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rtems-docs/c_user/timespec_helpers.rst
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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.
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