Re: [Haskell-beginners] Linking C++ in Cabal

26 May 2020

      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  wrote:
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1. Re:  Linking C++ in Cabal - repository Link (Leonhard Applis)
   2. Re:  Linking C++ in Cabal (鲍凯文)
----------------------------------------------------------------------
Message: 1
Date: Mon, 25 May 2020 13:00:03 +0000
From: Leonhard Applis 
To: "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,  wrote:
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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
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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

鲍凯文

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