Re: [Haskell-beginners] Linking C++ in Cabal
Hi, So I tried it out, and there're a couple of things: - Declaring a function as `static` will hide it from anything other than the current translation unit, AFAIK, which means that you won't be able to link it to anything else; the symbol won't be visible (you can verify using `objdump`). - For C++ functions, you need to suppress name-mangling since Haskell's FFI doesn't know about that. With your repo as is, the symbol corresponding to the `multiply` function is not...`multiply`: ``` dist-newstyle/build/x86_64-linux/ghc-8.6.5/HaskellForeignCPP-0.0.1/build/cpp-src/some.o: file format elf64-x86-64 SYMBOL TABLE: 0000000000000000 l df *ABS* 0000000000000000 some.cpp 0000000000000000 l d .text 0000000000000000 .text 0000000000000000 l d .data 0000000000000000 .data 0000000000000000 l d .bss 0000000000000000 .bss 0000000000000000 l d .note.GNU-stack 0000000000000000 .note.GNU-stack 0000000000000000 l d .eh_frame 0000000000000000 .eh_frame 0000000000000000 l d .comment 0000000000000000 .comment 0000000000000000 g F .text 0000000000000006 _Z8multiplyii ``` so you just need to wrap every exposed function in `extern "C"` blocks, like it's explained here: https://www.geeksforgeeks.org/extern-c-in-c/. With `extern "C"` and removing `static`: ``` dist-newstyle/build/x86_64-linux/ghc-8.6.5/HaskellForeignCPP-0.0.1/build/cpp-src/some.o: file format elf64-x86-64 SYMBOL TABLE: 0000000000000000 l df *ABS* 0000000000000000 some.cpp 0000000000000000 l d .text 0000000000000000 .text 0000000000000000 l d .data 0000000000000000 .data 0000000000000000 l d .bss 0000000000000000 .bss 0000000000000000 l d .note.GNU-stack 0000000000000000 .note.GNU-stack 0000000000000000 l d .eh_frame 0000000000000000 .eh_frame 0000000000000000 l d .comment 0000000000000000 .comment 0000000000000000 g F .text 0000000000000006 multiply ``` ...and everything builds fine. Best, toz On Tue, May 26, 2020 at 5:03 AM <beginners-request@haskell.org> wrote:
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Today's Topics:
1. Re: Linking C++ in Cabal - repository Link (Leonhard Applis) 2. Re: Linking C++ in Cabal (鲍凯文)
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Message: 1 Date: Mon, 25 May 2020 13:00:03 +0000 From: Leonhard Applis <Leonhard.Applis@protonmail.com> To: "beginners@haskell.org" <beginners@haskell.org> Subject: Re: [Haskell-beginners] Linking C++ in Cabal - repository Link Message-ID:
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Ups, in my mailing list the links have disappeared. Just in case:
The minimal reproducing repository:
https://github.com/Twonki/HaskellForeingCPPMinimal
The repository from Aditya Siram with C and makefile:
https://github.com/deech/CPlusPlusBindings
best Leonhard
‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐ On Monday, May 25, 2020 2:00 PM, <beginners-request@haskell.org> wrote:
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Today's Topics:
1. Linking C++ in Cabal (Leonhard Applis)
Message: 1 Date: Sun, 24 May 2020 16:30:22 +0000 From: Leonhard Applis Leonhard.Applis@protonmail.com To: "beginners@haskell.org" beginners@haskell.org Subject: [Haskell-beginners] Linking C++ in Cabal Message-ID:
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Hello,
I want to use a foreign C++ Function in a Haskell Library, and the Library in a Haskell Program. The C++ function is separate in a file and I have a header file. I made a minimal reproducing repository.
The closes I have found is this repository which wraps the c++ in c and makes a full library, needing a makefile etc. So while I think I'd get it working like that, it looks to be quite a lot to do for my simple task.
the repository structure looks as follows:
Projekt
- cpp-src - some.cpp - some.h - lib-src - CPPLib.hs - program-src - program.hs - project.cabal
And my (currently failing) project.cabal the following:
cabal-version: 3.0 name: cpp_lib [...] library exposed-modules: CPPLib other-extensions: ForeignFunctionInterface build-depends: base hs-source-dirs: lib-src default-language: Haskell2010 include-dirs: ./cpp-src cxx-sources: ./cpp-src/some.cpp install-includes: ./cpp-src/some.h
executable CPPApp main-is: program.hs default-language: Haskell2010 build-depends: HaskellForeignCPP, base hs-source-dirs: program-src
The some.cpp is trivial for this case lets assume its a function
multiply :: Int -> Int -> Int, and both .cpp and .h are well-formed and don't have further dependencies.
The library compiles fine, but when building the executable it fails telling me
collect2: error: ld returned 1 exit status `gcc' failed in phase`Linker'. (Exit code: 1)
Can someone help me out? I find very little documentation on this topic.
I know there is Inline-CPP which would help as well, but I just very much like the idea of having a nice separate file and interface for modularity.
best Leonhard
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