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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
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<title>The GHC Commentary - foreign export</title>
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<h1>The GHC Commentary - foreign export</h1>
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.
<p>
<b>(1) static export of an IO-typed function from some module <code>MMM</code></b>
<p>
<code>foreign export foo :: Int -> Int -> IO Int</code>
<p>
For this we generate no Haskell code. However, a C stub is
generated, and it looks like this:
<p>
<pre>
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));
}
</pre>
<p>
This does the obvious thing: builds in the heap the expression
<code>(foo a1 a2)</code>, calls <code>rts_evalIO</code> to run it,
and uses <code>rts_getInt</code> to fish out the result.
<p>
<b>(2) static export of a non-IO-typed function from some module <code>MMM</code></b>
<p>
<code>foreign export foo :: Int -> Int -> Int</code>
<p>
This is identical to case (1), with the sole difference that the
stub calls <code>rts_eval</code> rather than
<code>rts_evalIO</code>.
<p>
<b>(3) dynamic export of an IO-typed function from some module <code>MMM</code></b>
<p>
<code>foreign export mkCallback :: (Int -> Int -> IO Int) -> IO (FunPtr a)</code>
<p>
Dynamic exports are a whole lot more complicated than their static
counterparts.
<p>
First of all, we get some Haskell code, which, when given a
function <code>callMe :: (Int -> Int -> IO Int)</code> to be made
C-callable, IO-returns a <code>FunPtr a</code>, 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 <code>callMe</code>.
<p>
The generated Haskell function looks like this:
<p>
<pre>
mkCallback f
= do sp <- mkStablePtr f
r <- ccall "createAdjustorThunk" sp (&"run_mkCallback")
return r
</pre>
<p>
<code>createAdjustorThunk</code> 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 <code>f</code>.
<p>
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 <code>run_mkCallback</code>, passing all the args given
to it but also conveying <code>sp</code>, which is a stable
pointer
to the Haskell function to run. So:
<p>
<pre>
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
}
</pre>
<p>
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 <code>sp</code>,
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 <code>createAdjustorThunk</code> implementation for Sparc
and Alpha, which handles only the cases where all args, including
the extra one, fit in registers.
<p>
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:
<pre>
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));
}
</pre>
<p>
Note how this function has a purely made-up name
<code>Main_d1kv</code>, since unlike the f-x-static case, this
function is never called from user code, only from the adjustor
thunk.
<p>
Note also how the function takes a bogus parameter
<code>original_return_addr</code>, 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.
<p>
Finally, there is some extra trickery, detailed in
<code>ghc/rts/Adjustor.c</code>, to get round the following
problem: the adjustor thunk lives in mallocville. It is
quite possible that the Haskell code will actually
call <code>free()</code> 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 <code>free()</code>.
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.
<p>
That's how the <code>stdcall</code> convention works. If the
adjustor thunk has been called using the <code>ccall</code>
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 <code>ccall</code>-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.
<p>
You didn't really want to know all this stuff, did you?
<p>
<b>(4) dynamic export of an non-IO-typed function from some module <code>MMM</code></b>
<p>
<code>foreign export mkCallback :: (Int -> Int -> Int) -> IO (FunPtr a)</code>
<p>
(4) relates to (3) as (2) relates to (1), that is, it's identical,
except the C stub uses <code>rts_eval</code> instead of
<code>rts_evalIO</code>.
<p>
<h2>Some perspective on f-x-dynamic</h2>
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.
<p>
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.
<p>
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.
<p>
<b>OS-thread-safety</b> 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
<code>deRefStablePtr</code> call and when <code>rts_eval[IO]</code>
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.
<p><small>
<!-- hhmts start -->
Last modified: Weds 27 Feb 02
<!-- hhmts end -->
</small>
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