Date: Wed, 26 Sep 2007 11:06:59 -0700 (PDT)
From: Davide Libenzi <davidel@xmailserver.org>
To: Michael Kerrisk <mtk-manpages@gmx.net>
cc: lkml <linux-kernel@vger.kernel.org>, Jonathan Corbet <corbet@lwn.net>,
       Subrata Modak <subrata@linux.vnet.ibm.com>,
       Ulrich Drepper <drepper@redhat.com>,
       =?X-UNKNOWN?Q?David_H=E4rdeman?= <david@hardeman.nu>,
       Thomas Gleixner <tglx@linutronix.de>, geoff@gclare.org.uk,
       Christoph Hellwig <hch@lst.de>,
       Andrew Morton <akpm@linux-foundation.org>,
       Randy Dunlap <rdunlap@xenotime.net>
Subject: Re: Man page for revised timerfd API
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Michael, SCB ...


On Wed, 26 Sep 2007, Michael Kerrisk wrote:

> .TH TIMERFD_CREATE 2 2007-09-26 Linux "Linux Programmer's Manual"
> .SH NAME
> timerfd_create, timerfd_settime, timer_gettime \-
> timers that notify via file descriptors
> .SH SYNOPSIS
> .\" FIXME . This header file may well change
> .\" FIXME . Probably _GNU_SOURCE will be required
> .\" FIXME . May require: Link with \fI\-lrt\f
> .nf
> .B #include <sys/timerfd.h>
> .sp
> .BI "int timerfd_create(int " clockid );
> .sp
> .BI "int timerfd_settime(int " fd ", int " flags ,
> .BI "                    const struct itimerspec *" new_value ,
> .BI "                    struct itimerspec *" curr_value );
> .sp
> .BI "int timerfd_gettime(int " fd ", struct itimerspec *" curr_value );
> .fi
> .SH 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
> .BR setitimer (2)
> or
> .BR timer_create (3),
> with the advantage that the file descriptor may be monitored by
> .BR poll (2)
> and
> .BR select (2).

epoll, no?


> The use of these three system calls is analogous to the use of
> .BR timer_create (2),
> .BR timer_settime (2),
> and
> .BR timer_gettime (2).
> .\"
> .SS timerfd_create()
> .BR timerfd_create ()
> creates a new timer object,
> and returns a file descriptor that refers to that timer.
> The
> .I clockid
> argument specifies the clock that is used to mark the progress
> of the timer, and must be either
> .B CLOCK_REALTIME
> or
> .BR CLOCK_MONOTONIC .
> .B CLOCK_REALTIME
> is a settable system-wide clock.
> .B CLOCK_MONOTONIC
> is a non-settable 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
> .BR clock_gettime (3).
> .\"
> .SS timerfd_settime()
> .BR timerfd_settime ()
> arms (starts) or disarms (stops)
> the timer referred to by the file descriptor
> .IR fd .
> 
> The
> .I new_value
> argument specifies the initial expiration and interval for the timer.
> The
> .I itimer
> structure used for this argument contains two fields,
> each of which is in turn a structure of type
> .IR timespec :
> .in +0.25i
> .nf
> 
> 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 */
> };
> .fi
> .in
> .PP
> .I new_value.it_value
> specifies the initial expiration of the timer,
> in seconds and nanoseconds.
> Setting either field of
> .I new_value.it_value
> to a non-zero value arms the timer.
> Setting both fields of
> .I new_value.it_value
> to zero disarms the timer.
> 
> Setting one or both fields of
> .I new_value.it_interval
> to non-zero values specifies the period, in seconds and nanoseconds,
> for repeated timer expirations after the initial expiration.
> If both fields of
> .I new_value.it_interval
> are zero, the timer expires just once, at the time specified by
> .IR new_value.it_value .
> 
> The
> .I flags
> argument is either 0, to start a relative timer
> .RI ( new_value.it_interval
> specifies a time relative to the current value of the clock specified by
> .IR clockid ),
> or
> .BR TFD_TIMER_ABSTIME ,
> to start an absolute timer
> .RI ( new_value.it_interval
> specifies an absolute time for the clock specified by
> .IR clockid ;
> that is, the timer will expire when the value of that
> clock reaches the value specified in
> .IR new_value.it_interval ).
> 
> The
> .I curr_value
> argument returns a structure containing the setting of the timer that
> was current at the time of the call; see the description of
> .BR timerfd_gettime ()
> following.
> .\"
> .SS timerfd_gettime()
> .BR timerfd_gettime ()
> returns, in
> .IR curr_value ,
> an
> .IR itimerspec
> that contains the current setting of the timer
> referred to by the file descriptor
> .IR fd .
> 
> The
> .I 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
> .BR TFD_TIMER_ABSTIME
> flag was specified when setting the timer.
> 
> The
> .I 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
> .IR curr_value.it_value .
> .SS Operating on a timer file descriptor
> The file descriptor returned by
> .BR timerfd_create (2)
> supports the following operations:
> .TP
> .BR read (2)
> If the timer has already expired one or more times since it was created,
> or since the last
> .BR read (2),
> then the buffer given to
> .BR read (2)
> returns an unsigned 8-byte integer
> .RI ( uint64_t )
> containing the number of expirations that have occurred.
> .IP
> If no timer expirations have occurred at the time of the
> .BR read (2),
> then the call either blocks until the next timer expiration,
> or fails with the error
> .B EAGAIN
> if the file descriptor has been made non-blocking
> (via the use of the
> .BR fcntl (2)
> .B F_SETFL
> operation to set the
> .B O_NONBLOCK
> flag).
> .IP
> A
> .BR read (2)
> will fail with the error
> .B EINVAL
> if the size of the supplied buffer is less than 8 bytes.
> .TP
> .BR poll "(2), " select "(2) (and similar)"
> The file descriptor is readable
> (the
> .BR select (2)
> .I readfds
> argument; the
> .BR poll (2)
> .B POLLIN
> flag)
> if one or more timer expirations have occurred.
> .IP
> The file descriptor also supports the other file-descriptor
> multiplexing APIs:
> .BR pselect (2),
> .BR ppoll (2),
> and
> .BR epoll (7).
> .TP
> .BR 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.
> .\"
> .SS fork(2) semantics
> .\" FIXME Davide, is the following correct?

Yes.


> After a
> .BR fork (2),
> the child inherits a copy of the file descriptor created by
> .BR timerfd_create ().
> The file descriptor refers to the same underlying
> timer object as the corresponding file descriptor in the parent,
> and
> .BR read (2)s
> in the child will return information about
> expirations of the timer.
> .\"
> .SS execve(2) semantics
> .\" FIXME Davide, is the following correct?

Yes.


> A file descriptor created by
> .BR timerfd_create ()
> is preserved across
> .BR execve (2),
> and continues to generate timer expirations if the timer was armed.
> .SH "RETURN VALUE"
> On success,
> .BR timerfd_create ()
> returns a new file descriptor.
> On error, \-1 is returned and
> .I errno
> is set to indicate the error.
> 
> .BR timer_settime ()
> and
> .BR timer_gettime ()
> return 0 on success;
> on error they return \-1, and set
> .I errno
> to indicate the error.
> .SH ERRORS
> .\" FIXME -- there need to be errors for all syscalls here
> .BR tinerfd_create ()
> can fail with the following errors:
> .\" FIXME Davide, are there any other errors for timerfd_create()?

They seem all to me.


> .TP
> .B EINVAL
> The
> .I clockid
> argument is neither
> .B CLOCK_MONOTONIC
> nor
> .BR CLOCK_REALTIME .
> .TP
> .B EMFILE
> The per-process limit of open file descriptors has been reached.
> .TP
> .B ENFILE
> The system-wide limit on the total number of open files has been
> reached.
> .TP
> .B ENODEV
> Could not mount (internal) anonymous i-node device.
> .TP
> .B ENOMEM
> There was insufficient kernel memory to create the timer.
> .PP
> .BR timer_settime ()
> and
> .BR timer_gettime ()
> can fail with the following errors:
> .\" FIXME Davide, are there any other errors for timerfd_[gs]ettime()?
> .TP
> .B EBADF
> .I fd
> is not a valid file descriptor.
> .TP
> .B EINVAL
> .I fd
> is not a valid timerfd file descriptor.
> .I new_value
> is not properly initialized (one of the
> .I tv_nsec
> falls outside the range zero to 999,999,999).
> .SH VERSIONS
> These system calls are available on Linux since kernel 2.6.23.
> .\" FIXME . check later to see when glibc support is provided
> As at September 2007 (glibc 2.6), the details of the glibc interface
> have not been finalized, so that, for example,
> the eventual header file may be different from that shown on this page.
> .SH CONFORMING TO
> These system calls are Linux specific.
> .SH 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:
> .in +0.5i
> .nf
> 
> $ a.out 3 1 100
> 0.000: timer started
> 3.000: read: 1; total=1
> 4.000: read: 1; total=2
> [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
> [type control-C to terminate the program]
> .fi
> .in
> .nf
> 
> .\" FIXME . Check later what header file glibc uses for timerfd
> .\" FIXME . Probably glibc will require _GNU_SOURCE to be set
> .\"
> .\" The commented out code here is what we currently need until
> .\" the required stuff is in glibc
> .\"
> .\"
> .\"/* Link with -lrt */
> .\"#define _GNU_SOURCE
> .\"#include <sys/syscall.h>
> .\"#include <unistd.h>
> .\"#include <time.h>
> .\"#if defined(__i386__)
> .\"#define __NR_timerfd_create 322
> .\"#define __NR_timerfd_settime 325
> .\"#define __NR_timerfd_gettime 326
> .\"#endif
> .\"
> .\"static int
> .\"timerfd_create(int clockid)
> .\"{
> .\"    return syscall(__NR_timerfd_create, clockid);
> .\"}
> .\"
> .\"static int
> .\"timerfd_settime(int fd, int flags, struct itimerspec *new_value,
> .\"        struct itimerspec *curr_value)
> .\"{
> .\"    return syscall(__NR_timerfd_settime, fd, flags, new_value,
> .\"                   curr_value);
> .\"}
> .\"
> .\"static int
> .\"timerfd_gettime(int fd, struct itimerspec *curr_value)
> .\"{
> .\"    return syscall(__NR_timerfd_gettime, fd, curr_value);
> .\"}
> .\"
> .\"#define TFD_TIMER_ABSTIME (1 << 0)
> .\"
> .\"////////////////////////////////////////////////////////////
> #include <sys/timerfd.h>   /* May eventually be different in glibc */
> #include <time.h>
> #include <unistd.h>
> #include <stdlib.h>
> #include <stdio.h>
> #include <stdint.h>        /* Definition of uint64_t */
> 
> #define die(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)
>             die("clock_gettime");
>     }
> 
>     if (clock_gettime(CLOCK_MONOTONIC, &curr) == \-1)
>         die("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, tot_exp, fd;
>     struct timespec now;
>     uint64_t 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)
>         die("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);
>     if (fd == \-1)
>         die("timerfd_create");
> 
>     s = timerfd_settime(fd, TFD_TIMER_ABSTIME, &new_value, NULL);
>     if (s == \-1)
>         die("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))
>             die("read");
> 
>         tot_exp += exp;
>         print_elapsed_time();
>         printf("read: %llu; total=%d\\n", exp, tot_exp);
>     }
> 
>     exit(EXIT_SUCCESS);
> }
> .fi
> .SH "SEE ALSO"
> .BR eventfd (2),
> .BR poll (2),
> .BR read (2),
> .BR select (2),
> .BR setitimer (2),
> .BR signalfd (2),
> .BR timer_create (3),
> .BR timer_gettime (3),
> .BR timer_settime (3),
> .BR epoll (7),
> .BR time (7)
> .\" FIXME Create links for timerfd_settime.2 and timerfd_gettime.2.
> .\" FIXME have SEE ALSO in setitimer.2 refer to this page.
> .\" FIXME have SEE ALSO in time.7 refer to this page.


BTW: We need to re-look over the signalfd man page, since Linus, in an 
unexpected move, merged the simplification patch :)


- Davide


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