[Haskell-cafe] Question about touchForeignPtr
patperry at stanford.edu
Mon Jan 12 14:16:21 EST 2009
Thanks for your help, Duncan.
On Jan 12, 2009, at 6:10 AM, Duncan Coutts wrote:
>> Do the (ForeignPtr e) and the (Ptr e) point to the same thing? They
> appear to be related because you dereference p but touch f.
> It used to be the ForeignPtr was slower to dereference than a Ptr
> and so
> caching it like this could make it faster (but then requires lots of
> careful thought about touchForeignPtr). These days ForeignPtr is
> just as
> fast and so you can use withForeignPtr and never have to worry about
> touchForeignPtr. You'll notice ByteString works this way.
Often they point to the same thing, but not always. If they pointed to
the same thing, I wouldn't store p separately, but often p has a nonzero
offset from f. You either have to store the offset or cache the
I found that it was both faster and simpler to cache p rather than the
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