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man:timerfd_gettime

TIMERFD_CREATE(2) Linux Programmer's Manual TIMERFD_CREATE(2)

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 alter-
     native to the use of setitimer(2) or timer_create(2), with  the  advan-
     tage  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  capa-
            bility 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  capa-
            bility 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  con-
     tains  two  fields,  each of which is in turn a structure of type time-
     spec:
         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 under-
            goes 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  ECAN-
            CELED.
     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_get-
     time() 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  inte-
            gers 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  expi-
            ration,  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 sup-
            plied buffer is less than 8 bytes.
            If the associated clock is either CLOCK_REALTIME or  CLOCK_REAL-
            TIME_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 mul-
            tiplexing 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 expira-
                   tions.  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   check-
                   point/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  cre-
     ated  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 expira-
     tions 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_REAL-
            TIME;
     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  pro-
     gram  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 pro-
         gram
 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 4.16 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/.

Linux 2017-09-15 TIMERFD_CREATE(2)

/data/webs/external/dokuwiki/data/pages/man/timerfd_gettime.txt · Last modified: 2019/05/17 09:47 by 127.0.0.1

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