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@xxxxxxxxx>
> ---
> 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/
