By Alex Wayne


2009-02-26 02:31:34 8 Comments

What do atomic and nonatomic mean in property declarations?

@property(nonatomic, retain) UITextField *userName;
@property(atomic, retain) UITextField *userName;
@property(retain) UITextField *userName;

What is the operational difference between these three?

26 comments

@Suraj K Thomas 2019-01-29 06:12:16

Atomicity atomic (default)

Atomic is the default: if you don’t type anything, your property is atomic. An atomic property is guaranteed that if you try to read from it, you will get back a valid value. It does not make any guarantees about what that value might be, but you will get back good data, not just junk memory. What this allows you to do is if you have multiple threads or multiple processes pointing at a single variable, one thread can read and another thread can write. If they hit at the same time, the reader thread is guaranteed to get one of the two values: either before the change or after the change. What atomic does not give you is any sort of guarantee about which of those values you might get. Atomic is really commonly confused with being thread-safe, and that is not correct. You need to guarantee your thread safety other ways. However, atomic will guarantee that if you try to read, you get back some kind of value.

nonatomic

On the flip side, non-atomic, as you can probably guess, just means, “don’t do that atomic stuff.” What you lose is that guarantee that you always get back something. If you try to read in the middle of a write, you could get back garbage data. But, on the other hand, you go a little bit faster. Because atomic properties have to do some magic to guarantee that you will get back a value, they are a bit slower. If it is a property that you are accessing a lot, you may want to drop down to nonatomic to make sure that you are not incurring that speed penalty. Access

courtesy https://academy.realm.io/posts/tmi-objective-c-property-attributes/

Atomicity property attributes (atomic and nonatomic) are not reflected in the corresponding Swift property declaration, but the atomicity guarantees of the Objective-C implementation still hold when the imported property is accessed from Swift.

So — if you define an atomic property in Objective-C it will remain atomic when used by Swift.

courtesy https://medium.com/@YogevSitton/atomic-vs-non-atomic-properties-crash-course-d11c23f4366c

@Honey 2016-04-28 16:18:23

Before you begin: You must know that every object in memory needs to be deallocated from memory for a new writer to happen. You can't just simply write on top of something as you do on paper. You must first erase (dealloc) it and then you can write onto it. If at the moment that the erase is done (or half done) and nothing has yet been wrote (or half wrote) and you try to read it could be very problematic! Atomic and nonatomic help you treat this problem in different ways.

First read this question and then read Bbum's answer. In addition, then read my summary.


atomic will ALWAYS guarantee

  • If two different people want to read and write at the same time, your paper won't just burn! --> Your application will never crash, even in a race condition.
  • If one person is trying to write and has only written 4 of the 8 letters to write, then no can read in the middle, the reading can only be done when all 8 letters is written --> No read(get) will happen on 'a thread that is still writing', i.e. if there are 8 bytes to bytes to be written, and only 4 bytes are written——up to that moment, you are not allowed to read from it. But since I said it won't crash then it would read from the value of an autoreleased object.
  • If before writing you have erased that which was previously written on paper and then someone wants to read you can still read. How? You will be reading from something similar to Mac OS Trash bin ( as Trash bin is not still 100% erased...it's in a limbo) ---> If ThreadA is to read while ThreadB has already deallocated to write, you would get a value from either the final fully written value by ThreadB or get something from autorelease pool.

Retain counts are the way in which memory is managed in Objective-C. When you create an object, it has a retain count of 1. When you send an object a retain message, its retain count is incremented by 1. When you send an object a release message, its retain count is decremented by 1. When you send an object an autorelease message, its retain count is decremented by 1 at some stage in the future. If an objectʼs retain count is reduced to 0, it is deallocated.

  • Atomic doesn't guarantee thread safety, though it's useful for achieving thread safety. Thread Safety is relative to how you write your code/ which thread queue you are reading/writing from. It only guarantees non-crashable multithreading.

What?! Are multithreading and thread safety different?

Yes. Multithreading means: multiple threads can read a shared piece of data at the same time and we will not crash, yet it doesn't guarantee that you aren't reading from a non-autoreleased value. With thread safety, it's guaranteed that what you read is not auto-released. The reason that we don't make everything atomic by default is, that there is a performance cost and for most things don't really need thread safety. A few parts of our code need it and for those few parts, we need to write our code in a thread-safe way using locks, mutex or synchronization.


nonatomic

  • Since there is no such thing like Mac OS Trash Bin, then nobody cares whether or not you always get a value (<-- This could potentially lead to a crash), nor anybody cares if someone tries to read halfway through your writing (although halfway writing in memory is very different from halfway writing on paper, on memory it could give you a crazy stupid value from before, while on paper you only see half of what's been written) --> Doesn't guarantee to not crash, because it doesn't use autorelease mechanism.
  • Doesn't guarantee full written values to be read!
  • Is faster than atomic

Overall they are different in 2 aspects:

  • Crashing or not because of having or not having an autorelease pool.

  • Allowing to be read right in the middle of a 'not yet finished write or empty value' or not allowing and only allowing to read when the value is fully written.

@justin 2012-08-18 09:47:48

The syntax and semantics are already well-defined by other excellent answers to this question. Because execution and performance are not detailed well, I will add my answer.

What is the functional difference between these 3?

I'd always considered atomic as a default quite curious. At the abstraction level, we work at using atomic properties for a class as a vehicle to achieve 100% thread-safety is a corner case. For truly correct multithreaded programs, intervention by the programmer is almost certainly a requirement. Meanwhile, performance characteristics and execution have not yet been detailed in depth. Having written some heavily multithreaded programs over the years, I had been declaring my properties as nonatomic the entire time because atomic was not sensible for any purpose. During discussion of the details of atomic and nonatomic properties this question, I did some profiling encountered some curious results.

Execution

Ok. The first thing I would like to clear up is that the locking implementation is implementation-defined and abstracted. Louis uses @synchronized(self) in his example -- I have seen this as a common source of confusion. The implementation does not actually use @synchronized(self); it uses object level spin locks. Louis's illustration is good for a high-level illustration using constructs we are all familiar with, but it's important to know it does not use @synchronized(self).

Another difference is that atomic properties will retain/release cycle your objects within the getter.

Performance

Here's the interesting part: Performance using atomic property accesses in uncontested (e.g. single-threaded) cases can be really very fast in some cases. In less than ideal cases, the use of atomic accesses can cost more than 20 times the overhead of nonatomic. While the Contested case using 7 threads was 44 times slower for the three-byte struct (2.2 GHz Core i7 Quad-Core, x86_64). The three-byte struct is an example of a very slow property.

Interesting side note: User-defined accessors of the three-byte struct were 52 times faster than the synthesized atomic accessors; or 84% the speed of synthesized nonatomic accessors.

Objects in contested cases can also exceed 50 times.

Due to the number of optimizations and variations in implementations, it's quite difficult to measure real-world impacts in these contexts. You might often hear something like "Trust it unless you profile and find it is a problem". Due to the abstraction level, it's actually quite difficult to measure actual impact. Gleaning actual costs from profiles can be very time consuming, and due to abstractions, quite inaccurate. As well, ARC vs MRC can make a big difference.

So let's step back, not focussing on the implementation of property accesses, we'll include the usual suspects like objc_msgSend, and examine some real-world high-level results for many calls to an NSString getter in uncontested cases (values in seconds):

  • MRC | nonatomic | manually implemented getters: 2
  • MRC | nonatomic | synthesized getter: 7
  • MRC | atomic | synthesized getter: 47
  • ARC | nonatomic | synthesized getter: 38 (note: ARC's adding ref count cycling here)
  • ARC | atomic | synthesized getter: 47

As you have probably guessed, reference count activity/cycling is a significant contributor with atomics and under ARC. You would also see greater differences in contested cases.

Although I pay close attention to performance, I still say Semantics First!. Meanwhile, performance is a low priority for many projects. However, knowing the execution details and costs of technologies you use certainly doesn't hurt. You should use the right technology for your needs, purposes, and abilities. Hopefully, this will save you a few hours of comparisons, and help you make a better-informed decision when designing your programs.

@SDEZero 2012-08-27 15:20:46

MRC | atomic | synthesized getter: 47 ARC | atomic | synthesized getter: 47 What makes them the same? Should't ARC have more overhead?

@Kunal Balani 2013-09-17 20:44:59

So if atomic properties are bad y are they default. To increase the boilerplate code ?

@justin 2013-09-23 13:34:05

@LearnCocos2D i just tested on 10.8.5 on the same machine, targeting 10.8, for the single threaded uncontested case with an NSString which is not immortal: -ARC atomic (BASELINE): 100% -ARC nonatomic, synthesised: 94% -ARC nonatomic, user defined: 86% -MRC nonatomic, user defined: 5% -MRC nonatomic, synthesised: 19% -MRC atomic: 102% -- the results are a little different today. I wasn't doing any @synchronized comparisons. @synchronized is semantically different, and I don't consider it a good tool if you have nontrivial concurrent programs. if you need speed, avoid @synchronized.

@LearnCocos2D 2013-09-23 14:09:57

do you have this test online somewhere? I keep adding mine here: github.com/LearnCocos2D/LearnCocos2D/tree/master/…

@justin 2013-09-23 17:13:50

@LearnCocos2D i haven't prepared them for human consumption, sorry.

@Brad Thomas 2016-02-23 20:50:02

I always find it interesting that people will debate whether to making something incredibly fast, or 50 times faster but with no humanly noticeable difference between the two. It's like having a retina display and another display at 50 times the resolution. Why waste the resources to get that level of performance if it makes no difference to anyone? Especially when robust code can save days of debugging...

@Suryanarayan Sahu 2016-09-23 18:41:26

To simplify the entire confusion, let us understand mutex lock.

Mutex lock, as per the name, locks the mutability of the object. So if the object is accessed by a class, no other class can access the same object.

In iOS, @sychronise also provides the mutex lock .Now it serves in FIFO mode and ensures the flow is not affected by two classes sharing the same instance. However, if the task is on main thread, avoid accessing object using atomic properties as it may hold your UI and degrade the performance.

@ashish.surana 2018-08-04 11:26:43

Atomic properties :- When a variable assigned with atomic property that means it has only one thread access and it will be thread safe and will be good in performance perspective, will have default behaviour.

Non Atomic Properties :- When a variable assigned with atomic property that means it has multi thread access and it will not be thread safe and will be slow in performance perspective, will have default behaviour and when two different threads want to access variable at same time it will give unexpected results.

@Shourob Datta 2016-12-13 03:27:21

  • -Atomic means only one thread access the variable(static type).
  • -Atomic is thread safe.
  • -but it is slow in performance

How to declare:

As atomic is default so,

@property (retain) NSString *name;

AND in implementation file

self.name = @"sourov";

Suppose a task related to three properties are

 @property (retain) NSString *name;
 @property (retain) NSString *A;
 @property (retain) NSString *B;
 self.name = @"sourov";

All properties work parallelly (like asynchronously).

If you call "name" from thread A,

And

At the same time if you call

[self setName:@"Datta"]

from thread B,

Now If *name property is nonatomic then

  • It will return value "Datta" for A
  • It will return value "Datta" for B

Thats why non atomic is called thread unsafe But but it is fast in performance because of parallel execution

Now If *name property is atomic

  • It will ensure value "Sourov" for A
  • Then It will return value "Datta" for B

That's why atomic is called thread Safe and That's why it is called read-write safe

Such situation operation will perform serially. And Slow in performance

- Nonatomic means multiple thread access the variable(dynamic type).

- Nonatomic is thread unsafe.

- but it is fast in performance

-Nonatomic is NOT default behavior, we need to add nonatomic keyword in property attribute.

For In Swift Confirming that Swift properties are nonatomic in the ObjC sense. One reason is so you think about whether per-property atomicity is sufficient for your needs.

Reference: https://forums.developer.apple.com/thread/25642

Fro more info please visit the website http://rdcworld-iphone.blogspot.in/2012/12/variable-property-attributes-or.html

@Alejandro Iván 2017-02-15 13:31:12

As many many many maaaaany others have said, atomic is NOT thread-safe! It's more resistant to thread problems, but not thread-safe. It just ensures you will get a whole value, a.k.a. a "correct" value (binary level), but by no means it will ensure that it is the current and "correct" value for your business logic (it could be a past value and invalid by your logic).

@user3693546 2015-07-07 09:56:10

Atomic is thread safe, it is slow and it well-assures (not guaranteed) that only the locked value is provided no matter how many threads are attempting access over the same zone. When using atomic, a piece of code written inside this function becomes the part of the critical section, to which only one thread can execute at a time.

It only assures the thread safety; it does not guarantee that. What I mean is you hire an expert driver for you car, still it doesn't guarantees car won't meet an accident. However, probability remains the slightest.

Atomic - it can't be broken down, so the result is expected. With nonatomic - when another thread access the memory zone it can modify it, so the result is unexpected.

Code Talk :

Atomic make getter and setter of the property thread safe. for example if u have written :

self.myProperty = value;

is thread safe.

[myArray addObject:@"Abc"] 

is NOT thread safe.

@peak 2016-11-26 05:02:08

I don't know how the last paragraph comes, but it's simple wrong, there is no such thing like "private copy".

@Louis Gerbarg 2009-02-26 06:24:01

This is explained in Apple's documentation, but below are some examples of what is actually happening. Note that there is no "atomic" keyword, if you do not specify "nonatomic" then the property is atomic, but specifying "atomic" explicitly will result in an error.

//@property(nonatomic, retain) UITextField *userName;
//Generates roughly

- (UITextField *) userName {
    return userName;
}

- (void) setUserName:(UITextField *)userName_ {
    [userName_ retain];
    [userName release];
    userName = userName_;
}

Now, the atomic variant is a bit more complicated:

//@property(retain) UITextField *userName;
//Generates roughly

- (UITextField *) userName {
    UITextField *retval = nil;
    @synchronized(self) {
        retval = [[userName retain] autorelease];
    }
    return retval;
}

- (void) setUserName:(UITextField *)userName_ {
    @synchronized(self) {
      [userName_ retain];
      [userName release];
      userName = userName_;
    }
}

Basically, the atomic version has to take a lock in order to guarantee thread safety, and also is bumping the ref count on the object (and the autorelease count to balance it) so that the object is guaranteed to exist for the caller, otherwise there is a potential race condition if another thread is setting the value, causing the ref count to drop to 0.

There are actually a large number of different variants of how these things work depending on whether the properties are scalar values or objects, and how retain, copy, readonly, nonatomic, etc interact. In general the property synthesizers just know how to do the "right thing" for all combinations.

@Jonathan Sterling 2009-12-31 22:36:54

Not that the lock doesn't "guarantee thread safety".

@Florin 2010-08-12 09:29:41

@Louis Gerbarg: I believe your version of the (nonatomic, retain) setter will not work properly if you try to assign the same object (that is: userName == userName_)

@tc. 2010-12-01 18:20:56

Your code is slightly misleading; there is no guarantee on what atomic getters/setters are synchronized. Critically,@property (assign) id delegate; is not synchronized on anything (iOS SDK GCC 4.2 ARM -Os), which means there's a race between [self.delegate delegateMethod:self]; and foo.delegate = nil; self.foo = nil; [super dealloc];. See stackoverflow.com/questions/917884/…

@tc. 2013-12-10 11:09:34

@fyolnish I'm not sure what _val/val are, but no, not really. The getter for an atomic copy/retain property needs to ensure that it does not return an object whose refcount becomes zero due the setter being called in another thread, which essentially means it needs to read the ivar, retain it while ensuring that the setter hasn't overwritten-and-released it, and then autorelease it to balance the retain. That essentially means both the getter and setter have to use a lock (if the memory layout was fixed it should be doable with CAS2 instructions; alas -retain is a method call).

@Fjölnir 2013-12-11 05:47:41

@tc It's been quite a while but what I meant to write was probably this: gist.github.com/fjolnir/5d96b3272c6255f6baae But yes it is possible for the old value to be read by a reader before setFoo: returns, and released before the reader returns it. But maybe if the setter used -autorelease instead of -release, that would fix that.

@tc. 2013-12-11 13:18:15

@fyolnish Unfortunately, no: That autoreleases on the thread of the setter, while it needs to be autoreleased on the thread of the getter. It also looks like there's a (slim) chance of running out of stack because you're using recursion.

@kevlar 2015-09-14 21:35:34

Coudl we fix this answer to address @Florin's comment? I noticed the same thing and came here to comment about it.

@bbum 2009-02-26 06:40:50

The last two are identical; "atomic" is the default behavior (note that it is not actually a keyword; it is specified only by the absence of nonatomic -- atomic was added as a keyword in recent versions of llvm/clang).

Assuming that you are @synthesizing the method implementations, atomic vs. non-atomic changes the generated code. If you are writing your own setter/getters, atomic/nonatomic/retain/assign/copy are merely advisory. (Note: @synthesize is now the default behavior in recent versions of LLVM. There is also no need to declare instance variables; they will be synthesized automatically, too, and will have an _ prepended to their name to prevent accidental direct access).

With "atomic", the synthesized setter/getter will ensure that a whole value is always returned from the getter or set by the setter, regardless of setter activity on any other thread. That is, if thread A is in the middle of the getter while thread B calls the setter, an actual viable value -- an autoreleased object, most likely -- will be returned to the caller in A.

In nonatomic, no such guarantees are made. Thus, nonatomic is considerably faster than "atomic".

What "atomic" does not do is make any guarantees about thread safety. If thread A is calling the getter simultaneously with thread B and C calling the setter with different values, thread A may get any one of the three values returned -- the one prior to any setters being called or either of the values passed into the setters in B and C. Likewise, the object may end up with the value from B or C, no way to tell.

Ensuring data integrity -- one of the primary challenges of multi-threaded programming -- is achieved by other means.

Adding to this:

atomicity of a single property also cannot guarantee thread safety when multiple dependent properties are in play.

Consider:

 @property(atomic, copy) NSString *firstName;
 @property(atomic, copy) NSString *lastName;
 @property(readonly, atomic, copy) NSString *fullName;

In this case, thread A could be renaming the object by calling setFirstName: and then calling setLastName:. In the meantime, thread B may call fullName in between thread A's two calls and will receive the new first name coupled with the old last name.

To address this, you need a transactional model. I.e. some other kind of synchronization and/or exclusion that allows one to exclude access to fullName while the dependent properties are being updated.

@Daniel Dickison 2011-05-24 20:00:41

Given that any thread-safe code will be doing its own locking etc, when would you want to use atomic property accessors? I'm having trouble thinking of a good example.

@bbum 2011-05-24 21:51:31

If you have an accessor that'll be reading a structure (or an object) from multiple threads and your code is hardened against "is this really the current state", it is useful. I.e. if you are reading, say, a struct that has a few entries summating current status, atomic can enable safe reading.

@BBog 2012-01-18 10:48:03

Nevermind, I managed to find the answer in a comment here: stackoverflow.com/a/5047416/855738 , made by bbum: "Specifically, since the locking mechanism to implement atomic is not exposed, there is no way you can manually implement just the setter or just the getter and ensure atomicity with an @synthesized atomic setter/getter"

@Ben Flynn 2012-01-26 19:22:53

@bbum Makes sense. I like your comment to another answer that thread-safety is more a model-level concern. From an IBM thread safety definition: ibm.co/yTEbjY "If a class is correctly implemented, which is another way of saying that it conforms to its specification, no sequence of operations (reads or writes of public fields and calls to public methods) on objects of that class should be able to put the object into an invalid state, observe the object to be in an invalid state, or violate any of the class's invariants, preconditions, or postconditions."

@bugloaf 2013-02-15 15:52:30

Here's an example similar to @StevenKramer 's: I have a @property NSArray* astronomicalEvents; that lists data I want to display in the UI. When the application launches the pointer points to an empty array, then the app pulls data from the web. When the web request completes (in a different thread) the app builds a new array then atomically sets the property to a new pointer value. It's thread safe and I didn't have to write any locking code, unless I'm missing something. Seems pretty useful to me.

@bbum 2013-02-15 17:25:23

@bugloaf Yes -- you are correct (good point +1). However, most apps would typically use a completion block or handler to handle the "data is available" which would do significantly more work than just setting the property (tear down "loading", update other bits of state, etc). All the UI update has to be done on main thread anyway. So, why incur the overhead of atomic for all the accesses when the typical code already serializes on the main queue? Another case where atomic could be used, but real world patterns obviate the need.

@Anurag 2013-02-24 21:46:21

atomic has now become a keyword in the recent versions of clang.

@ma11hew28 2013-07-14 12:31:07

Why don't they make the default nonatomic since it's used more often? Then, you could just write: @property UIWindow *window instead of @property (nonatomic) UIWindow *window for example.

@Hot Licks 2013-11-23 19:43:25

@bugloaf - I don't understand how atomic serves any purpose there. If the property is only mutated in one thread, another thread examining the property pointer will find either the old pointer or the new one. I could see atomic being important if it were possible to have a partially mutated pointer, but I don't believe that's possible on any of the usual Objective-C platforms.

@bbum 2013-11-23 23:19:41

@HotLicks Another fun one; on certain architectures (Can't remember which one), 64 bit values passed as an argument might be passed half in a register and half on the stack. atomic prevents cross-thread half-value reads. (That was a fun bug to track down.)

@George 2013-12-06 06:35:36

To @bbum: What do you mean by "viable" in this comment: "@HotLicks With objects atomic ensures that the returned pointer will be viable in calling thread regardless of what other threads do. Basically, it does return [[foo retain] autorelease]; while guaranteeing that foo is viable"

@bbum 2013-12-06 07:13:36

@congliu Thread A returns an object without retain/autorelease dance. Thread B releases object. Thread A goes boom. atomic ensures that thread A has a strong reference (a +1 retain count) for the return value.

@George 2013-12-06 07:26:40

To @bbum: Then why we use so many nonatomic property? If you mean nonatomic property may not retain/autorelease the object before return. And where is the return [[foo retain] autorelease]; code for an atomic property object? Is it the implementation of the generated getter method?

@bbum 2014-06-01 20:14:25

@JoeBlow Massive overhead in context; compare atomic vs. nonatomic calls to a getter method. One is a handful of instructions more than a C function call, the other involves locking.

@Fattie 2014-06-03 07:25:17

hey @bbum -- thanks for that report; well, I'll keep an eagle-scout eye out. I might ask an SO question about this - can anyone bring to the fore specific examples of this. CHEERS

@2cupsOfTech 2015-03-26 16:08:55

i think @Duraiamuthan.H is way better and to the point

@2cupsOfTech 2015-03-27 00:43:03

@bbum it seem it sort of does I'm going to read further from WWDC & get it right: asciiwwdc.com/2010/sessions/138

@bbum 2016-02-18 19:28:11

@GrigoriJlavyan Copy/pasting a bit of weblog content that was written in 2010 is not an answer (and, btw, the contents you copied and pasted aren't quite correct anyway).

@Honey 2016-04-18 19:18:45

Another Important difference is if a thread has written the first two bytes of an atomic four byte number, and a read of that number is requested on another thread, that read has to wait until all four bytes have been written. Conversely, if a thread has written the first two bytes of a non-atomic four byte number, and a read of that number is requested on another thread at that moment, it will read the first two new data bytes, but will get old data from a previous write operation in the other two bytes. stackoverflow.com/a/21098554/5175709

@Honey 2016-04-29 14:41:39

I wanted to edit your post to this, but I decided to comment here instead: "that a whole (if you have wrote half of the value, it won't allow reading on that ‘exact halfway-wrote thread’, still you may be able to read from another whole value set by another thread)"...

@Swati 2019-08-09 05:14:26

Your answer is so great. I got the complete meaning of atomic and nonatomic for the first time. I am going for an interview. Thanks a lot.

@paul 2016-12-09 04:58:38

The truth is that they use spin lock to implement atomic property. The code as below:

 static inline void reallySetProperty(id self, SEL _cmd, id newValue, 
      ptrdiff_t offset, bool atomic, bool copy, bool mutableCopy) 
    {
        id oldValue;
        id *slot = (id*) ((char*)self + offset);

        if (copy) {
            newValue = [newValue copyWithZone:NULL];
        } else if (mutableCopy) {
            newValue = [newValue mutableCopyWithZone:NULL];
        } else {
            if (*slot == newValue) return;
            newValue = objc_retain(newValue);
        }

        if (!atomic) {
            oldValue = *slot;
            *slot = newValue;
        } else {
            spin_lock_t *slotlock = &PropertyLocks[GOODHASH(slot)];
            _spin_lock(slotlock);
            oldValue = *slot;
            *slot = newValue;        
            _spin_unlock(slotlock);
        }

        objc_release(oldValue);
    }

@Proton 2016-07-23 06:34:02

atomic (default)

Atomic is the default: if you don’t type anything, your property is atomic. An atomic property is guaranteed that if you try to read from it, you will get back a valid value. It does not make any guarantees about what that value might be, but you will get back good data, not just junk memory. What this allows you to do is if you have multiple threads or multiple processes pointing at a single variable, one thread can read and another thread can write. If they hit at the same time, the reader thread is guaranteed to get one of the two values: either before the change or after the change. What atomic does not give you is any sort of guarantee about which of those values you might get. Atomic is really commonly confused with being thread-safe, and that is not correct. You need to guarantee your thread safety other ways. However, atomic will guarantee that if you try to read, you get back some kind of value.

nonatomic

On the flip side, non-atomic, as you can probably guess, just means, “don’t do that atomic stuff.” What you lose is that guarantee that you always get back something. If you try to read in the middle of a write, you could get back garbage data. But, on the other hand, you go a little bit faster. Because atomic properties have to do some magic to guarantee that you will get back a value, they are a bit slower. If it is a property that you are accessing a lot, you may want to drop down to nonatomic to make sure that you are not incurring that speed penalty.

See more here: https://realm.io/news/tmi-objective-c-property-attributes/

@Andrew Grant 2009-02-26 02:41:49

Atomic :

Atomic guarantees that access to the property will be performed in an atomic manner. E.g. it always return a fully initialised objects, any get/set of a property on one thread must complete before another can access it.

If you imagine the following function occurring on two threads at once you can see why the results would not be pretty.

-(void) setName:(NSString*)string
{
  if (name)
  {
    [name release]; 
    // what happens if the second thread jumps in now !?
    // name may be deleted, but our 'name' variable is still set!
    name = nil;
  }

  ...
}

Pros : Return of fully initialised objects each time makes it best choice in case of multi-threading.

Cons : Performance hit, makes execution a little slower

Non-Atomic :

Unlike Atomic, it doesn't ensure fully initialised object return each time.

Pros : Extremely fast execution.

Cons : Chances of garbage value in case of multi-threading.

@Andrew Grant 2009-02-26 07:31:31

That comment doesn't make a lot of sense. Can you clarify? If you look at examples on the Apple site then the atomic keyword synchronizes on the object while updating its properties.

@satisharyan 2016-06-29 08:56:11

Atomic: Ensure thread-safety by locking the thread using NSLOCK.

Non atomic: Doesn't ensure thread-safety as there is no thread-locking mechanism.

@Preetha 2016-02-13 13:34:35

If you are using atomic, it means the thread will be safe and read-only. If you are using nonatomic, it means the multiple threads access the variable and is thread unsafe, but it is executed fast, done a read and write operations; this is a dynamic type.

@Kemo 2016-02-01 08:27:02

Atomic means only one thread can access the variable at a time (static type). Atomic is thread-safe, but it is slow.

Nonatomic means multiple threads can access the variable at same time (dynamic type). Nonatomic is thread-unsafe, but it is fast.

@Laxman Sahni 2015-10-23 15:41:25

The atomic property ensures to retain a fully initialised value irrespective of how many threads are doing getter & setter on it.

The nonatomic property specifies that synthesized accessors simply set or return a value directly, with no guarantees about what happens if that same value is accessed simultaneously from different threads.

@Ankul Gaur 2014-08-13 11:57:09

If you are using your property in multi-threaded code then you would be able to see the difference between nonatomic and atomic attributes. Nonatomic is faster than atomic and atomic is thread-safe, not nonatomic.

Vijayendra Tripathi has already given an example for a multi-threaded environment.

@Durai Amuthan.H 2013-07-10 13:07:39

Atomic = thread safety

Non-atomic = No thread safety

Thread safety:

Instance variables are thread-safe if they behave correctly when accessed from multiple threads, regardless of the scheduling or interleaving of the execution of those threads by the runtime environment, and with no additional synchronization or other coordination on the part of the calling code.

In our context:

If a thread changes the value of the instance the changed value is available to all the threads, and only one thread can change the value at a time.

Where to use atomic:

if the instance variable is gonna be accessed in a multithreaded environment.

Implication of atomic:

Not as fast as nonatomic because nonatomic doesn't require any watchdog work on that from runtime .

Where to use nonatomic:

If the instance variable is not gonna be changed by multiple threads you can use it. It improves the performance.

@Fattie 2014-06-01 08:37:19

Everything you say here is correct, but the last sentence is essentially "wrong", Dura, for today's programming. It's really inconceivable you would bother to try to "improve performance" this way. (I mean, before you got within lightyears of that, you would be "not using ARC", "not using NSString because it is slow!" and so on.) To make an extreme example, it would be like saying "team, don't put any comments in the code, as it slows us down." There is no realistic development pipeline where you would want the (nonexistent) theoretical performance gains at the sake of unreliability.

@Durai Amuthan.H 2014-06-01 13:47:24

@swiftBoy 2013-03-21 07:10:56

After reading so many articles, Stack Overflow posts and making demo applications to check variable property attributes, I decided to put all the attributes information together:

  1. atomic // Default
  2. nonatomic
  3. strong = retain // Default
  4. weak = unsafe_unretained
  5. retain
  6. assign // Default
  7. unsafe_unretained
  8. copy
  9. readonly
  10. readwrite // Default

In the article Variable property attributes or modifiers in iOS you can find all the above-mentioned attributes, and that will definitely help you.

  1. atomic

    • atomic means only one thread access the variable (static type).
    • atomic is thread safe.
    • But it is slow in performance
    • atomic is the default behavior
    • Atomic accessors in a non garbage collected environment (i.e. when using retain/release/autorelease) will use a lock to ensure that another thread doesn't interfere with the correct setting/getting of the value.
    • It is not actually a keyword.

    Example:

        @property (retain) NSString *name;
    
        @synthesize name;
    
  2. nonatomic

    • nonatomic means multiple thread access the variable (dynamic type).
    • nonatomic is thread-unsafe.
    • But it is fast in performance
    • nonatomic is NOT default behavior. We need to add the nonatomic keyword in the property attribute.
    • It may result in unexpected behavior, when two different process (threads) access the same variable at the same time.

    Example:

        @property (nonatomic, retain) NSString *name;
    
        @synthesize name;
    

@BangOperator 2016-07-29 16:06:14

How can assign and strong/retain both be default ?

@abdullahselek 2017-04-05 13:21:24

strong comes with ARC, retain was default before ARC

@IOS Rocks 2012-11-22 11:20:22

Atomic means only one thread accesses the variable (static type). Atomic is thread-safe, but it is slow.

Nonatomic means multiple threads access the variable (dynamic type). Nonatomic is thread-unsafe, but it is fast.

@raw3d 2012-05-25 10:56:12

Atomic

  • is the default behavior
  • will ensure the present process is completed by the CPU, before another process accesses the variable
  • is not fast, as it ensures the process is completed entirely

Non-Atomic

  • is NOT the default behavior
  • faster (for synthesized code, that is, for variables created using @property and @synthesize)
  • not thread-safe
  • may result in unexpected behavior, when two different process access the same variable at the same time

@Vishal16 2019-02-08 07:59:05

short n sweet :)

@Vijayendra 2012-01-31 18:36:08

The best way to understand the difference is using the following example.

Suppose there is an atomic string property called "name", and if you call [self setName:@"A"] from thread A, call [self setName:@"B"] from thread B, and call [self name] from thread C, then all operations on different threads will be performed serially which means if one thread is executing a setter or getter, then other threads will wait.

This makes property "name" read/write safe, but if another thread, D, calls [name release] simultaneously then this operation might produce a crash because there is no setter/getter call involved here. Which means an object is read/write safe (ATOMIC), but not thread-safe as another threads can simultaneously send any type of messages to the object. The developer should ensure thread-safety for such objects.

If the property "name" was nonatomic, then all threads in above example - A,B, C and D will execute simultaneously producing any unpredictable result. In case of atomic, either one of A, B or C will execute first, but D can still execute in parallel.

@Deepak 2011-11-08 05:41:12

There is no such keyword "atomic"

@property(atomic, retain) UITextField *userName;

We can use the above like

@property(retain) UITextField *userName;

See Stack Overflow question I am getting issues if I use @property(atomic,retain)NSString *myString.

@Matthijn 2013-02-25 14:51:17

"There is such keyword", That the keyword is not required by default and even is the default value does not mean the keyword does not exist.

@sethfri 2015-05-20 00:00:42

This is incorrect. The keyword does exist. This answer is misleading, and I would encourage taking it down.

@Mrug 2015-07-02 05:03:49

Please update your knowledge.. Atomic does exist.

@tipycalFlow 2012-02-24 05:17:25

I found a pretty well put explanation of atomic and non-atomic properties here. Here's some relevant text from the same:

'atomic' means it cannot be broken down. In OS/programming terms an atomic function call is one that cannot be interrupted - the entire function must be executed, and not swapped out of the CPU by the OS's usual context switching until it's complete. Just in case you didn't know: since the CPU can only do one thing at a time, the OS rotates access to the CPU to all running processes in little time-slices, to give the illusion of multitasking. The CPU scheduler can (and does) interrupt a process at any point in its execution - even in mid function call. So for actions like updating shared counter variables where two processes could try to update the variable at the same time, they must be executed 'atomically', i.e., each update action has to finish in its entirety before any other process can be swapped onto the CPU.

So I'd be guessing that atomic in this case means the attribute reader methods cannot be interrupted - in effect meaning that the variable(s) being read by the method cannot change their value half way through because some other thread/call/function gets swapped onto the CPU.

Because the atomic variables can not be interrupted, the value contained by them at any point is (thread-lock) guaranteed to be uncorrupted, although, ensuring this thread lock makes access to them slower. non-atomic variables, on the other hand, make no such guarantee but do offer the luxury of quicker access. To sum it up, go with non-atomic when you know your variables won't be accessed by multiple threads simultaneously and speed things up.

@Rob 2019-03-10 16:25:54

Link is broken. ;(

@Binarian 2013-09-27 09:43:38

The default is atomic, this means it does cost you performance whenever you use the property, but it is thread safe. What Objective-C does, is set a lock, so only the actual thread may access the variable, as long as the setter/getter is executed.

Example with MRC of a property with an ivar _internal:

[_internal lock]; //lock
id result = [[value retain] autorelease];
[_internal unlock];
return result;

So these last two are the same:

@property(atomic, retain) UITextField *userName;

@property(retain) UITextField *userName; // defaults to atomic

On the other hand does nonatomic add nothing to your code. So it is only thread safe if you code security mechanism yourself.

@property(nonatomic, retain) UITextField *userName;

The keywords doesn't have to be written as first property attribute at all.

Don't forget, this doesn't mean that the property as a whole is thread-safe. Only the method call of the setter/getter is. But if you use a setter and after that a getter at the same time with 2 different threads, it could be broken too!

@Luca Torella 2016-08-08 07:35:08

atomic is not thread safe

@Jay O'Conor 2009-02-26 02:56:47

Easiest answer first: There's no difference between your second two examples. By default, property accessors are atomic.

Atomic accessors in a non garbage collected environment (i.e. when using retain/release/autorelease) will use a lock to ensure that another thread doesn't interfere with the correct setting/getting of the value.

See the "Performance and Threading" section of Apple's Objective-C 2.0 documentation for some more information and for other considerations when creating multi-threaded apps.

@Louis Gerbarg 2009-02-26 06:34:41

Two reasons. First off, for synthesized code it generates faster (but not threadsafe code). Second, if you are writing customer accessors that are not atomic it lets you annotate for any future user that the code is not atomic when they are reading its interface, without making them implementation.

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