SafeArrayLock Why is it Required

Chetan Raj · Joined: 04 Oct 2007 Posts: 2

Hi All,

Basic questions...
Why do we need SafeArrayLock? Can't we directly access the pointer
pvdata? What issues will arise if I directly use pvdata as shown...

(m_pSa->pvData)[index] = nDirection

where m_pSa is a SafeArray of integer?

Thanks,
Chetan

Archived from group: microsoft>public>vc>atl

Igor Tandetnik · Joined: 04 Oct 2007 Posts: 26

"Chetan Raj" wrote in message@posting.google.com
> Basic questions...
> Why do we need SafeArrayLock?

Almost never needed. I never had a chance to use it. A locked array
cannot be freed or resized, which guarantees that the raw data pointer
does not change under you. However, this is only useful if you cache the
raw pointer, which is quite an unusual thing to do.

> Can't we directly access the pointer
> pvdata?

You can. Personally, I prefer to treat SAFEARRAY as an opaque data
structure, and use public APIs to manipulate it. Also, I'm not sure what
SafeArrayLock has to do with pvData - are you maybe thinking of
SafeArrayAccessData?

> What issues will arise if I directly use pvdata as shown...
>
> (m_pSa->pvData)[index] = nDirection
>
> where m_pSa is a SafeArray of integer?

It won't compile - you cannot apply subscript operator to a void*
pointer. You need to cast it appropriately.
--
With best wishes,
Igor Tandetnik

"For every complex problem, there is a solution that is simple, neat,
and wrong." H.L. Mencken

Chetan Raj · Joined: 04 Oct 2007 Posts: 2

Thanks for your reply,

<<<
Can't we directly access the pointer pvdata?
>>>

Here, I had meant that is it safe enough to access raw pvdata without
applying any locks either through SafeArracyAccessData or
SafeArrayLock APIs?

Now does,

((int*)(m_pSa->pvData))[index] = nDirection

have any undefined behaviour?

Thanks,
Chetan

"Igor Tandetnik" wrote in message news:...
> "Chetan Raj" wrote in message
> @posting.google.com
> > Basic questions...
> > Why do we need SafeArrayLock?
>
> Almost never needed. I never had a chance to use it. A locked array
> cannot be freed or resized, which guarantees that the raw data pointer
> does not change under you. However, this is only useful if you cache the
> raw pointer, which is quite an unusual thing to do.
>
> > Can't we directly access the pointer
> > pvdata?
>
> You can. Personally, I prefer to treat SAFEARRAY as an opaque data
> structure, and use public APIs to manipulate it. Also, I'm not sure what
> SafeArrayLock has to do with pvData - are you maybe thinking of
> SafeArrayAccessData?
>
> > What issues will arise if I directly use pvdata as shown...
> >
> > (m_pSa->pvData)[index] = nDirection
> >
> > where m_pSa is a SafeArray of integer?
>
> It won't compile - you cannot apply subscript operator to a void*
> pointer. You need to cast it appropriately.

Igor Tandetnik · Joined: 04 Oct 2007 Posts: 26

"Chetan Raj" wrote in message@posting.google.com
> <<<
> Can't we directly access the pointer pvdata?
>>>>
>
> Here, I had meant that is it safe enough to access raw pvdata without
> applying any locks either through SafeArracyAccessData or
> SafeArrayLock APIs?
>
> Now does,
>
> ((int*)(m_pSa->pvData))[index] = nDirection
>
> have any undefined behaviour?

I don't see any problem with this. Again, personally I prefer using
SafeArrayAccessData, but YMMV.
--
With best wishes,
Igor Tandetnik

"For every complex problem, there is a solution that is simple, neat,
and wrong." H.L. Mencken

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