Need to understand memory management when using "unsigned char" to point to raw data buffer

Question

My question is in the code below. I'd like to understand if there's such a thing as "retaining" when it comes to "unsigned char" pointers. Please explain.

// MyObject.h

@interface myObject : NSObject {

    unsigned char   *myData;
}

// MyObject.m

-(void)makeNewData
{

    if (myData) { free(myData); }
    myData = [self createBitmapContextData:myCGImageRef];

    //Here is my question: do I need a "retain" call equivalent on the next line?
    //[myData retain];

}

- (unsigned char*)createBitmapContextData:(CGImageRef)fromImage
{

    CGContextRef cgctx = [self createARGBBitmapContextFromImage:myCGImage];
    if (cgctx == NULL) { return nil; }

    size_t w = CGImageGetWidth(myCGImage);
    size_t h = CGImageGetHeight(myCGImage);
    CGRect rect = {{0,0},{w,h}}; 

    CGContextDrawImage(cgctx, rect, myCGImage); 

    unsigned char* data = CGBitmapContextGetData (cgctx);

    CGContextRelease(cgctx);

    return data;

}

Answer 1

No, there is no such thing as “retaining” a raw pointer.

As mackross says, NSData can be used to hold on to data when you allocate it yourself. However, in this case, you don’t own the data, you‘re merely “getting” it from the CGContext, which owns it. Your reference becomes invalid when the CGContext is released. In this case, you need to own a reference to the context until you no longer need the pointer.