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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.

1 Compiling FFI-using modules

2 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