Bad idea

gistfile1.m

MyCrazyBlock block = ^(BOOL *stop) {
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
        
        // do some work
        
        dispatch_async(dispatch_get_main_queue(), ^{
            *stop = YES;
        });
    });
};



// in your object

if (block) block(&self.shouldStop);

When your object is released and deallocated (might happen during "some work") the pointer to the stop BOOL is still held but points to invalid (maybe even some other objects) memory. When you write to its location you might end up writing to some other objects memory.

Result: Funky crashes (EXC_BAD_ACCESS & SIGSEGV) all over the place.

just a wild guess, but what if you call the block with a weak pointer to self?

__weak typeof(self) weakSelf = self;
if (block) block(&weakSelf.shouldStop);

Please ignore the fact that setting the stop BOOL asynchronously never worked anyway ;)

@hiDominik: should not make any difference, as ARC only manages object retain counts. we're dealing with a pointer into an objects memory space here. Nothing that could be retained.