bionic (2) timerfd_create.2.gz

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NAME

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

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 *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 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.

       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 new open file description.  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 contains two fields, each of which  is  in  turn  a  structure  of  type
       timespec:

           struct timespec {
               time_t tv_sec;                /* Seconds */
               long   tv_nsec;               /* Nanoseconds */
           };

           struct itimerspec {
               struct timespec it_interval;  /* Interval for periodic timer */
               struct timespec it_value;     /* Initial expiration */
           };

       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 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.)

       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 argument is neither CLOCK_MONOTONIC nor CLOCK_REALTIME;

       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.

       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 valid a pointer.

       EINVAL fd is not a valid timerfd file descriptor.

       timerfd_settime() can also fail with the following errors:

       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.

VERSIONS

       These  system  calls  are  available  on Linux since kernel 2.6.25.  Library support is provided by glibc
       since version 2.8.

CONFORMING TO

       These system calls are Linux-specific.

BUGS

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

EXAMPLE

       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 <sys/timerfd.h>
       #include <time.h>
       #include <unistd.h>
       #include <stdlib.h>
       #include <stdio.h>
       #include <stdint.h>        /* Definition of uint64_t */

       #define handle_error(msg) \
               do { perror(msg); exit(EXIT_FAILURE); } while (0)

       static void
       print_elapsed_time(void)
       {
           static struct timespec start;
           struct timespec curr;
           static int first_call = 1;
           int secs, nsecs;

           if (first_call) {
               first_call = 0;
               if (clock_gettime(CLOCK_MONOTONIC, &start) == -1)
                   handle_error("clock_gettime");
           }

           if (clock_gettime(CLOCK_MONOTONIC, &curr) == -1)
               handle_error("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[])
       {
           struct itimerspec new_value;
           int max_exp, fd;
           struct timespec now;
           uint64_t exp, tot_exp;
           ssize_t s;

           if ((argc != 2) && (argc != 4)) {
               fprintf(stderr, "%s init-secs [interval-secs max-exp]\n",
                       argv[0]);
               exit(EXIT_FAILURE);
           }

           if (clock_gettime(CLOCK_REALTIME, &now) == -1)
               handle_error("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_exp = 1;
           } else {
               new_value.it_interval.tv_sec = atoi(argv[2]);
               max_exp = atoi(argv[3]);
           }
           new_value.it_interval.tv_nsec = 0;

           fd = timerfd_create(CLOCK_REALTIME, 0);
           if (fd == -1)
               handle_error("timerfd_create");

           if (timerfd_settime(fd, TFD_TIMER_ABSTIME, &new_value, NULL) == -1)
               handle_error("timerfd_settime");

           print_elapsed_time();
           printf("timer started\n");

           for (tot_exp = 0; tot_exp < max_exp;) {
               s = read(fd, &exp, sizeof(uint64_t));
               if (s != sizeof(uint64_t))
                   handle_error("read");

               tot_exp += exp;
               print_elapsed_time();
               printf("read: %llu; total=%llu\n",
                       (unsigned long long) exp,
                       (unsigned long long) tot_exp);
           }

           exit(EXIT_SUCCESS);
       }

SEE ALSO

       eventfd(2), poll(2), read(2), select(2), setitimer(2),  signalfd(2),  timer_create(2),  timer_gettime(2),
       timer_settime(2), epoll(7), time(7)

COLOPHON

       This  page  is  part  of  release  4.15  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/.