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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |STANDARDS |HISTORY |NOTES |BUGS |EXAMPLES |SEE ALSO |COLOPHON | |
timerfd_create(2) System Calls Manualtimerfd_create(2)timerfd_create, timerfd_settime, timerfd_gettime - timers that notify via file descriptors
Standard C library (libc,-lc)
#include <sys/timerfd.h>int timerfd_create(intclockid, intflags);int timerfd_settime(intfd, intflags,const struct itimerspec *new_value,struct itimerspec *_Nullableold_value);int timerfd_gettime(intfd, struct itimerspec *curr_value);
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 ofsetitimer(2) ortimer_create(2), with the advantage that the file descriptor may be monitored byselect(2),poll(2), andepoll(7). The use of these three system calls is analogous to the use oftimer_create(2),timer_settime(2), andtimer_gettime(2). (There is no analog oftimer_getoverrun(2), since that functionality is provided byread(2), as described below.)timerfd_create()timerfd_create() creates a new timer object, and returns a file descriptor that refers to that timer. Theclockid 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) LikeCLOCK_MONOTONIC, this is a monotonically increasing clock. However, whereas theCLOCK_MONOTONICclock does not measure the time while a system is suspended, theCLOCK_BOOTTIMEclock does include the time during which the system is suspended. This is useful for applications that need to be suspend-aware.CLOCK_REALTIMEis 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 likeCLOCK_REALTIME, but will wake the system if it is suspended. The caller must have theCAP_WAKE_ALARMcapability in order to set a timer against this clock.CLOCK_BOOTTIME_ALARM(since Linux 3.11) This clock is likeCLOCK_BOOTTIME, but will wake the system if it is suspended. The caller must have theCAP_WAKE_ALARMcapability in order to set a timer against this clock. Seeclock_getres(2) for some further details on the above clocks. The current value of each of these clocks can be retrieved usingclock_gettime(2). Starting with Linux 2.6.27, the following values may be bitwise ORed inflags to change the behavior oftimerfd_create():TFD_NONBLOCK Set theO_NONBLOCKfile status flag on the open file description (seeopen(2)) referred to by the new file descriptor. Using this flag saves extra calls tofcntl(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 theO_CLOEXECflag inopen(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 descriptorfd. Thenew_value argument specifies the initial expiration and interval for the timer. Theitimerspec structure used for this argument is described initimerspec(3type).new_value.it_value specifies the initial expiration of the timer, in seconds and nanoseconds. Setting either field ofnew_value.it_value to a nonzero value arms the timer. Setting both fields ofnew_value.it_value to zero disarms the timer. Setting one or both fields ofnew_value.it_interval to nonzero values specifies the period, in seconds and nanoseconds, for repeated timer expirations after the initial expiration. If both fields ofnew_value.it_interval are zero, the timer expires just once, at the time specified bynew_value.it_value. By default, the initial expiration time specified innew_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 byclockid). An absolute timeout can be selected via theflags argument. Theflags argument is a bit mask that can include the following values:TFD_TIMER_ABSTIME Interpretnew_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 innew_value.it_value.TFD_TIMER_CANCEL_ON_SET If this flag is specified along withTFD_TIMER_ABSTIMEand the clock for this timer isCLOCK_REALTIMEorCLOCK_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 futureread(2) from the file descriptor will fail with the errorECANCELED. If theold_value argument is not NULL, then theitimerspec 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 oftimerfd_gettime() following.timerfd_gettime()timerfd_gettime() returns, incurr_value, anitimerspec structure that contains the current setting of the timer referred to by the file descriptorfd. Theit_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 theTFD_TIMER_ABSTIMEflag was specified when setting the timer. Theit_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 bycurr_value.it_value.Operating on a timer file descriptor The file descriptor returned bytimerfd_create() supports the following additional operations:read(2) If the timer has already expired one or more times since its settings were last modified usingtimerfd_settime(), or since the last successfulread(2), then the buffer given toread(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 theread(2), then the call either blocks until the next timer expiration, or fails with the errorEAGAINif the file descriptor has been made nonblocking (via the use of thefcntl(2)F_SETFLoperation to set theO_NONBLOCKflag). Aread(2) fails with the errorEINVALif the size of the supplied buffer is less than 8 bytes. If the associated clock is eitherCLOCK_REALTIMEorCLOCK_REALTIME_ALARM, the timer is absolute (TFD_TIMER_ABSTIME), and the flagTFD_TIMER_CANCEL_ON_SET was specified when callingtimerfd_settime(), thenread(2) fails with the errorECANCELEDif 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 eitherCLOCK_REALTIMEorCLOCK_REALTIME_ALARM, the timer is absolute (TFD_TIMER_ABSTIME), and the flagTFD_TIMER_CANCEL_ON_SET wasnot specified when callingtimerfd_settime(), then a discontinuous negative change to the clock (e.g.,clock_settime(2)) may causeread(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 theread(2) on the file descriptor.poll(2)select(2) (and similar) The file descriptor is readable (theselect(2)readfds argument; thepoll(2)POLLINflag) if one or more timer expirations have occurred. The file descriptor also supports the other file-descriptor multiplexing APIs:pselect(2),ppoll(2), andepoll(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 theCONFIG_CHECKPOINT_RESTOREoption.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 afork(2), the child inherits a copy of the file descriptor created bytimerfd_create(). The file descriptor refers to the same underlying timer object as the corresponding file descriptor in the parent, andread(2)s in the child will return information about expirations of the timer.execve(2) semantics A file descriptor created bytimerfd_create() is preserved acrossexecve(2), and continues to generate timer expirations if the timer was armed.
On success,timerfd_create() returns a new file descriptor. On error, -1 is returned anderrno is set to indicate the error.timerfd_settime() andtimerfd_gettime() return 0 on success; on error they return -1, and seterrno to indicate the error.
timerfd_create() can fail with the following errors:EINVALTheclockid is not valid.EINVALflags is invalid; or, in Linux 2.6.26 or earlier,flags is nonzero.EMFILEThe per-process limit on the number of open file descriptors has been reached.ENFILEThe system-wide limit on the total number of open files has been reached.ENODEVCould not mount (internal) anonymous inode device.ENOMEMThere was insufficient kernel memory to create the timer.EPERMclockid wasCLOCK_REALTIME_ALARMorCLOCK_BOOTTIME_ALARM but the caller did not have theCAP_WAKE_ALARMcapability.timerfd_settime() andtimerfd_gettime() can fail with the following errors:EBADFfd is not a valid file descriptor.EFAULTnew_value,old_value, orcurr_value is not a valid pointer.EINVALfd is not a valid timerfd file descriptor.timerfd_settime() can also fail with the following errors:ECANCELED See NOTES.EINVALnew_value is not properly initialized (one of thetv_nsec falls outside the range zero to 999,999,999).EINVALflags is invalid.
Linux.
Linux 2.6.25, glibc 2.8.
Suppose the following scenario forCLOCK_REALTIMEorCLOCK_REALTIME_ALARMtimer that was created withtimerfd_create(): (1) The timer has been started (timerfd_settime()) with theTFD_TIMER_ABSTIMEandTFD_TIMER_CANCEL_ON_SETflags; (2) A discontinuous change (e.g.,settimeofday(2)) is subsequently made to theCLOCK_REALTIMEclock; and (3) the caller once more callstimerfd_settime() to rearm the timer (without first doing aread(2) on the file descriptor). In this case the following occurs: • Thetimerfd_settime() returns -1 witherrno set toECANCELED. (This enables the caller to know that the previous timer was affected by a discontinuous change to the clock.) • The timeris successfully rearmed with the settings provided in the secondtimerfd_settime() call. (This was probably an implementation accident, but won't be fixed now, in case there are applications that depend on this behaviour.)
Currently,timerfd_create() supports fewer types of clock IDs thantimer_create(2).
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 programProgram 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); }eventfd(2),poll(2),read(2),select(2),setitimer(2),signalfd(2),timer_create(2),timer_gettime(2),timer_settime(2),timespec(3),epoll(7),time(7)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-07-19timerfd_create(2)Pages that refer to this page:alarm(2), eventfd(2), getitimer(2), read(2), signalfd(2), syscalls(2), timer_create(2), itimerspec(3type), pcap_get_required_select_timeout(3pcap), sd-event(3), sd_event_add_time(3), timespec(3type), proc_pid_fd(5), proc_pid_fdinfo(5), time(7), time_namespaces(7)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |