FFI Introduction

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

Compiling FFI-using modules

GHC

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? ]

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:

termops.c:

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

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


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

termops.h:

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

Termios.hs:

{-# 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 ()

FfiEx.hs:

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')

makefile:

_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
%