stdout is not flushed using custom main

According to the using your own main() guide, Prepare two files.

main.c:

#include <stdio.h>
#include "HsFFI.h"

#ifdef __GLASGOW_HASKELL__
#include "Vomit_stub.h"
#endif

int main(int argc, char *argv[])
{
    int i;

    hs_init(&argc, &argv);

    vomit();

    hs_exit();
    return 0;
}

Vomit.hs:

{-# LANGUAGE ForeignFunctionInterface #-}
{-# LANGUAGE OverloadedStrings #-}

module Vomit where

import Data.ByteString
import Data.ByteString.Char8 ()
import Prelude hiding (putStr)

foreign export ccall vomit :: IO ()
vomit = putStr "Wrrreerrerek\n"

Compile the code:

$ ghc -c Vomit.hs
$ ghc --make -no-hs-main -optc-O main.c Vomit -o main

Run:

$ ./main
Wrrreerrerek
$

Looks fine. However:

$ ./main > output.txt
$ cat output.txt
$

output.txt is empty.

It seems when the output is done by some Haskell code called by a C code, and the stdout is redirected to a file, the output is not properly flushed. Changing Vomit.hs to:

{-# LANGUAGE ForeignFunctionInterface #-}
{-# LANGUAGE OverloadedStrings #-}

module Vomit where

import Data.ByteString
import Data.ByteString.Char8 ()
import Prelude hiding (putStr)

import System.IO (hFlush, stdout)

foreign export ccall vomit :: IO ()
vomit = putStr "Wrrreerrerek\n" >> hFlush stdout

which manually flushes stdout, fixes the behavior.

Since hs_exit() on the C side is supposed to terminate the RTS and flush all output, this seems to be a bug.

Tested with GHC 7.0.4 on Debian GNU/Linux for x86.

Trac metadata

Trac field	Value
Version	7.0.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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