#5594: stdout is not flushed using custom main
--------------------------+-------------------------------------------------
Reporter: handonson | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.0.4 | Keywords:
Testcase: | Blockedby:
Os: Linux | Blocking:
Architecture: x86 | Failure: Incorrect result at runtime
--------------------------+-------------------------------------------------
According to the [http://www.haskell.org/ghc/docs/latest/html/users_guide
/ffi-ghc.html#using-own-main 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.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5594>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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