TIMER_CREATE(2) | Linux Programmer's Manual | TIMER_CREATE(2) |
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:
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:
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.
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:
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:
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)
This page is part of release 4.16 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
2017-09-15 | Linux |