Re: [Haskell-cafe] question concerning ANSI "void *"

basically I am trying to implement ioctl for the Posix library .. so a possible signtaure would be: fdIoctl :: Fd -> Int -> Ptr Word 8 -> IO( Ptr Word8) ???? Vasili On 2/8/08, Galchin Vasili wrote:

a couple of concrete examples:

typedef struct {char a; int b; char str[8]} type1;

typedef struct {long c; char d} type2;

So to pthread_create (just an example function) we could be passing a struct of type1 or a struct of type2 .. i.e. arbitrary length and content ... I am trying to better understand this. I see some of the poke functions mentioned in the FFI. Which one are you alluding to?

Regards, Vasili

On 2/8/08, Adam Langley wrote:

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Let's take a concrete but "made up" case .. suppose we want to call

On Feb 8, 2008 9:13 AM, Galchin Vasili wrote: through

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to pthread_create and pass the (void *) argument to pthread_create which in turn gets interpreted by the pthread that is launched. How would one populate the C struct that is passed to the launched pthread keeping in mind that this C struct is variable in length? From the FFI how would one model this C struct?

It tough to be helpful with such a generic request. Here are some options that you can consider:

1) Write a wrapper function in C which has a nicer FFI interface to call from Haskell. Using cabal this is pretty painless and, if the interface suits it, it probably the easiest solution. 2) Call pthread_create directly with the FFI. You can give the FFI function a Haskell type with 'Ptr ()' or 'Ptr X', it doesn't really matter. However the type system serves you best, do it that way. This means that you need to populate the struct yourself in Haskell. The issue with this is that the local system defines lots of things like padding and alignment which mean that the layout of the structure in memory is complex and platform specific. Tools like hsc2hs[1] or c2hs will be very helpful here. Dealing with the variable length probably isn't an issue. Usually variable length structures have a fixed header and a variable tail, where the tail is an array of primitives. You can malloc the correct sized region, use one of the previous tools to fill in the fixed header and then use poke to complete the tail.

I might be able to be more helpful if you give the actual struct and function prototype that you're trying to wrap.

Cheers

[1] http://therning.org/magnus/archives/tag/hsc2hs [2] http://www.cse.unsw.edu.au/~chak/haskell/c2hs/

-- Adam Langley agl@imperialviolet.org http://www.imperialviolet.org 650-283-9641