使用EXC_BAD_ACCESS的Objective-C回调函数错误

Question

I have the following callback function:

void(^getImageCallback)(BOOL success,  UIImage *image , NSError *error);

- (void)getRemoteImageWithObjectId:(NSString *)objectId andType:(NSString *)type andSaveLocal:(BOOL)saveLocal withCallback:(GAGetImageCompletionBlock)callback{
    getImageCallback = callback;

    [self getRemoteImageWithObjectId:objectId andType:type shouldSaveLocal:saveLocal];

}

- (void)getRemoteImageWithObjectId:(NSString *)objectId andType:(NSString *)type shouldSaveLocal:(BOOL)saveLocal{

    UIImage *image = [UIImage imageNamed:@"running.png"];
    getImageCallback(YES, image, nil); //it errors out here
}

My app crashes here with error: Thread 1: EXC_BAD_ACCESS (code=1, address=0x10)

Does anyone know why this might happen?

Answer 1

You might call the 2nd getRemoteImageWithObjectId method instead of the 1st getRemoteImageWithObjectId method. So getImageCallback variable was nil at that time. The reason of EXC_BAD_ACCESS (code=1, address=0x10) might be calling the nil getImageCallback block.

For example, if you compile and execute the following Objective-C code on 64bit environment, you got the exactly same EXC_BAD_ACCESS with address=0x10.

void (^block)() = 0;
block();

You can double check this code in C++ with clang -rewrite-objc option. This Objective-C code was compiled as the following C++ code.

struct __block_impl {
    void *isa;      // offset:0x00
    int Flags;      // offset:0x08
    int Reserved;   // offset:0x0c
    void *FuncPtr;  // offset:0x10
};

void (*block)() = 0;
((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);

Now block variable is 0 (nil), so block->FuncPtr points address 0x10 . Thus EXC_BAD_ACCESS (code=1, address=0x10) was occurred.

LLVM document says the same thing. http://clang.llvm.org/docs/Block-ABI-Apple.html#high-level

struct Block_literal_1 {
    void *isa; // initialized to &_NSConcreteStackBlock or &_NSConcreteGlobalBlock
    int flags;
    int reserved;
    void (*invoke)(void *, ...);