Building a dynamic library with haskell and using it from C++

Question

I want to build a dynamic library containing haskell functions. I work on linux and want to call this dynamic library from C++ code.

I used the example at http://wiki.python.org/moin/PythonVsHaskell and have the following files:

Test.hs:

{-# LANGUAGE ForeignFunctionInterface #-}
module Test where

import Foreign.C.Types

hsfun :: CInt -> IO CInt
hsfun x = do
    putStrLn "Hello World"
    return (42 + x)

foreign export ccall
    hsfun :: CInt -> IO CInt

module_init.c:

#define CAT(a,b) XCAT(a,b)
#define XCAT(a,b) a ## b
#define STR(a) XSTR(a)
#define XSTR(a) #a

#include <HsFFI.h>

extern void CAT (__stginit_, MODULE) (void);

static void library_init (void) __attribute__ ((constructor));
static void
library_init (void)
{
  /* This seems to be a no-op, but it makes the GHCRTS envvar work. */
  static char *argv[] = { STR (MODULE) ".so", 0 }, **argv_ = argv;
  static int argc = 1;

  hs_init (&argc, &argv_);
  hs_add_root (CAT (__stginit_, MODULE));
}

static void library_exit (void) __attribute__ ((destructor));
static void
library_exit (void)
{
  hs_exit ();
}

Now I compile this files to a dynamic library:

$ ghc -dynamic -shared -fPIC -optc '-DMODULE=Test' Test.hs module_init.c -o libTest.so
[1 of 1] Compiling Test             ( Test.hs, Test.o )
Linking libTest.so ...

This creates among other things the file Test_stub.h:

#include "HsFFI.h"
#ifdef __cplusplus
extern "C" {
#endif
extern HsInt32 hsfun(HsInt32 a1);
#ifdef __cplusplus
}
#endif

and Test_stub.c:

#define IN_STG_CODE 0
#include "Rts.h"
#include "Stg.h"
#ifdef __cplusplus
extern "C" {
#endif

extern StgClosure Test_zdfhsfunzua165_closure;
HsInt32 hsfun(HsInt32 a1)
{
Capability *cap;
HaskellObj ret;
HsInt32 cret;
cap = rts_lock();
cap=rts_evalIO(cap,rts_apply(cap,(HaskellObj)runIO_closure,rts_apply(cap,&Test_zdfhsfunzua165_closure,rts_mkInt32(cap,a1))) ,&ret);
rts_checkSchedStatus("hsfun",cap);
cret=rts_getInt32(ret);
rts_unlock(cap);
return cret;
}
static void stginit_export_Test_zdfhsfunzua165() __attribute__((constructor));
static void stginit_export_Test_zdfhsfunzua165()
{getStablePtr((StgPtr) &Test_zdfhsfunzua165_closure);}
#ifdef __cplusplus
}
#endif

Then I create a cpp file main.cpp:

#include "Test_stub.h"

#include <iostream>

using namespace std;

int main()
{
    cout << hsfun(5);
}

and want to compile and link it. But when I call g++, it says:

$ g++ -I/usr/lib/ghc-7.0.3/include -L. -lTest main.cpp
/tmp/ccFP2AuB.o: In function `main':
main.cpp:(.text+0xa): undefined reference to `hsfun'
collect2: ld gab 1 als Ende-Status zurück

So I added the Test_stub.o file to the command line (although I think the hsfun function should already be defined in libTest.so which is added via the -lTest parameter. I don't think, I should link the Test_stub.o file into the executable because I want to use dynamic linking), but this also doesn't work:

$ g++ -I/usr/lib/ghc-7.0.3/include -L. -lTest main.cpp Test_stub.o
Test_stub.o: In function `hsfun':
Test_stub.c:(.text+0x9): undefined reference to `rts_lock'
Test_stub.c:(.text+0x16): undefined reference to `rts_mkInt32'
Test_stub.c:(.text+0x1d): undefined reference to `Test_zdfhsfunzua165_closure'
Test_stub.c:(.text+0x28): undefined reference to `rts_apply'
Test_stub.c:(.text+0x2f): undefined reference to `base_GHCziTopHandler_runIO_closure'
Test_stub.c:(.text+0x3a): undefined reference to `rts_apply'
Test_stub.c:(.text+0x4a): undefined reference to `rts_evalIO'
Test_stub.c:(.text+0x5c): undefined reference to `rts_checkSchedStatus'
Test_stub.c:(.text+0x66): undefined reference to `rts_getInt32'
Test_stub.c:(.text+0x70): undefined reference to `rts_unlock'
Test_stub.o: In function `stginit_export_Test_zdfhsfunzua165':
Test_stub.c:(.text.startup+0x3): undefined reference to `Test_zdfhsfunzua165_closure'
Test_stub.c:(.text.startup+0x8): undefined reference to `getStablePtr'
collect2: ld gab 1 als Ende-Status zurück

Do I have to link the Test_stub.o? If yes, why? And which arguments should I pass to the linker?

I don't know the details; but you certainly need to link the Haskell runtime too (in particular its garbage collector). — Basile Starynkevitch
– Basile Starynkevitch, Commented Dec 31, 2011 at 19:27
"$ g++ -I/usr/lib/ghc-7.0.3/include -L. -lTest main.cpp" perhaps it works if you put the linker flags at the end of the command line? — Daniel Fischer
– Daniel Fischer, Commented Dec 31, 2011 at 19:58
@Daniel: That's the second time I see someone suggest to put linker flags at the end, and the first time it seemed to solve the problem too. Why's that? I thought the positioning of the flags doesn't matter? — Xeo
– Xeo, Commented Dec 31, 2011 at 20:28
@Xeo I don't know if the flag order is important for g++ (except later -O options overriding earlier and such), but it's for gcc, so I thought it's worth a try. As far as I can tell, it didn't solve the problem, what worked was using ghc to do the job, that knows how to call the linker. — Daniel Fischer
– Daniel Fischer, Commented Dec 31, 2011 at 20:35

Daniel Fischer · Accepted Answer · 2011-12-31 20:25:03Z

9

Probably easier than wrestling with g++ is letting ghc do the work,

ghc main.cpp -o hithere -L. -lTest -lstdc++

did the job for me after creating the shared lib the way you did. I have tested it with 7.2.2 and 7.0.2, both worked here.

answered Dec 31, 2011 at 20:25

Daniel Fischer

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3 Comments

Heinzi Over a year ago

The haskell interface is one module of a larger project and I don't want to compile the whole C++ project with ghc.

Daniel Fischer Over a year ago

That makes sense. You could try capturing the necessary linker flags by having ghc compile the example with high enough verbosity and redirecting stderr to a file. It will give you a pretty long list of to-be-linked items, though.

Heinzi Over a year ago

I got it working using your solution. When I pass the parameter -v to ghc, it prints the used linker parameters on the command line.

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