FFI complete examples

This is a repository of COMPLETE WORKING EXAMPLES on how to do FFI. The examples here don't require any tools other than their respective compilers (ghc or g++). In particular, no make files or helper tools such as c2hs, hsc2hs, or cabal are necessary to build and run these examples.

A complete novice with no experience in any of Haskell, C, C++, or Python should be able to get all of these examples working. All of the build commands are explicitly given for each example.

I am using ghc version 6.12.3.

When main is in Haskell

The following examples all involve a Haskell program calling into libraries in other languages. These are the simplest case to build, because ghc can usually do the heavy lifting for us.

Calling standard library functions

This example is adapted from FFI_Introduction.

Main.hs:

{-# LANGUAGE ForeignFunctionInterface #-}
module Main where

import Prelude hiding (sin)

import Foreign.C -- get the C types
import Foreign.Ptr (Ptr,nullPtr)

-- pure function
foreign import ccall "sin" c_sin :: CDouble -> CDouble
sin :: Double -> Double
sin d = realToFrac (c_sin (realToFrac d))

-- impure function
foreign import ccall "time" c_time :: Ptr a -> IO CTime
getTime :: IO CTime
getTime = c_time nullPtr

main = do
  print . sin =<< readLn
  print =<< getTime

To run:

$ ghc --make Main.hs
[1 of 1] Compiling Main             ( Main.hs, Main.o )
Linking Main ...
$ ./Main
3.14159265358
9.793177720293495e-12
1291494446

Calling C functions when you have the source code

The FFI_Introduction page has a nice example of wrapping the termios libraries. Here is how to build the program without a make file:

$ ghc --make -main-is FfiEx FfiEx.hs termops.c
[1 of 2] Compiling Termios          ( Termios.hs, Termios.o )

Termios.hs:1:11:
    Warning: -#include and INCLUDE pragmas are deprecated: They no longer have any effect

Termios.hs:2:11:
    Warning: -#include and INCLUDE pragmas are deprecated: They no longer have any effect
[2 of 2] Compiling FfiEx            ( FfiEx.hs, FfiEx.o )
Linking FfiEx ...
$ ./FfiEx
? ayou typed a
? byou typed b
? cyou typed c
? qyou typed q

If you are running GHC 6.10 or higher (I am running 6.12.3), you can get rid of the warnings by removing the INCLUDE lines from Termios.hs.

Calling C functions when you don't have the source code

See the FFI_Introduction page for how to create the files termops.c, termops.h, Termios.hs, and FfiEx.hs. This example is essentially the same as the one on the FFI_Introduction, but we are running by hand the commands that make would normally run for us.

$ mkdir c
$ mkdir hs
$ cd c
$ $EDITOR termops.c
$ $EDITOR termops.h
$ gcc -c -o termops.o termops.c
$ cp termops.o ../hs
$ cd ../hs

Now to build FfiEx.hs without access to the original c file:

$ $EDITOR Termios.hs
$ $EDITOR FfiEx.hs
$ ghc --make -main-is FfiEx -o ffi_ex FfiEx.hs termops.o
[1 of 2] Compiling Termios          ( Termios.hs, Termios.o )

Termios.hs:1:11:
    Warning: -#include and INCLUDE pragmas are deprecated: They no longer have any effect

Termios.hs:2:11:
    Warning: -#include and INCLUDE pragmas are deprecated: They no longer have any effect
[2 of 2] Compiling FfiEx            ( FfiEx.hs, FfiEx.o )
Linking ffi_ex ...
$ ./ffi_ex 
? ayou typed a
? byou typed b
? cyou typed c
? qyou typed q

When main is in C++

These examples are for building Haskell libraries to be used in a C++ program. Because g++ is not Haskell aware, building is substantially more complicated, so you would normally use a tool for this.

Reading structs in Haskell

Foo.hs:

{-# LANGUAGE ForeignFunctionInterface #-}
module Foo where

import Foreign.Ptr
import Foreign.Storable
import Foreign.C.Types

-- pure function
foreign export ccall foo :: MyStruct -> Int

foo :: MyStruct -> Int
foo = const 42

-- impure function
foreign export ccall showStruct :: MyStruct -> IO ()

showStruct :: MyStruct -> IO ()
showStruct ss = peek ss >>= print

data MyStructType = MyStructType CInt CChar
  deriving (Show, Read, Eq)
type MyStruct = Ptr MyStructType

instance Storable MyStructType where
  sizeOf _ = 8
  alignment = sizeOf
  peek ptr = do
    a <- peekByteOff ptr 0
    b <- peekByteOff ptr 4
    return (MyStructType a b)

test.cpp:

#include <iostream>
#include "Foo_stub.h"

struct MyStruct {
    int foo;
    char bar;
};

int main(int argc, char *argv[])
{
    MyStruct myStruct;

    myStruct.foo = 7;
    myStruct.bar = 'x';

    hs_init(&argc, &argv);
    std::cout << foo(&myStruct) << "\n";
    showStruct(&myStruct);
    hs_exit();
    return 0;
}

Here's where it gets a bit hacky. We need to capture all of Haskell's library dependencies, so we will compile Foo.hs verbosely.

$ ghc Foo.hs -v >ghc_output 2>&1

This should produce Foo.hi, Foo.o, Foo_stub.c, Foo_stub.h, and Foo_stub.o. Take a look at ghc_output and look for "*** Linker" to see all of the libraries that we will need to let g++ know about.

$ grep -A 1 "*** Linker" ghc_output | tail -n 1 | grep -o -- "-L.*" > link_options

Now to build test.cpp:

$ g++ -c test.cpp -I`ghc --print-libdir`/include
$ g++ -o test Foo.o Foo_stub.o test.o `cat link_options`
$ ./test 
42
MyStructType 7 120

When main is in Python

Using ctypes

This is described at the Python wiki page PythonVsHaskell, look for "Using both Python & Haskell with ctypes"

Using swig (and a c wrapper)

This example is adapted from The Swig Tutorial. The factorial function has been modified to call into Haskell.

Foo.hs:

{-# LANGUAGE ForeignFunctionInterface #-}
module Foo where

import Foreign.C.Types

foreign export ccall hs_fact :: CInt -> CInt
hs_fact :: CInt -> CInt
hs_fact n = product [1..n]

example.c:

#include "Foo_stub.h"

void py_init(int argc, char *argv[]) {
    hs_init(&argc, &argv);
}

void py_exit() {
    hs_exit();
}

int fact(int n) {
    return hs_fact(n);
}

example.i:

%module example
%{

/* Put header files here or function declarations like below */
extern int fact(int n);
extern void py_init(int argc, char** argv);
extern void py_exit();
%}

%typemap(in) (int argc, char **argv) {
  /* Check if is a list */
  if (PyList_Check($input)) {
    int i;
    $1 = PyList_Size($input);
    $2 = (char **) malloc(($1+1)*sizeof(char *));
    for (i = 0; i < $1; i++) {
      PyObject *o = PyList_GetItem($input,i);
      if (PyString_Check(o))
    $2[i] = PyString_AsString(PyList_GetItem($input,i));
      else {
    PyErr_SetString(PyExc_TypeError,"list must contain strings");
    free($2);
    return NULL;
      }
    }
    $2[i] = 0;
  } else {
    PyErr_SetString(PyExc_TypeError,"not a list");
    return NULL;
  }
}

%typemap(freearg) (int argc, char **argv) {
  free((char *) $2);
}

extern int fact(int n);
extern void py_init(int argc, char** argv);
extern void py_exit();

run.py:

import example
import sys
example.py_init(sys.argv)
print example.fact(5)
example.py_exit()

To build (you may need to change the include path if you are using a different installation of python):

$ ghc -c -dynamic -fPIC Foo.hs
$ swig -python example.i
$ gcc -fpic -c example.c example_wrap.c -I/usr/include/python2.6 -I`ghc --print-libdir`/include
$ ghc -o _example.so -shared -dynamic -fPIC example.o example_wrap.o Foo.o Foo_stub.o -lHSrts-ghc6.12.3
$ python run.py