Re: [Haskell-cafe] Wrapping a C++ library

Hi Arjun, I've been doing quite a bit of this kind of thing recently, but it took me a while to figure it out too. I've made a little example at https://github.com/ian-ross/cpp-ffi-example -- just clone the repo and follow the instructions in the README. The example has a C++ test library to build that doesn't do anything very interesting but provides some things to bind to. It uses a C wrapper around the C++ library and C2HS to write the Haskell FFI side of things (you can almost certainly use hsc2hs in the same kind of way). There's also a small test program so you see the bindings working end-to-end. I guess the most critical thing is using extern "C" and the __cplusplus compiler macro to control the visibility of C++ code to GCC -- take a look at cbits/a.h and cbits/a.cpp in the repo to see what I mean. Feel free to ask if you have any questions. Cheers, Ian. On 26 January 2015 at 23:55, Arjun Comar wrote:

Alexey, It's unfortunately been about a year since I've worked on the project that needed FFI support so I don't recall the precise issue any longer. From talking to people in #haskell, I discovered that it would be necessary to call g++ and not gcc because gcc is unable to locate some headers and libraries that inevitably get included by C++ projects.

Thanks, Arjun

On Mon, Jan 26, 2015 at 5:49 PM, Alexey Shmalko wrote:

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Arjun,

I'm not an expert in writing Haskell binding but pretty much experienced in C/low-level stuff.

You shouldn't force compilation of .c files with g++. Instead, you should keep your wrapping functions as extern "C" and place their implementation in .cpp file (which of course should be compiled with g++). Don't forget to link to C++ runtime library (-lstdc++).

There is also a wiki page[1] that describes how to call C++ from Haskell.

Best regards, Alexey

[1]: https://wiki.haskell.org/CPlusPlusFromHaskell

2015-01-26 23:27 GMT+02:00 Arjun Comar :

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Richard, If you create a straight C header that's able to call into the C++ library then what you're trying to do will work. This is the standard (though frustrating) approach to using C++ libraries within Haskell. I can send you an example cabal file for building the Haskell code against the C headers and archives if you'd like. There isn't much more to it than what you're already doing though. The key is that the headers have to be pure C and you have to force cabal to build the C code with g++ (since the implementing .c files will necessarily make C++ calls).

Thanks, Arjun

On Mon, Jan 26, 2015 at 11:29 AM, Donn Cave wrote:

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Quoth Richard Lewis , ...

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There's a specific question here which goes something like: how do I get hsc2hs to compile code that uses STL headers?

For me, that would be the first thing to fix. Use #ifdef __cplusplus, or just reorganize, so the hsc2hs-generated files are just C. You wrote your wrapper library for the C++ part, and if the callers need to be C++ it kind of defeats the purpose.

From there, I would have guessed "extra-libraries: stdc++" would do it, (assuming you have also arranged to get your wrapper library in there), but I'm no cabal expert. I use C++ more or less like you're doing, so there's hope, but I haven't gotten around to the cabal part of the exercise.

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