On 26/10/10 07:10, Antoine Latter wrote:

On Tue, Oct 26, 2010 at 12:21 AM, Johan Tibell wrote:

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On Sun, Oct 24, 2010 at 2:02 AM, Johan Tibell wrote:

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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.

Correct - the right way is to use byteArrayContents# to get the Addr# to the contents, and use touch# to keep the ByteArray# alive until you're finished with the Addr#. Cheers, Simon