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StaticMutex/ONCELOCK question [message #19927]

Tue, 03 February 2009 06:28

Novo
Messages: 1430
Registered: December 2006

Ultimate Contributor

I couldn't understand completely several things with StaticMutex and ONCELOCK.

StaticMutex will never call destructor of a contained Mutex object. Is this meant to be?

#define ONCELOCK \
for(static volatile bool o_b_; !ReadWithBarrier(o_b_);) \
	for(static StaticMutex o_ss_; !o_b_;) \
		for(Mutex::Lock o_ss_lock__(o_ss_); !o_b_; BarrierWrite(o_b_, true))

How the above code actually works? Rolling Eyes

TIA

Regards,
Novo

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Re: StaticMutex/ONCELOCK question [message #19928 is a reply to message #19927]

Tue, 03 February 2009 07:41

mirek
Messages: 14271
Registered: November 2005

Ultimate Member

Novo wrote on Tue, 03 February 2009 00:28

I couldn't understand completely several things with StaticMutex and ONCELOCK.

StaticMutex will never call destructor of a contained Mutex object. Is this meant to be?

Yes. OS will clean that up when program exits.

Quote:

#define ONCELOCK \
for(static volatile bool o_b_; !ReadWithBarrier(o_b_);) \
	for(static StaticMutex o_ss_; !o_b_;) \
		for(Mutex::Lock o_ss_lock__(o_ss_); !o_b_; BarrierWrite(o_b_, true))

How the above code actually works? Rolling Eyes

TIA

Do not get fooled by 3 'for' loops - these are just syntactic sugar to make ONCELOCK work on C statements and blocks - they in fact simulate the outer block

{
   static volatile bool o_b_;
   if(!ReadWithBarrier(o_b_)) {
       static StaticMutex mutex;
       mutex.Enter();
       {
           do_the_initialization - the statement 'body'
           BarrierWrite(o_b_);
       }
   }
}

The purpose is to avoid locking mutex in subsequent passes of ONCELOCK - you need the barrier code to do that.

Note that both compilers we use optimize the for loops away.

Mirek

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Re: StaticMutex/ONCELOCK question [message #19940 is a reply to message #19928]

Tue, 03 February 2009 20:28

Novo
Messages: 1430
Registered: December 2006

Ultimate Contributor

luzr wrote on Tue, 03 February 2009 01:41

Do not get fooled by 3 'for' loops - these are just syntactic sugar to make ONCELOCK work on C statements and blocks - they in fact simulate the outer block

Thanks. I understand the idea with loops. I'm using similar technique to handle transactions myself.

What I do not understand is how uninitialized o_b_ works.

{
   static volatile bool o_b_;
   if(!ReadWithBarrier(o_b_)) {
       ...
   }
}

Regards,
Novo

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Re: StaticMutex/ONCELOCK question [message #19984 is a reply to message #19940]

Sun, 08 February 2009 03:09

Novo
Messages: 1430
Registered: December 2006

Ultimate Contributor

Novo wrote on Tue, 03 February 2009 14:28

What I do not understand is how uninitialized o_b_ works.

{
   static volatile bool o_b_;
   if(!ReadWithBarrier(o_b_)) {
       ...
   }
}

I finally figured out that myself. ANSI-compatible compiler initializes all static POD data with zeroes before a very first function call. So, this static variable is always initialized in a thread-safe way.

There is always something to learn about C.

Regards,
Novo

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