timerfd

timerfd是Linux为用户程序提供的一个定时器接口。这个接口基于文件描述符，所以能够被用于select/poll的应用场景。
1.      使用方法

timerfd提供了如下接口供用户使用
timerfd_create
int timerfd_create(int clockid, int flags);
timerfd_create用于创建一个定时器文件。
参数clockid可以是CLOCK_MONOTONIC或者CLOCK_REALTIME。
参数flags可以是0或者O_CLOEXEC/O_NONBLOCK。
函数返回值是一个文件句柄fd。
timerfd_settime
int timerfd_settime(int ufd, int flags, const struct itimerspec * utmr, struct itimerspec * otmr);
此函数用于设置新的超时时间，并开始计时。
参数ufd是timerfd_create返回的文件句柄。
参数flags为1代表设置的是绝对时间；为0代表相对时间。
参数utmr为需要设置的时间。
参数otmr为定时器这次设置之前的超时时间。
函数返回0代表设置成功。
timerfd_gettime
int timerfd_gettime(int ufd, struct itimerspec * otmr);
此函数用于获得定时器距离下次超时还剩下的时间。如果调用时定时器已经到期，并且该定时器处于循环模式（设置超时时间时struct itimerspec::it_interval不为0），那么调用此函数之后定时器重新开始计时。
read
当timerfd为阻塞方式时，read函数将被阻塞，直到定时器超时。
函数返回值大于0，代表定时器超时；否则，代表没有超时（被信号唤醒，等等）。
poll/close
poll，close与标准文件操作相同。

2.      内核实现

timerfd的内核实现代码在kernel/fs/timerfd.c，它的实现基于Linux的hrtimer。
timerfd_create的实现
SYSCALL_DEFINE2(timerfd_create, int, clockid, int, flags)
l         做一些定时器的初始化工作
l         调用hrtimer_init初始化一个hrtimer
l         调用anon_inode_getfd分配一个dentry，并得到一个文件号fd，同时传入timerfd的文件操作指针struct file_operations timerfd_fops。anno_inode_getfd是文件系统anon_inodefs的一个帮助函数。anon文件系统比较简单，整个文件系统只有一个inode节点，其实现代码可以在fs/anon_inodes.c中找到。
timerfd_settime的实现
timerfd_settime最终会调用hrtimer_start启动定时器，其超时函数被设置为timerfd_tmrproc。
timerfd_tmrproc
timefd_tmrproc是timerfd的定时器超时函数。在timerfd超时时，该函数会设置定时器超时标记位；增加定时器超时次数（在设置定时器循环模式时，可能会出现多次超时没有被处理的情况）；唤醒一个等待队列，从而唤醒可能存在的正被阻塞的read、select。
timerfd_fops
static const struct file_operations timerfd_fops = {
       .release    = timerfd_release,
       .poll        = timerfd_poll,
       .read              = timerfd_read,
};
timerfd_read函数是文件操作read的内核实现，读到的是定时器的超时次数。该函数在阻塞模式下会把自身挂到timerfd的等待队列中，等待定时器超时时被唤醒。
timerfd_poll将timerfd的等待队列登记到一个poll_table，从而在定时器超时时能唤醒select系统调用。
timerfd_release

timerfd_release函数释放timerfd_create函数中申请的资源，删除已分配的定时器。

NAME         top

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

       #include 

       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         top

       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 either CLOCK_REALTIME or
       CLOCK_MONOTONIC.  CLOCK_REALTIME is a settable system-wide clock.
       CLOCK_MONOTONIC 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 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 itimer 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.

       The flags argument is either 0, to start a relative timer
       (new_value.it_interval specifies a time relative to the current value of the
       clock specified by clockid), or TFD_TIMER_ABSTIME, to start an absolute timer
       (new_value.it_value 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 new_value.it_value).

       The old_value argument returns a structure containing 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, 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 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) 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
              multiplexing 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 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         top

       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         top

       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 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         top

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

       These system calls are Linux-specific.
EXAMPLE         top

       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 
       #include 
       #include 
       #include 
       #include 
       #include         /* 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);
       }
BUGS         top

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

       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         top

       This page is part of release 3.32 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/.