c - SAL annotations for returning reference counted objects

Question

I'm trying to use SAL annotations to guard against certain possible errors when using reference counted objects. Annotations specific for this use case are not available (as far as I can tell), but using the locking annotations could work.

typedef struct RefCnt;

void
Reference(_Inout_ _Acquires_lock_(*_Curr_) RefCnt *R);

void
Unreference(_Inout_ _Releases_lock_(*_Curr_) RefCnt *R);

This will ensure that for each call to Reference there is a call to Unreference, which is exactly what I want.

However, this leaves some gaps. If I have a reference counted object and I have some way of getting that object I want to receive a warning if I'm not unreferencing it:

typedef struct
{
    RefCnt R;
} Object;

Object global_obj;

Object *
GetObject(void)
{
    Reference(&global_obj.R);
    return &global_obj;
}

This will trigger a warning: warning C26165: Possibly failing to release lock 'global_obj.R' in function 'GetObject'., which is to be expected. I can annotate the GetObject function so it becomes: _Acquires_lock_(_Curr_->R) Object *GetObject(void). This solves the problem, but I'm probably introducing a new one because I'm not getting any warnings for code like this:

void
Test(void)
{
    Object *o = GetObject();
    // The reference counter is never reset, this is a leak, I should get a warning
}

If I'm using output parameters it works, so void GetObject(_At_(*_Curr_, _Acquires_lock_(_Curr_->R)) Object **O) is a solution, but I'm trying to improve an existing API that directly returns the object, so doing this change is something I'd like to avoid.

Is there a way of doing this or is this something that SAL can not express?

Answer 1

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