Haskell how to work with extern FILE* from c? [closed] - haskell

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I want to foreign import a function from some c header, but how to deal with the stderr of type FILE* which defined as:
extern FILE* __stderrp;
#define stderr __stderrp
Maybe not precisely. I use c2hs for my ffi work, and already have:
{#pointer *FILE as File foreign finalizer fclose newtype#}
but I can not import stderr like this:
foreign import ccall "stdio.h stderr" stderr :: File
My c function has the signature:
void func(FILE*);
I can import func with c2hs:
{#fun func as ^ {`File'} -> `()'#}
I need to use stderr to run func:
func(stderr);
I am noob with the foreign import mechanism. It seems I can not import stderr in this way.
ps. Maybe I would wrap my func in a new function
void func2(void){func(stderr);}
This is a workaround, but seems not clean.

Answer 1

It's not unusual to require some kind of "shim" when writing FFI code for Haskell, and I'd encourage you to just write a helper function:
FILE* get_stderr() { return stderr; }
and use that (see example at the bottom of this answer).
However, I was able to get the following minimal example to work by using the vanilla FFI's support for static pointers -- it doesn't import stderr directly, but imports a pointer to the stderr pointer. This kind of import is not directly supported by c2hs, so the interface code is ugly, and I don't think there's any way to avoid having to fetch the stderr pointer value in the IO monad, independent of whether or not you use c2hs.
// file.h
#include <stdio.h>
void func(FILE*);
// file.c
#include "file.h"
void func(FILE *f) {
fputs("Output to stderr!\n", f);
}
// File.chs
{-# LANGUAGE ForeignFunctionInterface #-}
module Main where
import Foreign
#include "file.h"
{#pointer *FILE as File newtype#}
{#fun func as ^ { `File' } -> `()'#}
foreign import ccall "&stderr" stderr_ptr :: Ptr (Ptr File)
main :: IO ()
main = do stderr <- File <$> peek stderr_ptr
func stderr
For comparison, this minimal example with the helper function looks much cleaner at the Haskell level:
// file.h
#include <stdio.h>
void func(FILE*);
FILE* get_stderr(void);
// file.c
#include "file.h"
void func(FILE *f) {
fputs("Output to stderr!\n", f);
}
FILE* get_stderr(void) {return stderr; }
// File.chs
{-# LANGUAGE ForeignFunctionInterface #-}
module Main where
#include "file.h"
{#pointer *FILE as File newtype#}
{#fun func as ^ { `File' } -> `()'#}
{#fun pure get_stderr as ^ {} -> `File'#}
main :: IO ()
main = func getStderr
Note that in both these examples, I removed your fclose finalizer. You probably don't want Haskell arbitrarily deciding it's a good time to close stderr on you.

Answer 2

With c2hs of version 0.28.2, the following code works:
-- lib.chs
{#pointer *FILE as File newtype#}
foreign import ccall "stdio.h &__stderrp" c_stderr :: Ptr (Ptr File) -- can not just use "stdio.h &stderr", this may cause a reference error
-- main.hs
stderr <- File <$> peek c_stderr
func stderr