> At 2001-09-20 02:14, Simon Marlow wrote:
>
> >I'll just add that the docs have been updated for 5.02,
>
> Do you have a URL for that?
Not yet, but the release is imminent (really!) so it'll be up on the web
site shortly.
> >And don't forget that using foreignPtrToPtr is quite dangerous; much
> >better to use withForeignPtr instead, otherwise you might find the
> >ForeignPtr being finalised earlier than you expect.
>
> This is because foreignPtrToPtr is not in the IO monad, correct?
>
> foreignPtrToPtr :: ForeignPtr a -> Ptr a;
>
> A function like this:
>
> ioForeignPtrToPtr :: ForeignPtr a -> IO (Ptr a);
> ioForeignPtrToPtr fp = withForeignPtr fp return;
>
> ...would surely be safe? The 5.00 documentation claims that it isn't,
> however, but I don't see why.
No, it's not safe. The reason is that the compiler can track a
ForeignPtr to discover when it dies, in order to run the finalizer, but
it can't track a Ptr. As soon as you drop all references to the
ForeignPtr then the finalizer will run, even if you converted it to a
Ptr and you're still using it.
withForeignPtr is safe to use because it ensures that the ForeignPtr is
kept live until it returns. The documentation is a little terse on this
issue; I'll see if I can improve it.
Cheers,
Simon
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