getitimer, setitimer - get or set value of interval timer
int getitimer(int which, struct itimerval *value);
int setitimer(int which, const struct itimerval *value,
struct itimerval *ovalue);
The system provides each process with four interval timers, defined in
<sys/time.h>. The getitimer() function stores the current value of the
timer specified by which into the structure pointed to by value. The
setitimer() function call sets the value of the timer specified by
which to the value specified in the structure pointed to by value, and
if ovalue is not NULL, stores the previous value of the timer in the
structure pointed to by ovalue.
A timer value is defined by the itimerval structure (see gettimeof‐
day(3C)) for the definition of timeval), which includes the following
struct timeval it_interval; /* timer interval */
struct timeval it_value; /* current value */
The it_value member indicates the time to the next timer expiration.
The it_interval member specifies a value to be used in reloading
it_value when the timer expires. Setting it_value to 0 disables a
timer, regardless of the value of it_interval. Setting it_interval to 0
disables a timer after its next expiration (assuming it_value is non-
Time values smaller than the resolution of the system clock are rounded
up to the resolution of the system clock, except for ITIMER_REALPROF,
whose values are rounded up to the resolution of the profiling clock.
The four timers are as follows:
Decrements in real time. A SIGALRM signal is deliv‐
ered to the process when this timer expires.
Decrements in lightweight process (lwp) virtual
time. It runs only when the calling lwp is execut‐
ing. A SIGVTALRM signal is delivered to the calling
lwp when it expires.
Decrements both in lightweight process (lwp) virtual
time and when the system is running on behalf of the
lwp. 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 to the calling lwp.
Because this signal may interrupt in-progress func‐
tions, programs using this timer must be prepared to
restart interrupted functions.
Decrements in real time. It is designed to be used
for real-time profiling of multithreaded programs.
Each time the ITIMER_REALPROF timer expires, one
counter in a set of counters maintained by the sys‐
tem for each lightweight process (lwp) is incre‐
mented. The counter corresponds to the state of the
lwp at the time of the timer tick. All lwps execut‐
ing in user mode when the timer expires are inter‐
rupted into system mode. When each lwp resumes exe‐
cution in user mode, if any of the elements in its
set of counters are non-zero, the SIGPROF signal is
delivered to the lwp. The SIGPROF signal is deliv‐
ered before any other signal except SIGKILL. This
signal does not interrupt any in-progress function.
A siginfo structure, defined in <sys/siginfo.h>, is
associated with the delivery of the SIGPROF signal,
and includes the following members:
si_tstamp; /* high resolution timestamp */
si_syscall; /* current syscall */
si_nsysarg; /* number of syscall arguments */
si_sysarg; /* actual syscall arguments */
si_fault; /* last fault type */
si_faddr; /* last fault address */
si_mstate; /* ticks in each microstate */
The enumeration of microstates (indices into
si_mstate) is defined in <sys/msacct.h>.
Unlike the other interval timers, the ITIMER_REAL‐
PROF interval timer is not inherited across a call
to one of the exec(2) family of functions.
Upon successful completion, 0 is returned. Otherwise, −1 is returned
and errno is set to indicate the error.
The getitimer() and setitimer() functions will fail if:
The specified number of seconds is greater than 100,000,000,
the number of microseconds is greater than or equal to
1,000,000, or the which argument is unrecognized.
See attributes(5) for descriptions of the following attributes:
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
│Interface Stability │ Standard │
│MT-Level │ MT-Safe │
SEE ALSOalarm(2), exec(2), gettimeofday(3C), sleep(3C), sysconf(3C),
The setitimer() function is independent of the alarm(2) and sleep(3C)
The ITIMER_PROF and ITIMER_REALPROF timers deliver the same signal and
have different semantics. They cannot be used together.
The granularity of the resolution of alarm time is platform-dependent.
Jun 15, 2009 GETITIMER(2)