8.2. Using the FFI with GHC
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## 8.2. Using the FFI with GHC

The following sections also give some hints and tips on the use of the foreign function interface in GHC.

### 8.2.1. Using `foreign export` and `foreign import ccall "wrapper"` 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 Section 4.7.4, “Redirecting the compilation output(s)”.

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).

#### 8.2.1.1. Using your own `main()`

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_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()`[11].

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`, otherwise GHC will try to link to the `Main` Haskell module.

#### 8.2.1.2. Making a Haskell library that can be called from foreign code

The scenario here is much like in Section 8.2.1.1, “Using your own `main()`”, 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.

### 8.2.2. Using header files

C functions are normally declared using prototypes in a C header file. Earlier versions of GHC (6.8.3 and earlier) `#include`d 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.

### 8.2.3. Memory Allocation

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.

### 8.2.4. Multi-threading and the FFI

In order to use the FFI in a multi-threaded setting, you must use the `-threaded` option (see Section 4.11.6, “Options affecting linking”).

#### 8.2.4.1. Foreign imports and multi-threading

When you call a `foreign import`ed 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 Section 8.2.4.2, “The relationship between Haskell threads and OS threads”).

Note that foreign calls made by different Haskell threads may execute in parallel, even when the `+RTS -N` flag is not being used (Section 4.14.2, “RTS options for SMP parallelism”). 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.

#### 8.2.4.2. The relationship between Haskell threads and OS threads

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.

#### 8.2.4.3. Foreign exports and multi-threading

When the program is linked with `-threaded`, then you may invoke `foreign export`ed 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 export`ed functions.

#### 8.2.4.4. On the use of `hs_exit()`

`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).

### 8.2.5. Floating point and the FFI

The standard C99 `fenv.h` header provides operations for inspecting and modifying the state of the floating point unit. In particular, the rounding mode used by floating point operations can be changed, and the exception flags can be tested.

In Haskell, floating-point operations have pure types, and the evaluation order is unspecified. So strictly speaking, since the `fenv.h` functions let you change the results of, or observe the effects of floating point operations, use of `fenv.h` renders the behaviour of floating-point operations anywhere in the program undefined.

Having said that, we can document exactly what GHC does with respect to the floating point state, so that if you really need to use `fenv.h` then you can do so with full knowledge of the pitfalls:

- GHC completely ignores the floating-point environment, the runtime neither modifies nor reads it.

- The floating-point environment is not saved over a normal thread context-switch. So if you modify the floating-point state in one thread, those changes may be visible in other threads. Furthermore, testing the exception state is not reliable, because a context switch may change it. If you need to modify or test the floating point state and use threads, then you must use bound threads (`Control.Concurrent.forkOS`), because a bound thread has its own OS thread, and OS threads do save and restore the floating-point state.

- It is safe to modify the floating-point unit state temporarily during a foreign call, because foreign calls are never pre-empted by GHC.

[11] 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 Section 12.1.3, “Divergence from the FFI specification”.

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