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Re: Use same variable in different threads [message #30133 is a reply to message #30132]

Fri, 10 December 2010 14:08

gprentice is currently offline

gprentice
Messages: 260
Registered: November 2005
Location: New Zealand

Experienced Member

No expert here either but when you're sharing data between threads, I think you need both volatile and synchronization.

A mutex ensures that the compiler/linker can't optimise code across the mutex entry, that the cache is flushed and that only one thread can be executing the code the mutex protects (i.e. it's synchronized).

http://msdn.microsoft.com/en-us/library/ms686355(v=VS.85).aspx

volatile ensures that the compiler/linker can't optimise your code and use a cached value. With Microsoft, optimisation can occur at link time. On some platforms you can get away without volatile if you call a global function that the compiler can't see - the compiler has to assume that global function might modify the variable you're sharing so is forced to re-read the variable from memory, but that is't safe with Microsoft.

There's also thread local storage - see thread__

Regarding atomic - on Win32, 32 bits are atomic and on Win64, 64 bits are atomic. On Win32, the atomixXXX functions use the Interlocked... functions that allow you to read/write without being interrupted by another thread etc, and also provide a memory barrier.

Hence I think you need
volatile mySharedData data;
volatile Atomic threadnum;
Anyway, I don't think that using volatile would be wrong, even if it's not always necessary.

Graeme

[Updated on: Fri, 10 December 2010 14:09]

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[Message index]

		Use same variable in different threads By: koldo on Thu, 09 December 2010 18:26
		Re: Use same variable in different threads By: dolik.rce on Thu, 09 December 2010 18:57
		Re: Use same variable in different threads By: koldo on Thu, 09 December 2010 22:04
		Re: Use same variable in different threads By: Didier on Fri, 10 December 2010 00:26
		Re: Use same variable in different threads By: dolik.rce on Fri, 10 December 2010 06:49
		Re: Use same variable in different threads By: koldo on Fri, 10 December 2010 10:01
		Re: Use same variable in different threads By: dolik.rce on Fri, 10 December 2010 11:34
		Re: Use same variable in different threads By: koldo on Fri, 10 December 2010 13:19
		Re: Use same variable in different threads By: gprentice on Fri, 10 December 2010 14:08
		Re: Use same variable in different threads By: dolik.rce on Fri, 10 December 2010 14:16
		Re: Use same variable in different threads By: gprentice on Fri, 10 December 2010 14:34
		Re: Use same variable in different threads By: Didier on Fri, 10 December 2010 18:50
		Re: Use same variable in different threads By: gprentice on Sat, 11 December 2010 01:37
		Re: Use same variable in different threads By: Didier on Sat, 11 December 2010 23:13
		Re: Use same variable in different threads By: gprentice on Sun, 12 December 2010 01:57
		Re: Use same variable in different threads By: mirek on Sun, 12 December 2010 09:43
		Re: Use same variable in different threads By: koldo on Sun, 12 December 2010 23:16
		Re: Use same variable in different threads By: tojocky on Mon, 13 December 2010 11:35
		Re: Use same variable in different threads By: gprentice on Mon, 13 December 2010 11:46
		Re: Use same variable in different threads By: Didier on Mon, 13 December 2010 22:48
		Re: Use same variable in different threads By: koldo on Tue, 14 December 2010 09:02
		Re: Use same variable in different threads By: mirek on Sat, 25 December 2010 20:45
		Re: Use same variable in different threads By: tojocky on Sun, 02 January 2011 15:35
		Re: Use same variable in different threads By: mirek on Sun, 02 January 2011 18:38
		Re: Use same variable in different threads By: koldo on Sun, 02 January 2011 19:06
		Re: Use same variable in different threads By: mirek on Sun, 02 January 2011 22:56
		Re: Use same variable in different threads By: koldo on Mon, 03 January 2011 09:17
		Re: Use same variable in different threads By: koldo on Sun, 02 January 2011 01:12
		Re: Use same variable in different threads By: gprentice on Sun, 02 January 2011 03:08
		Re: Use same variable in different threads By: Didier on Sun, 02 January 2011 12:18
		Re: Use same variable in different threads By: gprentice on Sun, 02 January 2011 13:11

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