[Haskell-cafe] Dynamically Linking Foreign Functions
richard.warburton at gmail.com
Thu Dec 31 04:28:01 EST 2009
Apologies in advance for the length of this email, but I've tried to
be as clear as possible. Any help on the matter most appreciated.
Goal: I have a simple interpreter for a language that I've written,
and I wish to implement an FFI. My ideal api would be like 'ctypes'
in python. Here's an example:
from ctypes import *
libc = CDLL("libc.so.6")
libc.printf("An int %d, a double %f\n", 1234, c_double(3.14))
An int 1234, a double 3.140000
Implementation: I am currently trying to use the dynamic linker in
order to specify the library string, for example the following seems
import Monad (liftM)
type Fun = CString -> IO CInt
foreign import ccall unsafe "dynamic" c_printf :: FunPtr Fun -> Fun
printf :: String -> IO Int
printf str = do
withDL "libc.so.6" [RTLD_NOW] $ \dl -> do
printf_ptr <- dlsym dl "printf"
let fun = c_printf printf_ptr
liftM fromIntegral $ withCString str fun
main = printf "hello world"
Problem: I don't understand how I can generate the foreign import
statements at runtime. I am currently considering generating strings
of such expressions, and using hs-plugins in order to compile them.
This seems like a complete hack though - I'm dynamically loading a
haskell library in order to dynamically load a foreign library.
Another option is to write a dynamic loader in C, and then call this
from haskell, but how would I type the loading function - obviously I
can give it a CString for the library, and another for the function in
question - but what would the return type be?
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