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GHC (mostly) conforms to the Haskell Foreign Function Interface,
  whose definition is part of the Haskell Report on http://www.haskell.org/.
FFI support is enabled by default, but can be enabled or disabled explicitly with the -XForeignFunctionInterface flag.
GHC implements a number of GHC-specific extensions to the FFI Addendum. These extensions are described in Section 8.1, “GHC extensions to the FFI Addendum”, 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.
The FFI features that are described in this section are specific to GHC. Your code will not be portable to other compilers if you use them.
The following unboxed types may be used as basic foreign types
      (see FFI Addendum, Section 3.2): Int#,
      Word#, Char#,
      Float#, Double#,
      Addr#, StablePtr# a,
      MutableByteArray#, ForeignObj#,
      and ByteArray#.
The FFI spec requires the IO monad to appear in various  places,
      but it can sometimes be convenient to wrap the IO monad in a
      newtype, thus:
newtype MyIO a = MIO (IO a)
(A reason for doing so might be to prevent the programmer from calling arbitrary IO procedures in some part of the program.)
The Haskell FFI already specifies that arguments and results of foreign imports and exports will be automatically unwrapped if they are newtypes (Section 3.2 of the FFI addendum). GHC extends the FFI by automatically unwrapping any newtypes that wrap the IO monad itself. More precisely, wherever the FFI specification requires an IO type, GHC will accept any newtype-wrapping of an IO type. For example, these declarations are OK:
foreign import foo :: Int -> MyIO Int foreign import "dynamic" baz :: (Int -> MyIO Int) -> CInt -> MyIO Int
	  GHC extends the FFI with an additional calling convention
	  prim, e.g.:
foreign import prim "foo" foo :: ByteArray# -> (# Int#, Int# #)
This is used to import functions written in Cmm code that follow an internal GHC calling convention. This feature is not intended for use outside of the core libraries that come with GHC. For more details see the GHC developer wiki.
          This concerns the interaction of foreign calls
          with Control.Concurrent.throwTo.
          Normally when the target of a throwTo is
          involved in a foreign call, the exception is not raised
          until the call returns, and in the meantime the caller is
          blocked.  This can result in unresponsiveness, which is
          particularly undesirable in the case of user interrupt
          (e.g. Control-C).  The default behaviour when a Control-C
          signal is received (SIGINT on Unix) is to raise
          the UserInterrupt exception in the main
          thread; if the main thread is blocked in a foreign call at
          the time, then the program will not respond to the user
          interrupt.
        
          The problem is that it is not possible in general to
          interrupt a foreign call safely.  However, GHC does provide
          a way to interrupt blocking system calls which works for
          most system calls on both Unix and Windows.  When the
          InterruptibleFFI extension is enabled,
          a foreign call
          can be annotated with interruptible instead
          of safe or unsafe:
foreign import ccall interruptible "sleep" :: CUint -> IO CUint
          interruptible behaves exactly as
          safe, except that when
          a throwTo is directed at a thread in an
          interruptible foreign call, an OS-specific mechanism will be
          used to attempt to cause the foreign call to return:
          
                  The thread making the foreign call is sent
                  a SIGPIPE signal
                  using pthread_kill().  This is
                  usually enough to cause a blocking system call to
                  return with EINTR (GHC by default
                  installs an empty signal handler
                  for SIGPIPE, to override the
                  default behaviour which is to terminate the process
                  immediately).
                
                  [Vista and later only] The RTS calls the Win32
                  function CancelSynchronousIO,
                  which will cause a blocking I/O operation to return
                  with the
                  error ERROR_OPERATION_ABORTED.
                
          If the system call is successfully interrupted, it will
          return to Haskell whereupon the exception can be raised.  Be
          especially careful when
          using interruptible that the caller of
          the foreign function is prepared to deal with the
          consequences of the call being interrupted; on Unix it is
          good practice to check for EINTR always,
          but on Windows it is not typically necessary to
          handle ERROR_OPERATION_ABORTED.
        
          The CApiFFI extension allows a calling
          convention of capi to be used in foreign
          declarations, e.g.
foreign import capi "header.h f" f :: CInt -> IO CInt
          Rather than generating code to call f
          according to the platform's ABI, we instead call
          f using the C API defined in the header
          header.h. Thus f can be
          called even if it may be defined as a CPP
          #define rather than a proper function.
        
          When using capi, it is also possible to
          import values, rather than functions. For example,
foreign import capi "pi.h value pi" c_pi :: CDouble
          will work regardless of whether pi is
          defined as
const double pi = 3.14;
or with
#define pi 3.14
          In order to tell GHC the C type that a Haskell type
          corresponds to when it is used with the CAPI, a
          CTYPE pragma can be used on the type
          definition. The header which defines the type can optionally
          also be specified. The syntax looks like:
data    {-# CTYPE "unistd.h" "useconds_t" #-} T = ...
newtype {-# CTYPE            "useconds_t" #-} T = ...