timer_create — create a POSIX per-process timer
#include <signal.h> #include <time.h>
int timer_create( |
clockid_t clockid, |
struct sigevent *evp, | |
timer_t *timerid) ; |
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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.
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_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
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_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.
Temporary error during kernel allocation of timer structures.
Clock ID, sigev_notify
, sigev_signo
, or
sigev_notify_thread_id
is invalid.
Could not allocate memory.
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.
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
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 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
#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); }
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)
This page is part of release 3.25 of the Linux man-pages
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description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
Copyright (c) 2009 Linux Foundation, written by Michael Kerrisk <mtk.manpagesgmail.com> Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. |