I've struck a problem using the latest XCode beta (4.2 Build 4C114, iOS 5.0) and autorelease that I can't solve. The code needs to conditionally set a string which will be the message in an alert:
NSString* msg = ([result rangeOfString:#"Ok"].location == NSNotFound) ?
#"Upload failed" : #"Uploaded ok";
Running Analyze highlights the line saying "Object sent -autorelease too many times (2)".
And sho'nuf, running the app (under the Simulator) causes a SIGABRT double free.
I've tried coding the line as an if/else.
I've tried creating separate strings for the two messages and just assigning the appropriate pointer to a third pointer with the ternary and if/else.
Nothing I do makes this go away!
Creating a string with #"string contents" will always be autoreleased automatically. You don't need to specifically release it yourself.
In most cases you would only need to release an object if you called "alloc" or "new" on it to begin with.
Related
My app is crashing when I try to set value in NSMutableDictionary.
Here is the code below which demostrate the crash, I am not able to find out any crash log also in the console.
NSArray *b =[[a objectAtIndex:1] valueForKey:#"value"];
NSMutableDictionary *b1 =[b objectAtIndex:0];
NSString *str = self.tes;
[b1 setValue:str forKey:#"value"];
Please help me regarding this.
b1 Dictionary log
{
question = "vale";
type = a;
}
setValue:forKey: is part of the NSKeyValueCoding protocol, which among other things, lets you access object properties from the likes of Interface Builder. setValue:forKey: is implemented in classes other than NSDictionary.
setObject:forKey: is NSMutableDictionary's reason to exist. Its signature happens to be quite similar to setValue:forKey:, but is more generic (e.g. any key type).
So in your case just replace all setValue:forKey with setObject:forKey and valueForKey: with objectForKey:
--
Difference between objectForKey and valueForKey?
So you said you are getting EXC_BAD_ACCESS crash. Try turning on Zombies in Xcode’s Scheme editor. See Enable and Debug Zombie objects in iOS using Xcode 5.1.1. That question explains how to use it, but you don’t need to run Instruments as mentioned there.
Then just run the app and Xcode will show you the reason of EXC_BAD_ACCESS. Don’t forget to turn Zombies off after you don’t need it.
The name Zombie comes from the fact that under this mode all deallocated objects will be marked and will be kept as special regions of memory (not live, not dead, zombies). Once your code attempts to use such deallocated object, Xcode will notice and print helpful message.
So here's the deal:
I am building an app that accesses the internal addressbook on the iPhone. Everything was working fine (and is working fine on the simulator still), but now on the device I get a ThreadBreak and lldb opens up with no error message in the console on this line:
tempPerson.firstname = ABRecordCopyValue(person, kABPersonFirstNameProperty)?.takeRetainedValue() as String? ?? ""
Again, this is still working on the simulator mind you and used to work just fine.
There is some multithread action going on because I am loading the addressbook into memory in the background using GCD above:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), {
My addressbook was loaded properly. I have deleted the app from my test device (iPhone 6), cleaned the project (figuring it could be a bad link between some Obj-C bridge file that didn't like the ABAddressBook datatype, and even hopped on one foot while holding the phone Northward for good luck. Nothing worked.
I just can't figure out why it would work on all the Simulators and not on the phone. I do, however, notice that the app is not asking permission to access the contacts when it firsts opens after deleting (though the Authorization check is called and is granting authorization):
case .Authorized:
return self.createAddressBook()
I'm at a loss at even what to show you all since it's not returning much of anything, but here is a breakdown of the Thread Error (NOTE: Sometimes it happens on different threads...)
Thread 4
Queue: com.apple.root.background.-qos (concurrent)
0 swift_getObjectType
1 swift_dynamicCast
2 SomeApp.ViewController.(createAddressBook(SomeApp.ViewController) -> () -> Swift.Bool).(closure #1)
3_Dispatch_call_block_and_release
4_dispatch_client_callout
5_dispatch_root_queue_drain
6_dispatch_worker_thread3
7_pthread_wqthread
Enqueued from com.apple.main-thread (Thread 1)
0_dispatch_async_f_slow
*>> 1_SomeApp.ViewController.createAddressBook(SomeApp.ViewController)() -> Swift.Bool [inlined]
2_SomeApp.ViewController.determineStatus(SomeApp.ViewController)() -> Swift.Bool
3_SomeApp.ViewController.loadContacts(SomeApp.ViewController() -> ()
4_SomeApp.ViewController.viewDidLoad(SomeApp.ViewController() -> ()
... etc Viewloading stuff
On the breakpoint, the two variables given in the debugger are
The addressbookref
adbk = (AnyObject?) (instance_type = Builtin.RawPointer = 0x... EvaluatingTo: Some
and the ViewController called "Self"
self(SomeApp.ViewController)
containing all the variables my app uses elsewhere.
Sorry for being so verbose, but I really don't know what of this is relevant and what isn't..
Thanks for your help in advance!
Update: I turned off multithreading/ dispatching and it didn't work. So it must be something to do with that tempPerson.firstname bit.
Update 2: It does go around the loop twice and get two contacts before failing. I think it is hitting a nil value that isn't quite nil for some of the contacts and then crashing.. I do have the following check for it though and it is passing through:
if ABRecordCopyValue(person, kABPersonFirstNameProperty).takeRetainedValue() as AnyObject? != nil{
&
if ABRecordCopyValue(person, kABPersonLastNameProperty).takeRetainedValue() as AnyObject? != nil{
respectively.
Update 3: Okay. So Now it seems whenever there is a blank field for first or last name it crashes (on the simulator now too). Which really sucks because I thought I put in a good check to make sure I am not accessing nil valued properties. Whats even more annoying is that I haven't changed this code and it was working fine until I removed some derived data and changed the test file to avoid a linker error. Anything that could happen in the build process that is mucking up my code?
Basically, you need to really check your type casts when accessing the addressbook, see:
tempPerson.lastname = ABRecordCopyValue(person, kABPersonLastNameProperty).takeRetainedValue() as NSObject as? String ?? ""
The problem is that some people have really dirty addressbooks for a variety of reasons and even have some mixed up properties like having a last name property that contains "LastName, FirstName SPACE Birthday" with no firstname property.
For whatever reason, typecasting first as an NSObject BEFORE as a String seems to work well.
I am keeping my data in a property called practiceRecords (an NSArray of dictionaries).
I check to see if the data already exists in the documents folder.
If yes, I load the data into self.practiceRecords.
If not, I build the array of dictionaries (using literal syntax), keeping this data in the self.practiceRecords property, and then write the data out to the documents folder.
(I am NOT reloading the data after writing it out)
As far as I am able to tell, there are no problems occurring during this process.
Then I have a step where I modify my data as follows ...
-(void)incNumberOfTriesFor:(NSString *)stringOfIndex {
if (self.practiceRecords)
{
int index = [stringOfIndex intValue];
int numberOfTries = [(NSNumber *)(self.practiceRecords[index][#"tries"]) intValue] + 1;
//CRASHING on this next line.
self.practiceRecords[index][#"tries"] = #(numberOfTries);
//message to helper method
[self writePracticeRecords];
}
}
So the first time through (when the array is built and written out) I get a crash at the indicated line.
The error is:
-[__NSDictionaryI setObject:forKeyedSubscript:]: unrecognized selector sent to instance
I quit the app, check the documents folder and see the data file written out with no issues.
I re-run the app, and then get no crash and the data file still looks great.
This is repeatable.
If the data file exists, no crash.
If the data first needs to be created, then a crash.
(In all cases, I manually look inside the resulting data file and see exactly what I expect to see - no issues there)
I'm not sure where to even begin squashing this bug, and would really like to understand the details of why this is happening.
Thanks very much for any help!
Just to recap the correct comments above:
-[__NSDictionaryI setObject:forKeyedSubscript:]: unrecognized selector sent to instance
NSDictionary does not implement any of the set... methods because it is immutable. You state that you're creating with literals syntax when the data is not found on disk. The literal syntax creates immutable containers
Instead, try...
// try to initialize from disk, but if not
// we can still use literal (immutable) syntax, but in a mutable container
self.practiceRecords = [NSMutableDictionary
dictionaryWithDictionary:#{ #"key" : #"value" }];
I found a strange behavior with NSString. I tried to run the below code and noticed this.
NSString *str = [[NSString alloc] initwithstring : #"hello"];
[str release];
NSLog(#" Print the value : %#", str);
Here, in the third line app should crash because we are accessing an object which is released. But it is printing the value of str. It is not crashing. But with NSArray i observed different behavior.
NSArray *array = [[NSArray alloc] initwithobjects : #"1", #"2", nil];
[array release];
NSLog(#"Print : %#", [array objectatindex : 0]);
NSLog(#"Print : %#", [array objectatindex : 0]);
The code has two NSLog statements used for NSArray. Here after releasing when the first NSLog is executed, it is printing value. But when second NSLog is executed, app crashes. App crash is acceptable because the array accessed was released already. But it should crash when the first NSLog is executed. Not the second one.
Help me with this behaviors. How release works in these cases.
Thanks
Jithen
The first example doesn't crash because string literals are never released. The code is really:
NSString *str = #"hello";
[str release];
People get burned with string literals on memory management and mistakenly using == to compare them instead of isEqualToString:. The compiler does some optimizations that lead to misleading results.
Update:
The following code proves my point:
NSString *literal = #"foo";
NSString *second = [NSString stringWithString:literal];
NSString *third = [NSString stringWithString:#"foo"]; // <-- this gives a compiler warning for being redundant
NSLog(#"literal = %p", literal);
NSLog(#"second = %p", second);
NSLog(#"third = %p", third);
This code gives the following output:
2013-02-28 22:03:35.663 SelCast[85617:11303] literal = 0x359c
2013-02-28 22:03:35.666 SelCast[85617:11303] second = 0x359c
2013-02-28 22:03:35.668 SelCast[85617:11303] third = 0x359c
Notice that all three variable point to the same memory.
Your second example crashes at the second NSLog because at the first log, the memory where array was hasn't been re-used, but that first log causes enough activity on the heap to cause the memory to become used by something else. Then, when you try to access it again, you get a crash.
Whenever an object is deallocated and its memory marked as free, there is going to be some period of time where that memory still stores what's left of that object. During this time you can still call methods on such objects and so forth, without crashing. This time is extremely short, and if you're running a lot of threads it may not even be enough to get your method call in. So clearly, don't rely on this implementation detail for any behavior.
As others have said, regarding your first question, NSString literals aren't going to be deallocated. This is true for some other Foundation classes (NSNumber comes to mind) but is an implementation detail as well. If you need to do experiments on memory management, use an NSObject instance instead, as it will not show the unusual behaviors.
When you send a release message on an object, the object is actually not being removed from the memory. The release message simply decrements the reference count by one only. If the reference count is zero the object is marked as free. Then the system remove it from the memory. Until this deallocation happens you can access your object. Even if you release the object your object pointer still points to the object unless you are assigning nil to the pointer.
The first example doesn't crash because string literals are never released. Where the second totally depends on release and retain counter.
Read this article. Its contains short-and-sweet explanation for your query
You Should read this apple guideline
You seem to assume that release should destroy the object immediately. I don't think that's the guarantee that the language makes. What release means is: I have finished using this object and I promise not to use it again. From that point onwards it's up to the system to decide when to actually deallocate the memory.
Any behaviour you see beyond that is not defined and may change from one version of the Objective C runtime to the next.
That's to say that the other answers that suggest the difference is string literals and re-use of memory are currently correct but assuming that the behaviour will always be like this would likely be a mistake.
I've written the following code:
NSString *string = [[NSString alloc] initWithFormat:#"test"];
[string release];
NSLog(#"string lenght = %d", [string length]);
//Why I don't get EXC_BAD_ACCESS at this point?
I should, it should be released. The retainCount should be 0 after last release, so why is it not?
P.S.
I am using latest XCode.
Update:
NSString *string = [[NSString alloc] initWithFormat:#"test"];
NSLog(#"retainCount before = %d", [string retainCount]);// => 1
[string release];
NSLog(#"retainCount after = %d", [string retainCount]);// => 1 Why!?
In this case, the frameworks are likely returning the literal #"test" from NSString *string = [[NSString alloc] initWithFormat:#"test"];. That is, it determines the literal may be reused, and reuses it in this context. After all, the input matches the output.
However, you should not rely on these internal optimizations in your programs -- just stick with the reference counting rules and well-defined behavior.
Update
David's comment caused me to look into this. On the system I tested, NSString *string = [[NSString alloc] initWithFormat:#"test"]; returns a new object. Your program messages an object which should have been released, and is not eligible for the immortal string status.
Your program still falls into undefined territory, and happens to appear to give the correct results in some cases only as an artifact of implementation details -- or just purely coincidence. As David pointed out, adding 'stuff' between the release and the log can cause string to really be destroyed and potentially reused. If you really want to know why this all works, you could read the objc runtime sources or crawl through the runtime's assembly as it executes. Some of it may have an explanation (runtime implementation details), and some of it is purely coincidence.
Doing things to a released object is an undefined behavior. Meaning - sometimes you get away with it, sometimes it crashes, sometimes it crashes a minute later in a completely different spot, sometimes a variable ten files away gets mysteriously modified.
To catch those issues, use the NSZombie technique. Look it up. That, and some coding discipline.
This time, you got away because the freed up memory hasn't been overwritten by anything yet. The memory that string points at still contains the bytes of a string object with the right length. Some time later, something else will be there, or the memory address won't be valid anymore. And there's no telling when this happens.
Sending messages to nil objects is, however, legitimate. That's a defined behavior in Objective C, in fact - nothing happens, 0 or nil is returned.
Update:
Ok. I'm tired and didn't read your question carefully enough.
The reason you are not crashing is pure luck. At first I though that you were using initWithString: in which case all the answers (including my original one (below)) about string literals would be valid.
What I mean by "pure luck"
The reason this works is just that the object is released but your pointer still points to where it used to be and the memory is not overwritten before you read it again. So when you access the variable you read from the untouched memory which means that you get a valid object back. Doing the above is VERY dangerous and will eventually cause a crash in the future!
If you start creating more object in between the release and the log then there is a chance that one of them will use the same memory as your string had and then you would crash when trying to read the old memory.
It is even so fragile that calling log twice in a row will cause a crash.
Original answer:
String literals never get released!
Take a look at my answer for this question for a description of why this is.
This answer also has a good explanation.
One possible explanation: You're superfluously dynamically allocating a string instead of just using the constant. Probably Cocoa already knows that's just a waste of memory (if you're not creating a mutable string), so it maybe releases the allocated object and returns the constant string instead. And on a constant string, release and retain have no effect.
To prove this, it's worth comparing the returned pointer to the constant string itself:
int main()
{
NSString *s = #"Hello World!";
NSString *t = [[NSString alloc] initWithFormat:s];
if (s == t)
NSLog(#"Strings are the same");
else
NSLog(#"Not the same; another instance was allocated");
return 0;
}