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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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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)):
timer_create():
_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:
CLOCK_REALTIME
A settable system-wide real-time clock.
CLOCK_MONOTONIC
A nonsettable monotonically increasing clock that measures
time from some unspecified point in the past that does not
change after system startup.
CLOCK_PROCESS_CPUTIME_ID (since Linux 2.6.12)
A clock that measures (user and system) CPU time consumed
by (all of the threads in) the calling process.
CLOCK_THREAD_CPUTIME_ID (since Linux 2.6.12)
A clock that measures (user and system) CPU time consumed
by the calling thread.
CLOCK_BOOTTIME (Since Linux 2.6.39)
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.
CLOCK_REALTIME_ALARM (since Linux 3.0)
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.
CLOCK_BOOTTIME_ALARM (since Linux 3.0)
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.
CLOCK_TAI (since Linux 3.10)
A system-wide clock derived from wall-clock time but
counting 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(3type).
The sevp.sigev_notify field can have the following values:
SIGEV_NONE
Don't asynchronously notify when the timer expires.
Progress of the timer can be monitored using
timer_gettime(2).
SIGEV_SIGNAL
Upon timer expiration, generate the signal sigev_signo for
the process. See sigevent(3type) 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.
SIGEV_THREAD
Upon timer expiration, invoke sigev_notify_function as if
it were the start function of a new thread. See
sigevent(3type) for details.
SIGEV_THREAD_ID (Linux-specific)
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.
EAGAIN Temporary error during kernel allocation of timer
structures.
EINVAL Clock ID, sigev_notify, sigev_signo, or
sigev_notify_thread_id is invalid.
ENOMEM Could not allocate memory.
ENOTSUP
The kernel does not support creating a timer against this
clockid.
EPERM clockid was CLOCK_REALTIME_ALARM or CLOCK_BOOTTIME_ALARM
but the caller did not have the CAP_WAKE_ALARM capability.
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.
POSIX.1-2008.
Linux 2.6. POSIX.1-2001.
Prior to Linux 2.6, 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.
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:
timer_create()
Create a timer.
timer_settime(2)
Arm (start) or disarm (stop) a timer.
timer_gettime(2)
Fetch the time remaining until the next expiration of a
timer, along with the interval setting of the timer.
timer_getoverrun(2)
Return the overrun count for the last timer expiration.
timer_delete(2)
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.
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 CLOCKID CLOCK_REALTIME
#define SIG SIGRTMIN
#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)
errExit("timer_getoverrun");
else
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);
print_siginfo(si);
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",
argv[0]);
exit(EXIT_FAILURE);
}
/* Establish handler for timer signal. */
printf("Establishing handler for signal %d\n", SIG);
sa.sa_flags = SA_SIGINFO;
sa.sa_sigaction = handler;
sigemptyset(&sa.sa_mask);
if (sigaction(SIG, &sa, NULL) == -1)
errExit("sigaction");
/* Block timer signal temporarily. */
printf("Blocking signal %d\n", SIG);
sigemptyset(&mask);
sigaddset(&mask, SIG);
if (sigprocmask(SIG_SETMASK, &mask, NULL) == -1)
errExit("sigprocmask");
/* 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)
errExit("timer_create");
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)
errExit("timer_settime");
/* Sleep for a while; meanwhile, the timer may expire
multiple times. */
printf("Sleeping for %d seconds\n", atoi(argv[1]));
sleep(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)
errExit("sigprocmask");
exit(EXIT_SUCCESS);
}
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(3type), signal(7), time(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 timer_create(2)
Pages that refer to this page: alarm(2), clock_getres(2), clock_nanosleep(2), execve(2), fork(2), getitimer(2), gettid(2), nanosleep(2), seccomp(2), sigaction(2), syscalls(2), timer_delete(2), timerfd_create(2), timer_getoverrun(2), timer_settime(2), clock_getcpuclockid(3), clockid_t(3type), pthread_getcpuclockid(3), sigevent(3type), timer_t(3type), ualarm(3), usleep(3), proc_pid_timers(5), systemd.exec(5), nptl(7), pthreads(7), time(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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