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c-user: Clarify time formats

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Update #4572.
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Sebastian Huber 2021-12-16 08:20:27 +01:00
parent 732d8bca8f
commit 786b89ac86
1 changed files with 26 additions and 11 deletions
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c-user/clock/background.rst
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@ -1,5 +1,6 @@
.. SPDX-License-Identifier: CC-BY-SA-4.0
.. Copyright (C) 2021 embedded brains GmbH (http://www.embedded-brains.de)
.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR)
Background
@ -29,7 +30,7 @@ and date format:
.. code-block:: c
struct rtems_tod_control {
typedef struct {
uint32_t year; /* greater than 1987 */
uint32_t month; /* 1 - 12 */
uint32_t day; /* 1 - 31 */
@ -37,20 +38,34 @@ and date format:
uint32_t minute; /* 0 - 59 */
uint32_t second; /* 0 - 59 */
uint32_t ticks; /* elapsed between seconds */
};
typedef struct rtems_tod_control rtems_time_of_day;
} rtems_time_of_day;
The native date and time format is the only format supported when setting the
system date and time using the ``rtems_clock_set`` directive. Some
system date and time using the :ref:`InterfaceRtemsClockSet` directive. Some
applications expect to operate on a *UNIX-style* date and time data structure.
The ``rtems_clock_get_tod_timeval`` always returns the date and time in
``struct timeval`` format.
For example, the :ref:`InterfaceRtemsClockGetTodTimeval` returns the date and
time in ``struct timeval`` format.
The ``struct timeval`` data structure has two fields: ``tv_sec`` and
``tv_usec`` which are seconds and microseconds, respectively. The ``tv_sec``
field in this data structure is the number of seconds since the POSIX epoch of
*January 1, 1970* but will never be prior to the RTEMS epoch of *January 1,
1988*.
.. index:: struct timeval
.. index:: struct timespec
Some directives use data structures defined by :term:`POSIX`. The ``struct
timeval`` data structure has two members: ``tv_sec`` and ``tv_usec`` which are
seconds and microseconds, respectively. The ``struct timespec`` data structure
has two members: ``tv_sec`` and ``tv_nsec`` which are seconds and nanoseconds,
respectively. For :term:`CLOCK_REALTIME` time points, the ``tv_sec`` member in
these data structures is the number of seconds since the :term:`Unix epoch` but
will never be prior to the :term:`RTEMS epoch`.
.. index:: struct bintime
.. index:: sbintime_t
The ``struct bintime`` and ``sbintime_t`` time formats used by some directives
originate in FreeBSD. The ``struct bintime`` data structure which represents
time in a binary time format has two members: ``sec`` and ``frac`` which are
seconds and fractions of a second in units of :math:`1 / 2^{64}` seconds,
respectively. The ``sbintime_t`` type is a signed 64-bit integer type used to
represent time in units of :math:`1 / 2^{32}` seconds.
.. index:: timeslicing