FFI Introduction

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Haskell's FFI is used to call functions from other languages (basically C at this point), and for C to call haskell functions.


Short version

There are many more useful examples in the FFICookBook, but here's a few basic ones:

{-# INCLUDE <math.h> #-}
{-# LANGUAGE ForeignFunctionInterface #-}
module FfiExample where
import Foreign.C -- get the C types

-- pure function
-- "unsafe" means it's slightly faster but can't callback to haskell
foreign import ccall unsafe "sin" c_sin :: CDouble -> CDouble
sin :: Double -> Double
sin d = realToFrac (c_sin (realToFrac d))

Note that the FFI document recommends putting the header in the double quotes, like

foreign import ccall unsafe "math.h sin" c_sin :: CDouble -> CDouble

However, the GHC docs say the pragma is "the Right Way" [ any technical reasons for this? ], and in practice most foreign imports will come from a small set of headers and it's easier to write them once at the top of the file.

Notice that C types are not the same as haskell types, and you have to import them from Foreign.C. Notice also that, as usual in haskell, you have to explicitly convert to and from haskell types. Using c_<name_of_c_function> for the raw C function is just my convention.

The haskell report only guarantees that Int has 30 bits of signed precision, so converting CInt to Int is not safe! On the other hand, many classes have instances for Int and Integer but not CInt, so it's generally more convenient to convert from the C types. To convert, I suppose you could either write a checkedFromIntegral function if you're sure it's small or just use Integer.

For details on impure functions, pointers to objects, etc., see the cookbook.

Marshalling and unmarshalling arguments

See the cookbook. It's nicer to do the marshalling and unmarshalling in haskell, but it's still low-level repetetive stuff. The functions are all available below Foreign, which supports memory allocation and pointers (and hence C arrays and "out" parameters). One thing it doesn't support is structs.

Tools like GreenCard were created to help with this (as well as the low-level boilerplate thing).

[ TODO: more detail here? examples in greencard? ]

Compiling FFI-using modules


Here's a makefile fragment to compile an FfiExample module that uses C functions from c_functions.c, which uses library functions from libcfuncs:

HFLAGS=-I/path/to/lib/include -L/path/to/lib

_dummy_target: c_functions.o c_functions.h
  ghc $(HFLAGS) -main-is FfiExample --make -o ffi_example c_functions.o -lcfuncs

Notice the use of _dummy_target and --make. The idea is that you get make to compile what is necessary for C, and then always run ghc with --make, at which point it will figure out what is necessary to compile for haskell.

Actually, this is broken, because ghc --make will not notice if a .o file has changed!

[ this is just my hack, anyone have a better way to do this? ]

Other compilers

Fill me in!

Complete example with GHC

GHC's libs don't (apparently?) support generic termios stuff. I could implement the whole tcgetattr / tcsetattr thing, but let's just turn ICANON on and off, so IO.getChar doesn't wait for a newline:


#include <termios.h>
#include "termops.h"

set_icanon(int fd)
        struct termios term;
        tcgetattr(0, &term);
        term.c_lflag |= ICANON;
        tcsetattr(fd, TCSAFLUSH, &term);

unset_icanon(int fd)
        struct termios term;
        tcgetattr(0, &term);
        term.c_lflag &= ~ICANON;
        tcsetattr(fd, TCSAFLUSH, &term);


void set_icanon(int fd);
void unset_icanon(int fd);


{-# INCLUDE <termios.h> #-}
{-# INCLUDE "termops.h" #-}
{-# LANGUAGE ForeignFunctionInterface #-}
module Termios where
import Foreign.C

foreign import ccall "set_icanon" set_icanon :: CInt -> IO ()
foreign import ccall "unset_icanon" unset_icanon :: CInt -> IO ()


module FfiEx where
import Control.Exception
import System.IO
import qualified Termios
import Control.Monad (when)

main = bracket_ (Termios.unset_icanon 0) (Termios.set_icanon 0)
    (while_true prompt)
while_true op = do
    continue <- op
    when continue (while_true op)
prompt = do
    putStr "? "
    hFlush stdout
    c <- getChar
    putStrLn $ "you typed " ++ [c]
    return (c /= 'q')


_ffi_ex: termops.o
    ghc --make -main-is FfiEx -o ffi_ex FfiEx.hs termops.o

[this only worked for me when I omitted termops.o at the end of the `ghc --make` command. Seems like it searches for and finds the .o automatically? --lodi ]

And now:

% make
gcc -c -o termops.o termops.c
ghc --make -main-is FfiEx -o ffi_ex FfiEx.hs termops.o
[1 of 2] Compiling Termios          ( Termios.hs, Termios.o )
[2 of 2] Compiling FfiEx            ( FfiEx.hs, FfiEx.o )
Linking ffi_ex ...
% ./ffi_ex
? you typed a
? you typed b
? you typed q