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

Let's take a concrete but "made up" case .. suppose we want to call through 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? Thanks, Vasili On 2/7/08, Adam Langley wrote:

2008/2/7 Galchin Vasili :

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Ok .. I am writing a Haskell function that will call down into the

ANSI

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C library .. blah :: .... -> Ptr Word8 ....-> .... The underlying C function that "blah" is calling has a "void *" so I am using "Ptr Word 8" to model the "void *".

Depending on the context, "void *" is generally either taken as a Ptr () (for an opaque pointer) or Ptr Word8 (for calls like memcpy).

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I propose to have the callers of function "blah" to populate a data structure something like "Ptr Buz" where "data Buz = { ........} and then do a "recast :: Ptr Word 8 -> Ptr Buz" when invoking function "blah". Does this seem reasonable? Is there a better way?

Generally, Ptr x is only used where x is either a shadow type or a Bits type. Having a Ptr Baz where Baz is an ADT seems a little odd. If you need to translate a structure from Haskell to C code, probably you are better off having callers pass in a Baz then, internal to the wrapping, fill out the C structure and call the FFI function with a Ptr CBaz (where CBaz is a shadow type).

AGL

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