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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 *sevp,
                      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 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.

     CLOCK_BOOTTIME (Since Linux 2.6.39)
            Like  CLOCK_MONOTONIC, this is a monotonically increasing clock.
            However, whereas the CLOCK_MONOTONIC clock does not measure  the
            time  while a system is suspended, the CLOCK_BOOTTIME clock does
            include the time during which the system is suspended.  This  is
            useful   for   applications   that  need  to  be  suspend-aware.
            CLOCK_REALTIME is not suitable for such applications, since that
            clock  is affected by discontinuous changes to the system clock.

     CLOCK_REALTIME_ALARM (since Linux 3.0)
            This clock is like CLOCK_REALTIME, but will wake the  system  if
            it  is suspended.  The caller must have the CAP_WAKE_ALARM capa-
            bility in order to set a timer against this clock.

     CLOCK_BOOTTIME_ALARM (since Linux 3.0)
            This clock is like CLOCK_BOOTTIME, but will wake the  system  if
            it  is suspended.  The caller must have the CAP_WAKE_ALARM capa-
            bility in order to set a timer against this clock.

     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 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:

     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.  See sigevent(7)  for  general  details.   The  si_code
            field  of  the  siginfo_t structure will be set to SI_TIMER.  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.   See  sigevent(7)  for
            details.

     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
            intended only for use by threading libraries.

     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

     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, POSIX.1-2008.

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 cre-
     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.

     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.

 C library/kernel differences
     Part of the implementation of the  POSIX  timers  API  is  provided  by
     glibc.  In particular:

Much of the functionality for SIGEV_THREAD is implemented within

glibc, rather than the kernel. (This is necessarily so, since the

        thread  involved  in  handling  the notification is one that must be
        managed by the C library POSIX  threads  implementation.)   Although
        the  notification  delivered  to the process is via a thread, inter-
        nally  the  NPTL  implementation  uses  a  sigev_notify   value   of
        SIGEV_THREAD_ID  along  with  a real-time signal that is reserved by
        the implementation (see nptl(7)).

The implementation of the default case where evp is NULL is handled

inside glibc, which invokes the underlying system call with a suit-

        ably populated sigevent structure.

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   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.

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 1 100 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

 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 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); }