On 02 November 2005 11:15, Rene de Visser wrote:
Where is the documentation on how pinning works in the GHC garbage collector (from a GHC users point of view).
I have copied the following code from array/IO.hs and am thinking that it is assuming that the array is pinned? What triggers the pinning?
Actually this code does not assume that any memory is pinned. It is ok to pass the underlying ByteArr# directly to C, as long as the C call is annotated "unsafe", which means that GC cannot happen while the call is running. If you want to pass ByteArr# to a "safe" C call, then you have to allocate the ByteArr# using newPinnedByteArray#. This is the only way to get a pinned object in GHC, and the only kind of pinned object that is supported is a MutByteArr# or ByteArr# (this is to simplify the GC; it doesn't need to traverse pinned objects because they don't contain any pointers, all it needs to do is remember that the memory block they occupy is still alive). Note that all this is GHC-specific; the right high-level interface to allocating pinned memory is mallocForeignPtr.
On a second note. Why is the type signiture so constricted. The code below works on any IOUArray (which is very usefull, not just on Int Word8). Naturally this assumes the particular in memory array layout that GHC uses on a particular platform, so would not be compatible (probably) with other Haskell compilers.
I think the type is right - it makes it clear that the representation being written to the file is an array of bytes. You can use castIOUArray, although that isn't ideal (it doesn't change the bounds). We should do something better here. Cheers, Simon