Unaligned word-sized reads on ByteArray#

[Antoine Latter]

On Tue, Oct 26, 2010 at 12:21 AM, Johan Tibell <johan.tibell at gmail.com> wrote:
> On Sun, Oct 24, 2010 at 2:02 AM, Johan Tibell <johan.tibell at gmail.com> wrote:
>> I noticed that indexWordArray# only allows for aligned reads (by
>> forcing the offset to be in words, rather than in bytes.) Is it
>> possible to perform unaligned reads on a ByteArray# e.g. going via
>> Addr#? There's the byteArrayContent# primitive but I don't know how to
>> force the ByteArray# to stay pinned in memory while performing my
>> reads. The ByteArray# is not guaranteed to be pinned in my case.
>
> It seems like I should be able to use (the undocumented) touch#
> function to achieve this. Is that the case?
>

I don't know much about GHC internals, but in the interest of keeping
the conversation going I'll respond.

I had thought that touch# prevented it from being collected - that is,
it forces the GC to believe there is a reference to the region in
memory so it doesn't disappear out from under the foreign call. So it
is necessary for what you want, but I do not think sufficient.

Whenever folks need a region to remain pinned in memory during a call
use the various 'pined' allocation functions.

All of the functions in Foreign.Marshal.Alloc use the pinned byte
array allocation functions (aside from the ones which call malloc),
which I take as the prototype of creating buffers to pass to foreign
calls.

Antoine