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