GHC (mostly) conforms to the Haskell 98 Foreign Function Interface Addendum 1.0, whose definition is available from http://www.haskell.org/. To enable FFI support in GHC, give the -XForeignFunctionInterface-XForeignFunctionInterface flag. GHC implements a number of GHC-specific extensions to the FFI Addendum. These extensions are described in , 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. The following sections also give some hints and tips on the use of the foreign function interface in GHC. foreign export with GHC When GHC compiles a module (say M.hs) which uses foreign export or foreign import "wrapper", it generates two additional files, M_stub.c and M_stub.h. GHC will automatically compile M_stub.c to generate M_stub.o at the same time. For a plain foreign export, the file M_stub.h contains a C prototype for the foreign exported function, and M_stub.c contains its definition. For example, if we compile the following module: module Foo where foreign export ccall foo :: Int -> IO Int foo :: Int -> IO Int foo n = return (length (f n)) f :: Int -> [Int] f 0 = [] f n = n:(f (n-1)) Then Foo_stub.h will contain something like this: #include "HsFFI.h" extern HsInt foo(HsInt a0); and Foo_stub.c contains the compiler-generated definition of foo(). To invoke foo() from C, just #include "Foo_stub.h" and call foo(). The foo_stub.c and foo_stub.h files can be redirected using the -stubdir option; see . When linking the program, remember to include M_stub.o in the final link command line, or you'll get link errors for the missing function(s) (this isn't necessary when building your program with ghc ––make, as GHC will automatically link in the correct bits). Normally, GHC's runtime system provides a main(), which arranges to invoke Main.main in the Haskell program. However, you might want to link some Haskell code into a program which has a main function written in another language, say C. In order to do this, you have to initialize the Haskell runtime system explicitly. Let's take the example from above, and invoke it from a standalone C program. Here's the C code: #include <stdio.h> #include "HsFFI.h" #ifdef __GLASGOW_HASKELL__ #include "foo_stub.h" #endif #ifdef __GLASGOW_HASKELL__ extern void __stginit_Foo ( void ); #endif int main(int argc, char *argv[]) { int i; hs_init(&argc, &argv); #ifdef __GLASGOW_HASKELL__ hs_add_root(__stginit_Foo); #endif for (i = 0; i < 5; i++) { printf("%d\n", foo(2500)); } hs_exit(); return 0; } We've surrounded the GHC-specific bits with #ifdef __GLASGOW_HASKELL__; the rest of the code should be portable across Haskell implementations that support the FFI standard. The call to hs_init() initializes GHC's runtime system. Do NOT try to invoke any Haskell functions before calling hs_init(): bad things will undoubtedly happen. We pass references to argc and argv to hs_init() so that it can separate out any arguments for the RTS (i.e. those arguments between +RTS...-RTS). Next, we call hs_add_rooths_add_root , a GHC-specific interface which is required to initialise the Haskell modules in the program. The argument to hs_add_root should be the name of the initialization function for the "root" module in your program - in other words, the module which directly or indirectly imports all the other Haskell modules in the program. In a standalone Haskell program the root module is normally Main, but when you are using Haskell code from a library it may not be. If your program has multiple root modules, then you can call hs_add_root multiple times, one for each root. The name of the initialization function for module M is __stginit_M, and it may be declared as an external function symbol as in the code above. Note that the symbol name should be transformed according to the Z-encoding: Character Replacement . zd _ zu ` zq Z ZZ z zz After we've finished invoking our Haskell functions, we can call hs_exit(), which terminates the RTS. There can be multiple calls to hs_init(), but each one should be matched by one (and only one) call to hs_exit()The outermost hs_exit() will actually de-initialise the system. NOTE that currently GHC's runtime cannot reliably re-initialise after this has happened, see . . NOTE: when linking the final program, it is normally easiest to do the link using GHC, although this isn't essential. If you do use GHC, then don't forget the flag -no-hs-main-no-hs-main , otherwise GHC will try to link to the Main Haskell module. The scenario here is much like in , except that the aim is not to link a complete program, but to make a library from Haskell code that can be deployed in the same way that you would deploy a library of C code. The main requirement here is that the runtime needs to be initialized before any Haskell code can be called, so your library should provide initialisation and deinitialisation entry points, implemented in C or C++. For example: HsBool mylib_init(void){ int argc = ... char *argv[] = ... // Initialize Haskell runtime hs_init(&argc, &argv); // Tell Haskell about all root modules hs_add_root(__stginit_Foo); // do any other initialization here and // return false if there was a problem return HS_BOOL_TRUE; } void mylib_end(void){ hs_exit(); } The initialisation routine, mylib_init, calls hs_init() and hs_add_root() as normal to initialise the Haskell runtime, and the corresponding deinitialisation function mylib_end() calls hs_exit() to shut down the runtime. C calls, function headers C functions are normally declared using prototypes in a C header file. Earlier versions of GHC (6.8.3 and earlier) #included the header file in the C source file generated from the Haskell code, and the C compiler could therefore check that the C function being called via the FFI was being called at the right type. GHC no longer includes external header files when compiling via C, so this checking is not performed. The change was made for compatibility with the native code backend (-fasm) and to comply strictly with the FFI specification, which requires that FFI calls are not subject to macro expansion and other CPP conversions that may be applied when using C header files. This approach also simplifies the inlining of foreign calls across module and package boundaries: there's no need for the header file to be available when compiling an inlined version of a foreign call, so the compiler is free to inline foreign calls in any context. The -#include option is now deprecated, and the include-files field in a Cabal package specification is ignored. The FFI libraries provide several ways to allocate memory for use with the FFI, and it isn't always clear which way is the best. This decision may be affected by how efficient a particular kind of allocation is on a given compiler/platform, so this section aims to shed some light on how the different kinds of allocation perform with GHC. alloca and friends Useful for short-term allocation when the allocation is intended to scope over a given IO computation. This kind of allocation is commonly used when marshalling data to and from FFI functions. In GHC, alloca is implemented using MutableByteArray#, so allocation and deallocation are fast: much faster than C's malloc/free, but not quite as fast as stack allocation in C. Use alloca whenever you can. mallocForeignPtr Useful for longer-term allocation which requires garbage collection. If you intend to store the pointer to the memory in a foreign data structure, then mallocForeignPtr is not a good choice, however. In GHC, mallocForeignPtr is also implemented using MutableByteArray#. Although the memory is pointed to by a ForeignPtr, there are no actual finalizers involved (unless you add one with addForeignPtrFinalizer), and the deallocation is done using GC, so mallocForeignPtr is normally very cheap. malloc/free If all else fails, then you need to resort to Foreign.malloc and Foreign.free. These are just wrappers around the C functions of the same name, and their efficiency will depend ultimately on the implementations of these functions in your platform's C library. We usually find malloc and free to be significantly slower than the other forms of allocation above. Foreign.Marshal.Pool Pools are currently implemented using malloc/free, so while they might be a more convenient way to structure your memory allocation than using one of the other forms of allocation, they won't be any more efficient. We do plan to provide an improved-performance implementation of Pools in the future, however. In order to use the FFI in a multi-threaded setting, you must use the -threaded option (see ). When you call a foreign imported function that is annotated as safe (the default), and the program was linked using -threaded, then the call will run concurrently with other running Haskell threads. If the program was linked without -threaded, then the other Haskell threads will be blocked until the call returns. This means that if you need to make a foreign call to a function that takes a long time or blocks indefinitely, then you should mark it safe and use -threaded. Some library functions make such calls internally; their documentation should indicate when this is the case. If you are making foreign calls from multiple Haskell threads and using -threaded, make sure that the foreign code you are calling is thread-safe. In particularly, some GUI libraries are not thread-safe and require that the caller only invokes GUI methods from a single thread. If this is the case, you may need to restrict your GUI operations to a single Haskell thread, and possibly also use a bound thread (see ). Note that foreign calls made by different Haskell threads may execute in parallel, even when the +RTS -N flag is not being used (). The +RTS -N flag controls parallel execution of Haskell threads, but there may be an arbitrary number of foreign calls in progress at any one time, regardless of the +RTS -N value. If a call is annotated as interruptible and the program was multithreaded, the call may be interrupted in the event that the Haskell thread receives an exception. The mechanism by which the interrupt occurs is platform dependent, but is intended to cause blocking system calls to return immediately with an interrupted error code. The underlying operating system thread is not to be destroyed. Normally there is no fixed relationship between Haskell threads and OS threads. This means that when you make a foreign call, that call may take place in an unspecified OS thread. Furthermore, there is no guarantee that multiple calls made by one Haskell thread will be made by the same OS thread. This usually isn't a problem, and it allows the GHC runtime system to make efficient use of OS thread resources. However, there are cases where it is useful to have more control over which OS thread is used, for example when calling foreign code that makes use of thread-local state. For cases like this, we provide bound threads, which are Haskell threads tied to a particular OS thread. For information on bound threads, see the documentation for the Control.Concurrent module. When the program is linked with -threaded, then you may invoke foreign exported functions from multiple OS threads concurrently. The runtime system must be initialised as usual by calling hs_init() and hs_add_root, and these calls must complete before invoking any foreign exported functions. hs_exit() normally causes the termination of any running Haskell threads in the system, and when hs_exit() returns, there will be no more Haskell threads running. The runtime will then shut down the system in an orderly way, generating profiling output and statistics if necessary, and freeing all the memory it owns. It isn't always possible to terminate a Haskell thread forcibly: for example, the thread might be currently executing a foreign call, and we have no way to force the foreign call to complete. What's more, the runtime must assume that in the worst case the Haskell code and runtime are about to be removed from memory (e.g. if this is a Windows DLL, hs_exit() is normally called before unloading the DLL). So hs_exit() must wait until all outstanding foreign calls return before it can return itself. The upshot of this is that if you have Haskell threads that are blocked in foreign calls, then hs_exit() may hang (or possibly busy-wait) until the calls return. Therefore it's a good idea to make sure you don't have any such threads in the system when calling hs_exit(). This includes any threads doing I/O, because I/O may (or may not, depending on the type of I/O and the platform) be implemented using blocking foreign calls. The GHC runtime treats program exit as a special case, to avoid the need to wait for blocked threads when a standalone executable exits. Since the program and all its threads are about to terminate at the same time that the code is removed from memory, it isn't necessary to ensure that the threads have exited first. (Unofficially, if you want to use this fast and loose version of hs_exit(), then call shutdownHaskellAndExit() instead).