ARC下dataWithBytesNoCopy的内存泄漏？

Question

I have an NSData instance but I'm only interested in the data at a given offset. I considered using dataWithBytesNoCopy:length:freeWhenDone: to prevent copying the data, but it occurred to me I don't really realise what's happening:

self.data = [NSMutableData dataWithBytesNoCopy:(void *)(self.data.bytes + offset)
                                        length:self.data.length - offset
                                  freeWhenDone:NO];

I no longer have a pointer to the beginning of the data block, so I'm not sure how ARC handles this. The data block now points to the same block, but not at the beginning. Does ARC deallocate the whole block when I'm done? Is the data before the offset kept in the meanwhile?

Answer 1

You're not leaking memory but, even worse, referring to freed memory: The new NSData instance will point to the storage maintained by the original NSData . But the original instance will be released as soon as the new one is assigned. From that point the new NSData contains a dangling pointer. Accessing the data is actually undefined behavior.

You should create a copy of the bytes:

self.data = [self.data subdataWithRange:(NSRange){ offset, self.data.length - offset }];

This way you're sure the new NSData actually owns the bytes it's pointing to.

Answer 2

ARC can handle some situations involving pointers into an object, but there are restrictions. Read Clang documentation on interior pointers . HTH