.\" Copyright (c) 2009 Linux Foundation, written by Michael Kerrisk
.\"     <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH timer_create 2 2024-06-15 "Linux man-pages 6.9.1"
.SH NAME
timer_create \- create a POSIX per-process timer
.SH LIBRARY
Real-time library
.RI ( librt ", " \-lrt )
.SH SYNOPSIS
.nf
.BR "#include <signal.h>" "           /* Definition of " SIGEV_* " constants */"
.B  #include <time.h>
.P
.BI "int timer_create(clockid_t " clockid ,
.BI "                 struct sigevent *_Nullable restrict " sevp ,
.BI "                 timer_t *restrict " timerid );
.fi
.P
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.P
.BR timer_create ():
.nf
    _POSIX_C_SOURCE >= 199309L
.fi
.SH DESCRIPTION
.BR timer_create ()
creates a new per-process interval timer.
The ID of the new timer is returned in the buffer pointed to by
.IR 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.
.P
The
.I clockid
argument specifies the clock that the new timer uses to measure time.
It can be specified as one of the following values:
.TP
.B CLOCK_REALTIME
A settable system-wide real-time clock.
.TP
.B CLOCK_MONOTONIC
A nonsettable monotonically increasing clock that measures time
from some unspecified point in the past that does not change
after system startup.
.\" Note: the CLOCK_MONOTONIC_RAW clock added for clock_gettime()
.\" in Linux 2.6.28 is not supported for POSIX timers -- mtk, Feb 2009
.TP
.BR 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.
.TP
.BR CLOCK_THREAD_CPUTIME_ID " (since Linux 2.6.12)"
A clock that measures (user and system) CPU time consumed by
the calling thread.
.\" The CLOCK_MONOTONIC_RAW that was added in Linux 2.6.28 can't be used
.\" to create a timer -- mtk, Feb 2009
.TP
.BR CLOCK_BOOTTIME " (Since Linux 2.6.39)"
.\" commit 70a08cca1227dc31c784ec930099a4417a06e7d0
Like
.BR CLOCK_MONOTONIC ,
this is a monotonically increasing clock.
However, whereas the
.B CLOCK_MONOTONIC
clock does not measure the time while a system is suspended, the
.B CLOCK_BOOTTIME
clock does include the time during which the system is suspended.
This is useful for applications that need to be suspend-aware.
.B CLOCK_REALTIME
is not suitable for such applications, since that clock is affected
by discontinuous changes to the system clock.
.TP
.BR CLOCK_REALTIME_ALARM " (since Linux 3.0)"
.\" commit 9a7adcf5c6dea63d2e47e6f6d2f7a6c9f48b9337
This clock is like
.BR CLOCK_REALTIME ,
but will wake the system if it is suspended.
The caller must have the
.B CAP_WAKE_ALARM
capability in order to set a timer against this clock.
.TP
.BR CLOCK_BOOTTIME_ALARM " (since Linux 3.0)"
.\" commit 9a7adcf5c6dea63d2e47e6f6d2f7a6c9f48b9337
This clock is like
.BR CLOCK_BOOTTIME ,
but will wake the system if it is suspended.
The caller must have the
.B CAP_WAKE_ALARM
capability in order to set a timer against this clock.
.TP
.BR CLOCK_TAI " (since Linux 3.10)"
A system-wide clock derived from wall-clock time but counting leap seconds.
.P
See
.BR clock_getres (2)
for some further details on the above clocks.
.P
As well as the above values,
.I clockid
can be specified as the
.I clockid
returned by a call to
.BR clock_getcpuclockid (3)
or
.BR pthread_getcpuclockid (3).
.P
The
.I sevp
argument points to a
.I sigevent
structure that specifies how the caller
should be notified when the timer expires.
For the definition and general details of this structure, see
.BR sigevent (3type).
.P
The
.I sevp.sigev_notify
field can have the following values:
.TP
.B SIGEV_NONE
Don't asynchronously notify when the timer expires.
Progress of the timer can be monitored using
.BR timer_gettime (2).
.TP
.B SIGEV_SIGNAL
Upon timer expiration, generate the signal
.I sigev_signo
for the process.
See
.BR sigevent (3type)
for general details.
The
.I si_code
field of the
.I siginfo_t
structure will be set to
.BR SI_TIMER .
At any point in time,
at most one signal is queued to the process for a given timer; see
.BR timer_getoverrun (2)
for more details.
.TP
.B SIGEV_THREAD
Upon timer expiration, invoke
.I sigev_notify_function
as if it were the start function of a new thread.
See
.BR sigevent (3type)
for details.
.TP
.BR SIGEV_THREAD_ID " (Linux-specific)"
As for
.BR SIGEV_SIGNAL ,
but the signal is targeted at the thread whose ID is given in
.IR sigev_notify_thread_id ,
which must be a thread in the same process as the caller.
The
.I sigev_notify_thread_id
field specifies a kernel thread ID, that is, the value returned by
.BR clone (2)
or
.BR gettid (2).
This flag is intended only for use by threading libraries.
.P
Specifying
.I sevp
as NULL is equivalent to specifying a pointer to a
.I sigevent
structure in which
.I sigev_notify
is
.BR SIGEV_SIGNAL ,
.I sigev_signo
is
.BR SIGALRM ,
and
.I sigev_value.sival_int
is the timer ID.
.SH RETURN VALUE
On success,
.BR timer_create ()
returns 0, and the ID of the new timer is placed in
.IR *timerid .
On failure, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EAGAIN
Temporary error during kernel allocation of timer structures.
.TP
.B EINVAL
Clock ID,
.IR sigev_notify ,
.IR sigev_signo ,
or
.I sigev_notify_thread_id
is invalid.
.TP
.B ENOMEM
.\" glibc layer: malloc()
Could not allocate memory.
.TP
.B ENOTSUP
The kernel does not support creating a timer against this
.IR clockid .
.TP
.B EPERM
.I clockid
was
.B CLOCK_REALTIME_ALARM
or
.B CLOCK_BOOTTIME_ALARM
but the caller did not have the
.B CAP_WAKE_ALARM
capability.
.SH VERSIONS
.SS C library/kernel differences
Part of the implementation of the POSIX timers API is provided by glibc.
.\" See nptl/sysdeps/unix/sysv/linux/timer_create.c
In particular:
.IP \[bu] 3
Much of the functionality for
.B 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
.I sigev_notify
value of
.B SIGEV_THREAD_ID
along with a real-time signal that is reserved by the implementation (see
.BR nptl (7)).
.IP \[bu]
The implementation of the default case where
.I evp
is NULL is handled inside glibc,
which invokes the underlying system call with a suitably populated
.I sigevent
structure.
.IP \[bu]
The timer IDs presented at user level are maintained by glibc,
which maps these IDs to the timer IDs employed by the kernel.
.\" See the glibc source file kernel-posix-timers.h for the structure
.\" that glibc uses to map user-space timer IDs to kernel timer IDs
.\" The kernel-level timer ID is exposed via siginfo.si_tid.
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
Linux 2.6.
POSIX.1-2001.
.P
Prior to Linux 2.6,
glibc provided an incomplete user-space implementation
.RB ( CLOCK_REALTIME
timers only) using POSIX threads,
and before glibc 2.17,
.\" glibc commit 93a78ac437ba44f493333d7e2a4b0249839ce460
the implementation falls back to this technique on systems
running kernels older than Linux 2.6.
.SH NOTES
A program may create multiple interval timers using
.BR timer_create ().
.P
Timers are not inherited by the child of a
.BR fork (2),
and are disarmed and deleted during an
.BR execve (2).
.P
The kernel preallocates a "queued real-time signal"
for each timer created using
.BR timer_create ().
Consequently, the number of timers is limited by the
.B RLIMIT_SIGPENDING
resource limit (see
.BR setrlimit (2)).
.P
The timers created by
.BR timer_create ()
are commonly known as "POSIX (interval) timers".
The POSIX timers API consists of the following interfaces:
.TP
.BR timer_create ()
Create a timer.
.TP
.BR timer_settime (2)
Arm (start) or disarm (stop) a timer.
.TP
.BR timer_gettime (2)
Fetch the time remaining until the next expiration of a timer,
along with the interval setting of the timer.
.TP
.BR timer_getoverrun (2)
Return the overrun count for the last timer expiration.
.TP
.BR timer_delete (2)
Disarm and delete a timer.
.P
Since Linux 3.10, the
.IR /proc/ pid /timers
file can be used to list the POSIX timers for the process with PID
.IR pid .
See
.BR proc (5)
for further information.
.P
Since Linux 4.10,
.\" baa73d9e478ff32d62f3f9422822b59dd9a95a21
support for POSIX timers is a configurable option that is enabled by default.
Kernel support can be disabled via the
.B CONFIG_POSIX_TIMERS
option.
.SH EXAMPLES
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.
.P
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.
.P
.in +4n
.EX
$ \fB./a.out 1 100\fP
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
.EE
.in
.SS Program source
\&
.\" SRC BEGIN (timer_create.c)
.EX
#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); \[rs]
                        } 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\[rs]n", (uintmax_t) *tidp);
\&
    or = timer_getoverrun(*tidp);
    if (or == \-1)
        errExit("timer_getoverrun");
    else
        printf("    overrun count = %d\[rs]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\[rs]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>\[rs]n",
                argv[0]);
        exit(EXIT_FAILURE);
    }
\&
    /* Establish handler for timer signal. */
\&
    printf("Establishing handler for signal %d\[rs]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\[rs]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\[rs]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\[rs]n", atoi(argv[1]));
    sleep(atoi(argv[1]));
\&
    /* Unlock the timer signal, so that timer notification
       can be delivered. */
\&
    printf("Unblocking signal %d\[rs]n", SIG);
    if (sigprocmask(SIG_UNBLOCK, &mask, NULL) == \-1)
        errExit("sigprocmask");
\&
    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.ad l
.nh
.BR clock_gettime (2),
.BR setitimer (2),
.BR timer_delete (2),
.BR timer_getoverrun (2),
.BR timer_settime (2),
.BR timerfd_create (2),
.BR clock_getcpuclockid (3),
.BR pthread_getcpuclockid (3),
.BR pthreads (7),
.BR sigevent (3type),
.BR signal (7),
.BR time (7)