GHC (mostly) conforms to the Haskell 98 Foreign Function Interface Addendum 1.0, whose definition is available from http://haskell.org/. The FFI support in GHC diverges from the Addendum in the following ways:
Syntactic forms and library functions proposed in earlier versions of the FFI are still supported for backwards compatibility.
GHC implements a number of GHC-specific extensions to the FFI Addendum. These extensions are described in Section 8.1, but please note that programs using these features are not portable. Hence, these features should be avoided where possible.
The FFI libraries are documented in the accompanying library documentation; see for example the Foreign module.