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TIMER_SETTIME(2) System Calls Manual TIMER_SETTIME(2)

timer_getoverrun, timer_gettime, timer_settime — per-process timers (REALTIME)

POSIX Real-time Library (librt, -lrt)

#include <time.h>

int
timer_getoverrun(timer_t timerid);

int
timer_gettime(timer_t timerid, struct itimerspec *value);

int
timer_settime(timer_t timerid, int flags, const struct itimerspec *restrict value, struct itimerspec *restrict ovalue);

The () system call stores the amount of time until the specified timer, timerid, expires and the reload value of the timer into the space pointed to by the value argument. The it_value member of this structure contains the amount of time before the timer expires, or zero if the timer is disarmed. This value is returned as the interval until timer expiration, even if the timer was armed with absolute time. The it_interval member of value contains the reload value last set by timer_settime().

The () system call sets the time until the next expiration of the timer specified by timerid from the it_value member of the value argument and arms the timer if the it_value member of value is non-zero. If the specified timer was already armed when timer_settime() is called, this call resets the time until next expiration to the value specified. If the it_value member of value is zero, the timer is disarmed. If the timer is disarmed, then pending signal is removed.

If the flag TIMER_ABSTIME is not set in the argument flags, () behaves as if the time until next expiration is set to be equal to the interval specified by the it_value member of value. That is, the timer expires in it_value nanoseconds from when the call is made. If the flag TIMER_ABSTIME is set in the argument flags, timer_settime() behaves as if the time until next expiration is set to be equal to the difference between the absolute time specified by the it_value member of value and the current value of the clock associated with timerid. That is, the timer expires when the clock reaches the value specified by the it_value member of value. If the specified time has already passed, the system call succeeds and the expiration notification is made.

The reload value of the timer is set to the value specified by the it_interval member of value. When a timer is armed with a non-zero it_interval, a periodic (or repetitive) timer is specified.

Time values that are between two consecutive non-negative integer multiples of the resolution of the specified timer are rounded up to the larger multiple of the resolution. Quantization error will not cause the timer to expire earlier than the rounded time value.

If the argument ovalue is not NULL, the () system call stores, in the location referenced by ovalue, a value representing the previous amount of time before the timer would have expired, or zero if the timer was disarmed, together with the previous timer reload value. Timers do not expire before their scheduled time.

Only a single signal is queued to the process for a given timer at any point in time. When a timer for which a signal is still pending expires, no signal is queued, and a timer overrun will occur. When a timer expiration signal is accepted by a process, the () system call returns the timer expiration overrun count for the specified timer. The overrun count returned contains the number of extra timer expirations that occurred between the time the signal was generated (queued) and when it was accepted, up to but not including an maximum of {DELAYTIMER_MAX}. If the number of such extra expirations is greater than or equal to {DELAYTIMER_MAX}, then the overrun count is set to {DELAYTIMER_MAX}. The value returned by timer_getoverrun() applies to the most recent expiration signal acceptance for the timer. If no expiration signal has been delivered for the timer, the return value of timer_getoverrun() is unspecified.

If the timer_getoverrun() system call succeeds, it returns the timer expiration overrun count as explained above. Otherwise the value -1 is returned, and the global variable errno is set to indicate the error.


The timer_gettime() and timer_settime() functions return the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

The timer_settime() system call will fail if:

[]
A value structure specified a nanosecond value less than zero or greater than or equal to 1000 million, and the it_value member of that structure did not specify zero seconds and nanoseconds.

These system calls may fail if:

[]
The timerid argument does not correspond to an ID returned by timer_create() but not yet deleted by timer_delete().

The timer_settime() system call may fail if:

[]
The it_interval member of value is not zero and the timer was created with notification by creation of a new thread (sigev_sigev_notify was SIGEV_THREAD) and a fixed stack address has been set in the thread attribute pointed to by sigev_notify_attributes.

The timer_gettime() and timer_settime() system calls may fail if:

[]
Any arguments point outside the allocated address space or there is a memory protection fault.

clock_getres(2), timer_create(2), siginfo(3)

The timer_getoverrun(), timer_gettime(), and timer_settime() system calls conform to IEEE Std 1003.1-2004 (“POSIX.1”).

Support for POSIX per-process timer first appeared in FreeBSD 7.0.

September 11, 2000 Debian