# The GHC Commentary - foreign export

The implementation scheme for foreign export, as of 27 Feb 02, is as follows. There are four cases, of which the first two are easy.

(1) static export of an IO-typed function from some module `MMM`

`foreign export foo :: Int -> Int -> IO Int`

For this we generate no Haskell code. However, a C stub is generated, and it looks like this:

    extern StgClosure* MMM_foo_closure;

    HsInt foo (HsInt a1, HsInt a2)
    {
       SchedulerStatus rc;
       HaskellObj ret;
       rc = rts_evalIO(
               rts_apply(rts_apply(MMM_foo_closure,rts_mkInt(a1)),
                         rts_mkInt(a2)
                        ),
               &ret
            );
       rts_checkSchedStatus("foo",rc);
       return(rts_getInt(ret));
    }

This does the obvious thing: builds in the heap the expression `(foo a1 a2)`, calls `rts_evalIO` to run it, and uses `rts_getInt` to fish out the result.

(2) static export of a non-IO-typed function from some module `MMM`

`foreign export foo :: Int -> Int -> Int`

This is identical to case (1), with the sole difference that the stub calls `rts_eval` rather than `rts_evalIO`.

(3) dynamic export of an IO-typed function from some module `MMM`

`foreign export mkCallback :: (Int -> Int -> IO Int) -> IO (FunPtr a)`

Dynamic exports are a whole lot more complicated than their static counterparts.

First of all, we get some Haskell code, which, when given a function `callMe :: (Int -> Int -> IO Int)` to be made C-callable, IO-returns a `FunPtr a`, which is the address of the resulting C-callable code. This address can now be handed out to the C-world, and callers to it will get routed through to `callMe`.

The generated Haskell function looks like this:

    mkCallback f
      = do sp <- mkStablePtr f
           r  <- ccall "createAdjustorThunk" sp (&"run_mkCallback")
           return r

`createAdjustorThunk` is a gruesome, architecture-specific function in the RTS. It takes a stable pointer to the Haskell function to be run, and the address of the associated C wrapper, and returns a piece of machine code, which, when called from the outside (C) world, eventually calls through to `f`.

This machine code fragment is called the "Adjustor Thunk" (don't ask me why). What it does is simply to call onwards to the C helper function `run_mkCallback`, passing all the args given to it but also conveying `sp`, which is a stable pointer to the Haskell function to run. So:

    createAdjustorThunk ( StablePtr sp, CCodeAddress addr_of_helper_C_fn )
    {
       create malloc'd piece of machine code "mc", behaving thusly:

       mc ( args_to_mc )
       {
          jump to addr_of_helper_C_fn, passing sp as an additional
          argument
       }

This is a horrible hack, because there is no portable way, even at the machine code level, to function which adds one argument and then transfers onwards to another C function. On x86s args are pushed R to L onto the stack, so we can just push `sp`, fiddle around with return addresses, and jump onwards to the helper C function. However, on architectures which use register windows and/or pass args extensively in registers (Sparc, Alpha, MIPS, IA64), this scheme borders on the unviable. GHC has a limited `createAdjustorThunk` implementation for Sparc and Alpha, which handles only the cases where all args, including the extra one, fit in registers.

Anyway: the other lump of code generated as a result of a f-x-dynamic declaration is the C helper stub. This is basically the same as in the static case, except that it only ever gets called from the adjustor thunk, and therefore must accept as an extra argument, a stable pointer to the Haskell function to run, naturally enough, as this is not known until run-time. It then dereferences the stable pointer and does the call in the same way as the f-x-static case:

    HsInt Main_d1kv ( StgStablePtr the_stableptr,
                      void* original_return_addr,
                      HsInt a1, HsInt a2 )
    {
       SchedulerStatus rc;
       HaskellObj ret;
       rc = rts_evalIO(
               rts_apply(rts_apply((StgClosure*)deRefStablePtr(the_stableptr),
                                   rts_mkInt(a1)
                         ),
                         rts_mkInt(a2)
               ),
               &ret
            );
       rts_checkSchedStatus("Main_d1kv",rc);
       return(rts_getInt(ret));
    }

Note how this function has a purely made-up name `Main_d1kv`, since unlike the f-x-static case, this function is never called from user code, only from the adjustor thunk.

Note also how the function takes a bogus parameter `original_return_addr`, which is part of this extra-arg hack. The usual scheme is to leave the original caller's return address in place and merely push the stable pointer above that, hence the spare parameter.

Finally, there is some extra trickery, detailed in `ghc/rts/Adjustor.c`, to get round the following problem: the adjustor thunk lives in mallocville. It is quite possible that the Haskell code will actually call `free()` on the adjustor thunk used to get to it -- because otherwise there is no way to reclaim the space used by the adjustor thunk. That's all very well, but it means that the C helper cannot return to the adjustor thunk in the obvious way, since we've already given it back using `free()`. So we leave, on the C stack, the address of whoever called the adjustor thunk, and before calling the helper, mess with the stack such that when the helper returns, it returns directly to the adjustor thunk's caller.

That's how the `stdcall` convention works. If the adjustor thunk has been called using the `ccall` convention, we return indirectly, via a statically-allocated yet-another-magic-piece-of-code, which takes care of removing the extra argument that the adjustor thunk pushed onto the stack. This is needed because in `ccall`-world, it is the caller who removes args after the call, and the original caller of the adjustor thunk has no way to know about the extra arg pushed by the adjustor thunk.

You didn't really want to know all this stuff, did you?

(4) dynamic export of an non-IO-typed function from some module `MMM`

`foreign export mkCallback :: (Int -> Int -> Int) -> IO (FunPtr a)`

(4) relates to (3) as (2) relates to (1), that is, it's identical, except the C stub uses `rts_eval` instead of `rts_evalIO`.

## Some perspective on f-x-dynamic

The only really horrible problem with f-x-dynamic is how the adjustor thunk should pass to the C helper the stable pointer to use. Ideally we would like this to be conveyed via some invisible side channel, since then the adjustor thunk could simply jump directly to the C helper, with no non-portable stack fiddling.

Unfortunately there is no obvious candidate for the invisible side-channel. We've chosen to pass it on the stack, with the bad consequences detailed above. Another possibility would be to park it in a global variable, but this is non-reentrant and non-(OS-)thread-safe. A third idea is to put it into a callee-saves register, but that has problems too: the C helper may not use that register and therefore we will have trashed any value placed there by the caller; and there is no C-level portable way to read from the register inside the C helper.

In short, we can't think of a really satisfactory solution. I'd vote for introducing some kind of OS-thread-local-state and passing it in there, but that introduces complications of its own.

OS-thread-safety is of concern in the C stubs, whilst building up the expressions to run. These need to have exclusive access to the heap whilst allocating in it. Also, there needs to be some guarantee that no GC will happen in between the `deRefStablePtr` call and when `rts_eval[IO]` starts running. At the moment there are no guarantees for either property. This needs to be sorted out before the implementation can be regarded as fully safe to use.

Last modified: Weds 27 Feb 02