Marshalling with get and set

by Edward Z. Yang

This part five of a six part introduction to c2hs. Today, we explain how to marshal data to and from C structures.

An important note. There is a difference between struct foo and foo; c2hs only considers the latter a type, so you may need to add some typedefs of the form typedef struct foo foo in order to get c2hs to recognize these structs.

Get. The Haskell FFI has no knowledge of C structs; Haskell's idea of reading a member of a struct is to peek at some byte offset of a memory location, which you calculated manually. This is horrid, and hsc2hs has #peek to relieve you of this non-portable drudgery. c2hs has something even simpler: you can specify {#get StructName->struct_field #} and c2hs will replace this with a lambda that does the correct peek with the correct type: (\ptr -> do {peekByteOff ptr 12 ::IO CInt}) (in the IO monad!) Note the following gotchas:

  • You will need to manually convert the resulting primitive C type into a more friendly Haskell type, and
  • The left hand side of the expression is a type or a struct name, not the Haskell variable containing the pointer/struct you want to peek at. That will usually go to the right of the lambda.

The get directive is actually more general than just struct access: it can dereference pointers (*StructName) or access a member without dereferencing (StructName.struct_field).

Set. The opposite of get, set lets you poke values into arbitrary memory locations. Unlike get, the value passed in is required to be a pointer (and the syntax uses periods). {#set StructName.struct_field #} expands to (\ptr val -> do {pokeByteOff ptr 12 (val::CInt)}); the pointer is the first argument and the value is the second. You also need to marshal the input value manually.

Defining Storable. If you're not individually getting and setting fields in the struct in an opaque pointer, creating a Storable instance is a good thing to do. However, since all of the lambdas that get and set create are in the IO monad, composing them can be slightly tricky. Judicious use of monadic lifting and applicative instances can make the code a lot simpler, however:

data StructName = StructName
  { struct_field1'StructName :: Int
  , struct_field2'StructName :: Int
  }
instance Storable StructName where
  sizeOf _ = {#sizeof StructName #}
  alignment _ = 4
  peek p = StructName
    <$> liftM fromIntegral ({#get StructName->struct_field1 #} p)
    <*> liftM fromIntegral ({#get StructName->struct_field2 #} p)
  poke p x = do
    {#set StructName.struct_field1 #} p (fromIntegral $ struct_field1'StructName x)
    {#set StructName.struct_field2 #} p (fromIntegral $ struct_field2'StructName x)

The odd naming convention in StructName is to account for the fact that different structures can share field names, while Haskell field names may not.

Note. c2hs recently got support added for an alignment directive, which computes the alignment for a C datastructure. Unfortunately, as of 0.6.12, this has not yet been released to the general public.

Request. The paper describing c2hs states the following: “[Marshaling of compound C values to Haskell values] is more generally useful; however, often we do not really want to marshal entire C structures to Haskell.” Unfortunately, current incarnations of c2hs do not offer any optional functionality to reduce the drudgery of writing the “straightforward” Storable instance, which would be absolutely lovely. bindings-dsl and GreenCard appear to fare better in this respect.

Next time. Call and fun: marshalling redux