[plt-scheme] Testing a pointer for null (FFI)

On Oct  2, Noel Welsh wrote:
> Hi all,
> 
> How does one test a pointer for null using the foreign library?

A null pointer is represented as #f, but...

> I've printed out the following:
> 
>  the pointer: #<cpointer>
>  (ptr-equal? the-pointer #f): #f
>  (ptr-ref the-pointer _int): 0

... it seems that you have a pointer to a pointer, so you have to do
that `ptr-ref'.  You can either do the above, or use
(ptr-ref the-pointer _pointer) and check if it's #f or not.  Both are
fine since NULL is 0, and using _pointer will cost one malloc if the
pointer is not NULL (but who's counting?)...

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