How am I supposed to free the memory the system allocates in the SetPrivateObjectSecurity function?

Raymond Chen

July 27th, 20170 0

A customer noted that the Set­Private­Object­Security function updates a pointer provided by the Objects­Security­Descriptor parameter. Since it may allocate a new security descriptor, that means that it needs to deallocate the old one. But what function does it use to free the old one? After all, the allocation function must match the deallocation function. Similarly, how should the new security descriptor be freed? (I say “similarly” because the two answers had better be the same!)

The system allocates and frees the security descriptor from the proess heap, as reported by the Get­Process­Heap function. The allocation function is Heap­Alloc and the deallocation function is Heap­Free. That means that the security descriptor you pass in must have been allocated with

    SecurityDescriptor = HeapAlloc(GetProcessHeap(), flags, size);

and then you pass the pointer like this:

    SetPrivateObjectSecurity(..., &SecurityDescriptor, ...);
    // or
    SetPrivateObjectSecurityEx(..., &SecurityDescriptor, ...);

and after the Set­Private­Object­Security function is done, you must free the memory with

    HeapFree(GetProcessHeap(), SecurityDescriptor);

I wrote this post the same day that I submitted the change request to add this essential information to the documentation. We’ll see who wins.

Raymond Chen