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TIMER_CREATE(2) Linux Programmer's Manual TIMER_CREATE(2)
NAME
timer_create - create a POSIX per-process timer
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 >= 199309L
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 fol-
lowing 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.
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 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 */
int sigev_signo; /* Timer expiration signal */
union sigval sigev_value; /* Value accompanying signal or
passed to thread function */
void (*sigev_notify_function) (union sigval);
/* Function used for thread
notifications (SIGEV_THREAD) */
void *sigev_notify_attributes;
/* Attributes for notification thread
(SIGEV_THREAD) */
pid_t sigev_notify_thread_id;
/* ID of thread to signal (SIGEV_THREAD_ID) */
};
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_get-
time(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 signal-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
implementation 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_SIG-
NAL, 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
This system call is available since Linux 2.6.
CONFORMING TO
POSIX.1-2001
NOTES
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 num-
ber 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.
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 imple-
mentation (CLOCK_REALTIME timers only) using POSIX threads, and current glibc falls back to this implementation on sys-
tems running pre-2.6 Linux kernels.
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 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 10
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 <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
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);
}
/* 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);
}
SEE ALSO
clock_gettime(2), setitimer(2), timer_delete(2), timer_settime(2), timer_getoverrun(2), timerfd_create(2), clock_getcpu-
clockid(3), pthread_getcpuclockid(3), pthreads(7), signal(7), time(7)
COLOPHON
This page is part of release 3.25 of the Linux man-pages project. A description of the project, and information about
reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.
Linux 2009-02-20 TIMER_CREATE(2)
