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Subject: Re: [PATCH man-pages 1/2] clock_getres.2: Explain dynamic clocks.
To:     Richard Cochran <richardcochran@gmail.com>
References: <20171219165811.6ahuquuf5hq74zg3@localhost>
 <f85030cb44354de97f3b7822b23ec34fd21d1f28.1514787752.git.richardcochran@gmail.com>
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Message-ID: <aba829df-3da2-dd9e-dd19-808a46e09035@gmail.com>
Date:   Mon, 20 Apr 2020 13:13:57 +0200
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Hello Richard,

On 1/1/18 7:28 AM, Richard Cochran wrote:
> Linux has allowed passing open file descriptors to clock_gettime() and
> friends since v2.6.39.  This patch documents these "dynamic" clocks
> and adds a brief example of how to use them.

This fell on the floor, I'm sorry. It's applied now.

Thanks you!

Michael

> Signed-off-by: Richard Cochran <richardcochran@gmail.com>
> ---
>  man2/clock_getres.2 | 39 ++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 38 insertions(+), 1 deletion(-)
> 
> diff --git a/man2/clock_getres.2 b/man2/clock_getres.2
> index 0812d159a..30cbfe46a 100644
> --- a/man2/clock_getres.2
> +++ b/man2/clock_getres.2
> @@ -183,6 +183,35 @@ Per-process CPU-time clock
>  .TP
>  .BR CLOCK_THREAD_CPUTIME_ID " (since Linux 2.6.12)"
>  Thread-specific CPU-time clock.
> +.PP
> +Linux also implements dynamic clock instances as described below.
> +.SH DYNAMIC CLOCKS " (since Linux 2.6.39)"
> +In addition to hard coded SYS-V style clock ids, Linux also supports
> +POSIX clock operations on certain character devices.  Such devices are
> +called "dynamic" clocks.  Using the appropriate macros, open file
> +descriptors may be converted into clock ids and passed to
> +.BR clock_gettime (),
> +.BR clock_settime (),
> +and
> +.BR clock_adj (2).
> +The follow example shows how to convert a file descriptor into a
> +dynamic clock id.
> +.PP
> +.in +4n
> +.EX
> +#define CLOCKFD 3
> +#define FD_TO_CLOCKID(fd)       ((~(clockid_t) (fd) << 3) | CLOCKFD)
> +#define CLOCKID_TO_FD(clk)      ((unsigned int) ~((clk) >> 3))
> +
> +struct timeval tv;
> +clockid_t clkid;
> +int fd;
> +
> +fd = open("/dev/ptp0", O_RDWR);
> +clkid = FD_TO_CLOCKID(fd);
> +clock_gettime(clkid, &tv);
> +.EE
> +.in
>  .SH RETURN VALUE
>  .BR clock_gettime (),
>  .BR clock_settime (),
> @@ -200,11 +229,19 @@ points outside the accessible address space.
>  .B EINVAL
>  The
>  .I clk_id
> -specified is not supported on this system.
> +specified is invalid for one of two reasons.  Either the SYS-V style
> +hard coded positive value is out of range, or the dynamic clock id
> +does not refer to a valid instance of a clock object.
>  .\" Linux also gives this error on attempts to set CLOCK_PROCESS_CPUTIME_ID
>  .\" and CLOCK_THREAD_CPUTIME_ID, when probably the proper error should be
>  .\" EPERM.
>  .TP
> +.B ENODEV
> +The hot-plugable device (like USB for example) represented by a
> +dynamic
> +.I clk_id
> +has disappeared after its character device was opened.
> +.TP
>  .B EPERM
>  .BR clock_settime ()
>  does not have permission to set the clock indicated.
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/