gmane.comp.lang.haskell.cafe

handles and foreign C functions

Hello,

suppose a simple C function like this:

void printStuff(FILE *f)
{
    fprintf (f, "Hello World\n");
}

In order to use it I need to import fopen and pass to it a Ptr CFile:

foreign import ccall unsafe "fopen"  fopen  :: CString -> CString -> IO (Ptr CFile)
foreign import ccall unsafe "fclose" fclose :: Ptr CFile -> IO CInt

foreign import ccall unsafe "printStuff" printStuff :: Ptr CFile -> IO ()

main =
  withCString "tmp.txt" $ \cpath  ->
  withCString "w"       $ \cmode  -> do
    cfile <- throwErrnoIfNull "fopen: " (fopen cpath cmode)
    printStuff cfile
    fclose cfile

How can I pass to printStuff a stdout FILE pointer?

If, for instance, I do:

foreign import ccall unsafe "stdout" c_stdout :: Ptr CFile
main =
  withCString "tmp.txt" $ \cpath  ->

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Bulat Ziganshin | 3 Dec 17:08

Re: handles and foreign C functions

Hello Andrea,

Wednesday, December 3, 2008, 5:09:21 PM, you wrote:

> How can I pass to printStuff a stdout FILE pointer?

afair, "&stdout" syntax used to import variables. it was discussed
just a day or two ago :)

--

-- 
Best regards,
 Bulat                            mailto:Bulat.Ziganshin <at> gmail.com

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Andrea Rossato | 3 Dec 17:23

Re: handles and foreign C functions

On Wed, Dec 03, 2008 at 07:08:00PM +0300, Bulat Ziganshin wrote:
> Hello Andrea,
> 
> Wednesday, December 3, 2008, 5:09:21 PM, you wrote:
> 
> > How can I pass to printStuff a stdout FILE pointer?
> 
> afair, "&stdout" syntax used to import variables. it was discussed
> just a day or two ago :)

you mean this, I think:

foreign import ccall unsafe "&stdout" c_stdout :: Ptr CFile

(my fault with the cut&paste of the previous message).

This is causing the segmentation fault I was talking about.

Thanks,
Andrea

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Bulat Ziganshin | 3 Dec 17:26

Re[2]: handles and foreign C functions

Hello Andrea,

Wednesday, December 3, 2008, 7:23:20 PM, you wrote:

either some error in the code (i neevr used this feature) or stdout
may be defile by a macro. can you try to define function for it:

FILE *out() {return stdout;}

> On Wed, Dec 03, 2008 at 07:08:00PM +0300, Bulat Ziganshin wrote:
>> Hello Andrea,
>> 
>> Wednesday, December 3, 2008, 5:09:21 PM, you wrote:
>> 
>> > How can I pass to printStuff a stdout FILE pointer?
>> 
>> afair, "&stdout" syntax used to import variables. it was discussed
>> just a day or two ago :)

> you mean this, I think:

> foreign import ccall unsafe "&stdout" c_stdout :: Ptr CFile

> (my fault with the cut&paste of the previous message).

> This is causing the segmentation fault I was talking about.

> Thanks,
> Andrea
> _______________________________________________

(Continue reading)

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Andrea Rossato | 3 Dec 17:39

Re: handles and foreign C functions

Hello Bulat,

On Wed, Dec 03, 2008 at 07:26:39PM +0300, Bulat Ziganshin wrote:
> either some error in the code (i neevr used this feature) or stdout
> may be defile by a macro.

the second you said:

/* C89/C99 say they're macros.  Make them happy.  */

(from stdio.h)

> can you try to define function for it:
> 
> FILE *out() {return stdout;}

This does the trick.

Thank you once again.

Andrea