[Haskell-cafe] Fwd: c2hs: E.g. #include <iostream> OK in *.cpp's, but not in *.h's

[Ian Ross]

More or less.  Basically, C2HS is *C* FFI tool only.  If you get into a
situation where C2HS can see C++ header files, it's not going to work.  How
you get around that is up to you.  If your C++ library isn't too
complicated, a C wrapper as used in hsqml seems like a good choice, or you
can write the Haskell FFI import declarations yourself.  If your C++
library is complicated and you really want to get at all of its public
interface, you could try using a real C++ FFI tool (the only one I've
really heard anything about is fficxx: http://ianwookim.org/fficxx/).


On 7 April 2014 15:33, Nick Rudnick <nick.rudnick at gmail.com> wrote:

>
>
> ---------- Forwarded message ----------
> From: Nick Rudnick <nick.rudnick at gmail.com>
> Date: 2014-04-07 15:28 GMT+02:00
> Subject: Re: [Haskell-cafe] c2hs: E.g. #include <iostream> OK in *.cpp's,
> but not in *.h's
> To: Ian Ross <ian at skybluetrades.net>
>
>
> Hi Ian,
>
> thanks for clearing up :-)
>
> So you mean the trick of hsqml primarily is
>
> (A) dummy typedefs to char, e.g.:
>
>  typedef char HsQMLStringHandle;
>
> (B) 'extern' sigantures, e.g.:
> extern void hsqml_init_string(HsQMLStringHandle*);
>
> THIS WORKS...
>
> Occasionally, one might use two headers, e.g. a *.hpp as actual C++ header
> neither listed in the *.cabal nor the *.chs, where instead it would be
> represented by a *.h, as described above.
>
> No more...??
>
> Thanks and cheers, Nick
>
>
>
>
>
>
>
> 2014-04-07 14:04 GMT+02:00 Ian Ross <ian at skybluetrades.net>:
>
> Hi Nick,
>>
>> As far as I can tell from a quick look at hsqml, the stuff in Setup.hs
>> isn't related to C2HS.  What's more relevant is that hsqml defines a C
>> library wrapper around the C++ library it uses -- C2HS is only used on that
>> C wrapper, not on the C++ library itself.  In general, C2HS doesn't support
>> C++ at all, because it needs to be able to parse the header files that it
>> includes and we use the C parser from the languagge-c package to do that.
>>  Any C++ constructs will just cause breakage.  And your "old school" FFI
>> definition doesn't look in any header files, which is why it has no trouble.
>>
>> Basically, if you want to do something like what hsqml does, just take a
>> look in the cbits directory there, in particular at hsqml.h which is the
>> header that defines the C wrapper around the C++ library.  This hsqml.h
>> header is the one that's included in the C2HS files.  If you want to wrap a
>> C++ library directly, C2HS probably isn't the tool for the job.  I know
>> there was some work during the last GSOC on a C++ FFI wrapper, but I don't
>> know if anything concrete came of it.
>>
>> Cheers,
>>
>> Ian.
>>
>>
>>
>> On 6 April 2014 20:22, Nick Rudnick <nick.rudnick at gmail.com> wrote:
>>
>>> Dear all,
>>>
>>> maybe this is a piece of c2hs behaviour I missed to understand, but it
>>> appears happening with C++ style includes deep in the header call hierarchy
>>> - in case of a such backend C++ library, if one wishes to refer to by *.h's
>>> in c2hs (e.g. for access to types of it), one might have a problem.
>>>
>>> - 8< cSample.h ---------------------------------------------
>>> #include <iostream> ///////// here, with c2hs, leading to build abort
>>> #ifdef __cplusplus
>>> extern "C" {
>>> #endif
>>> void sampleFunction();
>>> #ifdef __cplusplus
>>> }
>>> #endif
>>> - 8< cSample.cpp ---------------------------------------------
>>> #include "cSample.h"
>>>  #include <iostream> ///////// here no problem
>>>
>>> void sampleFunction(){
>>>   std::cout << "OK" << std::endl;
>>> }
>>> - 8< ----------------------------------------------------------------
>>>
>>> While 'old school' FFI (foreign import ccall) handles this effortlessly,
>>> c2hs uses to abort with an exception like
>>> C/cSample.h:3:20: fatal error: iostream: No such file or directory
>>> compilation terminated.
>>>
>>> I see that hsqml gets around this, but apparently by tweaking Setup.hs
>>> in a way not quite trivial - would this be necessary in any case, or are
>>> there simpler alternatives?
>>>
>>> Please excuse if I oversaw something, but I am afraid I didn't see this
>>> interesting issue covered elsewhere.
>>>
>>> Thank you a lot in advance & cheers, Nick
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> _______________________________________________
>>> Haskell-Cafe mailing list
>>> Haskell-Cafe at haskell.org
>>> http://www.haskell.org/mailman/listinfo/haskell-cafe
>>>
>>>
>>
>>
>> --
>> Ian Ross   Tel: +43(0)6804451378   ian at skybluetrades.net
>> www.skybluetrades.net
>>
>
>
>
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-- 
Ian Ross   Tel: +43(0)6804451378   ian at skybluetrades.net
www.skybluetrades.net
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