# timerfd_create

## NAME



           timerfd_create, timerfd_settime, timerfd_gettime - timers that
           notify via file descriptors
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <sys/timerfd.h>**
    
           **int timerfd_create(int** _clockid_**, int** _flags_**);**
    
           **int timerfd_settime(int** _fd_**, int** _flags_**,**
                               **const struct itimerspec ***_new_value_**,**
                               **struct itimerspec *_Nullable** _old_value_**);**
           **int timerfd_gettime(int** _fd_**, struct itimerspec ***_curr_value_**);**
    


## DESCRIPTION



           These system calls create and operate on a timer that delivers
           timer expiration notifications via a file descriptor.  They
           provide an alternative to the use of setitimer(2) or
           timer_create(2), with the advantage that the file descriptor may
           be monitored by select(2), poll(2), and epoll(7).
    
           The use of these three system calls is analogous to the use of
           timer_create(2), timer_settime(2), and timer_gettime(2).  (There
           is no analog of timer_getoverrun(2), since that functionality is
           provided by read(2), as described below.)
    
       **timerfd_create()**
           **timerfd_create**() creates a new timer object, and returns a file
           descriptor that refers to that timer.  The _clockid_ argument
           specifies the clock that is used to mark the progress of the
           timer, and must be one of the following:
    
           **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_BOOTTIME**(Since Linux 3.15)
                  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 3.11)
                  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 3.11)
                  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.
    
           See clock_getres(2) for some further details on the above clocks.
    
           The current value of each of these clocks can be retrieved using
           clock_gettime(2).
    
           Starting with Linux 2.6.27, the following values may be bitwise
           ORed in _flags_ to change the behavior of **timerfd_create**():
    
           **TFD_NONBLOCK**
                  Set the **O_NONBLOCK** file status flag on the open file
                  description (see open(2)) referred to by the new file
                  descriptor.  Using this flag saves extra calls to fcntl(2)
                  to achieve the same result.
    
           **TFD_CLOEXEC**
                  Set the close-on-exec (**FD_CLOEXEC**) flag on the new file
                  descriptor.  See the description of the **O_CLOEXEC** flag in
                  open(2) for reasons why this may be useful.
    
           In Linux versions up to and including 2.6.26, _flags_ must be
           specified as zero.
    
       **timerfd_settime()**
           **timerfd_settime**() arms (starts) or disarms (stops) the timer
           referred to by the file descriptor _fd_.
    
           The _new_value_ argument specifies the initial expiration and
           interval for the timer.  The _itimerspec_ structure used for this
           argument is described in itimerspec(3type).
    
           _new_value.it_value_ specifies the initial expiration of the timer,
           in seconds and nanoseconds.  Setting either field of
           _new_value.it_value_ to a nonzero value arms the timer.  Setting
           both fields of _new_value.it_value_ to zero disarms the timer.
    
           Setting one or both fields of _new_value.it_interval_ to nonzero
           values specifies the period, in seconds and nanoseconds, for
           repeated timer expirations after the initial expiration.  If both
           fields of _new_value.it_interval_ are zero, the timer expires just
           once, at the time specified by _new_value.it_value_.
    
           By default, the initial expiration time specified in _new_value_ is
           interpreted relative to the current time on the timer's clock at
           the time of the call (i.e., _new_value.it_value_ specifies a time
           relative to the current value of the clock specified by _clockid_).
           An absolute timeout can be selected via the _flags_ argument.
    
           The _flags_ argument is a bit mask that can include the following
           values:
    
           **TFD_TIMER_ABSTIME**
                  Interpret _new_value.it_value_ as an absolute value on the
                  timer's clock.  The timer will expire when the value of the
                  timer's clock reaches the value specified in
                  _new_value.it_value_.
    
           **TFD_TIMER_CANCEL_ON_SET**
                  If this flag is specified along with **TFD_TIMER_ABSTIME** and
                  the clock for this timer is **CLOCK_REALTIME** or
                  **CLOCK_REALTIME_ALARM** , then mark this timer as cancelable if
                  the real-time clock undergoes a discontinuous change
                  (settimeofday(2), clock_settime(2), or similar).  When such
                  changes occur, a current or future read(2) from the file
                  descriptor will fail with the error **ECANCELED**.
    
           If the _old_value_ argument is not NULL, then the _itimerspec_
           structure that it points to is used to return the setting of the
           timer that was current at the time of the call; see the
           description of **timerfd_gettime**() following.
    
       **timerfd_gettime()**
           **timerfd_gettime**() returns, in _curr_value_ , an _itimerspec_ structure
           that contains the current setting of the timer referred to by the
           file descriptor _fd_.
    
           The _it_value_ field returns the amount of time until the timer will
           next expire.  If both fields of this structure are zero, then the
           timer is currently disarmed.  This field always contains a
           relative value, regardless of whether the **TFD_TIMER_ABSTIME** flag
           was specified when setting the timer.
    
           The _it_interval_ field returns the interval of the timer.  If both
           fields of this structure are zero, then the timer is set to expire
           just once, at the time specified by _curr_value.it_value_.
    
       **Operating on a timer file descriptor**
           The file descriptor returned by **timerfd_create**() supports the
           following additional operations:
    
           read(2)
                  If the timer has already expired one or more times since
                  its settings were last modified using **timerfd_settime**(), or
                  since the last successful read(2), then the buffer given to
                  read(2) returns an unsigned 8-byte integer (_uint64_t_)
                  containing the number of expirations that have occurred.
                  (The returned value is in host byte order—that is, the
                  native byte order for integers on the host machine.)
    
                  If no timer expirations have occurred at the time of the
                  read(2), then the call either blocks until the next timer
                  expiration, or fails with the error **EAGAIN** if the file
                  descriptor has been made nonblocking (via the use of the
                  fcntl(2) **F_SETFL** operation to set the **O_NONBLOCK** flag).
    
                  A read(2) fails with the error **EINVAL** if the size of the
                  supplied buffer is less than 8 bytes.
    
                  If the associated clock is either **CLOCK_REALTIME** or
                  **CLOCK_REALTIME_ALARM** , the timer is absolute
                  (**TFD_TIMER_ABSTIME**), and the flag **TFD_TIMER_CANCEL_ON_SET**
                  was specified when calling **timerfd_settime**(), then read(2)
                  fails with the error **ECANCELED** if the real-time clock
                  undergoes a discontinuous change.  (This allows the reading
                  application to discover such discontinuous changes to the
                  clock.)
    
                  If the associated clock is either **CLOCK_REALTIME** or
                  **CLOCK_REALTIME_ALARM** , the timer is absolute
                  (**TFD_TIMER_ABSTIME**), and the flag **TFD_TIMER_CANCEL_ON_SET**
                  was _not_ specified when calling **timerfd_settime**(), then a
                  discontinuous negative change to the clock (e.g.,
                  clock_settime(2)) may cause read(2) to unblock, but return
                  a value of 0 (i.e., no bytes read), if the clock change
                  occurs after the time expired, but before the read(2) on
                  the file descriptor.
    
           poll(2)
           select(2)
           (and similar)
                  The file descriptor is readable (the select(2) _readfds_
                  argument; the poll(2) **POLLIN** flag) if one or more timer
                  expirations have occurred.
    
                  The file descriptor also supports the other file-descriptor
                  multiplexing APIs: pselect(2), ppoll(2), and epoll(7).
    
           ioctl(2)
                  The following timerfd-specific command is supported:
    
                  **TFD_IOC_SET_TICKS**(since Linux 3.17)
                         Adjust the number of timer expirations that have
                         occurred.  The argument is a pointer to a nonzero
                         8-byte integer (_uint64_t_ *) containing the new number
                         of expirations.  Once the number is set, any waiter
                         on the timer is woken up.  The only purpose of this
                         command is to restore the expirations for the
                         purpose of checkpoint/restore.  This operation is
                         available only if the kernel was configured with the
                         **CONFIG_CHECKPOINT_RESTORE** option.
    
           close(2)
                  When the file descriptor is no longer required it should be
                  closed.  When all file descriptors associated with the same
                  timer object have been closed, the timer is disarmed and
                  its resources are freed by the kernel.
    
       **fork(2) semantics**
           After a fork(2), the child inherits a copy of the file descriptor
           created by **timerfd_create**().  The file descriptor refers to the
           same underlying timer object as the corresponding file descriptor
           in the parent, and read(2)s in the child will return information
           about expirations of the timer.
    
       **execve(2) semantics**
           A file descriptor created by **timerfd_create**() is preserved across
           execve(2), and continues to generate timer expirations if the
           timer was armed.
    


## RETURN VALUE



           On success, **timerfd_create**() returns a new file descriptor.  On
           error, -1 is returned and _errno_ is set to indicate the error.
    
           **timerfd_settime**() and **timerfd_gettime**() return 0 on success; on
           error they return -1, and set _errno_ to indicate the error.
    


## ERRORS



           **timerfd_create**() can fail with the following errors:
    
           **EINVAL** The _clockid_ is not valid.
    
           **EINVAL** _flags_ is invalid; or, in Linux 2.6.26 or earlier, _flags_ is
                  nonzero.
    
           **EMFILE** The per-process limit on the number of open file
                  descriptors has been reached.
    
           **ENFILE** The system-wide limit on the total number of open files has
                  been reached.
    
           **ENODEV** Could not mount (internal) anonymous inode device.
    
           **ENOMEM** There was insufficient kernel memory to create the timer.
    
           **EPERM** _clockid_ was **CLOCK_REALTIME_ALARM** or **CLOCK_BOOTTIME_ALARM**
                  but the caller did not have the **CAP_WAKE_ALARM** capability.
    
           **timerfd_settime**() and **timerfd_gettime**() can fail with the
           following errors:
    
           **EBADF** _fd_ is not a valid file descriptor.
    
           **EFAULT** _new_value_ , _old_value_ , or _curr_value_ is not a valid pointer.
    
           **EINVAL** _fd_ is not a valid timerfd file descriptor.
    
           **timerfd_settime**() can also fail with the following errors:
    
           **ECANCELED**
                  See NOTES.
    
           **EINVAL** _new_value_ is not properly initialized (one of the _tv_nsec_
                  falls outside the range zero to 999,999,999).
    
           **EINVAL** _flags_ is invalid.
    


## STANDARDS



           Linux.
    


## NOTES



           Suppose the following scenario for **CLOCK_REALTIME** or
           **CLOCK_REALTIME_ALARM** timer that was created with **timerfd_create**():
    
           (1)  The timer has been started (**timerfd_settime**()) with the
                **TFD_TIMER_ABSTIME** and **TFD_TIMER_CANCEL_ON_SET** flags;
    
           (2)  A discontinuous change (e.g., settimeofday(2)) is
                subsequently made to the **CLOCK_REALTIME** clock; and
    
           (3)  the caller once more calls **timerfd_settime**() to rearm the
                timer (without first doing a read(2) on the file descriptor).
    
           In this case the following occurs:
    
           •  The **timerfd_settime**() returns -1 with _errno_ set to **ECANCELED**.
              (This enables the caller to know that the previous timer was
              affected by a discontinuous change to the clock.)
    
           •  The timer _is successfully rearmed_ with the settings provided in
              the second **timerfd_settime**() call.  (This was probably an
              implementation accident, but won't be fixed now, in case there
              are applications that depend on this behaviour.)
    


## BUGS



           Currently, **timerfd_create**() supports fewer types of clock IDs than
           timer_create(2).
    


## EXAMPLES



           The following program creates a timer and then monitors its
           progress.  The program accepts up to three command-line arguments.
           The first argument specifies the number of seconds for the initial
           expiration of the timer.  The second argument specifies the
           interval for the timer, in seconds.  The third argument specifies
           the number of times the program should allow the timer to expire
           before terminating.  The second and third command-line arguments
           are optional.
    
           The following shell session demonstrates the use of the program:
    
               $ **a.out 3 1 100**
               0.000: timer started
               3.000: read: 1; total=1
               4.000: read: 1; total=2
               **^Z** # type control-Z to suspend the program
               [1]+  Stopped                 ./timerfd3_demo 3 1 100
               $ **fg** # Resume execution after a few seconds
               a.out 3 1 100
               9.660: read: 5; total=7
               10.000: read: 1; total=8
               11.000: read: 1; total=9
               **^C** # type control-C to suspend the program
    
       **Program source**
    
           #include <err.h>
           #include <inttypes.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <sys/timerfd.h>
           #include <sys/types.h>
           #include <time.h>
           #include <unistd.h>
    
           static void
           print_elapsed_time(void)
           {
               int                     secs, nsecs;
               static int              first_call = 1;
               struct timespec         curr;
               static struct timespec  start;
    
               if (first_call) {
                   first_call = 0;
                   if (clock_gettime(CLOCK_MONOTONIC, &start) == -1)
                       err(EXIT_FAILURE, "clock_gettime");
               }
    
               if (clock_gettime(CLOCK_MONOTONIC, &curr) == -1)
                   err(EXIT_FAILURE, "clock_gettime");
    
               secs = curr.tv_sec - start.tv_sec;
               nsecs = curr.tv_nsec - start.tv_nsec;
               if (nsecs < 0) {
                   secs--;
                   nsecs += 1000000000;
               }
               printf("%d.%03d: ", secs, (nsecs + 500000) / 1000000);
           }
    
           int
           main(int argc, char *argv[])
           {
               int                fd;
               ssize_t            s;
               uint64_t           expir, tot_expir, max_expir;
               struct timespec    now;
               struct itimerspec  new_value;
    
               if (argc != 2 && argc != 4) {
                   fprintf(stderr, "%s init-secs [interval-secs max-num-expir]\n",
                           argv[0]);
                   exit(EXIT_FAILURE);
               }
    
               if (clock_gettime(CLOCK_REALTIME, &now) == -1)
                   err(EXIT_FAILURE, "clock_gettime");
    
               /* Create a CLOCK_REALTIME absolute timer with initial
                  expiration and interval as specified in command line. */
    
               new_value.it_value.tv_sec = now.tv_sec + atoi(argv[1]);
               new_value.it_value.tv_nsec = now.tv_nsec;
               if (argc == 2) {
                   new_value.it_interval.tv_sec = 0;
                   max_expir = 1;
               } else {
                   new_value.it_interval.tv_sec = atoi(argv[2]);
                   max_expir = atoi(argv[3]);
               }
               new_value.it_interval.tv_nsec = 0;
    
               fd = timerfd_create(CLOCK_REALTIME, 0);
               if (fd == -1)
                   err(EXIT_FAILURE, "timerfd_create");
    
               if (timerfd_settime(fd, TFD_TIMER_ABSTIME, &new_value, NULL) == -1)
                   err(EXIT_FAILURE, "timerfd_settime");
    
               print_elapsed_time();
               printf("timer started\n");
    
               for (tot_expir = 0; tot_expir < max_expir;) {
                   s = read(fd, &expir, sizeof(uint64_t));
                   if (s != sizeof(uint64_t))
                       err(EXIT_FAILURE, "read");
    
                   tot_expir += expir;
                   print_elapsed_time();
                   printf("read: %" PRIu64 "; total=%" PRIu64 "\n",
                          expir, tot_expir);
               }
    
               exit(EXIT_SUCCESS);
           }
    


* * *