iOS Atomic and Non-Atomic

[nashyo]

@property (strong) NSURL * fileURL;

When would I not want to specify 'write atomically' or write 'non-atomically'?

 

[Reason077]

"Atomic" means that the whole value of a variable will always be written or read in one go, so that other threads can not see a partially-written value.

Most modern CPU architectures guarantee atomicity anyway, for variables that are smaller or equal in size to a machine word. So it probably only changes the generated code for, say, double values on a 32-bit system.

Note that "atomic" is different from say, "volatile" in Java. It doesn't make any guarantees about the memory ordering of reads and writes.

 

[MattInOz]

good blog generally but this post touches on the subject.

okay_atomic

 

[nashyo]

"Atomic" means that the whole value of a variable will always be written or read in one go, so that other threads can not see a partially-written value.

Most modern CPU architectures guarantee atomicity anyway, for variables that are smaller or equal in size to a machine word. So it probably only changes the generated code for, say, double values on a 32-bit system.

Note that "atomic" is different from say, "volatile" in Java. It doesn't make any guarantees about the memory ordering of reads and writes.

Objective C is my first and only language at this point and I don't come from an extensive computer science background. I've only been coding for 8months.

Typically I write properties for objects that specify nonatomic, and I avoid atomic/nonatomic when primitive types.

I think I understand that nonatomic has something to do with checking something has written to disk properly, is that right?

But my original question is, when would I not specify either when creating an object property?

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good blog generally but this post touches on the subject.

okay_atomic

Ha, I like it.

 

[Reason077]

I think I understand that nonatomic has something to do with checking something has written to disk properly, is that right?

But my original question is, when would I not specify either when creating an object property?

No, nothing to do with disk access.

atomic is only relevant if multiple threads are accessing your object simultaneously. 99.9% of time you don't have to worry about this, so "nonatomic" is almost always the right thing to do, because it can be faster.

(it was a mistake, IMO, that they made "atomic" the default in Objective C)

 

[solderguy1]

There's two atomics in Obj-C
1. Properties specified as nonatomic do not have extra code added to make them safe in a multi-thread environment. There's no keyword for atomic, it's simply the default case

2. A file write specified as atomic will write out to a temporary name first. Once all the data is transferred, it will overwrite the previous file in a single swoop so there's no chance of a partial write producing a garbage file.