When I make a function generator with "foreign export dynamic", GHC spits
out stub files which it apparently then needs for the link. What are
these for?
For instance:
--
foreign export dynamic mkCallback :: (Int -> IO Int) -> IO Addr
foreign import registerCallback :: Addr -> IO ()
exportCallback :: (Int -> IO Int) -> IO ()
exportCallback f = do
fx <- mkCallback f
registerCallback fx
--
And then I expect the C side of things to look like this:
--
typedef HsInt (*mkCallback_FunPtr) (HsInt arg1);
void registerCallback (HsAddr f)
{
mkCallback_FunPtr callBack = reinterpret_cast(f);
...
// actually call the callback
HsInt arg = 3;
HsInt result = (*callBack)(arg);
...
}
--
Is this correct? When I compile the Haskell Test.hs I get these strange
stub files, "Test_stub.h", "Test_stub.c" and "Test_stub.o". The _stub.h
one looks like this:
--
#include "HsFFI.h"
extern HsInt Test_dvD(HsStablePtr a0, HsAddr a_, HsInt a1);
--
...while the _stub.c looks like this:
--
#include "Stg.h"
#include "HsStd.h"
#include "HsLang.h"
#include "RtsAPI.h"
extern HaskellObj Test_zdfmkCallback_closure;
HsInt Test_dvD(HsStablePtr a0, HsAddr a_, HsInt a1)
{
SchedulerStatus rc;
HaskellObj ret;
rc=rts_evalIO(rts_apply(rts_apply((StgClosure*)&Test_zdfmkCallback_closure,
rts_mkStablePtr(a0)),rts_mkInt(a1)),&ret);
rts_checkSchedStatus("Test_dvD",rc);
return(rts_getInt(ret));
}
--
What are the stubs for? The GHC manual, sec. 8.4.1 says this:
# An FFI implementation is encouraged to generate the C
# typedef corresponding to a foreign export dynamic
# declaration, but isn't required to do so.
It doesn't say anything about funny little stub functions. And curiously,
it doesn't say exactly how GHC's FFI implementation responds to that
non-requirement encouragement. There were no typedefs in the stub files.
--
Ashley Yakeley, Seattle WA
