timer_create
SYNOPSIS
#include <signal.h>
#include <time.h>
int timer_create(clockid_t clockid, struct sigevent *evp,
timer_t *timerid);
Link with -lrt.
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
timer_create(): _POSIX_C_SOURCE >= 199309
DESCRIPTION
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 non-settable 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.
As well as the above values, clockid can be specified as the clockid
returned by a call to clock_getcpuclockid(3) or pthread_getcpu-
clockid(3).
The evp argument points to a sigevent structure that specifies how the
caller should be notified when the timer expires. This structure is
defined something like the following:
union sigval {
int sival_int;
void *sival_ptr;
};
struct sigevent {
int sigev_notify; /* Notification method */
Some of these fields may be defined as part of a union: a program
should only employ those fields relevant to the value specified in
sigev_notify. This 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. If sigev_signo is a real-time signal, then it will be
accompanied by the data specified in sigev_value (like the sig-
nal-accompanying data for sigqueue(2)). 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. (Among the implementa-
tion possibilities here are that each timer notification could
result in the creation of a new thread, or that a single thread
is created to receive all notifications.) The function is
invoked with sigev_value as its sole argument. If
sigev_notify_attributes is not NULL, it should point to a
pthread_attr_t structure that defines attributes for the new
thread (see pthread_attr_init(3)).
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
only intended for use by threading libraries.
Specifying evp 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.
RETURN VALUE
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.
ERRORS
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.
VERSIONS
ated using timer_create(). Consequently, the number of timers is lim-
ited 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.
Part of the implementation of the POSIX timers API is provided by
glibc. In particular:
* The functionality for SIGEV_THREAD is implemented within glibc,
rather than the kernel.
* 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 userspace implementation
(CLOCK_REALTIME timers only) using POSIX threads, and current glibc
falls back to this implementation on systems running pre-2.6 Linux ker-
nels.
EXAMPLE
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 invoca-
tion 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 mil-
lion overruns.
$ ./a.out 1 10
Establishing handler for signal 34
Blocking signal 34
timer ID is 0x804c008
#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
strictly correct, since printf() is not async-signal-safe;
see signal(7) */
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);
}
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);
}
SEE ALSO
clock_gettime(2), setitimer(2), timer_delete(2), timer_settime(2),
timer_getoverrun(2), timerfd_create(2), clock_getcpuclockid(3),
pthread_getcpuclockid(3), pthreads(7), signal(7), time(7)
COLOPHON
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