Haskell FFI local c header

Question

I want to call a C single header library with FFI.

Here's the Nuk.hs

{-# LANGUAGE CPP, ForeignFunctionInterface #-}
module Main where

import Foreign
import Foreign.C.Types

foreign import ccall unsafe "nuklear.h nk_sin"
 c_nk_sin:: IO CFloat

main = print $ c_nk_sin (5)

In the same directory, I have the nuklear.h

When I do stack ghc Nuk.hs, I get

[1 of 1] Compiling Main             ( Nuk.hs, Nuk.o )
Linking Nuk ...
Nuk.o:r1Rq_info: error: undefined reference to 'nk_sin'
collect2: error: ld returned 1 exit status
`gcc' failed in phase `Linker'. (Exit code: 1)

How should I solve this?

Answer 1

If it’s a header-only library, you need a C compiler to produce an object file for it so that your Haskell program can link to the definitions. According to the documentation for Nuklear, you can create a stub C file:

// nuklear.c
#define NK_IMPLEMENTATION
#include "nuklear.h"

As part of your build, compile this (e.g. with gcc -c nuklear.c -o nuklear.o) to produce an object file nuklear.o, use ar (e.g. ar -csr libnuklear.a nuklear.o) to create a static library libnuklear.a, then supposing this archive is in libs, you would add:

extra-lib-dirs: libs
extra-libraries: nuklear

To the executable section in your Cabal file. (Or the corresponding section in package.yml for hpack.)

To coordinate this, you might have a Makefile that builds this library and also invokes stack/cabal/ghc to produce the final build result. This tutorial covers the process in a little more explicit detail. But for this simple use case, you may be able to just use the c-sources section to do this from within Cabal:

c-sources: nuklear.c

You may also need to specify includes and include-dirs, e.g.:

includes: nuklear.h