timer_create - create a POSIX per-process timer
#include <signal.h>
#include <time.h>
int timer_create(clockid_t clockid, struct sigevent *sevp,
timer_t *timerid);
Link with
-lrt.
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.
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:
- 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(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.
- SIGEV_THREAD
- Upon timer expiration, invoke sigev_notify_function as if it were
the start function of a new thread. See sigevent(7) 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.
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:
- *
- 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.
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 in glibc versions before 2.17, the
implementation falls back to this technique on systems running pre-2.6 Linux
kernels.
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
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <signal.h>
#include <time.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)
{
timer_t *tidp;
int or;
tidp = si->si_value.sival_ptr;
printf(" sival_ptr = %p; ", si->si_value.sival_ptr);
printf(" *sival_ptr = 0x%lx\n", (long) *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;
struct sigevent sev;
struct itimerspec its;
long long freq_nanosecs;
sigset_t mask;
struct sigaction sa;
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 0x%lx\n", (long) 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(7),
signal(7),
time(7)