Hi Viktor, hi everyone, Thanks again for the many useful answers. I'll try to answers the questions asked in one email: 1. did you consider using `hsc2hs` to create these bindings? I have to admit I stopped at c2hs when I found out it could not produce correct alignments for libsodium: https://github.com/haskell/c2hs/issues/272 I am now being told that hsc2hs asks the compiler directly so it's not problem, but that's eleven months after the fact :) 2. What's the use case here? The use case is certainly my own partial worldview of how it all works. That being said one interesting thing is that peek & poke allow you to do IO, whereas Binary's Put and Get do not seem to allow me to do it (without cheating at least). And I couldn't find any instruction that said that it was okay to use unsafeDupablePerformIO (or similar) in Binary. Have a nice rest of your day! Cheers, Hécate Le 28/11/2022 à 17:42, Viktor Dukhovni a écrit :
On Mon, Nov 28, 2022 at 05:04:32PM +0100, Hécate wrote:
In a fit of madness I have found myself writing Haskell code where I need to implement a Storable instance. However, by virtue of not being a C programmer, I'm fairly lost on some of the details, especially the value of the sizeOf and alignment methods.
My Haskell-level record is the following:
data SignedMessage = SignedMessage { messageLength :: CSize , messageForeignPtr :: ForeignPtr CUChar , signatureForeignPtr :: ForeignPtr CUChar } This is not the sort of object for which `Storable` makes sense. It holds ephemeral pointers to variable sized external data, and so cannot be serialised in a modest fixed-size memory block. The `Storable` class is for primitive data (Ints, Words, ...) and simple fixed layout structures consisting of same (e.g. various structures passed to, or returned by C system calls). When structures contain pointers to data, nested `peek` or `poke` calls (with associated memory allocations) may be needed to read or write the structure.
To serialise your "SignedMessage" object you may need a higher-level serialisation format. ASN.1, protobufs, ... or (for a Haskell-only format) an instance of `Data.Binary.Binary` (rather than Storable).
https://hackage.haskell.org/package/binary-0.10.0.0/docs/Data-Binary.html
What's the use case here?
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