[Ace-users] [ace-users] RE : Notify in handle_signal

Douglas C. Schmidt schmidt at dre.vanderbilt.edu
Tue Feb 12 07:48:44 CST 2008

Hi Philippe,

> Does anyone agree/disagree there is an issue here ?
> I would really appreciate your help about this.

Ways to address this situation are covered in Chapters 3 and 4 of
C++NPv2 <www.cs.wustl.edu/~schmidt/ACE/book2/>.  I'll let Steve
H. address it in the context of the APG example.



> Regards,
> Philippe David
> 	 -----Original Message-----
> 	From: 	ZXERASOGE002, Ext  
> 	Sent:	Friday, February 08, 2008 11:42 AM
> 	To:	ace-users at cse.wustl.edu
> 	Subject:	Notify in handle_signal
> 	Hi,
> 	I have a question relative to the sample code in section 7.4 of
> the ACE programmer's guide p158.
> 	In a handle_signal callback, there is the following code:
> 	ACE_Reactor::instance ()->notify(this)
> 	There is a potential deadlock here. If for some reason the
> process receives a lot of signals, and you don't process the
> notifications quickly enough, the pipe can be full. Since this call is
> blocking, and you are processing a signal, there is no chance your
> process will unlock from this situation.
> 	What is the best way to solve this ? Making a blocking call in
> handle_signal looks like a mistake to me.
> 	In the case of notify, if the pipe is full there's is no need to
> wait, it will stay in this state as long as we are in handle_signal. I
> see that we can add a timeout to the notify call, so at least it won't
> wait forever. But as I said, if the pipe is full there is no need to
> wait, so what is the best value for this timeout ? 1ms ? Are we 100%
> sure this low value won't skip a write when the pipe is not full ?
> 	Still, it means that every signal received when the pipe is full
> will lock the process for 1ms, so we are slowing down the process
> exactly when it is not fast enough to handle all the signals received.
> Plus each handle_signal will be 1ms long, which is too much.
> 	So is there a way to make a non blocking call in the notify ? or
> a way to know if the pipe is full before trying to make a write on the
> pipe ?
> 	---
> 	Philippe David
> 	SOGETI High Tech
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