Chapter 8.  Foreign function interface (FFI)

Table of Contents

8.1. GHC extensions to the FFI Addendum
8.1.1. Unboxed types
8.1.2. Newtype wrapping of the IO monad
8.1.3. Primitive imports
8.1.4. Interruptible foreign calls
8.1.5. The CAPI calling convention
8.1.6. hs_thread_done()
8.2. Using the FFI with GHC
8.2.1. Using foreign export and foreign import ccall "wrapper" with GHC Using your own main() Making a Haskell library that can be called from foreign code
8.2.2. Using header files
8.2.3. Memory Allocation
8.2.4. Multi-threading and the FFI Foreign imports and multi-threading The relationship between Haskell threads and OS threads Foreign exports and multi-threading On the use of hs_exit()
8.2.5. Floating point and the FFI

GHC (mostly) conforms to the Haskell Foreign Function Interface, whose definition is part of the Haskell Report on

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.

8.1. GHC extensions to the FFI Addendum

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.

8.1.1. Unboxed types

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#.

8.1.2. Newtype wrapping of the IO monad

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

8.1.3. Primitive imports

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.

8.1.4. Interruptible foreign calls

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" sleepBlock :: 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:

Unix systems

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).

Windows systems

[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.

8.1.5. The CAPI calling convention

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 = ...

8.1.6. hs_thread_done()

void hs_thread_done(void);

GHC allocates a small amount of thread-local memory when a thread calls a Haskell function via a foreign export. This memory is not normally freed until hs_exit(); the memory is cached so that subsequent calls into Haskell are fast. However, if your application is long-running and repeatedly creates new threads that call into Haskell, you probably want to arrange that this memory is freed in those threads that have finished calling Haskell functions. To do this, call hs_thread_done() from the thread whose memory you want to free.

Calling hs_thread_done() is entirely optional. You can call it as often or as little as you like. It is safe to call it from a thread that has never called any Haskell functions, or one that never will. If you forget to call it, the worst that can happen is that some memory remains allocated until hs_exit() is called. If you call it too often, the worst that can happen is that the next call to a Haskell function incurs some extra overhead.