timer_create(2) - Linux manual page (original) (raw)
timercreate(2) System Calls Manual timercreate(2)
NAME top
timer_create - create a POSIX per-process timerLIBRARY top
Real-time library (_librt_, _-lrt_)SYNOPSIS top
**#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 >= 199309LDESCRIPTION top
**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)](../man2/clock%5Fgetres.2.html) 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)](../man3/clock%5Fgetcpuclockid.3.html) or
[pthread_getcpuclockid(3)](../man3/pthread%5Fgetcpuclockid.3.html).
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)](../man3/sigevent.3type.html).
The _sevp.sigevnotify_ 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)](../man2/timer%5Fgettime.2.html).
**SIGEV_SIGNAL**
Upon timer expiration, generate the signal _sigevsigno_ for
the process. See [sigevent(3type)](../man3/sigevent.3type.html) for general details. The
_sicode_ field of the _siginfot_ 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)](../man2/timer%5Fgetoverrun.2.html) for more details.
**SIGEV_THREAD**
Upon timer expiration, invoke _sigevnotifyfunction_ as if
it were the start function of a new thread. See
[sigevent(3type)](../man3/sigevent.3type.html) for details.
**SIGEV_THREAD_ID** (Linux-specific)
As for **SIGEV_SIGNAL**, but the signal is targeted at the
thread whose ID is given in _sigevnotifythreadid_, which
must be a thread in the same process as the caller. The
_sigevnotifythreadid_ field specifies a kernel thread ID,
that is, the value returned by [clone(2)](../man2/clone.2.html) or [gettid(2)](../man2/gettid.2.html). 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 _sigevnotify_ is **SIGEV_SIGNAL**,
_sigevsigno_ is **SIGALRM**, and _sigevvalue.sivalint_ is the timer ID.RETURN VALUE top
On success, **timer_create**() returns 0, and the ID of the new timer
is placed in _*timerid_. On failure, -1 is returned, and _[errno](../man3/errno.3.html)_ is
set to indicate the error.ERRORS top
**EAGAIN** Temporary error during kernel allocation of timer
structures.
**EINVAL** Clock ID, _sigevnotify_, _sigevsigno_, or
_sigevnotifythreadid_ 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.VERSIONS top
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 _sigevnotify_ value of **SIGEV_THREAD_ID** along with a real-
time signal that is reserved by the implementation (see
[nptl(7)](../man7/nptl.7.html)).
• 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.STANDARDS top
POSIX.1-2008.HISTORY top
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.NOTES top
A program may create multiple interval timers using
**timer_create**().
Timers are not inherited by the child of a [fork(2)](../man2/fork.2.html), and are
disarmed and deleted during an [execve(2)](../man2/execve.2.html).
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)](../man2/setrlimit.2.html)).
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)](../man2/timer%5Fsettime.2.html)
Arm (start) or disarm (stop) a timer.
[timer_gettime(2)](../man2/timer%5Fgettime.2.html)
Fetch the time remaining until the next expiration of a
timer, along with the interval setting of the timer.
[timer_getoverrun(2)](../man2/timer%5Fgetoverrun.2.html)
Return the overrun count for the last timer expiration.
[timer_delete(2)](../man2/timer%5Fdelete.2.html)
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)](../man5/proc.5.html) 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.EXAMPLES top
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 = 10004886Program 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);
}SEE ALSO top
[clock_gettime(2)](../man2/clock%5Fgettime.2.html), [setitimer(2)](../man2/setitimer.2.html), [timer_delete(2)](../man2/timer%5Fdelete.2.html),
[timer_getoverrun(2)](../man2/timer%5Fgetoverrun.2.html), [timer_settime(2)](../man2/timer%5Fsettime.2.html), [timerfd_create(2)](../man2/timerfd%5Fcreate.2.html),
[clock_getcpuclockid(3)](../man3/clock%5Fgetcpuclockid.3.html), [pthread_getcpuclockid(3)](../man3/pthread%5Fgetcpuclockid.3.html), [pthreads(7)](../man7/pthreads.7.html),
[sigevent(3type)](../man3/sigevent.3type.html), [signal(7)](../man7/signal.7.html), [time(7)](../man7/time.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 timercreate(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)