Chapter 8. Foreign function interface (FFI)
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# Chapter 8. Foreign function interface (FFI)

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.

## 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.

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