FFI example

ffi.cabal

name:                ffi
version:             0.1.0.0
license:             GPL-3
license-file:        LICENSE
author:              Mikhail Glushenkov
maintainer:          the.dead.shall.rise@gmail.com
category:            Testing
build-type:          Simple
cabal-version:       >=1.8
extra-source-files:  cbits/mean.h

executable ffi
  main-is: Main.hs
  build-depends:       base ==4.5.*
  c-sources:           cbits/mean.c
  cc-options:          -std=c99
  include-dirs:        cbits

Main.hs

{-# LANGUAGE ForeignFunctionInterface #-}

module Main
       where

import Foreign.C
import Foreign.Marshal.Safe
import Foreign.Ptr
import Foreign.Storable

foreign import ccall unsafe "mean" c_mean :: Ptr CInt -> CUInt -> IO CFloat

main :: IO ()
main = do
  buf <- mallocBytes (bufSize * sizeOfCInt)
  fillBuffer buf 0
  m <- c_mean buf (fromIntegral bufSize)
  print $ realToFrac m
    where
      sizeOfCInt = sizeOf (0::CInt)
      bufSize       = 100

      fillBuffer :: Ptr CInt -> Int -> IO ()
      fillBuffer buf i | i == bufSize = return ()
                       | otherwise    = do
        poke buf 123
        fillBuffer (buf `plusPtr` sizeOfCInt) (i+1)

mean.c

float mean(int* arr, unsigned n) {
  int acc = 0;

  for (unsigned i = 0; i < n; ++i) {
    acc += arr[i];
  }

  return (float) acc / (float) n;
}

mean.h

float mean(int*, unsigned);