Checking singleton property with nil value causes error

Question

When I check a singleton property that is nil, it kills my app. But when I check for nil on a class instance property, everything works fine.

This works fine:

self.MyProperty == nil

but this will kill the app with “EXC_BAD_ACCESS”

[MySingleton sharedManager].SomeProperty != nil

What is the difference with the singleton that I can't check for nil?

Here's the singleton implementation:

.h file:

@interface MySingleton : NSObject {
    NSString * SomeProperty;
}
@property (nonatomic, copy) NSString * SomeProperty;
+(MySingleton *)sharedManager;
@end

.m file:

#import "MySingleton"

static MySingleton *sharedManager = nil;

@implementation MySingleton
@synthesize SomeProperty;

- (void)dealloc {
     [SomeProperty dealloc];
     [super dealloc];
}

+(MySingleton *)sharedManager
{       
    if (!sharedManager){
         sharedManager = [[MySingleton alloc] init];
    }
    return sharedManager;   
}

This is what I find in the console when when trying to assign something to SomeProperty:

MyApp(51363,0xa0389500) malloc: *** mmap(size=2147487744) failed (error code=12)
*** error: can't allocate region  
*** set a breakpoint in malloc_error_break to debug
Current language:  auto; currently objective-c  
(gdb) break malloc_error_break  
Note: breakpoints 6 and 8 also set at pc 0x929c2072.
Breakpoint 11 at 0x929c2072  
(gdb) continue  
(gdb) po [MySingleton sharedManager].SomeProperty
Cannot access memory at address 0x0  
(gdb) po [MySingleton sharedManager]
<Session: 0x1938fa0>

I get the above only when trying to assign. When trying to read the variable is where the crash occurs.

Answer 1

Likely, whatever kind of object your +sharedManager method returns is not a valid Objective-C object, and trying to access a property of that invalid Objective-C object will cause EXC_BAD_ACCESS. Also, are you sure that EXC_BAD_ACCESS is not occurring within +sharedManager?

Answer 2

Are you ever releasing the singleton? It seems like the instance you are returning is no longer valid. I'd override "release" for the singleton to do nothing, and see if you still have a problem...

Answer 3

Given this:

- (void)dealloc {
[someKey dealloc];
[super dealloc];
}

And some of the other code, I'd go out on a limb and say it is likely that there are other problems with the code, the combination of which are leading to the problem you have hinted at. In particular, you should never call -dealloc directly (other than [super dealloc]). If you have that in other parts of your code and it is being executed, it could easily cause the symptom you describe.

If you want a more specific answer, post the backtrace of the crash.

MyApp(51363,0xa0389500) malloc: * mmap(size=2147487744) failed (error code=12) error: can't allocate region
** set a breakpoint in malloc_error_break to debug Current language: auto; currently objective-c

Set the breakpoint as indicated and then re-run the application. Once that error message happens, all bets are off as your app is already hosed. You need the backtrace of when that call is made.

What is happening, though, is that something is asking mmap() to map in 2GB of address space. Could be corruption. Could be bad code.

Answer 4

Change this code.

- (void)dealloc {
     [SomeProperty dealloc];
     [super dealloc];
}

Like this

- (void)dealloc {
     [SomeProperty release];
     [super dealloc];
}

DO NOT CALL dealloc method. Except [super dealloc]