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:

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.

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

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 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:

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 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.

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.

#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_getoverrun(2), timer_settime(2), timerfd_create(2), clock_getcpuclockid(3), pthread_getcpuclockid(3), pthreads(7), sigevent(7), signal(7), time(7)