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>Alarms</TITLE
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>eCos Reference Manual</TH
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><H1
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><A
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NAME="KERNEL-ALARMS">Alarms</H1
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><DIV
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CLASS="REFNAMEDIV"
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><A
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NAME="AEN1016"
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></A
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><H2
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>Name</H2
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>cyg_alarm_create, cyg_alarm_delete, cyg_alarm_initialize, cyg_alarm_enable, cyg_alarm_disable&nbsp;--&nbsp;Run an alarm function when a number of events have occurred</DIV
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><DIV
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CLASS="REFSYNOPSISDIV"
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><A
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NAME="AEN1023"><H2
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>Synopsis</H2
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><DIV
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NAME="AEN1024"><P
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></P
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><TABLE
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BORDER="5"
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BGCOLOR="#E0E0F0"
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WIDTH="70%"
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><TR
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><TD
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><PRE
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CLASS="FUNCSYNOPSISINFO"
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>#include &lt;cyg/kernel/kapi.h&gt;
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        </PRE
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></TD
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></TR
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></TABLE
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void cyg_alarm_create</CODE
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>(cyg_handle_t counter, cyg_alarm_t* alarmfn, cyg_addrword_t data, cyg_handle_t* handle, cyg_alarm* alarm);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void cyg_alarm_delete</CODE
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>(cyg_handle_t alarm);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void cyg_alarm_initialize</CODE
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>(cyg_handle_t alarm, cyg_tick_count_t trigger, cyg_tick_count_t interval);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void cyg_alarm_enable</CODE
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>(cyg_handle_t alarm);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void cyg_alarm_disable</CODE
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>(cyg_handle_t alarm);</CODE
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></P
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><P
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></P
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></DIV
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="KERNEL-ALARMS-DESCRIPTION"
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></A
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><H2
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>Description</H2
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><P
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>Kernel alarms are used together with counters and allow for action to
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be taken when a certain number of events have occurred. If the counter
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is associated with a clock then the alarm action happens when the
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appropriate number of clock ticks have occurred, in other words after
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a certain period of time.
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      </P
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><P
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>Setting up an alarm involves a two-step process. First the alarm must
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be created with a call to <TT
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CLASS="FUNCTION"
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>cyg_alarm_create</TT
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>. This
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takes five arguments. The first identifies the counter to which the
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alarm should be attached. If the alarm should be attached to the
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system's real-time clock then <TT
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CLASS="FUNCTION"
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>cyg_real_time_clock</TT
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>
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and <TT
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CLASS="FUNCTION"
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>cyg_clock_to_counter</TT
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> can be used to get hold
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of the appropriate handle. The next two arguments specify the action
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to be taken when the alarm is triggered, in the form of a function
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pointer and some data. This function should take the form:
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      </P
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><TABLE
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>void
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alarm_handler(cyg_handle_t alarm, cyg_addrword_t data)
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{
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    &#8230;
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}
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      </PRE
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></TD
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></TR
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></TABLE
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><P
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>The data argument passed to the alarm function corresponds to the
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third argument passed to <TT
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CLASS="FUNCTION"
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>cyg_alarm_create</TT
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>.
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The fourth argument to <TT
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CLASS="FUNCTION"
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>cyg_alarm_create</TT
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> is used
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to return a handle to the newly-created alarm object, and the final
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argument provides the memory needed for the alarm object and thus
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avoids any need for dynamic memory allocation within the kernel.
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      </P
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><P
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>Once an alarm has been created a further call to
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<TT
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CLASS="FUNCTION"
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>cyg_alarm_initialize</TT
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> is needed to activate it.
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The first argument specifies the alarm. The second argument indicates
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the number of events, for example clock ticks, that need to occur
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before the alarm triggers. If the third argument is 0 then the alarm
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will only trigger once. A non-zero value specifies that the alarm
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should trigger repeatedly, with an interval of the specified number of
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events.
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      </P
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><P
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>Alarms can be temporarily disabled and reenabled using
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<TT
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CLASS="FUNCTION"
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>cyg_alarm_disable</TT
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> and
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<TT
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CLASS="FUNCTION"
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>cyg_alarm_enable</TT
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>. Alternatively another call to
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<TT
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CLASS="FUNCTION"
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>cyg_alarm_initialize</TT
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> can be used to modify the
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behaviour of an existing alarm. If an alarm is no longer required then
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the associated resources can be released using
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<TT
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CLASS="FUNCTION"
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>cyg_alarm_delete</TT
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>.
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      </P
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><P
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>The alarm function is invoked when a counter tick occurs, in other
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words when there is a call to <TT
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CLASS="FUNCTION"
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>cyg_counter_tick</TT
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>,
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and will happen in the same context. If the alarm is associated with
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the system's real-time clock then this will be DSR context, following
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a clock interrupt. If the alarm is associated with some other
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application-specific counter then the details will depend on how that
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counter is updated.
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      </P
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><P
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>If two or more alarms are registered for precisely the same counter tick,
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the order of execution of the alarm functions is unspecified.
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      </P
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="KERNEL-ALARMS-CONTEXT"
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></A
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><H2
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>Valid contexts</H2
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><P
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><TT
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CLASS="FUNCTION"
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>cyg_alarm_create</TT
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>
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<TT
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CLASS="FUNCTION"
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>cyg_alarm_initialize</TT
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> is typically called during
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system initialization but may also be called in thread context. The
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same applies to <TT
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CLASS="FUNCTION"
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>cyg_alarm_delete</TT
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>.
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<TT
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CLASS="FUNCTION"
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>cyg_alarm_initialize</TT
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>,
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<TT
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CLASS="FUNCTION"
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>cyg_alarm_disable</TT
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> and
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<TT
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CLASS="FUNCTION"
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>cyg_alarm_enable</TT
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> may be called during
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initialization or from thread or DSR context, but
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<TT
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CLASS="FUNCTION"
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>cyg_alarm_enable</TT
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> and
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<TT
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CLASS="FUNCTION"
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>cyg_alarm_initialize</TT
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> may be expensive operations
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and should only be called when necessary.
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