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Multithreaded programming question (theoretical)

Started by SpaceRogue Jul 17, 2003 at 10:00 PM 4 replies 800+ views

Original Post

Jul 17, 2003 at 10:00 PM

Normally I know you would want to use synchronization objects like a mutex, or critical section if you need a variable modified by two or more threads, but is it ALWAYS necessary? If I have a simple integer variable that will be incremented periodically by different threads, is it really necessary to protect the variable with a synchronization object even if I have no conditionals depending on the variable's value (I'm using it purely for informational purposes)? In other words, is it possible two threads could 'corrupt' the value by accessing it simultaneously, or (since it is a simple increment) will it be ok? [edited by - SpaceRogue on July 17, 2003 11:19:35 PM]

SpaceRogue

Author

SpaceRogue

Jul 17, 2003 at 10:18 PM

I already know about InterlockedIncrement...thanks anyway.

Theoretically : I'd like to know what the possibilities are. Edit: It's my understanding that making the variable volatile should be all that is needed to prevent corruption. Am I correct?

[edited by - SpaceRogue on July 17, 2003 11:30:46 PM]

cmptrgear

Jul 17, 2003 at 10:36 PM

The biggest problem would be that it wouldnt catch all of the increments.

Say you have two threads and variable a=5
One thread increments the variable
a=6

Before the variable gets stored the second thread interrupts and increments the variable
a=6 because a in memory is still 5
a is stored in memory so a=6

the first thread finishes and stores a(6) in memory but since both threads incremented the variable it should be 7

"I may not agree with what you say but I will defend to the death your right to say it."
--Voltaire

Gizz

Jul 17, 2003 at 10:39 PM

I don''t think there should be any problem using a simple type variable which does not hold critical information. Even if you get "old" information it should not affect you.

As for the possibility of having corrupted data, on a single processor machine for a simple type, I don''t think it could happen ( only one operation to affect a value to an int ) .

Cheer

SpaceRogue

Author

SpaceRogue

Jul 17, 2003 at 10:39 PM

Thanks, hmm...that makes sense.

[edited by - SpaceRogue on July 17, 2003 11:39:56 PM]

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