I'm just trying the ffi - importing some Haskell function into C. I've got a module "HelloWorld" with an exported function "add". And I got this "undefined reference" error while linking:

undefined reference to `HelloWorld_zdfadd_closure'

Can anybody please help me, what's wrong here?

A second problem is the __stginit... function. I've defined void __stginit_HelloWorld (void) {} with an empty body. Is this o.k.? Or should I make ghc to generate it for me somehow?

It sounds like you're not linking the Haskell module into the binary. The compiled Haskell code should provide both HelloWorld_zdfadd_closure and __stginit_HelloWorld. Cheers, Simon