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<!-- Copyright (C) 2003 Red Hat, Inc.                                -->
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>Counters</TITLE
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>eCos Reference Manual</TH
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><HR
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WIDTH="100%"></DIV
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><H1
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><A
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NAME="KERNEL-COUNTERS">Counters</H1
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><DIV
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CLASS="REFNAMEDIV"
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><A
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NAME="AEN854"
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></A
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><H2
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>Name</H2
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>cyg_counter_create, cyg_counter_delete, cyg_counter_current_value, cyg_counter_set_value, cyg_counter_tick&nbsp;--&nbsp;Count event occurrences</DIV
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><DIV
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CLASS="REFSYNOPSISDIV"
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><A
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NAME="AEN861"><H2
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>Synopsis</H2
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><DIV
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CLASS="FUNCSYNOPSIS"
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NAME="AEN862"><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_counter_create</CODE
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>(cyg_handle_t* handle, cyg_counter* counter);</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_counter_delete</CODE
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>(cyg_handle_t counter);</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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>cyg_tick_count_t cyg_counter_current_value</CODE
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>(cyg_handle_t counter);</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_counter_set_value</CODE
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>(cyg_handle_t counter, cyg_tick_count_t new_value);</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_counter_tick</CODE
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>(cyg_handle_t counter);</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-COUNTERS-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 counters can be used to keep track of how many times a
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particular event has occurred. Usually this event is an external
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signal of some sort. The most common use of counters is in the
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implementation of clocks, but they can be useful with other event
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sources as well. Application code can attach <A
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HREF="kernel-alarms.html"
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>alarms</A
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> to counters, causing a function
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to be called when some number of events have occurred.
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      </P
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><P
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>A new counter is initialized by a call to
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<TT
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CLASS="FUNCTION"
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>cyg_counter_create</TT
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>. The first argument is used to
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return a handle to the new counter which can be used for subsequent
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operations. The second argument allows the application to provide the
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memory needed for the object, thus eliminating any need for dynamic
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memory allocation within the kernel. If a counter is no longer
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required and does not have any alarms attached then
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<TT
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CLASS="FUNCTION"
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>cyg_counter_delete</TT
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> can be used to release the
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resources, allowing the <SPAN
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CLASS="STRUCTNAME"
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>cyg_counter</SPAN
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> data
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structure to be re-used.
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      </P
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><P
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>Initializing a counter does not automatically attach it to any source
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of events. Instead some other code needs to call
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<TT
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CLASS="FUNCTION"
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>cyg_counter_tick</TT
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> whenever a suitable event
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occurs, which will cause the counter to be incremented and may cause
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alarms to trigger. The current value associated with the counter can
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be retrieved using <TT
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CLASS="FUNCTION"
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>cyg_counter_current_value</TT
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> and
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modified with <TT
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CLASS="FUNCTION"
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>cyg_counter_set_value</TT
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>. Typically
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the latter function is only used during initialization, for example to
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set a clock to wallclock time, but it can be used to reset a counter
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if necessary. However <TT
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CLASS="FUNCTION"
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>cyg_counter_set_value</TT
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> will
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never trigger any alarms. A newly initialized counter has a starting
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value of 0.
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      </P
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><P
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>The kernel provides two different implementations of counters. The
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default is <TT
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CLASS="VARNAME"
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>CYGIMP_KERNEL_COUNTERS_SINGLE_LIST</TT
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> which
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stores all alarms attached to the counter on a single list. This is
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simple and usually efficient. However when a tick occurs the kernel
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code has to traverse this list, typically at DSR level, so if there
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are a significant number of alarms attached to a single counter this
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will affect the system's dispatch latency. The alternative
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implementation, <TT
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CLASS="VARNAME"
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>CYGIMP_KERNEL_COUNTERS_MULTI_LIST</TT
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>,
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stores each alarm in one of an array of lists such that at most one of
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the lists needs to be searched per clock tick. This involves extra
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code and data, but can improve real-time responsiveness in some
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circumstances. Another configuration option that is relevant here
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is <TT
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CLASS="VARNAME"
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>CYGIMP_KERNEL_COUNTERS_SORT_LIST</TT
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>, which is
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disabled by default. This provides a trade off between doing work
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whenever a new alarm is added to a counter and doing work whenever a
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tick occurs. It is application-dependent which of these is more
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appropriate.
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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-COUNTERS-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_counter_create</TT
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> is typically called during
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system initialization but may also be called in thread context.
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Similarly <TT
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CLASS="FUNCTION"
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>cyg_counter_delete</TT
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> may be called during
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initialization or in thread context.
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<TT
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CLASS="FUNCTION"
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>cyg_counter_current_value</TT
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>,
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<TT
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CLASS="FUNCTION"
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>cyg_counter_set_value</TT
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> and
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<TT
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CLASS="FUNCTION"
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>cyg_counter_tick</TT
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> may be called during
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initialization or from thread or DSR context. In fact,
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<TT
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CLASS="FUNCTION"
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>cyg_counter_tick</TT
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> is usually called from inside a
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DSR in response to an external event of some sort.
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      </P
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>Exception handling</TD
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