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locking needed for simple addition?
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I am using multiple threads that are doing work. I need the threads to update an integer once they finish processing.
Then
My question is whether I need to use locks around i_Flag++ because it is a simple operation.
Thank you
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Then
C++ Syntax (Toggle Plain Text)
My question is whether I need to use locks around i_Flag++ because it is a simple operation.
Thank you
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>>My question is whether I need to use locks around
Yes, you need to synchronize access to that variable. There are a couple ways to do it: 1) semiphores, and 2) critical sections. Years ago I didn't do either because all it did was increment a global varialble. It cost me three days to figure out why my program randomily froze -- it was in a deadlock situation.
I would suggest you write a function to increment the variable. Inside that function you can add code to synchronize the threads. Then call that function instead of incrementing the variable all over the code.
Yes, you need to synchronize access to that variable. There are a couple ways to do it: 1) semiphores, and 2) critical sections. Years ago I didn't do either because all it did was increment a global varialble. It cost me three days to figure out why my program randomily froze -- it was in a deadlock situation.
I would suggest you write a function to increment the variable. Inside that function you can add code to synchronize the threads. Then call that function instead of incrementing the variable all over the code.
Last edited by Ancient Dragon; Sep 7th, 2009 at 1:11 am.
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Junior Poster
You reminded me one of my interviews (that did not go that well 
). Yes this is how you need to do it.
). Yes this is how you need to do it. I know I am. Therefore I am.
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Originally Posted by Ancient Dragon 
Years ago I didn't do either because all it did was increment a global varialble.
"One of the methods used by statists to destroy capitalism consists in establishing controls that tie a given industry hand and foot, making it unable to solve its problems, then declaring that freedom has failed and stronger controls are necessary." --Ayn Rand
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Originally Posted by Dave Sinkula
Just out of curiousity, what was the type of the variable, and on what platform? And related, ain't an integer increment on a PC on atomic operation?
. It is an integer as I mentioned in the code. •
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Originally Posted by dumrat
Just the reason I asked the question
[edit]I'm finding related info here. [edit]And here. Probably good 'nuff from that.
Last edited by Dave Sinkula; Sep 7th, 2009 at 3:06 am.
"One of the methods used by statists to destroy capitalism consists in establishing controls that tie a given industry hand and foot, making it unable to solve its problems, then declaring that freedom has failed and stronger controls are necessary." --Ayn Rand
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Originally Posted by Dave Sinkula
Just out of curiousity, what was the type of the variable, and on what platform? And related, ain't an integer increment on a PC on atomic operation?
Incrementing an int may take more than one assembly instruction, and a task switch could take place during the execution of any given assembly instruction. Without some sort of locking mechanism there is no guarantee that the increment operator will finish before another thread tries to read/write it.
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Some additional reading, which Dave's links allude to.
http://www.itis.mn.it/linux/quarta/x86/lock.htm
http://www.dlugosz.com/Repertoire/re...hitepaper.html
http://www.informationdelight.info/e...embly_language
You're not going to be able to do this in simple ANSI C, without getting your fingers grubby with assembler, or getting the OS involved in some way.
http://www.itis.mn.it/linux/quarta/x86/lock.htm
http://www.dlugosz.com/Repertoire/re...hitepaper.html
http://www.informationdelight.info/e...embly_language
You're not going to be able to do this in simple ANSI C, without getting your fingers grubby with assembler, or getting the OS involved in some way.
Yes you need a lock. Only atomic reads (32 bits on a 32 bit machine, 64 bits on a 64 bit machine) do not need locks.
.Net does provide a simple add and subract for simple data types. Reference here:
http://msdn.microsoft.com/en-us/libr...d_members.aspx
.Net does provide a simple add and subract for simple data types. Reference here:
http://msdn.microsoft.com/en-us/libr...d_members.aspx
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