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>More Features — Clocks and Alarm
Handlers</TITLE
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><DIV
CLASS="CHAPTER"
><H1
><A
NAME="CLOCKS-AND-ALARM-HANDLERS">Chapter 14. More Features — Clocks and Alarm
Handlers</H1
><P
>If a program wanted to execute a task at a given time, or
periodically, it could do it in an inefficient way by sitting in a
loop and checking the real-time clock to see if the proper amount of
time has elapsed. But operating systems usually provide system calls
which allow the program to be informed at the desired time.</P
><P
><SPAN
CLASS="PRODUCTNAME"
>eCos</SPAN
> provides a rich timekeeping formalism, involving
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>counters</I
></SPAN
>, <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>clocks</I
></SPAN
>,
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>alarms</I
></SPAN
>, and <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>timers</I
></SPAN
>. The
precise definition, relationship, and motivation of these features is
beyond the scope of this tutorial, but these examples illustrate how
to set up basic periodic tasks.</P
><P
>Alarms are events that happen at
a given time, either once or periodically. A thread associates an
alarm handling function with the alarm, so that the function will
be invoked every time the alarm “goes off”.</P
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="SAMPLE-ALARMS">A Sample Program with Alarms</H1
><P
><TT
CLASS="FILENAME"
>simple-alarm.c</TT
> (in
the examples directory) is a short program that creates a thread that
creates an alarm. The alarm is handled by the function
<TT
CLASS="FUNCTION"
>test_alarm_func()</TT
>, which sets a global
variable. When the main thread of execution sees that the variable has
changed, it prints a message.</P
><DIV
CLASS="EXAMPLE"
><A
NAME="AEN910"><P
><B
>Example 14-1. A sample program that creates an alarm</B
></P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>/* this is a very simple program meant to demonstrate
a basic use of time, alarms and alarm-handling functions in eCos */
#include <cyg/kernel/kapi.h>
#include <stdio.h>
#define NTHREADS 1
#define STACKSIZE 4096
static cyg_handle_t thread[NTHREADS];
static cyg_thread thread_obj[NTHREADS];
static char stack[NTHREADS][STACKSIZE];
static void alarm_prog( cyg_addrword_t data );
/* we install our own startup routine which sets up
threads and starts the scheduler */
void cyg_user_start(void)
{
cyg_thread_create(4, alarm_prog, (cyg_addrword_t) 0,
"alarm_thread", (void *) stack[0],
STACKSIZE, &thread[0], &thread_obj[0]);
cyg_thread_resume(thread[0]);
}
/* we need to declare the alarm handling function (which is
defined below), so that we can pass it to cyg_alarm_initialize() */
cyg_alarm_t test_alarm_func;
/* alarm_prog() is a thread which sets up an alarm which is then
handled by test_alarm_func() */
static void alarm_prog(cyg_addrword_t data)
{
cyg_handle_t test_counterH, system_clockH, test_alarmH;
cyg_tick_count_t ticks;
cyg_alarm test_alarm;
unsigned how_many_alarms = 0, prev_alarms = 0, tmp_how_many;
system_clockH = cyg_real_time_clock();
cyg_clock_to_counter(system_clockH, &test_counterH);
cyg_alarm_create(test_counterH, test_alarm_func,
(cyg_addrword_t) &how_many_alarms,
&test_alarmH, &test_alarm);
cyg_alarm_initialize(test_alarmH, cyg_current_time()+200, 200);
/* get in a loop in which we read the current time and
print it out, just to have something scrolling by */
for (;;) {
ticks = cyg_current_time();
printf("Time is %llu\n", ticks);
/* note that we must lock access to how_many_alarms, since the
alarm handler might change it. this involves using the
annoying temporary variable tmp_how_many so that I can keep the
critical region short */
cyg_scheduler_lock();
tmp_how_many = how_many_alarms;
cyg_scheduler_unlock();
if (prev_alarms != tmp_how_many) {
printf(" --- alarm calls so far: %u\n", tmp_how_many);
prev_alarms = tmp_how_many;
}
cyg_thread_delay(30);
}
}
/* test_alarm_func() is invoked as an alarm handler, so
it should be quick and simple. in this case it increments
the data that is passed to it. */
void test_alarm_func(cyg_handle_t alarmH, cyg_addrword_t data)
{
++*((unsigned *) data);
}</PRE
></TD
></TR
></TABLE
></DIV
><P
>When you run this program (by typing <B
CLASS="COMMAND"
>continue</B
> at
the (<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>gdb</I
></SPAN
>) prompt) the output should look like
this:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="SCREEN"
>Starting program: <TT
CLASS="REPLACEABLE"
><I
>BASE_DIR</I
></TT
>/examples/simple-alarm.exe
Time is 0
Time is 30
Time is 60
Time is 90
Time is 120
Time is 150
Time is 180
Time is 210
--- alarm calls so far: 1
Time is 240
Time is 270
Time is 300
Time is 330
Time is 360
Time is 390
Time is 420
--- alarm calls so far: 2
Time is 450
Time is 480</PRE
></TD
></TR
></TABLE
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>When running in a simulator the delays
might be quite long. On a hardware board (where the clock speed is 100
ticks/second) the delays should average to about 0.3 seconds (and 2
seconds between alarms). In simulation, the delay will depend on the
speed of the host processor and will almost always be much slower than
the actual board. You might want to reduce the delay parameter when
running in simulation.</P
></BLOCKQUOTE
></DIV
><P
>Here are a few things you might notice about this program:</P
><P
></P
><UL
><LI
><P
>It used the <TT
CLASS="FUNCTION"
>cyg_real_time_clock()</TT
> function;
this always returns a handle to the default system real-time clock. </P
></LI
><LI
><P
>Clocks are based on counters, so the function <TT
CLASS="FUNCTION"
>cyg_alarm_create()</TT
>
uses a counter handle. The program used the function
<TT
CLASS="FUNCTION"
>cyg_clock_to_counter()</TT
> to strip the clock handle
to the underlying counter handle. </P
></LI
><LI
><P
>Once the alarm is created it is
initialized with <TT
CLASS="FUNCTION"
>cyg_alarm_initialize()</TT
>, which
sets the time at which the alarm should go off, as well as the period
for repeating alarms. It is set to go off at the current time and
then to repeat every 200 ticks. </P
></LI
><LI
><P
>The alarm handler function
<TT
CLASS="FUNCTION"
>test_alarm_func()</TT
> conforms to the guidelines for
writing alarm handlers and other delayed service routines: it does not invoke any
functions which might lock the scheduler. This is discussed in detail
in the <I
CLASS="CITETITLE"
><SPAN
CLASS="PRODUCTNAME"
>eCos</SPAN
> Reference Manual</I
>, in the chapter
<I
CLASS="CITETITLE"
>The <SPAN
CLASS="PRODUCTNAME"
>eCos</SPAN
> Kernel</I
>.</P
></LI
><LI
><P
>There is a <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>critical region</I
></SPAN
> in this program:
the variable <TT
CLASS="LITERAL"
>how_many_alarms</TT
> is accessed in the
main thread of control and is also modified in the alarm handler. To
prevent a possible (though unlikely) race condition on this variable,
access to <TT
CLASS="LITERAL"
>how_many_alarms</TT
> in the principal thread
is protected by calls to <TT
CLASS="FUNCTION"
>cyg_scheduler_lock()</TT
> and
<TT
CLASS="FUNCTION"
>cyg_scheduler_unlock()</TT
>. When the scheduler is
locked, the alarm handler will not be invoked, so the problem is
averted. </P
></LI
></UL
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