Hi Amos, thank you very much for the clear and very informative reply.
It seems I need some learning before this implementation and I'll do that
right away based on your advises.
Rafi.
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] Behalf Of Amos Shapira
Sent: Tuesday, January 09, 2007 1:23 AM
To: Linux-IL
Subject: Re: implement timers in a multi thread application
On 09/01/07, Rafi Cohen <[EMAIL PROTECTED]> wrote:
Gilad, thank you very much for your advise.
I tend to implement it, but I still remain with the following question:
if a thread has various timeout cases, for example, one after 10 seconds
and another one after 40 seconds, how this could be implemented in the
timers thread waiting for the SIGALRM signal upon sigwait() call?
You should sort of implement this as a little OS scheduler - pass a timer
and a function to call to when the timer expires into a shared timer queue,
regardless of which thread the scheduling is done from. Once the timer
expires the main timer-handling thread should arrange that some "worker
thread" will execute the function handed over to handle that timer expiry.
When alarm is called, for example with 10 as parameters, I know that
SIGALRM will be received after (aproximately) 10 seconds.
But, if I don't use the alarm call, then what?
Is there a way to produce SIGALRM signal every constant number of
seconds so that I can count the number of received SIGALRM signals and
proceed according to the number that once sum up to 10 seconds and once sums
up to 40 seconds?
No, what you should do is to implement a timer-queue - a shared linked
list of timer events, each entry basically has a <time left from now,
function to call, void *> ttiplet. That list should be ordered by increasing
<time left from now>, each entry actually holding the difference in time
between it and the one before it. The "current alarm" will be the first in
the list and when the ALRM signal is received the "timer scheduling thread"
can check that it's the right time to run the first entry in the list
(passing it the <void *> given when the timer was added to the list) then
clear it from the list and set the next alarm for the next timer on the
list.
The <void *> will be extremely important to provide context for the
function which is being called when the timer expires.
(Small implementation note - take into account possibility of multiple
timers expiring at the same time, threads manipulating the list
concurrently, and the list being changed while the timer thread is
processing an alarm ( e.g. a function called from an expired timer adding a
timer to call itself in a few more seconds).
I've done this before (without threads, the POSIX standard wasn't commonly
available back then) and it's quite fun to program, but today I'd recommend
you to consider moving to C++ and maybe take advantage of libraries like
Boost ( boost.org) or ACE (http://www.cs.wustl.edu/~schmidt/ACE.html, look
for "Concurrency" in
http://www.dre.vanderbilt.edu/~schmidt/DOC_ROOT/ACE/docs/ACE-categories.html
for a start)
I hope I was clear here.
Thanks, Rafi.
HTH,
--Amos
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