Geek asks and answers

Import a “C” Delay function into Haskell using FFI

The wiringPi 'C' library has a function called delaywith type

void delay(unsigned int howLong);

This function delays code execution for howLongmilliseconds. I wrote the binding code in haskell to be able to call this function. The haskell code is as follows:

foreign import ccall "wiringPi.h delay" c_delay :: CUInt -> IO ()
hdelay :: Int -> IO ()
hdelay howlong = c_delay (fromIntegral howlong)

After that, I wrote a simple haskell program to call this function. Just haskell code looks like this.

- After importing the relevant libraries, I did

main = wiringPiSetup
    >> delay 5000

But there is no delay, or rather, the executable file generated by the ghc compiler immediately exits.

Can someone tell me what could go wrong here? A slight push in the right direction would help.

Greetings and greetings.

+5
1

, . . - - .

import unsafe, , (. @carl). , unsafe. , :

foreign import ccall unsafe "wiring.h delay" c_delay :: CUInt -> IO ()

, , GHC FFI . .

, , ( , GHC 7.4+, ). : http://hackage.haskell.org/trac/ghc/wiki/Commentary/Rts/Signals

: Signal handling differs between the threaded version of the runtime and the non-threaded version.

1 - FFI: , . Linux 2.6.18 ghc 7.6.1.

C-:

/** ctest.c **/
#include <unistd.h>
#include <stdio.h>
#include <time.h>

unsigned delay(unsigned sec)
{
  struct timespec req={0};
  req.tv_sec = sec;
  req.tv_nsec = 0;

  while (nanosleep(&req, &req) == -1) {
    printf("Got interrupt, continuing\n");
    continue;
    }
  return 1;
}

Haskell:

{-# LANGUAGE ForeignFunctionInterface #-}
-- Filename Test.hs
module Main (main) where
import Foreign.C.Types

foreign import ccall safe "delay" delay :: CUInt -> IO CUInt

main =  do
    putStrLn "Sleeping"
    n <- delay 2000
    putStrLn $ "Got return code from sleep: " ++ show n

, ghc 7.6.1 (: ghc Test.hs ctest.c), , :

./Test
Sleeping
Got interrupt, continuing
Got interrupt, continuing
Got interrupt, continuing
Got interrupt, continuing
....
....
Got return code from sleep: 1

2 - SIGVTALRM FFI :

, SIGVTALRM. , SIGVTALRM FFI, FFI. , FFI (, SIGVTALRM ).

{-# LANGUAGE ForeignFunctionInterface #-}
-- Test.hs
module Main (main) where
import Foreign.C.Types
import System.Posix.Signals

foreign import ccall safe "delay" delay :: CUInt -> IO CUInt

main =  do
    putStrLn "Sleeping"
    -- Block SIGVTALRM temporarily to avoid interrupts while sleeping
    blockSignals $ addSignal sigVTALRM emptySignalSet
    n <- delay 2
    putStrLn $ "Got return code from sleep: " ++ show n
    -- Unblock SIGVTALRM
    unblockSignals $ addSignal sigVTALRM emptySignalSet
    return ()
+2

More articles:


All Articles