# 10.3. Operations¶

## 10.3.1. Creating a Timer¶

The rtems_timer_create directive creates a timer by allocating a Timer Control Block (TMCB), assigning the timer a user-specified name, and assigning it a timer ID. Newly created timers do not have a timer service routine associated with them and are not active.

## 10.3.2. Obtaining Timer IDs¶

When a timer is created, RTEMS generates a unique timer ID and assigns it to the created timer until it is deleted. The timer ID may be obtained by either of two methods. First, as the result of an invocation of the rtems_timer_create directive, the timer ID is stored in a user provided location. Second, the timer ID may be obtained later using the rtems_timer_ident directive. The timer ID is used by other directives to manipulate this timer.

## 10.3.3. Initiating an Interval Timer¶

The rtems_timer_fire_after and rtems_timer_server_fire_after directives initiate a timer to fire a user provided timer service routine after the specified number of clock ticks have elapsed. When the interval has elapsed, the timer service routine will be invoked from a clock tick directive if it was initiated by the rtems_timer_fire_after directive and from the Timer Server task if initiated by the rtems_timer_server_fire_after directive.

## 10.3.4. Initiating a Time of Day Timer¶

The rtems_timer_fire_when and rtems_timer_server_fire_when directive initiate a timer to fire a user provided timer service routine when the specified time of day has been reached. When the interval has elapsed, the timer service routine will be invoked from a clock tick directive by the rtems_timer_fire_when directive and from the Timer Server task if initiated by the rtems_timer_server_fire_when directive.

## 10.3.5. Canceling a Timer¶

The rtems_timer_cancel directive is used to halt the specified timer. Once canceled, the timer service routine will not fire unless the timer is reinitiated. The timer can be reinitiated using the rtems_timer_reset, rtems_timer_fire_after, and rtems_timer_fire_when directives.

## 10.3.6. Resetting a Timer¶

The rtems_timer_reset directive is used to restore an interval timer initiated by a previous invocation of rtems_timer_fire_after or rtems_timer_server_fire_after to its original interval length. If the timer has not been used or the last usage of this timer was by the rtems_timer_fire_when or rtems_timer_server_fire_when directive, then an error is returned. The timer service routine is not changed or fired by this directive.

## 10.3.7. Initiating the Timer Server¶

The rtems_timer_initiate_server directive is used to allocate and start the execution of the Timer Server task. The application can specify both the stack size and attributes of the Timer Server. The Timer Server executes at a priority higher than any application task and thus the user can expect to be preempted as the result of executing the rtems_timer_initiate_server directive.

## 10.3.8. Deleting a Timer¶

The rtems_timer_delete directive is used to delete a timer. If the timer is running and has not expired, the timer is automatically canceled. The timer’s control block is returned to the TMCB free list when it is deleted. A timer can be deleted by a task other than the task which created the timer. Any subsequent references to the timer’s name and ID are invalid.