Appearing to succeed is a valid form of undefined behavior, but it's still undefined

Raymond Chen

A customer requested a clarification on the MSDN documentation for the HeapFree function.

The MSDN documentation says that if the lpMem parameter is NULL, then the behavior is undefined. Is this true?

As explicitly stated in MSDN, the behavior is undefined. Observe that the annotation on the lpMem parameter is __in, which means that the parameter must be a non-NULL value provided by the caller. (If NULL were permitted, the annotation would have been __in_opt.)

Undefined behavior means that anything can happen. The program might crash immediately. It might crash five minutes later. It might send email to your boss saying that you screwed up and then read you Vogon poetry. Or maybe not.

MSDN says don’t do it, so don’t do it.

The customer explained why they were interested in knowing more information about undefined behavior:

We were interested because there is a mismatch between the semantics of a function we are implementing (where NULL is valid and ignored) and the function HeapFree we are using as the implementation. It looks like Windows Vista returns TRUE if you pass NULL.

If there is a mismatch in semantics between the function you are implementing and the function you are calling, it is your responsibility as the programmer to bridge the gap. The customer didn’t say what function they were implementing, but I’m guessing it was something like operator delete. Since your function accepts NULL but HeapFree doesn’t, it is your responsibility to filter out NULL parameters.

void operator delete(void* ptr) throw ()
 if (ptr != NULL)
  HeapFree(CustomHeap, 0, ptr);

This concept goes by the fancy name of the Adapter Pattern. The less fancy name is wrapper function.

And the value returned by HeapFree on Windows Vista is irrelevant. Pretending to succeed is a valid form of undefined behavior, because anything qualifies as undefined behavior.

(Of course, you can’t assume that returning TRUE will always be the result of triggering undefined behavior. After all, if you could rely on it, then it wouldn’t be undefined any more!)


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