On Mon, Dec 17, 2007 at 02:12:31PM +0300, Bulat Ziganshin wrote: > Hello Simon, > > Monday, December 17, 2007, 1:33:19 PM, you wrote: > >>> My question is: what exactly does GHC.Prim.touch# do? This appears to > >> > >> it's a no-op (for ghc 6.6+ at least). its only task is to notify ghc > >> optimizer that data were accessed so it doesn't free the memory > > > Yes, exactly. touch# generates no code, but it is vitally important > > because if the ForeignPtr is referencing data in the heap (like > > mallocForeignPtrBytes does), it prevents the data from being GC'd before > > the operation completes. > > a bit more details for Scott: > > generated code is like this: > > ptr <- unsafeForeignPtrToPtr fptr > yourAction ptr > touch# fptr > > without touch, the *last* action where fptr involved is its conversion > to ptr. GHC Runtime (not optimizer as i said) have no idea that ptr > and fptr is the same object, so after conversion it feels free to > dispose object pointed by fptr if GC occurs. this means that during > execution of your action data pointed by fptr/ptr may be suddenly > freed, allocated by other object, bang!
I'd like to elaborate that ptr, as far as the GC is concerned is *not a pointer at all*, it is an integer memory address. So it doesn't keep anything alive, you need to hold on to fptr if you want the value to exist. Stefan
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