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583 lines
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ReStructuredText
583 lines
17 KiB
ReStructuredText
Timer Manager
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#############
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.. index:: timers
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Introduction
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============
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The timer manager provides support for timer
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facilities. The directives provided by the timer manager are:
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- ``rtems_timer_create`` - Create a timer
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- ``rtems_timer_ident`` - Get ID of a timer
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- ``rtems_timer_cancel`` - Cancel a timer
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- ``rtems_timer_delete`` - Delete a timer
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- ``rtems_timer_fire_after`` - Fire timer after interval
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- ``rtems_timer_fire_when`` - Fire timer when specified
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- ``rtems_timer_initiate_server`` - Initiate server for task-based timers
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- ``rtems_timer_server_fire_after`` - Fire task-based timer after interval
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- ``rtems_timer_server_fire_when`` - Fire task-based timer when specified
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- ``rtems_timer_reset`` - Reset an interval timer
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Background
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==========
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Required Support
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----------------
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A clock tick is required to support the functionality provided by this manager.
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Timers
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------
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A timer is an RTEMS object which allows the
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application to schedule operations to occur at specific times in
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the future. User supplied timer service routines are invoked by
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either the ``rtems_clock_tick`` directive or
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a special Timer Server task when the timer fires. Timer service
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routines may perform any operations or directives which normally
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would be performed by the application code which invoked the``rtems_clock_tick`` directive.
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The timer can be used to implement watchdog routines
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which only fire to denote that an application error has
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occurred. The timer is reset at specific points in the
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application to ensure that the watchdog does not fire. Thus, if
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the application does not reset the watchdog timer, then the
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timer service routine will fire to indicate that the application
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has failed to reach a reset point. This use of a timer is
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sometimes referred to as a "keep alive" or a "deadman" timer.
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Timer Server
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------------
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The Timer Server task is responsible for executing the timer
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service routines associated with all task-based timers.
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This task executes at a priority higher than any RTEMS application
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task, and is created non-preemptible, and thus can be viewed logically as
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the lowest priority interrupt.
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By providing a mechanism where timer service routines execute
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in task rather than interrupt space, the application is
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allowed a bit more flexibility in what operations a timer
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service routine can perform. For example, the Timer Server
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can be configured to have a floating point context in which case
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it would be safe to perform floating point operations
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from a task-based timer. Most of the time, executing floating
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point instructions from an interrupt service routine
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is not considered safe. However, since the Timer Server task
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is non-preemptible, only directives allowed from an ISR can be
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called in the timer service routine.
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The Timer Server is designed to remain blocked until a
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task-based timer fires. This reduces the execution overhead
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of the Timer Server.
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Timer Service Routines
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----------------------
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The timer service routine should adhere to C calling
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conventions and have a prototype similar to the following:.. index:: rtems_timer_service_routine
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.. code:: c
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rtems_timer_service_routine user_routine(
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rtems_id timer_id,
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void \*user_data
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);
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Where the timer_id parameter is the RTEMS object ID
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of the timer which is being fired and user_data is a pointer to
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user-defined information which may be utilized by the timer
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service routine. The argument user_data may be NULL.
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Operations
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==========
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Creating a Timer
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----------------
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The ``rtems_timer_create`` directive creates a timer by
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allocating a Timer Control Block (TMCB), assigning the timer a
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user-specified name, and assigning it a timer ID. Newly created
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timers do not have a timer service routine associated with them
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and are not active.
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Obtaining Timer IDs
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-------------------
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When a timer is created, RTEMS generates a unique
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timer ID and assigns it to the created timer until it is
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deleted. The timer ID may be obtained by either of two methods.
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First, as the result of an invocation of the``rtems_timer_create``
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directive, the timer ID is stored in a user provided location.
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Second, the timer ID may be obtained later using the``rtems_timer_ident`` directive. The timer ID
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is used by other directives to manipulate this timer.
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Initiating an Interval Timer
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----------------------------
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The ``rtems_timer_fire_after``
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and ``rtems_timer_server_fire_after``
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directives initiate a timer to fire a user provided
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timer service routine after the specified
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number of clock ticks have elapsed. When the interval has
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elapsed, the timer service routine will be invoked from the``rtems_clock_tick`` directive if it was initiated
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by the ``rtems_timer_fire_after`` directive
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and from the Timer Server task if initiated by the``rtems_timer_server_fire_after`` directive.
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Initiating a Time of Day Timer
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------------------------------
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The ``rtems_timer_fire_when``
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and ``rtems_timer_server_fire_when``
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directive initiate a timer to
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fire a user provided timer service routine when the specified
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time of day has been reached. When the interval has elapsed,
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the timer service routine will be invoked from the``rtems_clock_tick`` directive
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by the ``rtems_timer_fire_when`` directive
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and from the Timer Server task if initiated by the``rtems_timer_server_fire_when`` directive.
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Canceling a Timer
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-----------------
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The ``rtems_timer_cancel`` directive is used to halt the
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specified timer. Once canceled, the timer service routine will
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not fire unless the timer is reinitiated. The timer can be
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reinitiated using the ``rtems_timer_reset``,``rtems_timer_fire_after``, and``rtems_timer_fire_when`` directives.
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Resetting a Timer
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-----------------
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The ``rtems_timer_reset`` directive is used to restore an
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interval timer initiated by a previous invocation of``rtems_timer_fire_after`` or``rtems_timer_server_fire_after`` to
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its original interval length. If the
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timer has not been used or the last usage of this timer
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was by the ``rtems_timer_fire_when``
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or ``rtems_timer_server_fire_when``
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directive, then an error is returned. The timer service routine
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is not changed or fired by this directive.
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Initiating the Timer Server
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---------------------------
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The ``rtems_timer_initiate_server`` directive is used to
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allocate and start the execution of the Timer Server task. The
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application can specify both the stack size and attributes of the
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Timer Server. The Timer Server executes at a priority higher than
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any application task and thus the user can expect to be preempted
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as the result of executing the ``rtems_timer_initiate_server``
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directive.
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Deleting a Timer
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----------------
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The ``rtems_timer_delete`` directive is used to delete a timer.
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If the timer is running and has not expired, the timer is
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automatically canceled. The timer's control block is returned
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to the TMCB free list when it is deleted. A timer can be
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deleted by a task other than the task which created the timer.
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Any subsequent references to the timer's name and ID are invalid.
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Directives
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==========
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This section details the timer manager's directives.
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A subsection is dedicated to each of this manager's directives
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and describes the calling sequence, related constants, usage,
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and status codes.
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TIMER_CREATE - Create a timer
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-----------------------------
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.. index:: create a timer
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**CALLING SEQUENCE:**
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.. index:: rtems_timer_create
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.. code:: c
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rtems_status_code rtems_timer_create(
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rtems_name name,
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rtems_id \*id
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);
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**DIRECTIVE STATUS CODES:**
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``RTEMS_SUCCESSFUL`` - timer created successfully
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``RTEMS_INVALID_ADDRESS`` - ``id`` is NULL
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``RTEMS_INVALID_NAME`` - invalid timer name
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``RTEMS_TOO_MANY`` - too many timers created
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**DESCRIPTION:**
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This directive creates a timer. The assigned timer
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id is returned in id. This id is used to access the timer with
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other timer manager directives. For control and maintenance of
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the timer, RTEMS allocates a TMCB from the local TMCB free pool
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and initializes it.
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**NOTES:**
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This directive will not cause the calling task to be
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preempted.
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TIMER_IDENT - Get ID of a timer
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-------------------------------
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.. index:: obtain the ID of a timer
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**CALLING SEQUENCE:**
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.. index:: rtems_timer_ident
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.. code:: c
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rtems_status_code rtems_timer_ident(
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rtems_name name,
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rtems_id \*id
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);
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**DIRECTIVE STATUS CODES:**
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``RTEMS_SUCCESSFUL`` - timer identified successfully
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``RTEMS_INVALID_ADDRESS`` - ``id`` is NULL
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``RTEMS_INVALID_NAME`` - timer name not found
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**DESCRIPTION:**
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This directive obtains the timer id associated with
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the timer name to be acquired. If the timer name is not unique,
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then the timer id will match one of the timers with that name.
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However, this timer id is not guaranteed to correspond to the
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desired timer. The timer id is used to access this timer in
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other timer related directives.
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**NOTES:**
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This directive will not cause the running task to be
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preempted.
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TIMER_CANCEL - Cancel a timer
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-----------------------------
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.. index:: cancel a timer
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**CALLING SEQUENCE:**
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.. index:: rtems_timer_cancel
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.. code:: c
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rtems_status_code rtems_timer_cancel(
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rtems_id id
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);
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**DIRECTIVE STATUS CODES:**
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``RTEMS_SUCCESSFUL`` - timer canceled successfully
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``RTEMS_INVALID_ID`` - invalid timer id
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**DESCRIPTION:**
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This directive cancels the timer id. This timer will
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be reinitiated by the next invocation of ``rtems_timer_reset``,``rtems_timer_fire_after``, or``rtems_timer_fire_when`` with this id.
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**NOTES:**
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This directive will not cause the running task to be preempted.
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TIMER_DELETE - Delete a timer
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-----------------------------
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.. index:: delete a timer
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**CALLING SEQUENCE:**
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.. index:: rtems_timer_delete
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.. code:: c
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rtems_status_code rtems_timer_delete(
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rtems_id id
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);
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**DIRECTIVE STATUS CODES:**
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``RTEMS_SUCCESSFUL`` - timer deleted successfully
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``RTEMS_INVALID_ID`` - invalid timer id
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**DESCRIPTION:**
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This directive deletes the timer specified by id. If
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the timer is running, it is automatically canceled. The TMCB
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for the deleted timer is reclaimed by RTEMS.
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**NOTES:**
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This directive will not cause the running task to be
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preempted.
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A timer can be deleted by a task other than the task
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which created the timer.
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TIMER_FIRE_AFTER - Fire timer after interval
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--------------------------------------------
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.. index:: fire a timer after an interval
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**CALLING SEQUENCE:**
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.. index:: rtems_timer_fire_after
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.. code:: c
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rtems_status_code rtems_timer_fire_after(
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rtems_id id,
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rtems_interval ticks,
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rtems_timer_service_routine_entry routine,
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void \*user_data
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);
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**DIRECTIVE STATUS CODES:**
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``RTEMS_SUCCESSFUL`` - timer initiated successfully
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``RTEMS_INVALID_ADDRESS`` - ``routine`` is NULL
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``RTEMS_INVALID_ID`` - invalid timer id
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``RTEMS_INVALID_NUMBER`` - invalid interval
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**DESCRIPTION:**
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This directive initiates the timer specified by id.
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If the timer is running, it is automatically canceled before
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being initiated. The timer is scheduled to fire after an
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interval ticks clock ticks has passed. When the timer fires,
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the timer service routine routine will be invoked with the
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argument user_data.
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**NOTES:**
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This directive will not cause the running task to be
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preempted.
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TIMER_FIRE_WHEN - Fire timer when specified
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-------------------------------------------
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.. index:: fire a timer at wall time
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**CALLING SEQUENCE:**
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.. index:: rtems_timer_fire_when
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.. code:: c
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rtems_status_code rtems_timer_fire_when(
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rtems_id id,
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rtems_time_of_day \*wall_time,
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rtems_timer_service_routine_entry routine,
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void \*user_data
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);
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**DIRECTIVE STATUS CODES:**
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``RTEMS_SUCCESSFUL`` - timer initiated successfully
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``RTEMS_INVALID_ADDRESS`` - ``routine`` is NULL
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``RTEMS_INVALID_ADDRESS`` - ``wall_time`` is NULL
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``RTEMS_INVALID_ID`` - invalid timer id
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``RTEMS_NOT_DEFINED`` - system date and time is not set
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``RTEMS_INVALID_CLOCK`` - invalid time of day
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**DESCRIPTION:**
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This directive initiates the timer specified by id.
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If the timer is running, it is automatically canceled before
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being initiated. The timer is scheduled to fire at the time of
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day specified by wall_time. When the timer fires, the timer
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service routine routine will be invoked with the argument
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user_data.
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**NOTES:**
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This directive will not cause the running task to be
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preempted.
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TIMER_INITIATE_SERVER - Initiate server for task-based timers
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-------------------------------------------------------------
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.. index:: initiate the Timer Server
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**CALLING SEQUENCE:**
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.. index:: rtems_timer_initiate_server
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.. code:: c
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rtems_status_code rtems_timer_initiate_server(
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uint32_t priority,
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uint32_t stack_size,
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rtems_attribute attribute_set
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)
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);
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**DIRECTIVE STATUS CODES:**
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``RTEMS_SUCCESSFUL`` - Timer Server initiated successfully
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``RTEMS_TOO_MANY`` - too many tasks created
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**DESCRIPTION:**
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This directive initiates the Timer Server task. This task
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is responsible for executing all timers initiated via the``rtems_timer_server_fire_after`` or``rtems_timer_server_fire_when`` directives.
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**NOTES:**
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This directive could cause the calling task to be preempted.
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The Timer Server task is created using the``rtems_task_create`` service and must be accounted
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for when configuring the system.
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Even through this directive invokes the ``rtems_task_create``
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and ``rtems_task_start`` directives, it should only fail
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due to resource allocation problems.
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TIMER_SERVER_FIRE_AFTER - Fire task-based timer after interval
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--------------------------------------------------------------
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.. index:: fire task-based a timer after an interval
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**CALLING SEQUENCE:**
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.. index:: rtems_timer_server_fire_after
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.. code:: c
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rtems_status_code rtems_timer_server_fire_after(
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rtems_id id,
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rtems_interval ticks,
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rtems_timer_service_routine_entry routine,
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void \*user_data
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);
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**DIRECTIVE STATUS CODES:**
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``RTEMS_SUCCESSFUL`` - timer initiated successfully
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``RTEMS_INVALID_ADDRESS`` - ``routine`` is NULL
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``RTEMS_INVALID_ID`` - invalid timer id
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``RTEMS_INVALID_NUMBER`` - invalid interval
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``RTEMS_INCORRECT_STATE`` - Timer Server not initiated
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**DESCRIPTION:**
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This directive initiates the timer specified by id and specifies
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that when it fires it will be executed by the Timer Server.
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If the timer is running, it is automatically canceled before
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being initiated. The timer is scheduled to fire after an
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interval ticks clock ticks has passed. When the timer fires,
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the timer service routine routine will be invoked with the
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argument user_data.
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**NOTES:**
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This directive will not cause the running task to be
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preempted.
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TIMER_SERVER_FIRE_WHEN - Fire task-based timer when specified
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-------------------------------------------------------------
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.. index:: fire a task-based timer at wall time
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**CALLING SEQUENCE:**
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.. index:: rtems_timer_server_fire_when
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.. code:: c
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rtems_status_code rtems_timer_server_fire_when(
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rtems_id id,
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rtems_time_of_day \*wall_time,
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rtems_timer_service_routine_entry routine,
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void \*user_data
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);
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**DIRECTIVE STATUS CODES:**
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``RTEMS_SUCCESSFUL`` - timer initiated successfully
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``RTEMS_INVALID_ADDRESS`` - ``routine`` is NULL
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``RTEMS_INVALID_ADDRESS`` - ``wall_time`` is NULL
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``RTEMS_INVALID_ID`` - invalid timer id
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``RTEMS_NOT_DEFINED`` - system date and time is not set
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``RTEMS_INVALID_CLOCK`` - invalid time of day
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``RTEMS_INCORRECT_STATE`` - Timer Server not initiated
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**DESCRIPTION:**
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This directive initiates the timer specified by id and specifies
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that when it fires it will be executed by the Timer Server.
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If the timer is running, it is automatically canceled before
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being initiated. The timer is scheduled to fire at the time of
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day specified by wall_time. When the timer fires, the timer
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service routine routine will be invoked with the argument
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user_data.
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**NOTES:**
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This directive will not cause the running task to be
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preempted.
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TIMER_RESET - Reset an interval timer
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-------------------------------------
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.. index:: reset a timer
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**CALLING SEQUENCE:**
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.. index:: rtems_timer_reset
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.. code:: c
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rtems_status_code rtems_timer_reset(
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rtems_id id
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);
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**DIRECTIVE STATUS CODES:**
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``RTEMS_SUCCESSFUL`` - timer reset successfully
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``RTEMS_INVALID_ID`` - invalid timer id
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``RTEMS_NOT_DEFINED`` - attempted to reset a when or newly created timer
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**DESCRIPTION:**
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This directive resets the timer associated with id.
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This timer must have been previously initiated with either the``rtems_timer_fire_after`` or``rtems_timer_server_fire_after``
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directive. If active the timer is canceled,
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after which the timer is reinitiated using the same interval and
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timer service routine which the original``rtems_timer_fire_after````rtems_timer_server_fire_after``
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directive used.
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|
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**NOTES:**
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If the timer has not been used or the last usage of this timer
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was by a ``rtems_timer_fire_when`` or``rtems_timer_server_fire_when``
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directive, then the ``RTEMS_NOT_DEFINED`` error is
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returned.
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Restarting a cancelled after timer results in the timer being
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reinitiated with its previous timer service routine and interval.
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This directive will not cause the running task to be preempted.
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.. COMMENT: COPYRIGHT (c) 1988-2013.
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.. COMMENT: On-Line Applications Research Corporation (OAR).
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.. COMMENT: All rights reserved.
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.. COMMENT: Open Issues
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.. COMMENT: - nicen up the tables
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.. COMMENT: - use math mode to print formulas
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