NAME
timerfdcreate, timerfdsettime, timerfdgettime - timers that notify via file descriptors SYNOPSIS
#include
int timerfdcreate(int clockid, int flags); int timerfdsettime(int fd, int flags, const struct itimerspec *newvalue, struct itimerspec *oldvalue); int timerfdgettime(int fd, struct itimerspec *currvalue); 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 timercreate(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 timercreate(2), timersettime(2), and timergettime(2). (There is no analog of timergetoverrun(2), since that functionality is provided by read(2), as described below.) timerfdcreate() timerfdcreate() 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 either CLOCKREALTIME or CLOCKMONOTONIC. CLOCKREALTIME is a settable system-wide clock. CLOCKMONOTONIC is a nonsettable clock that is not affected by discontinuous changes in the system clock (e.g., manual changes to system time). The current value of each of these clocks can be retrieved using clockgettime(2). Starting with Linux 2.6.27, the following values may be bitwise ORed in flags to change the behavior of timerfdcreate(): TFDNONBLOCK Set the ONONBLOCK file status flag on the new open file description. Using this flag saves extra calls to fcntl(2) to achieve the same result.
TFDCLOEXEC Set the close-on-exec (FDCLOEXEC) flag on the new file descriptor. See the description of the OCLOEXEC 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. timerfdsettime() timerfdsettime() arms (starts) or disarms (stops) the timer referred to by the file descriptor fd. The newvalue argument specifies the initial expiration and interval for the timer. The itimer structure used for this argument contains two fields, each of which is in turn a structure of type timespec: struct timespec { timet tvsec; /* Seconds */ long tvnsec; /* Nanoseconds */ }; struct itimerspec { struct timespec itinterval; /* Interval for periodic timer */ struct timespec itvalue; /* Initial expiration */ }; newvalue.itvalue specifies the initial expiration of the timer, in seconds and nanoseconds. Setting either field of newvalue.itvalue to a nonzero value arms the timer. Setting both fields of newvalue.itvalue to zero disarms the timer. Setting one or both fields of newvalue.itinterval to nonzero values specifies the period, in seconds and nanoseconds, for repeated timer expirations after the initial expiration. If both fields of newvalue.itinterval are zero, the timer expires just once, at the time specified by newvalue.itvalue. The flags argument is either 0, to start a relative timer (newvalue.itvalue specifies a time relative to the current value of the clock specified by clockid), or TFDTIMERABSTIME, to start an absolute timer (newvalue.itvalue specifies an absolute time for the clock specified by clockid; that is, the timer will expire when the value of that clock reaches the value specified in newvalue.itvalue). If the oldvalue 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 timerfdget‐ time() following. timerfdgettime() timerfdgettime() returns, in currvalue, an itimerspec structure that contains the current setting of the timer referred to by the file descriptor fd. The itvalue 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 TFDTIMERABSTIME flag was specified when setting the timer. The itinterval 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 currvalue.itvalue. Operating on a timer file descriptor The file descriptor returned by timerfdcreate() supports the following operations: read(2) If the timer has already expired one or more times since its settings were last modified using timerfdsettime(), or since the last successful read(2), then the buffer given to read(2)
returns an unsigned 8-byte integer (uint64t) containing the number of expirations that have occurred. (The returned value is in host byte order, i.e., 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 expi‐ ration, or fails with the error EAGAIN if the file descriptor has been made nonblocking (via the use of the fcntl(2) FSETFL operation to set the ONONBLOCK flag). A read(2) will fail with the error EINVAL if the size of the supplied buffer is less than 8 bytes. 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). 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 timerfdcreate(). 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 timerfdcreate() is preserved across execve(2), and continues to generate timer expirations if the timer was armed. RETURN VALUE On success, timerfdcreate() returns a new file descriptor. On error,
-1 is returned and errno is set to indicate the error. timerfdsettime() and timerfdgettime() return 0 on success; on error
they return -1, and set errno to indicate the error. ERRORS timerfdcreate() can fail with the following errors: EINVAL The clockid argument is neither CLOCKMONOTONIC nor CLOCKREAL‐ TIME; EINVAL flags is invalid; or, in Linux 2.6.26 or earlier, flags is nonzero.
EMFILE The per-process limit 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. timerfdsettime() and timerfdgettime() can fail with the following errors: EBADF fd is not a valid file descriptor. EFAULT newvalue, oldvalue, or currvalue is not valid a pointer. EINVAL fd is not a valid timerfd file descriptor. timerfdsettime() can also fail with the following errors: EINVAL newvalue is not properly initialized (one of the tvnsec 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, timerfdcreate() supports fewer types of clock IDs than timercreate(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 ./timerfd3demo 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
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/* Definition of uint64t */ #define handleerror(msg) \ do { perror(msg); exit(EXITFAILURE); } while (0) static void printelapsedtime(void) { static struct timespec start; struct timespec curr; static int firstcall = 1; int secs, nsecs; if (firstcall) { firstcall = 0;
if (clockgettime(CLOCKMONOTONIC, &start) == -1) handleerror("clockgettime"); }
if (clockgettime(CLOCKMONOTONIC, &curr) == -1) handleerror("clockgettime");
secs = curr.tvsec - start.tvsec;
nsecs = curr.tvnsec - start.tvnsec; if (nsecs < 0) { secs; nsecs += 1000000000; }
printf("%d.%03d: ", secs, (nsecs + 500000) / 1000000); } int main(int argc, char *argv[]) { struct itimerspec newvalue; int maxexp, fd; struct timespec now; uint64t exp, totexp; ssizet s; if ((argc != 2) && (argc != 4)) {
fprintf(stderr, "%s init-secs [interval-secs max-exp]\n", argv[0]); exit(EXITFAILURE); }
if (clockgettime(CLOCKREALTIME, &now) == -1) handleerror("clockgettime"); /* Create a CLOCKREALTIME absolute timer with initial expiration and interval as specified in command line */ newvalue.itvalue.tvsec = now.tvsec + atoi(argv[1]); newvalue.itvalue.tvnsec = now.tvnsec; if (argc == 2) { newvalue.itinterval.tvsec = 0; maxexp = 1; } else { newvalue.itinterval.tvsec = atoi(argv[2]); maxexp = atoi(argv[3]); } newvalue.itinterval.tvnsec = 0; fd = timerfdcreate(CLOCKREALTIME, 0);
if (fd == -1) handleerror("timerfdcreate");
if (timerfdsettime(fd, TFDTIMERABSTIME, &newvalue, NULL) == -1) handleerror("timerfdsettime"); printelapsedtime(); printf("timer started\n"); for (totexp = 0; totexp < maxexp;) { s = read(fd, &exp, sizeof(uint64t)); if (s != sizeof(uint64t)) handleerror("read"); totexp += exp; printelapsedtime();
printf("read: %llu; total=%llu\n", (unsigned long long) exp, (unsigned long long) totexp); } exit(EXITSUCCESS); } SEE ALSO eventfd(2), poll(2), read(2), select(2), setitimer(2), signalfd(2), timercreate(2), timergettime(2), timersettime(2), epoll(7), time(7) COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2011-09-14 TIMERFDCREATE(2)