Roughly following this tutorial, I managed to get this toy project working. It calls a Haskell function from a C++ program.
Foo.hs
{-# LANGUAGE ForeignFunctionInterface #-}
module Foo where
foreign export ccall foo :: Int -> Int -> IO Int
foo :: Int -> Int -> IO Int
foo n m = return . sum $ f n ++ f m
f :: Int -> [Int]
f 0 = []
f n = n : f (n-1)
bar.c++
#include "HsFFI.h"
#include FOO // Haskell module (path defined in build script)
#include <iostream>
int main(int argc, char *argv[]) {
hs_init(&argc, &argv);
std::cout << foo(37, 19) << "\n";
hs_exit();
return 0;
}
call-haskell-from-cxx.cabal
name: call-haskell-from-cxx
version: 0.1.0.0
build-type: Simple
cabal-version: >=1.10
executable foo.so
main-is: Foo.hs
build-depends: base >=4.10 && <4.11
ghc-options: -shared -fPIC -dynamic
extra-libraries: HSrts-ghc8.2.1
default-language: Haskell2010
build script
#!/bin/bash
hs_lib="foo.so"
hs_obj="dist/build/$hs_lib/$hs_lib"
ghc_version="8.2.1" # May need to be tweaked,
ghc_libdir="/usr/local/lib/ghc-$ghc_version" # depending on system setup.
set -x
cabal build
g++ -I "$ghc_libdir/include" -D"FOO=\"${hs_obj}-tmp/Foo_stub.h\"" -c bar.c++ -o test.o
g++ test.o "$hs_obj" \
-L "$ghc_libdir/rts" "-lHSrts-ghc$ghc_version" \
-o test
env LD_LIBRARY_PATH="dist/build/$hs_lib:$ghc_libdir/rts:$LD_LIBRARY_PATH" \
./test
This works (Ubuntu 16.04, GCC 5.4.0), printing 893 – but it isn't really robust, namely, if I remove the actual invocation of the Haskell function, i.e. the std::cout << foo(37, 19) << "\n"; line, then it fails at the linking step, with the error message
/usr/local/lib/ghc-8.2.1/rts/libHSrts-ghc8.2.1.so: undefined reference to `base_GHCziTopHandler_flushStdHandles_closure'
/usr/local/lib/ghc-8.2.1/rts/libHSrts-ghc8.2.1.so: undefined reference to `base_GHCziStable_StablePtr_con_info'
/usr/local/lib/ghc-8.2.1/rts/libHSrts-ghc8.2.1.so: undefined reference to `base_GHCziPtr_FunPtr_con_info'
/usr/local/lib/ghc-8.2.1/rts/libHSrts-ghc8.2.1.so: undefined reference to `base_GHCziWord_W8zh_con_info'
/usr/local/lib/ghc-8.2.1/rts/libHSrts-ghc8.2.1.so: undefined reference to `base_GHCziIOziException_cannotCompactPinned_closure'
...
Apparently, the inclusion of the Haskell project pulls additional library files in that are needed. How do I explicitly depend on everything necessary, to avoid such brittleness?
Output of the build script when the foo call is included, with ldd on the final executable:
++ cabal build
Preprocessing executable 'foo.so' for call-haskell-from-C-0.1.0.0..
Building executable 'foo.so' for call-haskell-from-C-0.1.0.0..
Linking a.out ...
Linking dist/build/foo.so/foo.so ...
++ g++ -I /usr/local/lib/ghc-8.2.1/include '-DFOO="dist/build/foo.so/foo.so-tmp/Foo_stub.h"' -c bar.c++ -o test.o
++ g++ test.o dist/build/foo.so/foo.so -L /usr/local/lib/ghc-8.2.1/rts -lHSrts-ghc8.2.1 -o test
++ env LD_LIBRARY_PATH=dist/build/foo.so:/usr/local/lib/ghc-8.2.1/rts: sh -c 'ldd ./test; ./test'
linux-vdso.so.1 => (0x00007fff23105000)
foo.so => dist/build/foo.so/foo.so (0x00007fdfc5360000)
libHSrts-ghc8.2.1.so => /usr/local/lib/ghc-8.2.1/rts/libHSrts-ghc8.2.1.so (0x00007fdfc52f8000)
libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007fdfc4dbe000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fdfc49f4000)
libHSbase-4.10.0.0-ghc8.2.1.so => /usr/local/lib/ghc-8.2.1/base-4.10.0.0/libHSbase-4.10.0.0-ghc8.2.1.so (0x00007fdfc4020000)
libHSinteger-gmp-1.0.1.0-ghc8.2.1.so => /usr/local/lib/ghc-8.2.1/integer-gmp-1.0.1.0/libHSinteger-gmp-1.0.1.0-ghc8.2.1.so (0x00007fdfc528b000)
libHSghc-prim-0.5.1.0-ghc8.2.1.so => /usr/local/lib/ghc-8.2.1/ghc-prim-0.5.1.0/libHSghc-prim-0.5.1.0-ghc8.2.1.so (0x00007fdfc3b80000)
libgmp.so.10 => /usr/lib/x86_64-linux-gnu/libgmp.so.10 (0x00007fdfc3900000)
libffi.so.6 => /usr/local/lib/ghc-8.2.1/rts/libffi.so.6 (0x00007fdfc36f3000)
libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007fdfc33ea000)
librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007fdfc31e2000)
libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007fdfc2fde000)
libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fdfc2dc1000)
/lib64/ld-linux-x86-64.so.2 (0x00007fdfc5140000)
libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007fdfc2bab000)
Usually ghc links executables with -Wl,--no-as-needed option, and you should use it too. (You can check how ghc links executable e.g. using cabal build --ghc-options=-v3.)
You can find more details here. My understanding it the next: foo.so requires libHSbase-4.10.0.0-ghc8.2.1.so to be loaded at runtime as needed, i.e. when we need symbol from it (check readelf -a dist/build/foo.so/foo.so | grep NEEDED). So if you don't call foo, then base.so is not loaded. But ghc needs all libraries to be loaded (I don't know why). The --no-as-needed option forces all libraries to be loaded.
Note that --no-as-needed options is position-dependent, so put it before the shared library.