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Timespec Helpers
################

Introduction
============

The Timespec helpers manager provides directives to assist in manipulating
instances of the POSIX ``struct timespec`` structure.

The directives provided by the timespec helpers manager are:

- ``rtems_timespec_set`` - Set timespec's value

- ``rtems_timespec_zero`` - Zero timespec's value

- ``rtems_timespec_is_valid`` - Check if timespec is valid

- ``rtems_timespec_add_to`` - Add two timespecs

- ``rtems_timespec_subtract`` - Subtract two timespecs

- ``rtems_timespec_divide`` - Divide two timespecs

- ``rtems_timespec_divide_by_integer`` - Divide timespec by integer

- ``rtems_timespec_less_than`` - Less than operator

- ``rtems_timespec_greater_than`` - Greater than operator

- ``rtems_timespec_equal_to`` - Check if two timespecs are equal

- ``rtems_timespec_get_seconds`` - Obtain seconds portion of timespec

- ``rtems_timespec_get_nanoseconds`` - Obtain nanoseconds portion of timespec

- ``rtems_timespec_to_ticks`` - Convert timespec to number of ticks

- ``rtems_timespec_from_ticks`` - Convert ticks to timespec

Background
==========

Time Storage Conventions
------------------------

Time can be stored in many ways. One of them is the ``struct timespec``
format which is a structure that consists of the fields ``tv_sec``
to represent seconds and ``tv_nsec`` to represent nanoseconds.  The``struct timeval`` structure is simular and consists of seconds (stored
in ``tv_sec``) and microseconds (stored in ``tv_usec``). Either``struct timespec`` or ``struct timeval`` can be used to represent
elapsed time, time of executing some operations, or time of day.

Operations
==========

Set and Obtain Timespec Value
-----------------------------

A user may write a specific time by passing the desired seconds and
nanoseconds values and the destination ``struct timespec`` using the``rtems_timespec_set`` directive.

The ``rtems_timespec_zero`` directive is used to zero the seconds
and nanoseconds portions of a ``struct timespec`` instance.

Users may obtain the seconds or nanoseconds portions of a ``struct
timespec`` instance with the ``rtems_timespec_get_seconds`` or``rtems_timespec_get_nanoseconds`` methods, respectively.

Timespec Math
-------------

A user can perform multiple operations on ``struct timespec``
instances. The helpers in this manager assist in adding, subtracting, and
performing divison on ``struct timespec`` instances.

- Adding two ``struct timespec`` can be done using the``rtems_timespec_add_to`` directive. This directive is used mainly
  to calculate total amount of time consumed by multiple operations.

- The ``rtems_timespec_subtract`` is used to subtract two``struct timespecs`` instances and determine the elapsed time between
  those two points in time.

- The ``rtems_timespec_divide`` is used to use to divide one``struct timespec`` instance by another. This calculates the percentage
  with a precision to three decimal points.

- The ``rtems_timespec_divide_by_integer`` is used to divide a``struct timespec`` instance by an integer. It is commonly used in
  benchmark calculations to dividing duration by the number of iterations
  performed.

Comparing struct timespec Instances
-----------------------------------

A user can compare two ``struct timespec`` instances using the``rtems_timespec_less_than``, ``rtems_timespec_greater_than``
or ``rtems_timespec_equal_to`` routines.

Conversions and Validity Check
------------------------------

Conversion to and from clock ticks may be performed by using the``rtems_timespec_to_ticks`` and ``rtems_timespec_from_ticks``
directives.

User can also check validity of timespec with``rtems_timespec_is_valid`` routine.

Directives
==========

This section details the Timespec Helpers manager's directives.
A subsection is dedicated to each of this manager's directives
and describes the calling sequence, related constants, usage,
and status codes.

TIMESPEC_SET - Set struct timespec Instance
-------------------------------------------

**CALLING SEQUENCE:**

.. code:: c

    void rtems_timespec_set(
    struct timespec \*time,
    time_t           seconds,
    uint32_t         nanoseconds
    );

.. index:: rtems_timespec_set

**STATUS CODES:**

NONE

**DESCRIPTION:**

This directive sets the ``struct timespec`` ``time`` value to the
desired ``seconds`` and ``nanoseconds`` values.

**NOTES:**

This method does NOT check if ``nanoseconds`` is less than the
maximum number of nanoseconds in a second.

TIMESPEC_ZERO - Zero struct timespec Instance
---------------------------------------------

**CALLING SEQUENCE:**

.. code:: c

    void rtems_timespec_zero(
    struct timespec \*time
    );

.. index:: rtems_timespec_zero

**STATUS CODES:**

NONE

**DESCRIPTION:**

This routine sets the contents of the ``struct timespec`` instance``time`` to zero.

**NOTES:**

NONE

TIMESPEC_IS_VALID - Check validity of a struct timespec instance
----------------------------------------------------------------

**CALLING SEQUENCE:**

.. code:: c

    bool rtems_timespec_is_valid(
    const struct timespec \*time
    );

.. index:: rtems_timespec_is_valid

**STATUS CODES:**

This method returns ``true`` if the instance is valid, and ``false``
otherwise.

**DESCRIPTION:**

This routine check validity of a ``struct timespec`` instance. It
checks if the nanoseconds portion of the ``struct timespec`` instanceis
in allowed range (less than the maximum number of nanoseconds per second).

**NOTES:**

TIMESPEC_ADD_TO - Add Two struct timespec Instances
---------------------------------------------------

**CALLING SEQUENCE:**

.. code:: c

    uint32_t rtems_timespec_add_to(
    struct timespec       \*time,
    const struct timespec \*add
    );

.. index:: rtems_timespec_add_to

**STATUS CODES:**

The method returns the number of seconds ``time`` increased by.

**DESCRIPTION:**

This routine adds two ``struct timespec`` instances. The second argument is added to the first. The parameter ``time`` is the base time to which the ``add`` parameter is added.

**NOTES:**

NONE

TIMESPEC_SUBTRACT - Subtract Two struct timespec Instances
----------------------------------------------------------

**CALLING SEQUENCE:**

.. code:: c

    void rtems_timespec_subtract(
    const struct timespec \*start,
    const struct timespec \*end,
    struct timespec       \*result
    );

.. index:: rtems_timespec_subtract

**STATUS CODES:**

NONE

**DESCRIPTION:**

This routine subtracts ``start`` from ``end`` saves the difference
in ``result``. The primary use of this directive is to calculate
elapsed time.

**NOTES:**

It is possible to subtract when ``end`` is less than ``start``
and it produce negative ``result``. When doing this you should be
careful and remember that only the seconds portion of a ``struct
timespec`` instance is signed, which means that nanoseconds portion is
always increasing. Due to that when your timespec has seconds = -1 and
nanoseconds=500,000,000 it means that result is -0.5 second, NOT the
expected -1.5!

TIMESPEC_DIVIDE - Divide Two struct timespec Instances
------------------------------------------------------

**CALLING SEQUENCE:**

.. code:: c

    void rtems_timespec_divide(
    const struct timespec \*lhs,
    const struct timespec \*rhs,
    uint32_t              \*ival_percentage,
    uint32_t              \*fval_percentage
    );

.. index:: rtems_timespec_divide

**STATUS CODES:**

NONE

**DESCRIPTION:**

This routine divides the ``struct timespec`` instance ``lhs`` by
the ``struct timespec`` instance ``rhs``. The result is returned
in the ``ival_percentage`` and ``fval_percentage``, representing
the integer and fractional results of the division respectively.

The ``ival_percentage`` is integer value of calculated percentage and ``fval_percentage`` is fractional part of calculated percentage.

**NOTES:**

The intended use is calculating percentges to three decimal points.

When dividing by zero, this routine return both ``ival_percentage``
and ``fval_percentage`` equal zero.

The division is performed using exclusively integer operations.

TIMESPEC_DIVIDE_BY_INTEGER - Divide a struct timespec Instance by an Integer
----------------------------------------------------------------------------

**CALLING SEQUENCE:**

.. code:: c

    int rtems_timespec_divide_by_integer(
    const struct timespec \*time,
    uint32_t               iterations,
    struct timespec       \*result
    );

.. index:: rtems_timespec_divide_by_integer

**STATUS CODES:**

NONE

**DESCRIPTION:**

This routine divides the ``struct timespec`` instance ``time`` by the integer value ``iterations``.
The result is saved in ``result``.

**NOTES:**

The expected use is to assist in benchmark calculations where you
typically divide a duration (``time``) by a number of iterations what
gives average time.

TIMESPEC_LESS_THAN - Less than operator
---------------------------------------

**CALLING SEQUENCE:**

.. code:: c

    bool rtems_timespec_less_than(
    const struct timespec \*lhs,
    const struct timespec \*rhs
    );

.. index:: rtems_timespec_less_than

**STATUS CODES:**

This method returns ``struct true`` if ``lhs`` is less than``rhs`` and ``struct false`` otherwise.

**DESCRIPTION:**

This method is the less than operator for ``struct timespec`` instances. The first parameter is the left hand side and the second is the right hand side of the comparison.

**NOTES:**

NONE

TIMESPEC_GREATER_THAN - Greater than operator
---------------------------------------------

**CALLING SEQUENCE:**

.. code:: c

    bool rtems_timespec_greater_than(
    const struct timespec \*_lhs,
    const struct timespec \*_rhs
    );

.. index:: rtems_timespec_greater_than

**STATUS CODES:**

This method returns ``struct true`` if ``lhs`` is greater than``rhs`` and ``struct false`` otherwise.

**DESCRIPTION:**

This method is greater than operator for ``struct timespec`` instances.

**NOTES:**

NONE

TIMESPEC_EQUAL_TO - Check equality of timespecs
-----------------------------------------------

**CALLING SEQUENCE:**

.. code:: c

    bool rtems_timespec_equal_to(
    const struct timespec \*lhs,
    const struct timespec \*rhs
    );

.. index:: rtems_timespec_equal_to

**STATUS CODES:**

This method returns ``struct true`` if ``lhs`` is equal to``rhs`` and ``struct false`` otherwise.

**DESCRIPTION:**

This method is equality operator for ``struct timespec`` instances.

**NOTES:**

NONE

TIMESPEC_GET_SECONDS - Get Seconds Portion of struct timespec Instance
----------------------------------------------------------------------

**CALLING SEQUENCE:**

.. code:: c

    time_t rtems_timespec_get_seconds(
    struct timespec \*time
    );

.. index:: rtems_timespec_get_seconds

**STATUS CODES:**

This method returns the seconds portion of the specified ``struct
timespec`` instance.

**DESCRIPTION:**

This method returns the seconds portion of the specified ``struct timespec`` instance ``time``.

**NOTES:**

NONE

TIMESPEC_GET_NANOSECONDS - Get Nanoseconds Portion of the struct timespec Instance
----------------------------------------------------------------------------------

**CALLING SEQUENCE:**

.. code:: c

    uint32_t rtems_timespec_get_nanoseconds(
    struct timespec \*_time
    );

.. index:: rtems_timespec_get_nanoseconds

**STATUS CODES:**

This method returns the nanoseconds portion of the specified ``struct
timespec`` instance.

**DESCRIPTION:**

This method returns the nanoseconds portion of the specified timespec
which is pointed by ``_time``.

**NOTES:**

TIMESPEC_TO_TICKS - Convert struct timespec Instance to Ticks
-------------------------------------------------------------

**CALLING SEQUENCE:**

.. code:: c

    uint32_t rtems_timespec_to_ticks(
    const struct timespec \*time
    );

.. index:: rtems_timespec_to_ticks

**STATUS CODES:**

This directive returns the number of ticks computed.

**DESCRIPTION:**

This directive converts the ``time`` timespec to the corresponding number of clock ticks.

**NOTES:**

NONE

TIMESPEC_FROM_TICKS - Convert Ticks to struct timespec Representation
---------------------------------------------------------------------

**CALLING SEQUENCE:**

.. code:: c

    void rtems_timespec_from_ticks(
    uint32_t         ticks,
    struct timespec \*time
    );

.. index:: rtems_timespec_from_ticks

**STATUS CODES:**

NONE

**DESCRIPTION:**

This routine converts the ``ticks`` to the corresponding ``struct timespec`` representation and stores it in ``time``.

**NOTES:**

NONE

.. COMMENT: COPYRIGHT (c) 2011.

.. COMMENT: On-Line Applications Research Corporation (OAR).

.. COMMENT: All rights reserved.