NSInvocation returns value but makes app crash with EXC_BAD_ACCESS

Question

I have an array which I am iterating and looking for a particular flag. If the flag value is nil, I am calling a method which generates an invocation object and returns the result of invocation.

My code structure is as follows

for(NSString *key in [taxiPlanes allKeys])
{
        Plane *currentPlane = [taxiPlanes objectForKey:key];

        if(currentPlane.currentAction == nil)
        {
            NSString *selector = [[currentPlane planeTakeoffSequence] firstObject];
            currentPlane.currentAction = selector;

            // Calling for NSInvocation in [self ...]
            NSArray *action = [NSArray arrayWithArray:[self operationFromTakeoffAction:currentPlane.currentAction AtPoint:currentPlane.position]];

        NSLog(@"%@",action);
        }
 }

Method which generates NSInvocation

-(NSArray *) operationFromTakeoffAction:(NSString *) action AtPoint:(CGPoint) flightPoint
{
    NSMethodSignature *methodSignature = [FlightOperations instanceMethodSignatureForSelector:NSSelectorFromString(action)];
    NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:methodSignature];

    [invocation setTarget:fOps];
    [invocation setSelector:NSSelectorFromString(action)];
    [invocation setArgument:&flightPoint atIndex:2];

    NSArray *resultSet = [NSArray alloc]init];
    [invocation invoke];
    [invocation getReturnValue:&resultSet];

    return resultSet;
}

In the for loop, without the method call for NSInvocation ([self ....]), the loop just executes fine and not crashing. But when I introduce the method to invoke NSInvocation, I am able to see the NSLog in for loop prints expected NSArray result but it crashes with error message EXC_BAD_ACCESS.

I am not able to figure out why it fails even though NSInvocation returns proper result. Without NSInvocation, for loop is not getting crashed.

Any suggestions would be helpful.

Thanks

Answer 1

I am guessing you are using ARC?

The problem is with the line [invocation getReturnValue:&resultSet];. getReturnValue: just copies the bytes of the return value into the given memory buffer, regardless of type. It doesn't know or care about memory management if the return type is a retainable object pointer type. Since resultSet is a __strong variable of object pointer type, ARC assumes that any value that has been put into the variable has been retained, and thus will release it when it goes out of scope. That is not true in this case, so it crashes. (Also, the array that you had resultSet originally point to will be leaked, since getReturnValue: overwrites that value without releasing it. Why you even made that variable point to an object in the first place is beyond me.)

The solution is that you must give a pointer to a non-retained type to getReturnValue:. Either:

NSArray * __unsafe_unretained tempResultSet;
[invocation getReturnValue:&tempResultSet];
NSArray *resultSet = tempResultSet;

or:

void *tempResultSet;
[invocation getReturnValue:&tempResultSet];
NSArray *resultSet = (__bridge NSArray *)tempResultSet;