Foreign Function Interface (FFI)

HI: Reading Haskell 98 FFI report, one question arised: Imported and declared a foreign function , via foreign import ccall foo:: IO(CInt) Is it guaranteed to be executed in atomic way ? I mean , should it block the whole STG system ? Idea: To implement a monitor , in a "external context" , and threads around him to access it, bypassing the Mvar paramter-pass among them...