UIAlertView fails to show and results in “EXC_BAD_ACCESS” error

Question

A method is called when a return button on the keyboard is pressed. After calling another method which returns an integer a message is created based on that integer. The message is then passed into an UIAlterView and displayed to the user. The alert doesn't have any options (hence why I'm not calling a delegate), but simply notifies the user of what happened.

Edit: Below is the full method (previously displayed partial). When I comment out everything before the UIAlertView and substitute the string @"test" instead of passing message the Alert is shown successfully. Am I not handling memory correctly with my structure?

- (IBAction)joinButton {
    struct userInfo localUser;

    [emailAddress resignFirstResponder];

    //convert textField text to char array in structure
    localUser.firstName = [self convertStringtoCharArray:firstName.text];
    localUser.lastName = [self convertStringtoCharArray:lastName.text];
    localUser.username = [self convertStringtoCharArray:username.text];
    localUser.email = [self convertStringtoCharArray:emailAddress.text];
    localUser.ipAddress = [self convertStringtoCharArray:localIPAddress.text];
    localUser.latitude = currentLocation.coordinate.latitude;
    localUser.longitude = currentLocation.coordinate.longitude;

    //pass structure to be sent over socket
    int result = [myNetworkConnection registerWithServer:&localUser];

    NSString *message = nil;

    //process result of sending attempt
    if (result == 0) {
        //registration succesful
        message = [NSString stringWithString:@"Registration successful"];
    } else if (result == 1) {
        //server unavailable
        message = [NSString stringWithString:@"Server unavailable. Please check your wi-fi settings and try again."];
    } else if (result == 2) {
        //unable to establish connection
        message = [NSString stringWithString:@"Unable to communicate with server. Please check your wi-fi settings and try again."];
    } else if (result == 3) {
        //username already in use
        message = [NSString stringWithString:@"Username in use. Try another username."];
    }

    UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Registration"
                                                    message:message
                                                   delegate:nil 
                                          cancelButtonTitle:@"Ok" 
                                          otherButtonTitles:nil];

    [alert show];
    [alert release];
}

When I execute the code the iPhone greys out like it is about to display an alert but crashes. I get a EXC_BAD_ACCESS error in the console. Am I not releasing either the alert or the message correctly? Here is the console output:

Program received signal:  “EXC_BAD_ACCESS”.
(gdb) backtrace
#0  0x30011944 in objc_msgSend ()
#1  0x3054803e in NSPopAutoreleasePool ()
#2  0x3054c808 in -[NSAutoreleasePool release] ()
#3  0x30936ac4 in _UIApplicationHandleEvent ()
#4  0x3204696c in PurpleEventCallback ()
#5  0x30254a76 in CFRunLoopRunSpecific ()
#6  0x3025416a in CFRunLoopRunInMode ()
#7  0x320452a4 in GSEventRunModal ()
#8  0x308f037c in -[UIApplication _run] ()
#9  0x308eea94 in UIApplicationMain ()
#10 0x000020bc in main (argc=1, argv=0x2ffff508) at /Users/reu2009/Documents/iPhone Development/Development/BuddyTracker/main.m:14
(gdb) frame 10
#10 0x000020bc in main (argc=1, argv=0x2ffff508) at /Users/reu2009/Documents/iPhone Development/Development/BuddyTracker/main.m:14 14       int retVal = UIApplicationMain(argc, argv, nil, nil);

Edit: removed [message release]; and assigned strings using [NSString stringWithString]; based on answers.

Answer 1

i had an issue like this ...i was calling uiAlertView from a background thread ....call it from the main thread

Answer 2

Objects returned from convenience constructors are already set to autorelease. While you declared a pointer to "message", the "message" object itself doesn't belong to you, since you used the @"string" convenience constructor to create the NSString object. Thus, you don't need to release it.

When you release it manually, it then gets released too many times (once manually, and once when the autorelease process rolls around) and throws the error.

Here's some additional information from Apple:

http://developer.apple.com/documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmPractical.html

Good rule of thumb: unless you use one of the alloc or init or copy methods to create an object (or if you retain the object yourself) you don't need to release it, but can rely on the method that actually created it to do that work for you.

Answer 3

Try it with NSZombieEnabled = YES.

• Go into the Info of you Executable.
• Click on the Arguments tab.
• Click + on "Variables to be set in the environment."
• Type NSZombieEnable and YES.

When the memory is released that has already been released, NSZombie will display the address, then you can use Instruments to find the actual object. Corbin's Treehouse has a good overview of how to do this: Instruments on Leopard: How to debug those random crashes in your Cocoa app

Answer 4

Sean is right - you don't need to call [message release] here, because you're never actually retaining the message object.

Instead of just saying message = @"string", you need to say message = [NSString stringWithString:@"string"]; To be completely honest I'm not sure why (maybe someone can comment and I can improve this post!) but that should do the trick.

Answer 5

This could case due to updating UIKit from background thread

I solved it like this

UIAlertView *alertMSG = [[UIAlertView alloc] initWithTitle:nil
                        message:@"Your mnessage here"
                        delegate:self
                        cancelButtonTitle:@"Title here"
                        otherButtonTitles: nil];

    [[NSOperationQueue mainQueue] addOperationWithBlock:^{
         [alertMSG show];
    }];

Answer 6

I had the same problem here with a UIAlertView, in my case, I had another class implementing the alert, and, from another one I was calling a static method. Like the following:

ClassA

...

doSomething {
 ... some stuff ...

 [MyAlertView showAlert];

 ... some other stuff...

}

What I suspect is that, as the alertview is shown asynchronously when I clicked the button the object was already released.

To verify that, I changed the code to instantiate the alert and not release it. And everythong worked.

My final solution was to declare a variable in the parent view, and deallocate it with the other variables when the view is deallocated.