Haskell FFI for `getmntent(3)`, `setmntent(3)`

Sample.chs

{-# LANGUAGE ForeignFunctionInterface #-}

module MountUtils where

import Control.Monad (liftM)

import Foreign.C.Types
import Foreign.C.String
import Foreign.Ptr
import Foreign.Storable
import Foreign.Marshal.Alloc

#include "mntent.h"

#c
typedef struct mntent mntent_t;
#endc

-- FILE* is a opaque type
{#pointer *FILE as MntHandle newtype#}

data MntEnt = MntEnt
  { mntFSName :: String
  , mntDir :: String
  , mntType :: String
  , mntOpts :: String
  , mntFreq :: Integer
  , mntPassNum :: Integer
  } deriving Show

{#pointer *mntent_t as MntEntPtr -> MntEnt#}

instance Storable MntEnt where
  sizeOf _ = {#sizeof mntent_t #}
  alignment _ = 4
  peek p = do
    fsName <- ({#get mntent_t->mnt_fsname#} p)
    fsDir <- ({#get mntent_t->mnt_dir#} p)
    fsType <- ({#get mntent_t->mnt_type#} p)
    fsOpts <- ({#get mntent_t->mnt_opts#} p)
    fsFreq <- ({#get mntent_t->mnt_freq#} p)
    fsPassNum <- ({#get mntent_t->mnt_passno#} p)

    fsNameStr <- peekCString fsName
    fsDirStr <- peekCString fsDir
    fsTypeStr <- peekCString  fsType
    fsOptsStr <- peekCString fsOpts

    return $ MntEnt fsNameStr fsDirStr fsTypeStr fsOptsStr (fromIntegral fsFreq) (fromIntegral fsPassNum)

  poke p x = do
    fsName <- newCString $ mntFSName x
    fsDir <- newCString $ mntDir x
    fsType <- newCString $ mntType x
    fsOpts <- newCString $ mntOpts x
    {#set mntent_t.mnt_fsname#} p fsName
    {#set mntent_t.mnt_dir#} p fsDir
    {#set mntent_t.mnt_type#} p fsType
    {#set mntent_t.mnt_opts#} p fsOpts
    {#set mntent_t.mnt_freq#} p (fromIntegral $ mntFreq x)
    {#set mntent_t.mnt_passno#} p (fromIntegral $ mntPassNum x)
    mapM_ free [fsName, fsDir, fsType, fsOpts]


{#fun unsafe setmntent as ^ {`String', `String'} -> `MntHandle'#}

getmntent :: (MntHandle) -> IO ((MntEntPtr))
getmntent h = getmntent'_ h

foreign import ccall unsafe "Sample.chs.h getmntent"
  getmntent'_ :: ((MntHandle) -> (IO (C2HSImp.Ptr (MntEnt))))

To run this,

Install c2hs executable to generate Haskell bindings from C libs

stack install c2hs --> I installed it in my global package db

Command to generate Haskell binding module

c2hs --cppopts='-I/usr/include/' -l Sample.chs

Compile the generated Haskell binding module

ghc Sample.hs