How to assign dispatch_queue_t to variable in a structure

Question

I'm still relatively new to Objective C and easily confused by the various types. I am using code from the SquareCam sample project, incorporated into a larger project. It works fine, but now I want to save the videoDataOutputQueue, which is of type dispatch_queue_t so that I can use it elsewhere in the project. It has to pass through some C++ code before finally ending up back in Objective C code. Therefore I am trying to add it to a structure that I already have, as a void * (void *videoDataOutputQueue;)

However, I have not found the right way to assign it without getting a EXC_BAD_ACCESS runtime error. Since dispatch_queue_t is a C++ object, can't I just use it's address?

   declared in the interface for squarecamviewcontroller:
   @interface SquareCamViewController : UIViewController <UIGestureRecognizerDelegate, AVCaptureVideoDataOutputSampleBufferDelegate,UIActionSheetDelegate>
{
   AVCaptureVideoPreviewLayer *previewLayer;
   AVCaptureVideoDataOutput *dataOutput;
   AVCaptureVideoDataOutput *videoDataOutput;
   dispatch_queue_t videoDataOutputQueue;
   <other stuff>
}

later in the code:

- (void)setupAVCapture
{
   <other stuff from the sample code>
    MYSTRUCT  myStruct = (MYSTRUCT)struct;  // make a pointer to the structure
    myStruct->videoDataOutputQueue = (void *)videoDataOutputQueue;  <<<- bad access here at runtime

 <other stuff>
}

Clearly this is not the right way and I don't understand what I am doing. I have some hints from other posts but I'm missing something.

Thanks, Ken

Answer 1

You have made your question unnecessarily difficult to understand because the "code" you've presented has syntax errors. It's clearly not your real code, so we can't guess what's really supposed to be happening. For example, you use the struct reserved keyword as though it were a value.

Given where you say the bad access occurs, this has nothing to do with the dispatch queue. It looks like your myStruct variable is supposed to be a pointer to a structure of some kind but is just a garbage pointer. So, the attempt to assign a value to one of its fields ends up writing to an invalid memory address. It doesn't really matter what the nature of the field is.

The problem is apparently exactly in the code you omitted as "<other stuff from the sample code>". So, you need to show that. Indeed, you need to show your real code.

Beyond that, dispatch_queue_t is a C type. It's not specific to Objective-C. Therefore, you can use it across all C-based languages. There's no need to use a void*.

A dispatch queue, like all dispatch objects, is reference counted. If you're keeping a long-term reference to it, you need to make sure it stays alive. So, you need to retain it with dispatch_retain(). Likewise, you need to release it when you're done with it using dispatch_release(). (Don't forget to release the old value when you replace a reference you're keeping with another.)