Using the FFI with GHC

The Glasgow Haskell Compiler User's Guide, Version 6.2
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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()`.

### 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 languagem, 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()`: strange things will undoubtedly happen.

We pass `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.

After we've finished invoking our Haskell functions, we can call `hs_exit()`, which terminates the RTS. It runs any outstanding finalizers and generates any profiling or stats output that might have been requested.

There can be multiple calls to `hs_init()`, but each one should be matched by one (and only one) call to `hs_exit()`[1].

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. Using `foreign import ccall "wrapper"` with GHC

When `foreign import ccall "wrapper"` is used in a Haskell module, The C stub file `M_stub.c` generated by GHC contains small helper functions used by the code generated for the imported wrapper, so it must be linked in to the final program. 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).

## 8.2.2. Using function headers

When generating C (using the `-fvia-C` directive), one can assist the C compiler in detecting type errors by using the `-#include` directive (Section 4.10.5) to provide `.h` files containing function headers.

For example,

    #include "HsFFI.h"

    void         initialiseEFS (HsInt size);
    HsInt        terminateEFS (void);
    HsForeignObj emptyEFS(void);
    HsForeignObj updateEFS (HsForeignObj a, HsInt i, HsInt x);
    HsInt        lookupEFS (HsForeignObj a, HsInt i);

The types `HsInt`, `HsForeignObj` etc. are described in the H98 FFI Addendum.

Note that this approach is only essential for returning `float`s (or if `sizeof(int) != sizeof(int *)` on your architecture) but is a Good Thing for anyone who cares about writing solid code. You're crazy not to do it.

What if you are importing a module from another package, and a cross-module inlining exposes a foreign call that needs a supporting `-#include`? If the imported module is from the same package as the module being compiled, you should supply all the `-#include` that you supplied when compiling the imported module. If the imported module comes from another package, you won't necessarily know what the appropriate `-#include` options are; but they should be in the package configuration, which GHC knows about. So if you are building a package, remember to put all those `-#include` options into the package configuration. See the `c_includes` field in Section 4.8.4.

It is also possible, according the FFI specification, to put the `-#include` option in the foreign import declaration itself:

      foreign import "foo.h f" f :: Int -> IO Int

When compiling this module, GHC will generate a C file that includes the specified `-#include`. However, GHC disables cross-module inlinding for such foreign calls, because it doesn't transport the `-#include` information across module boundaries. (There is no fundamental reason for this; it was just tiresome to implement. The wrapper, which unboxes the arguments etc, is still inlined across modules.) So if you want the foreign call itself to be inlined across modules, use the command-line and package-configuration `-#include` mechanism.

### 8.2.2.1. Finding Header files

Header files named by the `-#include` option or in a `foreign import` declaration are searched for using the C compiler's usual search path. You can add directories to this search path using the `-I` option (see Section 4.10.3).

Note: header files are ignored unless compiling via C. If you had been compiling your code using the native code generator (the default) and suddenly switch to compiling via C, then you can get unexpected errors about missing include files. Compiling via C is enabled automatically when certain options are given (eg. `-O` and `-prof` both enable `-fvia-C`).

## 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` compuatation. 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 funcitons 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.Marhsal.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 implementaiton of Pools in the future, however.

### Notes

[1] | The outermost `hs_exit()` will actually de-initialise the system. NOTE that currently GHC's runtime cannot reliably re-initialise after this has happened.

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