[Haskell-cafe] building c-callable shared objects and dlls with Haskell

[Anton Tayanovskyy]

Hello,

What's the current best practice to build shared objects / dlls to
call Haskell from C? I think I figured out the basics, but I would
like to understand the picture completely and document  the current
state of affairs in the Haskell wiki. In particular, I'd like to make
sure that the shared libraries are usable from multiple applications,
and ideally that applications can dynamically link to multiple shared
libraries built from Haskell.

I was trying to do that to call Pandoc from C, and the information was
rather scattered.

Here are two links that I used:

http://www.haskell.org/haskellwiki/Calling_Haskell_from_C
http://www.haskell.org/ghc/docs/latest/html/users_guide/win32-dlls.html

Based on that information, I was able to compile and run small
examples. However, I'm not sure I completely understand how it works
in production.

Specifically, for the UNIX version, I followed a recipe for loading
and unloading the Haskell runtime with library
constructors/destructors:

#include <HsFFI.h>

static void library_init(void) __attribute__((constructor));
static void library_init(void) {  hs_init(..); }

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

Does this work correctly? Another concern is that every library
compiled this way will include its own copy of the Haskell runtime.
Will there be problems when using multiple libraries?

For the Windows counterpart, there is this very suspicious code I
adapted from the GHC user guide:

#include <windows.h>
#include <Rts.h>

extern void __stginit_LibPandoc(void);

BOOL STDCALL DllMain( HANDLE hModule, DWORD reason, void* reserved) {
 if (reason == DLL_PROCESS_ATTACH) {
   static char*  args[] = { "ghcDll", NULL };
   startupHaskell(1, args, __stginit_LibPandoc);
 }
 return TRUE;
}

It does not seem to unload the Haskell runtime, indeed the Guide warns
that it is unsafe to do so from DllMain. What are the implications of
this?

Thanks!


--A