On Jan 5, 2006, at 12:25 PM, Stéphane Letz wrote:
Hi,
We need to setup a 1 millisecond periodic timer used in a real-
time (time constraint) thread in a pure C console program. We
first tried to use the carbon Timer manager API (with NewTimerUPP
and a timeManagerTask ...) but without being able to access the
thread that call the timer task in order to setup its scheduling
property to be "time constraint". We try also low-level function
like mach_wait_until but had problems on Panther.
What is the recommended way (API...) to implement a timer?
Dunno if this is the recommended API but I would do it using the
POSIX API pthread_cond_timedwait (or alternatively
pthread_cond_timedwait_relative_np). Create your timer thread with
pthread_create, and call mach specific API to set the "time
constraint" scheduling policy. You could also use CoreAudio SDK
utilities CAPThread, CAMutex and CAGuard which are nice C++
encapsulations of POSIX thread, mutex and variable condition
concepts that allow you to do the same things in a simpler way.
"pthread_cond_timedwait" requires a mutex and a condvar. In your
case you'll use only the "time out" part of the API but you still
need to pass valid mutex and cond although the mutex will never be
signaled. Just in case, be aware that "pthread_cond_timedwait"
can spuriously return with a "no error (0)" status. To manage this
situation enclose the "pthread_cond_timedwait" call within a loop
such as:
pthread_mutex_lock(mutex);
int status;
while (1) {
status = pthread_cond_timedwait(cond, mutex, absolute_time);
if (status == EINVAL) break; // ERROR: some invalid parameters
if (status == ETIMEDOUT) break; // OK, the actual time-out
}
pthread_mutex_unlock(mutex);
// Check the status and do the timer thread job
It may not be necessary in your case to lock the mutex but I don't
know if the OS will correctly deal with that. It expects a locked
mutex at entry, unlocks it to put the caller thread to sleep and
relocks it when the thread has to wake up.
Thanks
Stephane Letz
