POSIX.1-2001 says that setitimer() should fail if a tv_usec value is specified that is outside of the range 0 to 999999. However, in kernels up to and including 2.6.21, Linux does not give an error, but instead silently adjusts the corresponding seconds value for the timer. From kernel 2.6.22 onward, this nonconformance has been repaired: an improper tv_usec value results in an EINVAL error.