# timer_create

## NAME



           timer_create - create a POSIX per-process timer
    


## LIBRARY



           Real-time library (_librt_ , _-lrt_)
    


## SYNOPSIS



           **#include <signal.h>           **/* Definition of **SIGEV_*** constants */
           **#include <time.h>**
    
           **int timer_create(clockid_t** _clockid_**,**
                            **struct sigevent *_Nullable restrict** _sevp_**,**
                            **timer_t *restrict** _timerid_**);**
    
       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** capability 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** capability in order to set a timer against
                  this clock.
    
           **CLOCK_TAI**(since Linux 3.10)
                  A system-wide clock derived from wall-clock time but
                  counting leap seconds.
    
           See clock_getres(2) for some further details on the above clocks.
    
           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(3type).
    
           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(3type) 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(3type) 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.
    
           **ENOTSUP**
                  The kernel does not support creating a timer against this
                  _clockid_.
    
           **EPERM** _clockid_ was **CLOCK_REALTIME_ALARM** or **CLOCK_BOOTTIME_ALARM**
                  but the caller did not have the **CAP_WAKE_ALARM** capability.
    


## VERSIONS



       **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, internally 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 suitably 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.
    


## STANDARDS



           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
           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.
    
           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.
    


## EXAMPLES



           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
    
       **Program source**
    
           #include <signal.h>
           #include <stdint.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <time.h>
           #include <unistd.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)
           {
               int      or;
               timer_t  *tidp;
    
               tidp = si->si_value.sival_ptr;
    
               printf("    sival_ptr = %p; ", si->si_value.sival_ptr);
               printf("    *sival_ptr = %#jx\n", (uintmax_t) *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;
               sigset_t           mask;
               long long          freq_nanosecs;
               struct sigevent    sev;
               struct sigaction   sa;
               struct itimerspec  its;
    
               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 %#jx\n", (uintmax_t) 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);
           }
    


* * *