Scroll to navigation

timer_create(2) System Calls Manual timer_create(2)


timer_create - create a POSIX per-process timer


Real-time library (librt, -lrt)


#include <signal.h>           /* Definition of SIGEV_* constants */
#include <time.h>
int timer_create(clockid_t clockid,
                 struct sigevent *_Nullable restrict sevp,
                 timer_t *restrict timerid);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):


_POSIX_C_SOURCE >= 199309L


timer_create() creates a new per-process interval timer. The ID of the new timer is returned in the buffer pointed to by timerid, which must be a non-null pointer. This ID is unique within the process, until the timer is deleted. The new timer is initially disarmed.

The clockid argument specifies the clock that the new timer uses to measure time. It can be specified as one of the following values:

A settable system-wide real-time clock.
A nonsettable monotonically increasing clock that measures time from some unspecified point in the past that does not change after system startup.
A clock that measures (user and system) CPU time consumed by (all of the threads in) the calling process.
A clock that measures (user and system) CPU time consumed by the calling thread.
Like CLOCK_MONOTONIC, this is a monotonically increasing clock. However, whereas the CLOCK_MONOTONIC clock does not measure the time while a system is suspended, the CLOCK_BOOTTIME clock does include the time during which the system is suspended. This is useful for applications that need to be suspend-aware. CLOCK_REALTIME is not suitable for such applications, since that clock is affected by discontinuous changes to the system clock.
This clock is like CLOCK_REALTIME, but will wake the system if it is suspended. The caller must have the CAP_WAKE_ALARM capability in order to set a timer against this clock.
This clock is like CLOCK_BOOTTIME, but will wake the system if it is suspended. The caller must have the CAP_WAKE_ALARM capability in order to set a timer against this clock.
A system-wide clock derived from wall-clock time but ignoring leap seconds.

See clock_getres(2) for some further details on the above clocks.

As well as the above values, clockid can be specified as the clockid returned by a call to clock_getcpuclockid(3) or pthread_getcpuclockid(3).

The sevp argument points to a sigevent structure that specifies how the caller should be notified when the timer expires. For the definition and general details of this structure, see sigevent(7).

The sevp.sigev_notify field can have the following values:

Don't asynchronously notify when the timer expires. Progress of the timer can be monitored using timer_gettime(2).
Upon timer expiration, generate the signal sigev_signo for the process. See sigevent(7) for general details. The si_code field of the siginfo_t structure will be set to SI_TIMER. At any point in time, at most one signal is queued to the process for a given timer; see timer_getoverrun(2) for more details.
Upon timer expiration, invoke sigev_notify_function as if it were the start function of a new thread. See sigevent(7) for details.
As for SIGEV_SIGNAL, but the signal is targeted at the thread whose ID is given in sigev_notify_thread_id, which must be a thread in the same process as the caller. The sigev_notify_thread_id field specifies a kernel thread ID, that is, the value returned by clone(2) or gettid(2). This flag is intended only for use by threading libraries.

Specifying sevp as NULL is equivalent to specifying a pointer to a sigevent structure in which sigev_notify is SIGEV_SIGNAL, sigev_signo is SIGALRM, and sigev_value.sival_int is the timer ID.


On success, timer_create() returns 0, and the ID of the new timer is placed in *timerid. On failure, -1 is returned, and errno is set to indicate the error.


Temporary error during kernel allocation of timer structures.
Clock ID, sigev_notify, sigev_signo, or sigev_notify_thread_id is invalid.
Could not allocate memory.
The kernel does not support creating a timer against this clockid.
clockid was CLOCK_REALTIME_ALARM or CLOCK_BOOTTIME_ALARM but the caller did not have the CAP_WAKE_ALARM capability.


This system call is available since Linux 2.6.


POSIX.1-2001, POSIX.1-2008.


A program may create multiple interval timers using timer_create().

Timers are not inherited by the child of a fork(2), and are disarmed and deleted during an execve(2).

The kernel preallocates a "queued real-time signal" for each timer created using timer_create(). Consequently, the number of timers is limited by the RLIMIT_SIGPENDING resource limit (see setrlimit(2)).

The timers created by timer_create() are commonly known as "POSIX (interval) timers". The POSIX timers API consists of the following interfaces:

Create a timer.
Arm (start) or disarm (stop) a timer.
Fetch the time remaining until the next expiration of a timer, along with the interval setting of the timer.
Return the overrun count for the last timer expiration.
Disarm and delete a timer.

Since Linux 3.10, the /proc/pid/timers file can be used to list the POSIX timers for the process with PID pid. See proc(5) for further information.

Since Linux 4.10, support for POSIX timers is a configurable option that is enabled by default. Kernel support can be disabled via the CONFIG_POSIX_TIMERS option.

C library/kernel differences

Part of the implementation of the POSIX timers API is provided by glibc. In particular:

Much of the functionality for SIGEV_THREAD is implemented within glibc, rather than the kernel. (This is necessarily so, since the thread involved in handling the notification is one that must be managed by the C library POSIX threads implementation.) Although the notification delivered to the process is via a thread, internally the NPTL implementation uses a sigev_notify value of SIGEV_THREAD_ID along with a real-time signal that is reserved by the implementation (see nptl(7)).
The implementation of the default case where evp is NULL is handled inside glibc, which invokes the underlying system call with a suitably populated sigevent structure.
The timer IDs presented at user level are maintained by glibc, which maps these IDs to the timer IDs employed by the kernel.

The POSIX timers system calls first appeared in Linux 2.6. Prior to this, glibc provided an incomplete user-space implementation (CLOCK_REALTIME timers only) using POSIX threads, and before glibc 2.17, the implementation falls back to this technique on systems running kernels older than Linux 2.6.


The program below takes two arguments: a sleep period in seconds, and a timer frequency in nanoseconds. The program establishes a handler for the signal it uses for the timer, blocks that signal, creates and arms a timer that expires with the given frequency, sleeps for the specified number of seconds, and then unblocks the timer signal. Assuming that the timer expired at least once while the program slept, the signal handler will be invoked, and the handler displays some information about the timer notification. The program terminates after one invocation of the signal handler.

In the following example run, the program sleeps for 1 second, after creating a timer that has a frequency of 100 nanoseconds. By the time the signal is unblocked and delivered, there have been around ten million overruns.

$ ./a.out 1 100
Establishing handler for signal 34
Blocking signal 34
timer ID is 0x804c008
Sleeping for 1 seconds
Unblocking signal 34
Caught signal 34

sival_ptr = 0xbfb174f4; *sival_ptr = 0x804c008
overrun count = 10004886

Program source

#include <signal.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>
#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \

} while (0) static void print_siginfo(siginfo_t *si) {
int or;
timer_t *tidp;
tidp = si->si_value.sival_ptr;
printf(" sival_ptr = %p; ", si->si_value.sival_ptr);
printf(" *sival_ptr = %#jx\n", (uintmax_t) *tidp);
or = timer_getoverrun(*tidp);
if (or == -1)
printf(" overrun count = %d\n", or); } static void handler(int sig, siginfo_t *si, void *uc) {
/* Note: calling printf() from a signal handler is not safe
(and should not be done in production programs), since
printf() is not async-signal-safe; see signal-safety(7).
Nevertheless, we use printf() here as a simple way of
showing that the handler was called. */
printf("Caught signal %d\n", sig);
signal(sig, SIG_IGN); } int main(int argc, char *argv[]) {
timer_t timerid;
sigset_t mask;
long long freq_nanosecs;
struct sigevent sev;
struct sigaction sa;
struct itimerspec its;
if (argc != 3) {
fprintf(stderr, "Usage: %s <sleep-secs> <freq-nanosecs>\n",
/* Establish handler for timer signal. */
printf("Establishing handler for signal %d\n", SIG);
sa.sa_flags = SA_SIGINFO;
sa.sa_sigaction = handler;
if (sigaction(SIG, &sa, NULL) == -1)
/* Block timer signal temporarily. */
printf("Blocking signal %d\n", SIG);
sigaddset(&mask, SIG);
if (sigprocmask(SIG_SETMASK, &mask, NULL) == -1)
/* Create the timer. */
sev.sigev_notify = SIGEV_SIGNAL;
sev.sigev_signo = SIG;
sev.sigev_value.sival_ptr = &timerid;
if (timer_create(CLOCKID, &sev, &timerid) == -1)
printf("timer ID is %#jx\n", (uintmax_t) timerid);
/* Start the timer. */
freq_nanosecs = atoll(argv[2]);
its.it_value.tv_sec = freq_nanosecs / 1000000000;
its.it_value.tv_nsec = freq_nanosecs % 1000000000;
its.it_interval.tv_sec = its.it_value.tv_sec;
its.it_interval.tv_nsec = its.it_value.tv_nsec;
if (timer_settime(timerid, 0, &its, NULL) == -1)
/* Sleep for a while; meanwhile, the timer may expire
multiple times. */
printf("Sleeping for %d seconds\n", atoi(argv[1]));
/* Unlock the timer signal, so that timer notification
can be delivered. */
printf("Unblocking signal %d\n", SIG);
if (sigprocmask(SIG_UNBLOCK, &mask, NULL) == -1)


clock_gettime(2), setitimer(2), timer_delete(2), timer_getoverrun(2), timer_settime(2), timerfd_create(2), clock_getcpuclockid(3), pthread_getcpuclockid(3), pthreads(7), sigevent(7), signal(7), time(7)

2023-02-05 Linux man-pages 6.03