Is reading from a int[] array while another thread updates that int[] array safe?

Question

Two threads. The first furiously reading elements from the array. The second is equally furious in updating elements by reading them and incrementing them by an arbitrary amount.

Is this safe? can anything go wrong in this situation? I don't mind that the reading thread reads an 'old' value while the updating thread is still in the process of updating. I just want to make sure the reader doesn't ever read a number that was not written and also that an exception cannot occur.

There will be no exception. There wouldn't even be an exception if "half-written" updates could happen, which they can't. — user555045
– user555045, Commented Sep 23, 2013 at 16:55
Do you have any requirement that a third thread that is also reading the array sees a consistent ordering of the operations of the other two threads? Because that is not guaranteed by the C# memory model without introducing barriers. — Eric Lippert
– Eric Lippert, Commented Sep 23, 2013 at 17:41

Hans Passant · Accepted Answer · 2013-09-23 16:55:08Z

9

An int update is atomic on all cpu architectures that can execute managed code. In other words, you won't read a value that has just a single byte modified by the writing thread. Value type values larger than 32-bit, like long and double are not guaranteed atomic. Object references are also always atomic.

answered Sep 23, 2013 at 16:55

Hans Passant

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2 Comments

Display Name Over a year ago

long must be atomic on x86-64, and also memory pointers are atomic

Eric Lippert Over a year ago

@SargeBorsch: To be picky: the C# language guarantees that pointers and references are atomic on all architectures when aligned. Though the runtime or the hardware might make a guarantee for longs, the C# language does not.

Daniel A. White · Accepted Answer · 2013-09-23 16:51:37Z

1

That isn't thread-safe - you aren't guaranteed that one thread gets the proper data.

answered Sep 23, 2013 at 16:51

Daniel A. White

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4 Comments

Matt Over a year ago

But can you be guaranteed that the reading thread will read either new or old data, rather than some other irrelevant data?

Preston Guillot Over a year ago

It doesn't appear that he's concerned about stale reads, only about potential run time exceptions, making me fear for the place this implementation is occuring.

Weyland Yutani Over a year ago

don't worry preston, it's just some autopilot code I am writing for the aircraft that flies around our nuclear power plant

Preston Guillot Over a year ago

If that's all, I suggest doing away with the array at all and returning Rand.Next() on every call.

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