does reading op need semaphore in multi-thread coding?

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fred - 26 Jul 2007 21:28 GMT

hi All,
I am working in a multi thread project. is it necessary to use
semaphore when doing reading operation. in my case I do loop to read
share memory data. I remember it should be fine to read without
protection.
Thank in advance.
Fred

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Jon Skeet [C# MVP] - 26 Jul 2007 22:14 GMT

> I am working in a multi thread project. is it necessary to use
> semaphore when doing reading operation. in my case I do loop to read
> share memory data. I remember it should be fine to read without
> protection.

No, it isn't. Depending on exactly what you're doing, the JIT or CPU
may decide to cache the results of an earlier read, not refreshing it
each iteration.

Either store your data in volatile variables, or only touch it inside a
lock, using the same lock to protect both the read and the write of the
data.

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Göran Andersson - 26 Jul 2007 22:22 GMT

> hi All,
> I am working in a multi thread project. is it necessary to use
[quoted text clipped - 3 lines]
> Thank in advance.
> Fred

Each single read is atomic, and is guaranteed not to give you a halfway
changed value, as long as it's a native data type.

If you are looping a list or collection that may change in size, though,
that needs to be synchronised.

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Jon Skeet [C# MVP] - 26 Jul 2007 23:10 GMT

> > I am working in a multi thread project. is it necessary to use
> > semaphore when doing reading operation. in my case I do loop to read
[quoted text clipped - 4 lines]
> Each single read is atomic, and is guaranteed not to give you a halfway
> changed value, as long as it's a native data type.

No - unless you deem "double" and "long" not to be native data types.

> If you are looping a list or collection that may change in size, though,
> that needs to be synchronised.

There's more to worry about than atomicity - there's volatility too.
Suppose the value of an int variable changes from 0 to 5. Without any
extra work, I know that I'll never see any value *other* than 0 or 5
(due to atomicity) but there's no guarantee that I'll *ever* see 5.
That's not good for most scenarios :)

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Göran Andersson - 27 Jul 2007 01:19 GMT

>>> I am working in a multi thread project. is it necessary to use
>>> semaphore when doing reading operation. in my case I do loop to read
[quoted text clipped - 5 lines]
>
> No - unless you deem "double" and "long" not to be native data types.

Right. "Native" wasn't exactly what I was after.

>> If you are looping a list or collection that may change in size, though,
>> that needs to be synchronised.
[quoted text clipped - 4 lines]
> (due to atomicity) but there's no guarantee that I'll *ever* see 5.
> That's not good for most scenarios :)

Good point.

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Göran Andersson
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does reading op need semaphore in multi-thread coding?

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