Can CreateFile ever return NULL?

Question

I know that the invalid value returned by CreateFile is INVALID_HANDLE_VALUE. But since I also like to use RAII it's very tempting to just stick the HANDLE in a shared_ptr (like this: shared_ptr<void> handle (CreateFile(args),&CloseHandle) ) to make sure that the handle is closed. My only concern with this quick and easy way to do RAII is if CreateFile can return NULL as the HANDLE value.

Answer 1

NULL is not a valid handle value. You can discern this from the fact that some Windows API functions return NULL to indicate a failure. Since there is a single function to dispose of handles, CloseHandle , it follows that NULL is not a valid HANDLE value. Hence CreateFile cannot ever return NULL .

Raymond Chen wrote a blog article touching on this topic: Why are HANDLE return values so inconsistent? .

Now, I know nothing about shared_ptr<> so would like to make no comment on whether or not your idea is appropriate. I am merely answering the direct question that you asked.

Answer 2

When testing a HANDLE for validity in a generic way, check for both NULL and INVALID_HANDLE_VALUE .

But I don't see how RAII has anything to do with whether CreateFile can return NULL . You will need to provide custom code for testing validity and deallocating in order to make HANDLE work with a shared pointer, so you are in control of these checks, not the shared pointer class.

In other words, it makes no difference whether it's in a shared pointer or you use a normal HANDLE , the checks are exactly the same, and you must provide them either way.

Answer 3

CreateFile never returns NULL . I suggest you to use already created wrapper ATL::CAtlFile and don't envent a new one based on shared_ptr .